Crude oil is what you have after natural gas has been separated from fluid pumped out of a well. It includes oil, other petroleum products such as paraffin, and water. The crude oil system and lines are everything in the tank battery after the separator, including stock tanks, heater-treaters, and other tanks and lines. When you’re laying out a tank battery for your operation, it’s helpful to keep a few standards and regulations, as well as some useful tips, in mind.

Standard practice is to have a line piping the separator’s output directly to the stock tank. You can use a diverter manifold to add line openings to specific tanks and equipment, like a gun barrel or heater-treater. That allows you to choose whether to send the oil to a stock tank, or first run it through some additional steps to remove more water.

Figure 1. A diagram of a tank battery with crude oil lines marked with an O.

Specialized Vessels

Once crude oil has been diverted into a specialized vessel like a heater-treater, it will often have to follow a separate path back to where it is held in the stock tanks. When you’re producing fluid that contains a low to medium weight oil and has heavier petroleum products, a heater-treater is most likely to be the first specialized vessel.

The Heater-Treater

As a heater-treater may sometimes burn gas to heat crude oil as part of the separation process, it has some special safety considerations. First, the heater-treater’s firebox should be fitted with a flame arrestor. In addition to this, a heater-treater should be at least 100 feet from the nearest hatch through which gas could escape. That means you’ll need to lay out a little more line when placing a heater-treater.

Figure 2. Heater-treaters and their inlet and outlet lines. Though it’s hard to tell in a black-and-white photo, the lines are color coded so they can be easily identified.

Figure 2 shows a couple of standard heater-treaters with their inlet and outlet lines. On the left is a heater-treater built according to a standard layout, where it’s fairly straightforward to identify the lines. The inlet would be the highest line on the right side, and the second highest line being an oil outlet. The water drain is near the bottom on the left, and the gas line is seen at the top. The placement of these lines is determined by their use and the operation of the vessel.

The heater-treater to the left has lines all in a single row. Starting at the right, the lines are: the inlet, the gas line, oil outlet, and the line leading to the water disposal tank. While it may be hard to figure out which line is which by just looking at the picture, where all you can see is the tank, it’s usually much easier to understand each line in the context of the tank battery. Often, you can follow a line from one vessel to the next to understand its purpose. Even when the line is buried, it’s usually possible to understand what it’s being used for. It may require you to access the buried line and, with a flashlight appropriate for working around oil and gas, trace the lines and outlets.

The operation of a standard heater-treater is fairly straightforward. Crude oil flows in through the inlet, and is directed through a downcomer near the bottom. Oil rises, past the heating element, and through a number of plates that act as baffles. At each step, water falls to the bottom where it is drained, and oil floats to the top and then flows out of the tank. Horizontal heater-treaters operate somewhat differently, using electricity to increase the vessel’s efficiency (don’t fret! GreaseBook has you covered!! To learn more about these horizontal heater-treaters, check out our post here: Basic Methods of Treating In Oil & Gas Production).

The Gun Barrel

A gun barrel is used to remove additional water from the oil. A line can be run to an inlet at the top of the gun barrel. Another arrangement that is popular, and increases the efficiency of a gun barrel somewhat, is to use an external side boot. Figure 3 shows an example of the side boot setup. With that, oil comes from the line, enters the bottom of the 2-inch pipe, and flows up to the larger container at the top. That’s the flash separator, where more gas is separated from the fluid. The fluid then travels down to enter the gun barrel about 12 inches from the bottom. In Figure 3, you can see that the side boot has a number of additional braces so that the gun barrel is supporting the top.

Figure 3. Gun barrel with a side boot inlet.

Once the oil enters the gun barrel, it hits a spreader which disperses the oil across the tank. Oil floats to the top while water sinks to the bottom and is drained.

The Stock Tanks

Once the crude oil has finished its trip through any specialized vessels, it is routed back to a stock tank for holding. Most often, the outlet line from the gun barrel will lead right to a stock tank. The distance for this is generally short, as it’s helpful to keep these tanks all in close proximity for a number of reasons. In Figure 4, you can see an example of a gravity feed line from a gun barrel to a stock tank. You’ll want to make sure this line is big enough to handle your production volumes.

Figure 4. A line flowing from a gun barrel to a stock tank. The line is closed in this case, as the valve’s handle is perpendicular to the pipe.

Stock Tanks and Selling

Stock tanks are where oil is held, the last place it’s routed to on the lease. It seems like it should just stay there and stop moving around at that point, but in reality it can be helpful to put equalizer lines across your stock tanks. These are lines that protrude from high on the side of the tank, and allow oil to flow between two tanks. If one stock tank is full, rather than overflowing, the oil is sent through the equalizer line to the next stock tank. These lines also make it easy to automatically top off each tank, as one tank will fill completely before oil flows to the next.

Figure 5. The first type of equalizer line, which runs directly to the next stock tank.

Figure 6. The second sort of equalizer line, where oil can be directed to specific tanks.

In the two pictures, you can see two methods of adding equalizer lines to your stock tanks. With the method Figure 5 shows, oil can only flow to the stock tank that’s directly connected. For the second arrangement, valves can be opened and closed to send oil to a specific tank. This method requires more room for piping and valves, and isn’t always possible due to the tight spaces created by walkways, lines, and other tanks.

The goal, ultimately, is to produce oil and get it in a state fit to be sold. When that happens, there’s a bit of a process that has to be followed. A number of valves are required to be locked closed to isolate the stock tank, a seal has to be inserted.

Seals come in two general types. One locks to itself, and the other uses wire and a soft lead seal. Both the inlet for oil and the drain line will most likely be sealed for the selling process. A seal will have some information on it, such as a serial number and the company buying the oil.

Once the oil in the sales tank has been sold, everything is returned to normal operating state, and a seal is again placed on the sales outlet. Keeping track of the seals is vital, as they will need to be kept track of and occasionally returned to the pipeline company.

Is your appetite for oil & gas operating knowledge insatiable like ours? 😀 If so, check out these related articles below – they’ll be sure to pump you up!!!

• The Basics of Setting Up an Oil & Gas Production Tank Battery
• Separators, Heater-treaters, and Pressure in Oil & Gas Production
• Tanks At Atmospheric Pressure in Oil & Gas Production
• Handling Emulsion in Oil & Gas Production
• Water Systems In Gas & Oil Production
• How To Sell Your Oil & Gas Production
• Tank Battery Design In Oil & Gas Production

Emulsion is the fluid produced from the well: the mixture of oil, water, gas, and other products. It generally refers specifically to the fluid that comes directly from the well, before any separation. After gas has been separated out, the oil and water that remain is called crude oil. Water will be separated next on it’s way to being sold.

Emulsion and the Tank Battery

The specifics of what is being pumped from the well determine what sort of tanks and lines you’ll need for a tank battery. The amount you’re pumping will determine the size of the tanks you’ll need. Other factors you’ll want to keep in mind when assembling a tank battery include:

• The number of wells feeding into the tank battery
• The crude oil’s API gravity
• The volume of water coming from the well daily
• Paraffin production
• How long the well is estimated to operate
• Other issues particular to wells and reservoirs

Separating water from oil is a major issue, and one of the more important factors in determining what vessels you’ll need. It will be easier to separate water from lighter oil, as lighter oil moves more quickly and leaves the heavier water behind. Heavier oil may require more tanks and more time to separate enough water for the oil to be sold. Once you’ve determined what vessels you’ll need you can layout the flow line and header, and start putting your tank battery together.

Figure 1. An example diagram outlining a tank battery, with emulsion lines labeled E.

Header and Lines

For a tank battery that receives from several wells, a header can be a good idea. This is an arrangement of flow lines, meters, and valves that will allow you to control the flow from different wells. When a lease is pumping only a small amount, the loss from shutting in a few wells to run a test may be small. You could use barrel or bucket tests, or an individual well tester. In that case, a header may not be necessary.

