Checking oil production is something that should be done on a regular schedule, if not daily. Each day of measurements adds to a record of production, which when taken as a whole can be an indicator of future oil production.
There will be days when the oil produced from the well is more or less than you were expecting. Obviously a rise in production can be exciting, and a fall worrying, but in truth production tends to be very steady over time. A big difference from what’s expected, if it’s confirmed after re-gauging, will more likely be an indicator of a problem. Whether it’s a drop or a rise, and the size of the change, can be a good indicator of where the problem is.
Figure 1. A worker gauging the tanks for the day.
Problems Leading To Overproduction
There are just a few things that can lead to a genuine rise in production, the most likely being an unusually successful waterflood system. In most cases, however, the increase is due to some other factor.
Out Of Balance Tank
Most of the tanks in the tank battery will hold some level of both water and oil. These two fluids are in a balance in the vessel to ensure that water is separated and that treated oil is routed, eventually, to the sales system. Tanks can fall out of balance, however, holding either more water or more oil than they should.
When the tank is holding more water than it should, more oil will be pushed up and over into the oil flow line. This is more likely to happen with three phase vessels like heater-treaters that use a water leg. The water leg can become plugged, slowing or stopping the flow to the water disposal system. The buildup forces the water level in the tank to rise. A valve that’s mistakenly been closed can also lead to a rise in water and therefore an out of balance tank. It’s a common enough problem that stock tanks should be large enough to hold overproduction without overflowing.
If you suspect that an out of balance tank is the problem, you can use Kolor Kut or another water gauging paste on the gauging line. A thief, a small graduated cylinder, can also be lowered into the tank and used to measure the level of the oil-water interface.
There will be times when a clogged line or closed valve is not the culprit, and there genuinely is additional oil coming from the well. That can often be the result of running the pumps for longer, which will lead to a temporary rise in production. However, wells can only produce so much, and production levels will fall back to their previous levels even if the pumps are run for longer periods.
A well may also have broken a gas lock. This is when a pump is working improperly, or has been serviced improperly, so that gas has become trapped in the pump. The pump will pump smaller amounts as the pump’s space can’t fill entirely. Eventually, enough liquid pressure will build to force the gas down the line, breaking the gas lock. When the lift bringing oil from downhole has a gas lock, oil will build up in the annular space until pressure has built to the point that the gas lock breaks. The oil in the annular space will be pumped to the surface, leading to a brief rise in production, but only until the annular space clears. Waiting for the gas lock to break is obviously not ideal, but the other option, unseating the pump, is not much better.
Problems Leading to Underproduction
Problems leading to production shortfalls can be a bit more concerning, as shortfalls can quickly add up to lost money. While there are a few more causes for production loss, it’s still usually possible to find the problem in one of a few areas.
You’re most likely to find the problem in the tank battery. That’s where most of the equipment is, and so there’s more there to go wrong. A quick look around can often provide a hint to the problem; leaks and overflows are easy to spot just by walking around the battery. If there’s no obvious signs of an issue, the problem is most likely going to be somewhere you can’t see, in one of the vessels.
Most tank batteries will at least include a separator. This vessel is used to break gas out of the produced oil before it’s sent either to stock tanks or to further treatment. The level of fluid in the separator is controlled by a float switch. If that switch breaks or stops working for some other reason, the separator can fill to the point that emulsion forces it’s way up and out the gas line. This can be diagnosed by checking the sight glass and by feeling the float switch to see if it’s still functioning. Faulty or leaking valves can also cause problems. There is also a small insect, called a mud dauber, native to many of the areas where oil reservoirs are found. They get their name by building nests out of mud, which when they select a pumping operation for their homes can clog vent holes.
As mentioned above, vessels can be put out of balance, though a fall of production usually indicates that oil has built up in a heater-treater or other vessel. If the oil outlet becomes clogged, the oil in the vessel will build up and force water out the drain. If the vessel becomes overfull enough, it can also push oil down through the water outlet. A sight glass will usually make this problem plain, as will checking the water disposal tank and pit.
Paraffin is a petroleum product that is very similar to wax in some respects. It is produced by many wells, and will often collect at the bottom of the oil in vessels, just about at the level of water. The wax can collect to the point that it forms a seal between the water and the oil, preventing additional oil from separating from the water and going out the oil outlet. Instead, it will be forced out of the water drain. This problem is a pain to fix, as it requires some special equipment or chemicals to break the wax dam. Vessels, and in particular gun barrels where oil can sit for longer periods, should be checked regularly so that this situation can be prevented.
