Basics of Hydraulic Lift
Hydraulic systems uses fluid pressure to power a pump. That is done by pumping fluids downhole using a triplex pump designed for extremely high pressure, usually between approximately 2,000 and 5,000 psi. Hydraulic lift pumps are flexible, and are useful for wells that are producing any volume, from low to high. In general, hydraulic lifts have higher production volumes than mechanical lift pumps.
Figure 1. A central triplex, high pressure pump.(courtesy of Trico Industries, Inc.)
The hydraulic, reciprocating pump is at the bottom of the well. New oil is pulled from the annulus by the pump. The newly produced oil and power oil are combined, then pumped back to the surface and then to the operation’s tank battery.
Figure 2. A hydraulic pump’s up and downstroke. (courtesy of Trico Industries, Inc.)
Fluid is recycled to operate the wells. For a rough guideline, for every three barrels pumped into the well as power oil, you can expect to see five barrels pumped back to the surface. The extra two barrels is new production. The pump will produce oil on the triplex pump’s upstroke and on its downstroke, and its speed can be adjusted using a valve.
Hydraulic Lift Pumps
There are a few different options when using hydraulic lift pumps. Among the different options are:
- Fixed casing.
- Free casing.
- Fixed insert.
- Free parallel.
- Jet pump.
- Closed power fluid.
- Commingled power fluids.
Some of the options are more complex. We’re going to take a look at some of the simpler options, free parallel and fixed insert pumps, as well as giving a brief overview of what a jet pump looks like.
When you decide to put a hydraulic lift on your lease, you’ll have to choose between a free parallel or a fixed insert system. The pump is similar with both options, but the choice between fixed insert and free parallel can make a big difference on which wellhead you choose, and how you decide to install the moveable pipe.
The Free Parallel Pump
The free parallel pump using two strings of tubing, one of which is a smaller string that is strapped to the outside of the larger tubing string. Once you’ve lowered the tubing down into the well and installed the wellhead, you can simply drop the pump into the tubing.
You can then open the hydraulic valve so that the power oil or water flows down into the well, carrying the pump with it to the bottom. When the pump hits the bottom and seats properly, it will begin to function as lower as a power fluid is being pumped.
That power fluid will flow over with the produced oil and be pumped up to the surface through the smaller tube on the outside of the string. As with any pumping well, natural gas that is produced will mix with the produced oil and power fluid, and travel back to the tank battery.
An important advantage with this sort of pump is that it’s much easier to replace the pump when there’s a problem. The system is designed to allow a single person to bring the pump to the surface by turning a valve on the wellhead. The pump can be retrieved once it’s reached the surface with a few simple pieces of equipment.
Free parallel pumps can sometimes become knocked out of the proper position by solid objects, known as trash. The same valve that brings it to the surface to change can also be used to hop the pump up briefly, which will clear the trash. Returning the valve to its original position allows the pump to reseat. This is just as common with free parallel pumps as with insert pumps.
The Insert Pump
The insert pump is inserted (hence the clever name) into larger diameter tubing, usually. around 2 ⅜ inch. Attached to the top of the pump is a smaller diameter string of tubing, which is also inside the larger tube. The bottom of the pumps seats against the the tubing seating nipple. The pump is designed to use it’s own weight to hold it seated and in place. There’s a packer, so gas is returned to the surface up through the annular space, as with a mechanical pumping well. It’s then combined with the produced fluid from the wellhead, where everything enters the flow line. A pulling unit is required to retrieve the smaller tubing string and change the hydraulic pump.
Figure 3. Four different hydraulic pump designs. The fixed insert design is shown at the far left, and the free parallel design is shown third from the left. (courtesy of Trico Industries, Inc.)
As with the free parallel pump, trash can collect under the pump seating, causing production to fall or stop altogether. This can cause the column of fluid inside the larger diameter tubing to fall back into the well. A lift piston can be placed at the top of the wellhead so that power oil can be pumped under the piston. That allows the insert pump to use the same ‘hop’ technique as with a free parallel pump to clear trash and reseat the pump. This will remove the trash, and the pump will begin to operate normally again. You’ll most likely have to do this regularly while this pump is in use.
The valve on a pumping wellhead is designed so that a quarter turn of the valve handle opens the valves the correct amount to get the pump to hop up. Returning the valve to its standard setting will allow the pump and smaller diameter tubing to fall back to the bottom and where the pump will reseat.
The Jet Pump
Jet pumps are more complex. The jet action is produced using a venturi tube, which has a particular cone shape intended to narrow the flow path. The shape creates an area of low pressure by increasing flow rate. Fluid is drawn into that low pressure area.
There are a few contexts where a jet pump is going to work well. It’s common in wells offshore, where space is tight, as a single triplex unit can power several wells at once. Jet pumps can also be used with continuous coiled tubing and in horizontal completions.
Figure 4. A jet pump’s basic components. (courtesy of Trico Industries, Inc.)
Should You Use A Hydraulic Lift?
Hydraulic lifts have a few advantages compared to other high volume production systems, but no production system is perfect.
A key advantage of using hydraulic production systems is that it’s easy to adjust the volume of the power fluid pumped. Hydraulic pumps can also handle a high daily production volume. Free pumps, in particular, can be replaced by one or two workers without needing a whole crew.
There are some chronic problems with hydraulic lifts systems, however. Keeping enough clean oil or water to use for power fluid can be difficult in some areas. When equipment fails, it can be time consuming to repair, with one or more wells shut in for long periods. There is also simply more equipment to monitor and maintain, as you’ll need both an additional tank for power fluid, and several tube strings in addition to power fluid lines for the hydraulic systems.
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