Generally, you’ll need two headers, one for production and one for testing. If one tank battery is serving a large number of wells, you may need a second production header. As the flow lines from the wells approach the tank battery they generally are routed so they run parallel and about 18 inches apart from each other. That distance may seem random, but it’s due to the arrangement of pipes needed to receive the flow lines. A standard arrangement uses a 12 inch nipple and two tees for the header of each flow line, which adds up to needing about 18 inches.

Flow lines can be steel pipe of various types, fiberglass, or a few different types of plastic. The material should be chosen to meet the demands of your particular operation; for example, if corrosion is a major concern, consider fiberglass or plastic over steel. As the lines enter the header, it passes through a 6 inch nipple, meets a union joint and then another 6 inch nipple. A check valve is required next, and then the pipe hits another 6 inch nipple before entering the header proper.

That check valve is an important part of the header, and without it you could lose all the oil produced by all the wells routed through the header if a flow line develops a leak. This can cause the line to lose pressure, and so the fluid will fall naturally back down into the well. The check valve prevents this by closing at a loss of pressure, so that only the fluid in the line is lost. When that happens, there may not be any obvious signs of a problem. The well will continue to show a flow and pumping action at the bleeder valve. Nothing is being produced, however, and the well is simply circulating the fluid.

Figure 2. Lines from several wells flowing into the header. Standard practice is to label the lines at this point.

One major consideration when designing your header is ease of identification. You’ll want your valves higher up, where they can be easily seen. Using quarter round valves is also advisable, as you can see if they are opened or closed at a glance. Using round opening valves is usually not the best option, as they can lead to mistakes. If the header is set up to be controlled automatically, there should be a telltale or indicator on the valves to show which are open and which are closed. Any header design that works for you is fine as long as it is clear where oil is flowing, and which valves are open and closed.

Figure 3. A header that controls three flow lines and a single four inch test line (on the far left).

Injecting Chemicals

Chemical separation is necessary for pumped fluid to become saleable oil. It’s common to add chemical to the fluid just after the header, but before the first vessel. For most tank batteries, that means adding a system for injecting chemicals between the header and separator. Adding chemical here makes sure that it is mixed evenly with all the oil. Adding chemical in a flow line before the header could mean that oil from some wells isn’t mixed well, though with high volume operations the flow is great enough this is less of a problem.

For some operations, a barrel on a stand with a method of adding chemicals is enough. Larger operations producing more fluid, and therefore adding more chemical, will need a larger and more involved setup.

Regulations require that tanks with a volume of 600 gallons or more have some backup method of containing its contents. For a chemical tank, a drip pan under the injector’s stand is usually a good way to go. There should be signs informing any crew of the chemical tank’s contents and any hazards it may pose, and more detailed information should also be available.

Figure 4. A basic injection system using a pump and barrel.

Figures 4 and 5 show a couple of example systems for adding chemicals. Figure 4 shows a setup that has been used for decades, and is still common with medium-production operations. Figure 5 shows a newer setup that is used by higher production operations. In that picture, you can see an example of a drip pan as well as signs (on the lower right) offering information about hazards.

Figure 5. More complex injection system for a higher production operation.

Is your appetite for oil & gas operating knowledge insatiable like ours? 😀 If so, check out these related articles below – they’ll be sure to pump you up!!!

• The Basics of Setting Up an Oil & Gas Production Tank Battery
• Separators, Heater-treaters, and Pressure in Oil & Gas Production
• Tanks At Atmospheric Pressure in Oil & Gas Production
• Equipment For Handling Crude Oil In Oil & Gas Production
• Water Systems In Gas & Oil Production
• How To Sell Your Oil & Gas Production
• Tank Battery Design In Oil & Gas Production

A tank battery will generally have just a few pressurized tanks, but may have a number of vessels at atmospheric pressure used for a variety of purposes. While pressurized vessels have some peculiarities, it’s important to understand that having a good understanding of the more basic atmospheric vessels is also extremely useful. These tanks are very versatile, and often one tank may be used several different ways on a lease.

There’s a few vessels that every lease pumping operation is likely to have, such as stock tanks and gun barrels, that we’ll cover in detail. Tanks also come in a few different styles and materials. First, though, it’s probably helpful to have a clear idea of what is meant by atmospheric pressure and pressure in general, particularly when you’re talking about lease pumping.

Figure 1. An example of atmospheric tanks in a tank battery. Seen here is a gun barrel, a few stock tanks for storing oil, and a couple of water tanks.

There’s a few vessels that every lease pumping operation is likely to have, such as stock tanks and gun barrels, that we’ll cover in detail. Tanks also come in a few different styles and materials. First, though, it’s probably helpful to have a clear idea of what is meant by atmospheric pressure and pressure in general, particularly when you’re talking about lease pumping.

A Few Basics About Pressure

Atmospheric pressure varies depending on your height above or below sea level. At sea level the pressure of the atmosphere pressing down is about 14.7 psi. As you get higher, there’s less atmosphere pressing down and so the pressure is lower. If you’re below sea level atmospheric pressure will be higher. Because pressure varies depending on altitude, it’s possible you may see pressure measured in pounds per square inch absolute (psia). That unit means that the pressure has been measured in comparison to a vacuum, meaning it was compared to a complete lack of pressure.

Pounds per square inch gauge (psig), on the other hand, measures pressure in comparison to the local atmosphere. As an example, imagine a car tire that has been pressurized to a standard 35 psi at sea level. The 35 psi is also 35 psig, as it’s in comparison to the local pressure. The tire would be at 49.7 psia, however, as that number includes the atmospheric pressure (35 psi of the tire + 14.7 psi of the atmosphere at sea level). It’s often helpful to know the local atmospheric pressure.

Whenever your produced fluid moves from a higher pressure to a lower pressure system, or has a lower pressure due to motion, additional gas is going to be released. This is true when the oil goes from the heater-treater to a stock tank, and to a lesser extent when water is separated and then routed to the water disposal tank. If that gas can be recovered, it can be sold, so most atmospheric vessels actually have a small amount of back pressure to reduce loss of gas and lighter weight components than produced oil through evaporation. It’s usually no more than a few ounces (between 2 and 8), however it’s enough that pressure safety measures are needed. Most tanks have a thief hatch, which is a hatch in the top of the tank that allows you to access the inside. These hatches will have a safety pressure relief, and the gas vent line will also have a relief valve.

In addition to the backpressure, these tanks have to also handle pressure from the weight of the fluids inside. A few rules of thumb can make calculating that pressure fairly straightforward. Oil weighs about ⅓ psi multiplied by the depth of the fluid in feet. As an example, say a tank is full to a depth of 9 feet. The oil itself would exert a pressure of 3 psi, but it’s important to remember the tank’s backpressure should also be included. If the backpressure is about 4 ounces, the total pressure on the bottom of the tank is about 3 pounds 4 ounces per square inch.

Water, on the other hand, has a pressure of about ½ multiplied by the depth. So, a water disposal tank full to about 8 feet would have a bottom pressure of about 4 psi, plus any backpressure.

Vessel Materials and Construction

As technology has advanced, so has the construction of tanks and vessels. In the past, they were often made of redwood. As you might imagine, there was a number of problems with using wood. Where they are used in a modern operation, they’re used for water disposal tanks or as gun barrels. Bolted steel tanks have been common, though they’re largely being replaced with welded steel tanks. Steel tanks are also now commonly being lined with fiberglass to prevent corrosion. Corrosion resistant paint is also available and might be a good investment for steel tanks.

Figure 2. You can see some example tanks here, with fiberglass tanks on the right and welded steel tanks on the left.

Fiberglass is another popular material for building tanks, though it’s light enough that empty tanks can sometimes face problems in high wind. Fiberglass is a popular choice for water disposal tanks, particularly where corrosion is a problem. With fiberglass, it’s not a bad idea to put guy wires at each corner of the tank to keep it stable.

Tanks are available in a few different sizes. You’ll want to pick the specific type and size that fits your operation’s needs. Here’s a few standard sizes, to give you an idea of what’s available.