At The Well And Downhole
After the tank battery, the most likely place to find a problem is at the well, either on the surface or downhole. It can be easier to diagnose problems on the surface, as you can simply look for leaks and other problems. A common issue is that a well has been turned off early or not turned on according to schedule. If the pump isn’t running, it’s obviously not producing oil. Making up the lost pumping time and gauging again should bring production up to the expected level.
A problem with the electrical system will also lead to lost pumping time. Often, replacing a fuse or resetting the system will solve the problem. It’s important to use the correct fuse in each case; control boxes may have a number of different fuses with different ratings. In some cases, rodents chewing on wires or otherwise getting into the system will cause an electrical failure.
Figure 2. An example of a swing check valve. (courtesy of Dandy Specialties, Inc.)
A failed or leaking check valve on the casing or flow lines may also cause a loss of production. With a valve open, oil won’t be produced to the tank battery. Instead, fluid will simply be pumped up to the top of the well and then allow to fall back. The pump will show good pressure, but a gauging of the tanks will show less production than expected. Cleaning the check valve can fix the issue. To diagnose a failed check valve on the casing, close the casing valve for a couple hours while the pump runs, then open it back up. Failed check valves at the tank battery could lead to losing production from other wells downhole.
Sometimes the problem is simple human error. Many operations require many valves to open and close. Forgetting one valve or other part of the sequence is not only possible, it’s almost certain to happen at one point.
When a valve that has been left closed is opened after the pump has been running, it should be opened slowly. There may be pressure on the line, and suddenly releasing that pressure can lead to unfortunate consequences down the line, such as a ruptured tank. Bleeding the valve and allowing pressure to drop before opening the valve completely is a safer way to do things.
When the problem is downhole, things can be a little more complex. A few problems that can cause a loss of production require a well servicing job. That might require extra equipment and cost you money. Other problems can be taken cared of from the surface.
Occasionally a pump valve may come unseated on the bottom. You can find out for sure if that’s the problem by opening the bleeder valve on the tubing while the pump is running. The rod clamp above the pump carrier bar can be raised, which will lower the pump. There may be rocks or other trash under the pump. Lowering the pump will cause it to start bottoming out. If the pump is powered by an engine, revving the engine will start the pump tapping.
Gas lock in the pump downhole can cause a drop in production, as mentioned up above. Salt bridges may also be a concern in operations that pump salty water. When the pump isn’t running, the salt water in the casing can rise. That water level drops when the pump is started, leaving a thin coating of salt behind. When that happens several times a day over a period of time, the casing space can be blocked by salt buildup to the point that it affects production. The solution is to pour fresh water down into the annular space to wash the salt away. That can be dangerous, as it may seal off some zones of the reservoir.
The casing perforations can sometimes become clogged with sand or sediment. As those openings are how oil flows into the pump to be sent to the surface, those clogs will cause a slowdown in oil production. The casing space may also fill with sand.
The pump used to lift oil to the surface will also eventually wear out, which will lead to a slowdown of production, or even bring it to a halt. Pumps generally wear out at predictable times, and checking the lease records can help you anticipate a failing pump.
The tubing string may fail to develop pressure when the pump is running. That can be an indication of a worn out pump or a split or crack in the tubing. You can test the pressure from the well by running the pump to put pressure on the tubing. Like many tasks around the lease, this can be dangerous if common sense and good safety habits aren’t used. A pressure gauge should be put on the bleeder valve. Once that’s in place, close the tubing wing valve that leads to the tank battery. Run the pump for one revolution and then turn it off. At this point you’ll want to check the pressure gauge, but it’s important to note that you should not look at the gauge directly. It may be under a great deal of pressure and a failing valve may be dangerous. If no pressure develops, run through that process a second time to double check. You may need to wait a few minutes for the pressure in the tube to settle before an accurate reading can be taken.
We go into more depth on downhole problems here Working Over Oil & Gas Production Wells, here Wire Lines In Oil & Gas Production, here Tubing String Basics And Maintenance In Oil & Gas Production, here Running And Maintaining Sucker Rod Strings In Oil & Gas Production, and here How To Service An Oil & Gas Production Well.
Flow lines should be checked regularly in any case, so problems are usually caught before they become more serious. However, they should be checked if production has fallen, just to be on the safe side. The whole length of surface lines should be walked to check for leaks or plugs. Leaks are generally obvious, and tapping the pipe will often reveal plugs. Empty line will sound more solid. Buried lines become plugged, though putting the line within conduit can reduce the chances of that.
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