Subsurface Hydraulic Pumping System

A complete hydraulic system consists of a fluid cleaning system, a power pump (usually a triplex), surface controls, a subsurface hydraulic pump, and tubing connecting the surface power pump to the subsurface pump. (See Section 3.1 for a complete description of power pumps.) Figure 6 is a schematic drawing of a complete multiple-well hydraulic system.

Because the pressurized power fluid supplies the energy needed to pump the well, the heavy sucker rod string and the heavy structure of the pumping unit required with the sucker rod pump are eliminated. Therefore, the hydraulic system can produce larger volumes of fluid from greater depths than the sucker rod pump.

The subsurface pump can be either a reciprocating piston pump or a jet pump. Both are usually made to be interchangeable in the same subsurface installation. Both pumps are

FIGURE 5 Winch pumping unit (Bethlehem Steel)
FIGURE 6 Multiple-well hydraulic installation: (A) power fluid tank, (B) triplex pump, (C) manifold, (D) wellhead, (E) tubing, (F) bypass valve (Kobe)

made in what is called the free pump configuration, which means that the pump assembly can be pumped down into the well or pumped out of it by the same surface power fluid system that powers the pump during normal operation with the pump on the bottom. The free pump feature gives the hydraulic pump a distinct servicing advantage over the sucker rod pump, which requires a mast or derrick over the well and a pulling unit to pull the rods one or two at a time when the pump needs servicing.

There are two basic types of hydraulic installations for the free pump: parallel free and casing free. Both result in a U-tube arrangement in which one leg of the U tube delivers the power fluid to the pump at the bottom and the other leg directs spent power fluid plus production back up to the surface. In the parallel free system, two parallel tubing strings* make up the U tube, and both are lowered into the well casing. In the casing free system, only one string is lowered into the well, and the annulus between this tubing and the well casing acts as the second leg of the U tube. Figure 7a illustrates installing a hydraulic pump in a parallel-free installation. A standing valve closes the bottom of the U tube, and the two strings are filled with fluid. The pump is inserted into the power fluid string, which is then securely capped, and the four-way valve is set to pump power fluid down the power fluid tubing. When the hydraulic pump reaches the bottom, a tapered bottom plug engages a seat on the standing valve and forms a pressure-tight seal. At the same time, an elastomer seal at the top of the pump enters a seal that now forces power fluid into the hydraulic pump (Figure 7b) to start it pumping (as described later). When it is desired to retrieve the pump to replace worn parts, the position of the four-way valve is reversed and power fluid is directed down the production tubing (Figure 7c.) The standing valve closes, the pump is unseated, and the circulation of fluid carries the pump to the surface, where it latches into the cap. The pressure is bled off from the U tube, and the wellhead cap with the worn pump is removed from the well.

*Individual joints of tubing coupled together to reach from the wellhead to the subsurface pump are called tubing strings.

FIGURE 7A through C Hydraulic parallel free pump installation

FIGURE 7A through C Hydraulic parallel free pump installation

FIGURE 8 Casing free installation

FIGURE 8 Casing free installation

The casing free installation (Figure 8) employs a packer that anchors and seals the tubing to the casing just below the pump assembly. The annulus between the tubing and casing above the packer takes the place of the production string in the parallel free installation. In operation, the power fluid is pumped down the power fluid string as before, but the mixture of exhaust power fluid plus production is returned to the surface in the tubing casing annulus. The annulus below the standing valve is at pump suction pressure. The casing free hydraulic pump installation is more economical because it requires only one string of tubing. It also has the advantage that the return fluid passage up the annu-lus is very large and fluid frictional losses are very low.

Natural gas separates from the crude oil as it enters the well bore. In the parallel free installation, this gas rises to the surface in the annulus between the tubing strings and the casing, entering the flow line with the produced fluid at the wellhead. In the casing free installation, however, this annulus is used to return the liquid discharged from the pump, and all of the produced gas must be pumped with the produced liquid. The hydraulic pump is not very efficient in pumping gas. Therefore, in gassy wells, the pump efficiencies for the casing free system will be lower.

Both the reciprocating hydraulic pump and the jet pump are designed to operate on fluid from the well. Either crude oil or water may be used as the power fluid. Sand particles and other abrasives contained in the produced fluid are removed either by gravity separation in surface settling tanks or by centrifugal force in cyclone separators. The cleaned

FIGURE 9 Typical central system (Kobe)
FIGURE 10 One-well unitized system (National Supply)

oil or water is then pressurized—typically to between 2500 and 4000 lb/in2 (172 and 276 bar)—by a power pump at the surface, and then the high-pressure fluid is returned down the power fluid tubing to operate the subsurface hydraulic pump. The exhausted power fluid from the hydraulic pump mixes with the produced fluid from the well, and the mixture is returned to the surface in the parallel side string or the casing tubing annulus.

Figure 9 illustrates a typical central system. The power fluid is separated by settling in tanks. A triplex pump or a battery of triplex pumps manifolded together supplies the power fluid to a number of oil wells. Figure 10 shows a one-well unitized system, in which the combined stream of well production and power fluid is brought into a pressure vessel for a brief period for gas separation and solids settling. The selected fluid is then passed through a cyclone for cleaning before going to the surface pump. The abrasive-laden underflow from the cyclone and the surplus fluid from the pressure vessel then go into the flow line and are carried to the lease tanks. This fluid constitutes the well production.

Subsurface Reciprocating Hydraulic Pumps A subsurface reciprocating hydraulic pump basically consists of an engine piston and cylinder with an engine reversing valve, and a pump barrel and plunger. These are assembled into one unit, and a polished rod connects the engine piston to the pump plunger so the two reciprocate together.