Standard welded steel tank capacity

Standard capacity for welded steel tanks.

Fiberglass tanks are similar in size and capacity to welded steel tanks.

Layout of a Standard Tank

Figure 3. Basic layout for a tank at atmospheric pressure.

Most tanks will have the same basic layout of openings. Whether the tank is used as a gun barrel, stock tank, or for some other use is going to depend on how the openings are used and what they’re hooked up to. We’ll look at each use in more depth, but it’s probably helpful to have a general understanding of what that basic layout is first. You can see an example layout in Figure 4.

On the very top of the tank is the gas outlet, which leads to a low pressure system for handling gas and has a valve that maintains that small backpressure discussed above. If this tank doesn’t have the backpressure, it’s likely you’ll see less oil produced than expected. The difference is due to the evaporation loss. The valve and backpressure also help keep oxygen out of the tank. Natural gas and air are a dangerous combination, and a mixture of the two can lead to an explosion. A tank filled entirely with natural gas is actually much safer.

On top of the tank, but not quite at the very top, is the emulsion inlet. There is usually a walkway or ladder to this inlet for easy access to the valve that opens and closes the inlet. Simply having fluid fall from the top of the tank to the fluid level can cause static electricity that encourages hydrolysis, and therefore corrosion. It can also lead to losing fluid to evaporation, so it’s common to have a tube, called a downcomer, leading down from the opening to near the bottom of the tank.

The highest opening on the side of the tank can be used for a couple different things. It can lead to an overflow line, which in most cases will lead to a lined pit or the water disposal tank. Alternatively, it can be an equalizer inlet. When several tanks are connected in series through an equalizer, one tank will be totally filled before the next begins to fill. The equalizer keeps the first tank topped off, even when you’re not there.

The oil outlet is generally near the top, but below the equalizer/overflow. When the tank is being used as a gun barrel, this is where the separated oil flows out. It’s height determines the level of fluid in the gun barrel. If the tank is being used as a skimmer tank, it can also be used as an oil outlet.

A side outlet is useful in a few different circumstances. It can be the opening leading to the water leg when the tank is being used as a gun barrel. It can be used as a water drain, and for tank maintenance like cleaning the bottom of the tank.

The very lowest opening is the drain. The most common style of tank is flat-bottomed with a drain on the side. Other types, discussed below, have conical bottoms.

Gun Barrel Tanks

A gun barrel (sometimes known as a wash tank) is another sort of separator, washing water from the oil before it’s sent to a stock tank. A gun barrel is driven entirely by gravity. Fluid flows into the gun barrel from a side inlet, and then is sent down through the tank using a spreader. The heavier water falls to the bottom where it’s sent to the water leg. The lighter oil floats to the top and flows out through the oil outlet. Some gas will also be separated and routed through the gas line at the top.

Figure 4. A gun barrel made from an older separator and mounted on a platform.

A water leg is a fairly standard setup for handling water disposal that is used with a number of vessels in the tank battery, like a heater-treater. Essentially, the water that drains from the bottom of the tank is forced by the weight of the fluid up into the water leg, which is a vertical, narrow pipe inside a taller, wider diameter one. The water flows up the inner pipe, then overflows into the outer pipe where it is then collected and sent to the water disposal tank. This helps keep the slight backpressure, and can also help control the level of the water in the tank.

Figure 5. The layout of a gun barrel using a side boot.

It’s possible and increasingly common to use a side boot to first separate more gas from the fluid. It first flows into the side boot, which is mounted outside and slightly higher than the tank. The fluid runs through a tube at the bottom of the boot to the bottom of the tank. From there, separation happens as normal, with the oil floating to the top and water remaining at the bottom. Because a gun barrel is operated entirely through gravity, to use a side boot the gun barrel needs to be mounted so that it sits about one foot above the fluid level in the stock tanks it feeds.

Stock Tanks

A stock tank is designed to hold produced and separated oil until it’s ready to be sold. Its layout is fairly similar to a gun barrel, and water will actually continue to fall out of the oil until it’s sent off either by pipe or truck, so some system for removing waste water is necessary with stock tanks, as well. You’ll usually need to accumulate enough oil to fill a 210 barrel transport tank. You don’t need the whole 210 barrels, more like 160 to 180. More than that, and a truck will weigh too much for most roads and highways.

One way that a stock tank differs is that the level of fluid inside needs to be frequently measured. A common way of doing this is to use a simple plumb bob and line. Dropping a metal plumb bob to the bottom of the tank multiple times a day can eventually lead to damage to the tank, however, so stock tanks will usually have an additional layer of metal, called a strike plate, placed under the thief hatch.

Figure 6. An example layout for a gun barrel and two stock tanks. (courtesy of National Tank Company)

There are several different stock tank designed. The simplest uses just a flat bottom. More specialized tanks have a conical bottom, which is very helpful in cleaning the stock tank. Paraffin and heavier oil can often collect water and become heavy enough to fall to the bottom (and in fact this mixture is commonly called bottoms), where it is difficult to remove. The conical shape collects most of these heavier elements at the very bottom, and encourages them to flow out through the drain.

There’s two different types of conical bottomed stock tank. The first has a conical bottom that ends in a narrow sump, as pictured in Figure 7.

Figure 7. The first type of conical-bottom tank.

This type of tank needs a small pit, with sides at the correct angle to support the tank’s bottom, and a small area dug out for the sump. The hole should be lined with gravel and tar paper before the tank is placed. This helps prevent ground condensation which leads to corrosion.

Figure 8. The second type of conical-bottom tank.

The second type of tank rests on a metal plate, which keeps it from sinking into the ground. The conical bottom is open to the air, though it’s often protected and not visible. It’s easy to recognize these tanks as the openings have to be slightly higher, as the tank sits higher to allow room for the conical bottom.

Figure 9. The second style that shows the higher outlet that’s necessary.

Most stock tanks have four openings in the side of the tank. One, which should be placed in the front of the tank about a foot off the bottom, is the sales outlet. This is used to send the produced oil to a pipeline or truck when it’s sold (diagrams of these two sales methods can be seen here: Preparing and Selling Oil & Gas Production).

The second is the drain, which is a 4 inch opening on the back of the tank and is about 4 inches from the bottom. At the top of the tank on either end will be openings to connect stock tanks together with equalizing lines, as explained above.

The drain opening, placed about 4 inches from the bottom, will usually connect to a pipe that runs along the bottom, then turns down so that it ends about an inch from the bottom. Paraffin and heavier oil often collect on the bottom into a layer multiple inches thick. The drain will help keep the area immediately around the intake clear. Drilling a series of small holes into the bottom of the pipe can help to keep a wider area clean.

The oil inlet for a stock tank will often have a downcomer so that the oil flows down directly to within a foot of the bottom. A couple of small holes should be drilled into the downcomer near the top to prevent creating a siphon effect and also allow gas to escape. The downcomer reduces static electricity in the oil, which can encourage hydrolysis and corrosion. It also reduces loss of lighter oil through evaporation. It also helps move the heavy layer that can collect on the bottom.

Water Disposal Tank

As you might imagine, the water disposal tank is used for holding water before it’s disposed of. In emergencies, it can also be used as an overflow tank. It should have as a minimum capacity of about 200 barrels or one transport load. The tank is usually short to make water disposal a little bit easier. It’s generally pumped down a water disposal well, or a well used for water flood (a further write up on tanks for water disposal and water floods can be read here: Second Stage Recovery Methods For Oil & Gas Production).

Figure 10. An example of a water disposal tank.

Tanks can be manufactured in two halves, to make shipping to the location easier. Due to regulations intended to protect the environment, the water tank has replaced the pit as a general disposal tank.

The Pit

The pit is basically an open tank with earthen walls built to hold water. The pit is usually has a plastic liner, which prevents the surrounding dirt from being contaminated with chemicals in the water. It may also have a net or mesh covering to keep animals out of the tank. In the past, the pit was called a slush pit. That tended to imply that it was filled with a slush of paraffin and a thicker fluid, which is no longer true. Pits can act as a holding tank or hold an emergency flair if necessary.

The Dike

Regulations may require a dike be built around all containers holding fluid that might contaminate the ground. Those regulations usually require that the dike be able to hold 1.5 times the amount of fluid all the vessels within it can contain. You’ll need to build a walkway over the dike in order to prevent it from being damaged by traffic.

Figure 11. A dike around a  tank battery, including a walkway over the dike.

Other Uses

A gun barrel, stock tank, pit, and other vessels we’ve already talked about are all standard vessels in a tank battery. However, it’s not unusual to use tanks for much more tailored uses. You may need to install a number of these tanks in different areas for a variety of purposes. You may arrive at a lease, and see some unique or apparently odd arrangements. It’s important to understand why these tanks have been placed there, and what problems they are being used to solve.

For example, a stock tank can easily be used as a gun barrel. A separator can be used for scrubbing liquid from gas for fueling a firebox. A line heater may be used to thin a thick emulsion, or between a separator and the header to prevent ice from forming. Some problems may require creative solutions.

Is your appetite for oil & gas operating knowledge insatiable like ours? 😀 If so, check out these related articles below – they’ll be sure to pump you up!!!

• The Basics of Setting Up an Oil & Gas Production Tank Battery
• Separators, Heater-treaters, and Pressure in Oil & Gas Production
• Handling Emulsion in Oil & Gas Production
• Equipment For Handling Crude Oil In Oil & Gas Production
• Water Systems In Gas & Oil Production
• How To Sell Your Oil & Gas Production
• Tank Battery Design In Oil & Gas Production

Pressurized tanks and other equipment require special handling. Gas or liquids under pressure can be dangerous to handle, and the vessels themselves are liable to some problems. Understanding the different parts of pressurized equipment and how they work, and how they work within the larger pumping system, is essential.

While different operations need different equipment, some pressurized equipment is going to be common to almost all pumping operations. These include the separator and the heater-treater, both of which are part of the process for separating oil from water and gas. Flow lines and the header used to control flow are also usually under some pressure.

Figure 1. Polyethylene piping for a flow line.

Pressurized Lines

The surface pipeline from the wellhead to the tank battery is called the flow line. The choice of material for the flow line is important, with different options being best suited to different situations.

Steel is a common option for operations with wells that flow naturally or have a high pressure flow. Various types of steel pipe can be used, including spare or older upset tubing that is not being used for the well. Heavy duty pipe is also common. Joints can be welded, or use groove clamps or pipeline coupling. While steel pipe can be prone to corrosion, it can be plasti-coated to protect it. Fiberglass piping can be used where corrosion is a major concern. For low to medium pressure operations, polyethylene has become very popular. It is also well suited to situations where conditions aren’t suitable for steel.

As the flow line is a surface pipeline, it’s important to keep weather conditions in mind when laying out its route. Whatever material you decide to use, it will expand in hotter weather and contract when it gets cold. Extreme cold weather can cause joints to fail and the pipe to separate. Extremely hot weather can cause the pipe to buckle. This is more likely to happen when the flow line is laid out in a straight line from the wellhead to the tank battery.

The solution is to run the pipe at a soft curve, so that the expansion and contraction won’t affect the operation of the line.

Figure 2. A buried flowline crossing under a road.

It is usually wise to provide some protection for the flow line. If the line has to pass under a roadway or needs to be buried for some other reason, you can protect it with casing. Sealed casing should always have a surface vent, to allow gas to dissipate if there’s a leak. It’s also important that, if you’re using steel pipe, it’s wrapped and coated properly.

The Header

The header is where flow lines from different wells come together into a single collection line. A system of check valves and choke valve are installed, so that the flow from each well can be controlled and measured independently.

It is possible to simply join all the flow lines together for all your wells together without using a header. However, that can cause some logistical problems. The advantage of using a header is that oil flow can be controlled and measured from a central location. Without a header, a well tester needs to be brought to each well. The other option is to shut in all but the well being tested, so that only that flow reaches the tank battery and is measured. That’s obviously less than desirable, as the shut in wells end up sitting idle.

The flow lines approach the tank battery parallel to each other, with about 20 inches of separation. They turn up, through a riser, where a check valve should be installed. The purpose of the check valve is to prevent the loss of oil down the well. If there’s a hole in the line and the check valve malfunctions, the loss of pressure can cause the flow line to drop into the well. A similar problem can lead to a line’s flow reversing itself, so that it pulls oil from other wells and the tank battery.

Processing of the oil usually begins at the header, where it’s possible to add chemicals. When adding processing chemicals at the tank battery, it’s standard practice to add them at the header. Another standard practice is to use quarter round valves, as they it’s easy to see if they’re open or closed quickly. It’s also important to have bypass lines for each vessel.

Common Lines and Openings

All pressure vessels have some fittings and openings in common, while some are specific to specialized vessels. Understanding of all your equipment’s specifications is essential, but in particular it’s helpful to know some basic information.

Figure 3. Common openings for pressurized tank battery vessels.

Fluid enters the vessel through the emulsion inlet. Emulsion in this case refers to the produced fluid, the combination of oil, water, and gas.

The inlet is located on the side of the vessel above the level of fluid. Some vessels, for example the separator, will have a diverter plate mounted inside of the inlet. This causes the fluid to mix and turn as it enters, which helps to separate the gas from the other fluids. The inlet itself is above the fluid level so that oil is not lost back down the fluid line.

At the very top of the vessel is the gas outlet, which allows the separated gas to exit. The drain outlet is at the bottom.

There can be several oil outlets. One will be at the level of the top of the fluid. The second one is toward the bottom of the vessel. This can also be used as a water outlet if the vessel is a three stage separator. If there’s also a fire tube, the vessel can be used as a heater-treater.

Floaters

A float is used inside a vessel attached to an arm or other sensor to control the volume of fluid by opening or shutting of its flow. They’re generally divided into indiscriminate and discriminate types.

Indiscriminate floats stay on top of both water and oil, and thus gauge the total volume of fluid in the vessel. Indiscriminate floats are most often ball floats, and their size can depend on a number of factors.

Discriminate floats are weighted to float only on oil, and thus rest on the meeting place between oil and water. As the density of oil can vary depending on the oil’s weight, and water density depends on the salinity of the water, floats of different weights are available. Oil generally weighs about 7 pounds per gallon, while water will weigh around 9 lbs, varying with the salt content.

Separator

The separator is usually the first vessel in the tank battery. It’s purpose is to separate the gas that is produced from the water and oil. Separators are not technically under high pressure, as their normal operating pressure is between 15 and 50 pounds. Their max pressure is usually up to 150 pounds, however. This vessel is pressurized primarily to push fluid to the next vessel in the tank battery.

Separators can be spherical, horizontal, or vertical, which refers to the shape and internal design. They can also be two phase or three phase separators. Two phase separators are more common, and separate gas from water and oil. Three phase separators separate all three fluids from each other. A third kind is called a metering separator. It can use either a two phase system, a three phase system, or both, and both separates the three fluids and measures each of their volumes.

The most common in lease pumping operations is the two stage vertical separator, which comes in three basic varieties. The right-hand separator has an emulsion inlet on its right side, and the left-hand separator has an inlet on its left. A two handed separator will have an inlet on either side. Generally, whichever is better placed is used, and the extra is plugged.

How A Separator Works

Fluid from the well enters at the emulsion inlet. Most separators, as mentioned earlier, will have a diverter plate near the emulsion inlet. The point of this plate is to set the fluid moving in a circular motion which encourages gas to separate from water and oil, helping to ensure that as little fluid is lost as mist is possible.

Figure 4. The interior of a vertical separator.

The gas rises to the top of the separator, where it passes through a mist extractor. From there, the gas will pass through a valve or two and then into the gas line, which is located at the very top of the vessel. The gas is under some pressure, normally between 20 and 50 pounds, so it’s not really high pressure.