Several designs of subsurface hydraulic reciprocating pumps are available. Figure 11a is a schematic of a double-acting pump. The engine valve directs high-pressure power fluid below the piston and opens the area above the piston to exhaust pressure on the upstroke. On the downstroke, it directs high-pressure power fluid above the piston and exhausts the power fluid below the piston to low pressure. The double-acting design exhausts an equal amount of power fluid from the engine and produces fluid from the pump on the upstroke and downstroke.

Figure 11b illustrates a balanced-design pump where the polished rod area is equal to half of the piston area. The underside of the piston is always connected to high-pressure power fluid. On the downstroke, the engine valve directs high pressure on top of the piston. Because this area is larger than the bottom area of the piston, the unit makes a down-stroke. On the upstroke, the engine valve exhausts the area above the piston to low pressure. The high pressure below the piston then causes the unit to make an upstroke. The balanced design exhausts all of the spent power fluid on the upstroke and all of the produced fluid displaced by the pump on the downstroke.

Figure 11c illustrates a single-acting pump where the polished rod area is small relative to the piston area. The underside of the piston is always connected to high-pressure power fluid. The engine valve directs high-pressure power fluid to the top of the piston on the downstroke and exhausts it to low pressure on the upstroke. The single-acting unit exhausts all of the exhaust power fluid and most of the produced fluid displaced by the pump on the upstroke. Only the displacement of the polished rod is exhausted on the downstroke.

In general, subsurface hydraulic reciprocating pumps are used in small- to mediumvolume wells. When they are installed in 2§-in (60-mm) OD tubing, they are most commonly used to produce 25 to 500 barrels/day (4 to 80 m3/day); in 2|-in (73-mm) OD tubing, from 50 to 1000 barrels/day (8 to 158 m3/day); and in 31-in (89-mm) OD tubing, from 100 to 1500 barrels/day (16 to 240 m3/day). Where conditions are right, these volumes can be exceeded, but in the higher volume ranges, the jet pump is usually a better application. Most subsurface hydraulic units have a maximum power fluid pressure rating of 4000 lb/in2 (278 bar).

Reciprocating hydraulic pumps are available in several pressure ratios. For moderate-depth wells, the engine cylinder and the pump barrel can be of the same diameter. In deeper wells, a pump plunger smaller than the engine cylinder is used; thus the operating pressure can be reduced proportionally, but a proportionally greater quantity of power fluid will be required. Hydraulic pumps are also offered with tandem engines and single pump for deep wells and with single engine and tandem pumps for shallow wells. The pump-to-engine-area ratio in these models varies from a low of 0.40 to a high of 2.00. There is no standardization of design among the various manufacturers, and the models of each are so diverse that no typical charts are offered here. Data can be obtained from the individual manufacturers.

As mentioned previously, the complete up and down cycle of the piston and plunger is called a stroke, and the distance traveled in one direction is called the stroke length. Stroke lengths offered vary from 1 to 5 ft (0.3 to 1.5 m). Stroke-per-minute ratings are from 200 on the shortest stroke to 50 on the longest stroke. The strokes are visible on the surface pressure gage because the pressure rises at each reversal when fluid flow is momentarily interrupted. Because the triplex power pump is positive displacement, the power fluid rate to the pump is usually controlled by bypassing the unneeded surplus. Some triplex pumps are equipped with a three-speed manual transmission that can be used to vary the speed of the surface pump. By selecting the proper rotational speed, the output volume of the surface pump can be adjusted closely to the requirements of the subsurface pump. The triplex pump normally runs continuously, although occasionally in small-volume wells, it will be run intermittently by a time clock.

Subsurface Jet Pumps A jet pump has no moving parts, but it pumps by transferring momentum from a very-high-velocity—1000 ft/s (300m/s)-fluid jet to the pumped fluid

FIGURE 11 Designs of subsurface hydraulic pumps: FIGURE 12 Jet free pump

(a) double-acting, (b) balanced, (c) single-acting

FIGURE 11 Designs of subsurface hydraulic pumps: FIGURE 12 Jet free pump

(a) double-acting, (b) balanced, (c) single-acting

(Figure 12). At the nozzle exit, the jet creates a low-pressure area, thereby drawing in the fluid and carrying it to the throat of the pump, where the momentum transfer is completed. Then the combined stream enters the expanding tapered section of the diffuser, where the velocity head is converted back to a static head. The static head at this point is sufficient to lift the combined stream back to the surface. For a complete description of the general theory of jet pumps, see Chapter 4.

The jet pump has a broad range of application. It has been applied in wells as shallow as 800 ft (240 m) and as deep as 15,000 ft (4800 m). In general, a jet pump is capable of producing wells in the following volume ranges: in 2§-in (60-mm) OD tubing, from 25 to 8000 barrels/day (4 to 475 m3/day); in 2g-in (73-mm) OD tubing, from 50 to 6000 barrels/day (8 to 950 m3/day); and in 3fin (89-mm) OD tubing, from 100 to 12,000 barrels/day (16 to 1900 m3/day).

Jet pumps are less energy-efficient than reciprocating pumps. A well-designed jet pump may perform at approximately 40% hydraulic efficiency, as compared with more than 90% efficiency for a reciprocating pump. However, jet pumps do have numerous advantages. With no moving parts, they have a high level of reliability, and sustained runs of several years are not uncommon. In addition to their high volume capability, jet pumps have a higher tolerance to abrasives in the produced fluid as well as in the power fluid. They are also more efficient in handling entrained gas in the pumped fluid.

PRODUCTION RATE, m3/d 200 300
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