After the gas has been separated, fluid is directed to the next vessel in the tank battery, most often a wash basin or heater-treater. This is controlled by a dump valve. Most newer dump valves use a small pipe or tub to draw fluid closer to the bottom of the separator, as the dump valve itself is usually higher on the vessel. That reduces the amount of water that sits in the separator, helping to reduce corrosion. The dump valve itself should be opened and closed by hand regularly to make sure it hasn’t gotten stuck.

Figure 5. An example of a standard two stage vertical separator.

A pressure gauge should be mounted above the dump valve. A sight glass is also usually helpful in keeping track of how much fluid is being held in the separator at once. As always keeping a record of standard operations can help you recognize when there’s a problem. A sight glass usually uses two valves, one at either end. These valves can become clogged with paraffin or other substances, so they’ll often have a reamer to automatically clean those sorts of clogs out. It’s a good idea to occasionally empty a sight glass, in a sense rinsing it out, so that readings remain accurate.

Figure 6. A pneumatically controlled separator.

Rather than using mechanical valves, some separators may use hydraulic controls. In this case, a gas line will need to be run to supply power to the dump valve and other controls.

Figure 7. A few different release valves and rupture discs.

Pressure Safety

As the separator is under pressure, there are a couple of safety devices to prevent overpressure. First, there is the pop-off, more technically known as a relief valve. The relief valve is set to open near, but still a bit below, the pressure limit for the separator. It’s usually designed to need little or no maintenance, and will open and close automatically to regulate pressure.

The second device is the rupture disc, also known as the safety head. This a thin dome of metal, usually steel or aluminum. Some older separators may use brass. The disc is designed to rupture at a pressure slightly higher than the relief valve opens. If the valve doesn’t open for some reason, the disc breaks instead, releasing the pressure. Unlike the relief valve, once the disk breaks it releases gas into the air until a valve is manually shut to stop it. It may be a good idea to run a pipe from the disc to a disposal pit so that escaping fluids and gas don’t contaminate the area. This pipe will rarely get used, but frozen water and corrosion can be serious problems when it is. A good route and grade that prevents the collection of water is essential.

Heater-Treater

These are three-phase vessels that are usually larger than separators while operating at around the same pressure of about 50 pounds. They’re also usually more expensive, as the larger size requires thicker walls to hold the same pressure. The heater-treater is usually the second vessel in the tank battery, just after the separator. If you use a higher pressure separator, it’s possible to use a lower pressure heater-treater and save a little on its cost.

Figure 8. An example of a standard vertical heater-heater. The firebox and site gauges are on the far side.

How A Heater-Treater Works

As the name implies, the heater-treater uses heat as part of the separation process. In many cases, particularly during warm summers, the heat from the sun warming the tank is enough to do the job. You can just add chemicals, and it will work without any additional expense. This effectively makes it a three stage separator. You can light the firebox if the weather turns cold, though that will use natural gas as fuel which could otherwise be sold. Cost effective use of a heater-treater depends on balancing the efficiency of using natural gas as fuel vs. selling it as an additional source of income.

Figure 9. Inside a heater-treater. (courtesy of Sivalls, Inc.)

A heater-treater is a three phase vessel, so it has three primary outlets. There is a gas line at the top of the heater-treater that collects natural gas. Somewhat below the top of tank is the oil outlet. This is also the level of total fluid in the heater-treater. There is also a water outlet for disposing of waste water.

The inlet leads to a smaller compartment at the top of the tank where any gas that was not removed in the separator is piped out through the gas line. The water and oil flow down through a tube to the bottom of the heater treater. Water flows out through the water leg, while oil continues up to the oil outlet. Controlling the height of the water column in the heater-treater is an important aspect of using the heater treater. The level of water should be about one foot above the fire tube. In other words, water fills the space from the bottom of the tank to one foot above the fire tube, with oil above that. As oil flows from the inlet tube and up, it will flow past and around the fire tube.

The water stays at the bottom of the tank and stays relatively cool. The oil absorbs most of the heat as it rises and leaves through the oil outlet. The inside of this part of the heater-treater has several horizontal plates with offset openings. Any water hits the plates and falls back to the bottom, while the heated oil continues upward. Any remaining gas is also separated at this point, through a tube at the top of the tank. This tube also helps maintain the pressure, and thus the water level, of the water in the water leg.

Figure 10. A back pressure valve that uses a diaphragm. (courtesy Kimray, Inc.)

Separated oil flows out through the oil outlet and into the oil line. Likewise, water flows through the water leg and out. Both the oil line and water disposal line should use back pressure valves, which only open when a certain amount of pressure is applied from the upstream side. As the oil line fills up above the valve, pressure grows until the valve opens. That usually happens when four or five feet of fluid has collected above the valve. Once the collected column of fluid has passed through the valve the pressure drops and the valve closes once more. Treater valves, like that shown in Figure 11 are good choices for these valve.

Figure 11. Examples of treater valves. (courtesy Kimray, Inc.)

Figure 12. An outline of the system for using a treater valve. (courtesy Kimray, Inc.)

The dump valve in Figure 13 is float controlled, and of a type that is popular with lease pumpers for its reliability and versatility. The pressure exerted below the valve seat is transferred to its top, which helps with the ease and reliability of the valve’s operation. It can also be turned to operate in the opposite direction.

Figure 13. A dump valve that is float controlled. (courtesy Kimray, Inc.)  

Controlling Water Height

The water leg is a name applied to the secondary tube on the right side of Figure 13. Rather than using floats and arms to open valves, the heater treater simply uses line height and gravity flow for operation. As the fluid enters from the highest opening in the tank, it continues to flow throughout the system to the slightly lower oil outlet. The height of water in the water leg will equal the height of the total column of oil and water in the heater-treater.

Water flows from the bottom of the heater-treater and up the interior tube of the water leg. It flows over the top of the inside tube and the collects in the outer tube until the pressure is enough to open the valve. The amount of water in the heater-treater can be controlled by raising or lowering the side boot on the water leg.

Is your appetite for oil & gas operating knowledge insatiable like ours? 😀 If so, check out these related articles below – they’ll be sure to pump you up!!!

• The Basics of Setting Up an Oil & Gas Production Tank Battery
• Tanks At Atmospheric Pressure in Oil & Gas Production
• Handling Emulsion in Oil & Gas Production
• Equipment For Handling Crude Oil In Oil & Gas Production
• Water Systems In Gas & Oil Production
• How To Sell Your Oil & Gas Production
• Tank Battery Design In Oil & Gas Production

The tank battery is the arrangement of storage and processing tanks, flow lines, and other equipment necessary to operate a well. Some tank batteries are connected to just one well, while others receive and process fluids from several different wells. When a single tank battery receives from a few different wells, those wells will usually all be close together which means they are all producing similar amounts and types of fluids. The different vessels and equipment that make up the tank battery will be chosen to store and treat the products from those type of wells. For example, wells in one area may be using hydraulic lift while wells in another use gas lift to up production. A tank battery will need to be equipped to handle the different requirements in each case.

As the equipment chosen for a tank battery depends largely on what is being produced, it’s important to keep a number of things in mind when designing a tank battery for a particular operation. Obviously, it’s important to understand what each vessel does and how it does it. Understanding the interior layout of a vessel, as well as its place in the overall tank battery, is also vital. Most of all, you’ll need to be able to spot and solve problems you encounter, as a tank battery will most likely evolve over the course of its use.

Assembling a Tank Battery

The composition of a tank battery will change and evolve over the life of the well. As the nature of production changes, different equipment will need to be brought in to meet different needs. Older equipment will be removed to make room. For example, a well may have sufficient natural pressure at first to produce a satisfactory flow. That pressure will fall, however, and eventually you may want to install a gas lift system, which requires specific, specialized equipment. Later, you may move to hydraulic lift, and have to add all of the equipment necessary for that. By the end of a well’s life, several different methods of production will probably have been used.

There are a few basic things that most tank batteries will have. As each tank battery has to be tailored to the needs of the well and operation, it’s important to understand how each of the basic components works.

Figure 1. A picture of a tank battery that includes water tanks (painted black), a wash tank, and two stock tanks for oil (painted gray).

Essential Vessels

Vessel is essentially a fancy name for the tanks and similar equipment that receive the produced fluid. These are mostly used either to simply store fluid until it can be treated or sold, or to separate oil from water and gas.

A stock tank is used for storing oil prior to treating or selling it. There’s also usually a tank for holding produced, separated water, as amounts have to be measured and recorded. These tanks are usually not under pressure. Tanks can be either round or rectangular.

Rectangular tanks usually don’t have a roof. This makes it easy to access the stored fluid for measuring and testing. A ladder may come with the tank. However, as with all things when lease pumping, a bit of ingenuity may be required; you may have to put together a simple one to access the fluid. A hoop at the top of the ladder allows you to use both hands to test and measure fluid. A safety belt is another option.

You’ll also need a separator, both a regular and a test one. This is usually the produced fluid’s first stop after leaving the well. Most often these are two phase separator, meaning the vessel will only separate gas from oil and water. They can sometimes be three phase separators, meaning that it also separates the oil and water. Unlike stock tanks, separators are usually under pressure.

Several vessels actually are involved in various steps of separating oil from other produced fluids and impurities. The heater-treater is another example, which is a three phase separator that uses heat. Heater-treaters can be either pressurized or at atmospheric pressure. A wash tank, sometimes known as a gun barrel, also separates oil from water and gas, making it another three phase separator.

Just about every tank battery will need some sort of circulation pump. It can be one of a bunch of different kinds, and is used to move fluids from one vessel in the tank battery to another.

Most tank batteries will require some sort of dike or firewall. These are required around vessels that are not pressurized, with fluids that are stored at atmospheric pressure. The firewall contains fluids in the case of leaks or other emergencies where oil may end up outside of a stock tank or other vessel. There’s some specific requirements regarding the size of the dike that you should check out. Usually, the dike has to be able to contain 1 ½ times as much fluid as can be stored in the tank.

Flow Lines

Lines can be simply be upset steel pipe like what’s used downhole. It can also be synthetic, like plastic or fiberglass. They can be joined however steel seems best for your operation. Steel lines can be threaded pipe and use appropriate fittings, or might use collars or grooved clamps.

Synthetic lines are used frequently in situations where steel would corrode too quickly. Polyethylene lines are also popular for their low cost and ease of use. However, polyethylene lines are best used with low pressure wells.

Figure 2. An example header.

Head lines flow from the wellhead to the tank battery. When a tank battery receives fluid from several wells, you’ll need to put together a header. This is an assembly of lines and valves that allows you to control the flow from each well to the tank battery, as well as to other equipment such as a meter.

In the example pictured, each well has it’s own set of flow lines and valves. Flow enters from lines on the bottom right. It then heads through a valve and then a check valve. Oil is then sent to different parts of the tank battery, either the production or test separator.

Chemical can be added in one of two places: either by injecting it after the header but before the emulsion reaches the separator or by injecting chemical all the wellhead. For more info about chemical treatment at the lease, check out the following write ups:

• Testing And Treating Oil & Gas Production
• Basic Methods of Treating In Oil & Gas Production
• Chemical Treatment And Pumps In Oil & Gas Production
• Preparing and Selling Oil & Gas Production

All valves are quarter round valves, so it’s easy to see at a glance which valves are open, closed, and which wells have been shut in. Valves and lines are also clearly labeled. This sort of clarity is important to the efficient running of a lease.

Figure 3. A drilling rig where a drill stem test is being performed. Gas is being flared off on the left.

Initial Production

The very first oil drawn from a well will almost always be through the drill stem, and used for testing purposes. Rather than having a full tank battery for such a small flow, a smaller test tank is usually used. If the test shows that the well may produce a profit, a large bore pipe will be set in place to serve as casing and then perforated. At the same time, the tank battery should be assembled so that production can start as soon as the well is prepared.

Some wells have sufficient bottom pressure to that flow will start as soon as the correct valve is opened. Other wells will require some further work. Many wells will have a column of water on the surface of the oil. The water will need to be swabbed out to so that the pressure in the tubing column is less that the bottomhole pressure. A column of oil can also be swabbed out to start flow.

The first flow will often be measure by the drilling company, but it will ultimately become the responsibility of the operator to keep track of what’s produced from the well.

Figure 4. The Natural Product Curve.

When a new well is opened for production, the pressure throughout the reservoir will be more or less equal. As fluid is drawn from the reservoir, the pressure around the wellbore will naturally drop. The oil in the reservoir will filter through the formation to the wellbore. However, oil will most likely be drawn much more quickly than it can flow through the formation, which leads to the drop in pressure. Over time, the production will fall according to the natural production curve. If no lift system is used, the production rate will follow this curve over time. Many different factors will determine the actual numbers.

What Are You Pumping?

Obviously, you’re most interested in the oil and natural gas that is produced from the well. However, you’ll be handling a few other products, some of which can also be sold to petroleum companies. Asphalt is used in road construction. Natural gas can be used in several industries. Paraffin and petroleum are also valuable byproducts.

BS&W, or basic sediment and water, is going to be the biggest byproduct by volume. Water can be used for some pumping operations, but there’s a number of byproducts which can be difficult to deal with, if not dangerous. Sulfur is often found in wells, which, when combined with produced water, can lead to the formation of acids that cause corrosion. Hydrogen sulfide can be particularly dangerous, and proper safety procedures should always be followed. For lease pumpers, specifically, those procedures should never be ignored, as you’ll often be working the lease solo.

Figure 5. An example tank chart.

Recording

You’ll be required to keep precise records about the well’s production, breaking down the volumes of natural gas and oil, as well as water. These records are required by a variety of regulations, and are reviewed by regulatory agencies. For any given well, the very first production will be recorded, as will all production up to the end of the well’s production lift. A yearly, weekly, and daily report is usually required.

These records are actually useful for the lease pumper as well, as they can be a guide to the amount and type of equipment needed to fully exploit a well. That decision will also be affected by many other factors, such as the volume of oil coming from the well, the lease size, and financial considerations. Fluid volumes should be measured frequently throughout the day when the well first is flowing.

Gauging and the procedures around it are so important in fact that we’ve dedicated several posts to it, they can be checked out by clicking the following links:

• Gauging And Gauging Equipment For Oil & Gas Production
• The Basics Of Keeping Records For Oil & Gas Production
• Well Records For Oil & Gas Production
• Operational Records For Oil & Gas Production Wells

Record keeping usually begins at the wellhead, and a basic meter is usually installed there. More detailed records can be taken from the various tanks that make up the tank battery. The use of a gauge line paste will help in determining the ratio of oil to water. While gas is usually vented to the air or lit to form a flare, the amount is still needed to be measured and recorded.

Is your appetite for oil & gas operating knowledge insatiable like ours? 😀 If so, check out these related articles below – they’ll be sure to pump you up!!!

• Separators, Heater-treaters, and Pressure in Oil & Gas Production
• Tanks At Atmospheric Pressure in Oil & Gas Production
• Handling Emulsion in Oil & Gas Production
• Equipment For Handling Crude Oil In Oil & Gas Production
• Water Systems In Gas & Oil Production
• How To Sell Your Oil & Gas Production
• Tank Battery Design In Oil & Gas Production

When using a gas lift system, mandrels are an integral part of the pumping system. Valves are installed in these pieces of hardware, which are an important part of regulating the flow of gas. Conventional mandrels are straightforward to use, but they have a significant downside. When one mandrel fails or needs maintenance, the entire tubing string has to be pulled.

Side pocket mandrels are an alternative that addresses this issue. Rather than pulling the entire tube string, side pocket mandrels can be pulled separately from the tubing string using a wireline machine. You won’t need a pulling unit and a whole crew, and wireline machines are useful to have around for a lot of reasons. Side pocket mandrels are particularly good choices for offshore rigs, having lesser requirements for crew and equipment.

Figure 1. A few different types of side pocket mandrels. (courtesy of CAMCO Products and Services Company)

Wireline Machines and Safety

Wireline machines can do more than simply pull side pocket mandrels. The tools used with a wireline machine can be used to do a variety of things, from removing sand to cleaning up scale. Paraffin can precipitate, for example, blocking tubing and slowing the flow of production oil. A wireline can cut through the paraffin and reduce the buildup.

Other types of maintenance are also possible with a wireline machine. Safety valves and other safety equipment could also be serviced and maintained. A wireline machine is useful for many reasons on an offshore well. One of the most important is that it allows the running of a second string of tubing into the well, in case of an emergency.

While it’s possible to add some safety equipment using a wireline machine, the machine itself can be dangerous. Fast moving wire cables are dangerous, and you should never approach or attempt to handle the wire while the machine is on.

Continuously Producing

Gas lift usually works best when the well is going to be continuously flowing. This is because the unloading sequence that is required to get a gas lift system flowing is complex and can be time consuming. For smaller wells, where the unloading sequence is easier to manage, running the well only intermittently could be prudent.

Figure 2. An example chart showing production from a gas lift system producing continuously. (courtesy of McMurry-Macco)

Figure 3. An example chart from a well producing intermittently using gas lift. (courtesy of McMurry-Macco)

Gas Lift Using The Annular Space

For most operations, the gas is injected through the annular space so that oil is produced up through the tubing. That’s the optimal arrangement for most wells, from those only producing a few dozen barrels a day to operations producing tens of thousands of barrels.

For high production wells, the standard system is actually reversed, with the gas being injected through the tubing and fluid produced through the casing. In that case, no packing is necessary in the casing, as it would be with the standard system. This is more common with wells producing more than 20,000 barrels daily. This is a particularly efficient in operations dependent on waterflood, as the amount of water pumped to the surface will tend to increase over time.

Is your appetite for oil & gas operating knowledge insatiable like ours? 😀 If so, check out these related articles, The Basics of Gas Lift Pumping in Oil & Gas Production and, Mandrels and Gas Lift Valves in Oil & Gas Production – they’ll be sure to pump you up!!!

A gas lift system normally requires valves in the production tubing down the well.  These valves open in sequence, injecting the gas that forces fluid in the tubing to the surface. The hardware that connects the valve to the tubing is called a mandrel. There’s two general categories of mandrels, and whichever you choose can have an impact on how your well operates and is maintained. The first variety of mandrel is the conventional mandrel.

Conventional Gas Lifts and Mandrels

Valves are attached to the outside of the mandrels, which are then inserted into the tubing string at regular intervals. The entire assembly of tubing, mandrel, and the attached valves are all run into the well together. That means that when a valve needs maintenance or to be replaced, the entire thing needs to be pulled, which requires a crew of workers.

Another component to be aware of with a conventional gas lift system is the packer above the tubing perforations, which seals the annular space. That space is closed at the bottom as well, so that gas that is fed into that area from the compressor on the surface will activate the system.

Figure 1. Four different conventional mandrels. (courtesy of Camco Products and Services Company)

Installing Mandrel and Valves

The valves will need to be installed at a specific, regular interval along the tubing. It’s best to measure the intervals and assemble the mandrel and valve before transporting the tubing to the well. The first lengths of tubing to be lowered down are all loaded together and marked with a 1. The second group is marked 2 and also loaded together, and so forth. Each mandrel is similarly assembled and marked for the order it’s going to be installed. When the tubing reaches the well location, it can be assembled with the mandrels in order.

When the tubing has to be pulled for servicing, the same ordering and numbering can be used. It’s vital that each valve be used only in its correct place in sequence, as otherwise the lifting system will not work correctly.

Figure 2. A few different gas lift valves. (courtesy of Camco)

The Basics of the Gas Lift Valve

A valve will automatically open under a specific pressure, the opening operated by a bellows that is gas operated or occasionally through a spring loaded mechanism. The gas lift valve is divided into several different chambers.

The top most chamber is filled with a pressurized neutral gas. It has a fill valve that’s similar to one you’d find on a tire. The fill valve is set to a specific pressure and then it’s sealed up.

The second chamber is a bellows, one end of which is round. The bellows is forced up against its seat by the internal pressure.

The lower chamber of the valve is exposed to the annular pressure at the bottom of the well. The difference in pressure in the annular space opens the valve so that the gas is injected into the fluid.

Gas enters the tubing from the uppermost valve first. That valve then closes, and the one below it opens. This sequence continues until the entire column has been injected with gas and the fluid begins to flow. As long as gas continues to enter the tubing, fluid will continue to flow.

Is your appetite for oil & gas operating knowledge insatiable like ours? 😀 If so, check out these related articles, The Basics of Gas Lift Pumping in Oil & Gas Production and, Using Side Pocket Mandrels in Oil & Gas Production – they’ll be sure to pump you up!!!

The production of any well is going to eventually drop. Even with a well that at first naturally flows rather than requiring pumping, the volume of oil produced will drop at some point. Production may drop to the point that the number of barrels produced each day falls a significant amount.

One of the methods you can use to bring production back up is gas lift, which involves pumping gas into the tube line below the surface of the liquid in the well. The gas forces the fluid through the tubing and to the surface. It’s a method that’s used commonly, particularly in wells that have lower production volumes, as gas lift can add a few barrels a day to production and still be cost effective. It’s also used offshore commonly, where space is at a premium. Gas lifts don’t take up much room, and it’s possible to use it with a few wells drilled close together. Additionally, no gas is lost in the pumping process.

Gas lift systems are also commonly used for a number of other situations. As mentioned above, it’s a good choice when a well needs a little additional force to produce satisfactorily. It can also be used with wells that have a head of water; the gas lift clears the water, allowing the well to flow as it had previously. In the same vein, it can be used to pump water for use in waterflood operations. Gas lifts are useful in a number of different contexts, and can also be used for injecting chemicals and other additives.

Gas Lift Basics

Gas lift uses gas pressure to, in effect, make the fluid weigh less. This reduces the pressure required to push fluid out of the well system to the point that bottomhole pressure is enough. Gas lift can be used to coax a natural flow, or increase an already existing flow. As long as the bottomhole pressure combined with the gas lift is enough to push fluid to the surface, production will continue.

Figure 1. A basic gas lift system. (courtesy of McMurry-Macco Lift Systems)

When using gas lift, there will be a number of valves on the tubing lowered below the fluid level. These valves open automatically at certain predetermined gas pressures, which happens as part of the sequence for unloading the well. Each of the tubing valves is opened in sequence, so that a column of fluid is injected with gas. The fluid is lifted to the surface, and the next valve in the sequence is opened, sending the next column up. More columns are injected and lifted until the total weight of the fluid column, from the bottom of the well to the surface, has been reduced to the point that the well begins flowing.

When Should You Use Gas Lift?

It’s obvious that gas lifts can be used in a wide variety of contexts, but they can also be used in wells producing a wide range of volumes as well, from tens of barrels a day to tens of thousands of barrels a day, making it a very flexible system. Maintenance and installation costs are also usually lower, and it’s generally easy to service. It’s also useful in contexts where sand might clog or damage other types of lifts. It’s particularly useful when a well is deviated, and the wear on rods may be a concern.

How To Get Started With A Gas Lift

A gas lifting system has three primary parts. The inlet provides a supply of high pressure gas. This leads downhole, the second part, to the system in place there. The outlet is where the produced volume is sent and the gas recaptured.

The best option for gas lift is a large supply of high pressure, dry natural gas. Ideally, produced natural gas, which will mostly likely be wet, is sent to a processing plant to strip way fluids. The dry gas can then be sent back to the lease for pumping; usually a central gas distribution system supplies all of the wells which can be placed near the processing plant.

It is possible to use wet gas to power a gas lift system. However, you’ll spend a great deal of time in maintenance, and you’ll need to install some additional equipment. You’ll need to install some sort of scrubber to strip out all the fluids from the gas. You’ll also need to install a compressor to put the gas under enough pressure to be useful. Under the higher pressure conditions of the flow line, water may separate from the fluids. Drop pits are required to keep the condensation contained.

The gas is sent to the well through a control valve that is usually near where a pipe from the compressor meets the wellhead. A choke valve is also installed at the wellhead to vary the amount of gas that is injected into the well. You’ll want to set the choke valve so that you’re using gas as efficiently as possible. Near the bottom of the tubing string, a packer is used to seal off the annular space below the casing perforations from the space above.

Figure 2. A two pin pressure recorder at the wellhead. (courtesy of McMurry-Macco Lift Systems)

As with other types of lifting systems, a simple recording device at the wellhead can be very helpful when it comes to diagnosing problems. For a gas lift system, a two pin pressure recorder is a reliable way to keep track of the pressures used in the gas lift valves during operation, and to keep track of the lifting system’s overall efficiency. Using the information the recorder provides, you can monitor how much gas is being injected, and adjust that amount so you’re hitting the production amounts you need, as well as diagnose a variety of problems.

It is important to consider the what changes or improvements you’ll need to make to your tank battery to use gas lift. Using gas lift can have an impact on the equipment needed for handling natural gas, water produced from the well, and crude oil, and you should plan for a higher level of production for all three.

Is your appetite for oil & gas operating knowledge insatiable like ours? 😀 If so, check out these related articles, Mandrels and Gas Lift Valves in Oil & Gas Production and, Using Side Pocket Mandrels in Oil & Gas Production – they’ll be sure to pump you up!!!

Using one central hydraulic system to power multiple wells is a popular setup. A single triplex pump, depending on the amount of power oil needed, could supply power oil to up to eight wells. The advantage of using such a system is that you only need to obtain and maintain a single hydraulic power system. The downside is that when that system is down for repairs or maintenance, all of the wells that depend on it stop producing.

Power Oil Tanks

Crude oil is the most common source of hydraulic power for lease pumping. This oil is stored in the power oil tank in the tank battery. The power oil tank is the last tank in the oil processing system and is located just before the crude oil stock or sales tanks. The power oil tank is usually taller than the sales tanks. This allows the supply from the power tank to be located about two feet below where the excess production equalizes over into the sales tanks.

Figure 1. An example of a central power system.

In addition, two feet below the standard supply line, there is an emergency supply line controlled by a separate valve. The valve can be opened to supply additional power oil in the case of an emergency. When everything goes back to normal, you can just shut that valve to stop the additional flow of power oil. It’s usually straightforward to pump oil in the sales tanks back to the power oil tank if it runs low.

Power Oil Lines

The other major component of using a central power oil system are the power oil lines. The hydraulic system should be placed near the tank battery. Once the header has been installed, a separate power oil supply line is run to each oil well; usually a 1-inch line is enough to do the job. A return flow line is then run from the well bank to the tank battery. Each well will have both of lines to link it to the overall system.

Figure 2. Power oil lines running from the central power system to each well. (courtesy of Trico Industries, Inc.)

Using Produced Water

It’s possible to use water produced from a well in place of crude oil. It’s not possible in every case, as the water has to be free of scale and corrosive compounds that damage equipment and are hard to eliminate.

However, in areas where it’s possible, it can be a wise choice. Water is easier to control than oil, and easier to neutralize. A special fitting can be added to the heater/treater that draws water from the system. Alternatively, it’s possible to tap directly into the produced water disposal system. That means that when using produced water, you don’t need to add a power oil tank to your tank battery.

Using A Closer Power Oil System

The power oil can be pumped and reused through its own system by adding a third line from the surface to the hydraulic pump. This sort of system would also require a special power oil tank be installed, as well as the additional line to the pump. However, it’s a good option when the produced fluids are too corrosive to be used for power.

Spotting Problems With Multiple Wells

When you’re using a single pump for a single well, problems are usually fairly straightforward to spot. When production starts to fall suddenly, you only have one set of equipment to check for issues.

If you’re operating multiple wells off a single power source, however, that multiplies the amount of work you have to do. It’s helpful to be able to narrow down the problem to a single well and its associated systems. To do that, you’ll have to put together a distribution manifold.

A standard layout is shown on the diagram. It controls five wells, each with a riser and set of valves, while the sixth valve is used in testing. It’s possible to test an individual well’s flow by opening (in this diagram) the first, lower valve on the right, which is the automatic bypass. By opening the second header from the right, which is the individual well test line, it’s possible to send the power oil through a meter which tracks the number of barrels that have gone through in a given period. After the oil goes through the back line and comes forward again to the selected well header, that valve is opened and the front valve is closed so that the test can begin. Returning the valves to their original settings will return the system to normal operation.

It’s possible to modify the manifold header depending on your needs. For example, it’s often helpful to add openings so you can add solubles or chemicals if you need to.

Figure 3. An example of a distribution manifold for five wells, including a system for testing individual wells using an automatic bypass. (courtesy of Trico Industries, Inc.)

Is your appetite for oil & gas operating knowledge insatiable like ours? 😀 If so, check out these related articles, Different Types of Hydraulic Lift Pumps in Oil & Gas Production and, Single Well Hydraulic Systems in Oil & Gas Production – they’ll be sure to pump you up!!!

Hydraulic Systems For A Single Well

Hydraulic systems can be used to pump a single well, or as a central power system for several wells. When using it with a single well, the hydraulic triplex system is placed on the edge of the location. The power line is run from the hydraulic unit to the wellhead. Fluid that has been produced from the well, including the power oil, is returned to the hydraulic system vessel.

The produced fluid is separated by the vessel in three stages. The water falls to the bottom, and by line height automation is duped into the flow line. The water and the produced gas are sent to the tank battery, where the gas comes off the top of the vessel.

The oil in the vessel is first utilized to operate the hydraulic lift system through a special line from the vessel to the triplex pump. The pump places the oil under high pressure, and it is piped to the wellhead and then downhole to operate the pump. Downhole pumps lift oil on both the upstroke and downstroke to pull fluid from the formation, combining it with the power oil before the fluid is pumped back to the hydraulic system vessel. Any oil above the amount needed to operate the pump automatically flows out and is commingled with the produced gas and water in the flow line, returning to the tank battery.

Operating a hydraulic system requires that enough oil be transported from the tank battery to the well to fill the tubing with oil, operate the system until it is full of fluid, and send the produced oil to the tank battery. A small truck can haul enough to oil to activate the system. It can be worth it, however, to install manifold bypass systems at that tank battery and at the well to make the transport unnecessary.

Figure 1. A unidraulic system for a single well. (courtesy of Trico Industries, Inc.)

After the system is initially activated, the produced gas and excess liquid are separated, with the excess liquid being dumped into the flow line. The vessel retains enough liquid in the vessel to allow the system to operate for a short time without running out of liquid. That’s a measure that’s intended to reduce the need for pumping or hauling more power oil from the tank battery.

Should You Use A Single Well Hydraulic System?

When problems occur with a triplex pump on a single well, only that well has to be shut in. Other wells on the lease are independent and continue as usual. Additionally, the length of the power oil line is reduced from the distance to the tank battery, just the distance across the location, and the triplex equipment in general is smaller.

However, using a triplex pump for each well is not always the best choice. It means that a pump has to be installed at each location, and also requires either an electrical or mechanical prime mover for every pump. If it’s electric, a power line must be run to each location. It also requires the purchasing and maintenance of many pieces of identical equipment that will require maintenance and repairs.

Is your appetite for oil & gas operating knowledge insatiable like ours? 😀 If so, check out these related articles, Different Types of Hydraulic Lift Pumps in Oil & Gas Production and, Using Hydraulic Systems to Power Multiple Wells in Oil & Gas Production – they’ll be sure to pump you up!!!