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BACKGROUND 
     Pumps can be used in wells to produce production fluids to the surface. One well known type of pump is a hydraulically actuated pump known as the PowerLift I, such as disclosed in U.S. Pat. Nos. 2,943,576; 4,118,154; and 4,214,854. Details of a system having this type of pump are reproduced in  FIG. 1 . The pump  30  deploys downhole in tubing  16  disposed in a wellbore casing  12 . Surface equipment  20  injects power fluid (e.g., produced water or oil) down the tubing  16  to the pump  30 . The power fluid enters the pump&#39;s inlet  32  and operates the pump  30  internally between upstrokes and downstrokes. In its upstroke, the pump  30  draws production fluid from below a packer  14  into the pump&#39;s intake  34 . As shown, the production fluid may enter the wellbore&#39;s casing  12  through perforations  13 . Subsequently operated in its downstroke, the pump  30  discharges the produced fluid and spent power fluid into the tubing  16  via ports  36 . The discharged fluid then passes through ports  18  in the production tubing  16  and eventually travels via the tubing-casing annulus to the surface equipment  20  for handling. 
     Internal details of the pump  30  and its operation are shown in  FIGS. 2A-2B . The pump  30  has an engine piston  50 , a reversing valve  60 , and a pump piston  70 . A rod  55  interconnects the engine piston  50  to the pump piston  70  so that the two pistons  50 / 70  move together in the pump  30 . Power fluid used to actuate the pump  30  enters the pump  30  via inlet  32  and travels into an engine barrel  40  via ports  42 . Inside the barrel  40 , the power fluid acts on the engine piston  50 . The reversing valve  60  within the engine piston  50  alternately directs the power fluid above and below the piston  50 , causing the piston  50  to reciprocate within the engine&#39;s barrel  40 . In the upstroke shown in  FIG. 2A , mechanical force from a push rod  62  initiates the shifting of the reversing valve  60  downward, after which hydraulic force from the fluid continues to shift the valve  60  downward. This shifting diverts the power fluid to the volume of the barrel  40  above the engine piston  50 , and the buildup of power fluid causes the engine piston  50  to move downward in the engine&#39;s barrel  40 . In the downstroke shown in  FIG. 2B , mechanical force and then hydraulic force shift the reversing valve  60  upward. The power fluid fills the barrel&#39;s volume below the engine piston  50  and causes the piston  50  to move upward. 
     The pump piston  70  connected to the engine piston  50  by rod  55  moves in tandem with the engine piston  50 . When moved, the pump piston  70  operates similar to a conventional sucker rod pump. At the start of the upstroke shown in  FIG. 2A , a traveling valve  75  closes, and a standing valve  35  opens. The fluid in the piston barrel  45  above the pump piston  70  is then displaced out of the pump&#39;s barrel  45  via port  36  as the pump piston  70  continues the upstroke. The fluid passes out tubing port  18  and then to the surface. 
     The upstroke reduces the pressure in the barrel  45  below the pump piston  70  so that the resulting suction allows production fluid to enter the barrel  45  through the open standing valve  34 . At the start of the downstroke shown in  FIG. 2B , the traveling valve  75  opens, and the standing valve  34  closes. This permits the production fluid that entered the lower part of the barrel  45  below the pump piston  70  to move above the piston  70  through the open traveling valve  75 . In this way, this moved production fluid can be discharged to the surface on the next upstroke. 
     The hydraulically actuated pump  30  is preferred in many installations because initial movement of the reversing valve  60  is mechanically actuated. This allows the pump  30  to operate at low speeds and virtually eliminates the chances that the pump  30  will stall during operation. Unfortunately, the pump  30  can suffer from problems with gas lock, especially in a wellbore that produces excessive compressible fluids, such as natural gas, along with incompressible liquids, such as oil and water. 
     During operation, for example, the pump  30  can easily draw gas through the standing valve  34  during the piston&#39;s upstroke. On the downstroke with the standing valve  34  closed, incompressible fluid in the lower volume of the piston barrel  45  is expected to force the traveling valve  75  open. Because gas between the traveling valve  75  and the standing valve  34  will compress, the hydrostatic head of the fluid above the traveling valve  75  may keep the traveling valve  75  from opening. On the upstroke, the gas and liquid above the standing valve  34  may then prevent any more fluid from being drawn into the pump barrel  45  because the compressed gas merely expands to fill the expanding volume. When this occurs, the pump  30  will alternatingly cycle through upstrokes and downstrokes, but it will simply compress and expand the gas in the pump barrel  45  caught between the standing valve  34  and the traveling valve  75 . When this gas lock occurs, the pump  30  fails to move any liquid to the surface. 
     Because gas lock can be an issue, operators may use other types of pumps that minimize the possibility of gas lock. One such pump is the Type F pump such as disclosed in U.S. Pat. No. Re 24,812. Functionally, the Type F pump operates in a similar way to the PowerLift I pump described above. To minimize gas lock, the Type F pump pressurizes produced fluid to discharge pressure. However, the Type F pump is entirely hydraulically shifted without the mechanical initiation found in the PowerLift I type pump so that the Type F pump can stall when operated at slow speeds. In addition, the Type F pump uses a bleed valve at the pump&#39;s discharge, which can be undesirable in some implementations. 
     What is needed is a hydraulically actuated pump that can operate at slow speeds but that can also reduce or prevent issues with gas lock conventionally found in such pumps. 
     SUMMARY 
     A hydraulic pump has an engine that is hydraulically actuated by power fluid communicated to the pump via tubing. A reversing valve in the engine controls the flow of the power fluid inside the engine and controls the flow of spent power fluid from the engine to a pump piston disposed in a pump barrel. Moved by the engine, the pump piston moves in upward and downward strokes and varies separate upper and lower pump volumes in the pump barrel. 
     The hydraulic pump disclosed herein avoids problems with gas lock found in conventional pumps. To do this, the pump compresses discharge fluid to a discharge pressure and expels an entire volume of the discharge fluid to the annulus during operation. During the upstroke, for example, the pump piston draws production fluid through an inlet valve into the pump&#39;s lower volume and discharges produced fluid and spent power fluid in the pump&#39;s upper volume through a discharge outlet to the annulus between the pump and the bottom hole assembly. During the downstroke, the produced fluid in the pump&#39;s lower volume is redirected through a first check valve to the pump&#39;s upper volume. During the upstroke, this first check valve prevents the produced fluid in the pump&#39;s upper volume from being redirected to the pump&#39;s lower volume. Instead, a second check valve controls flow of the fluid in the pump&#39;s upper volume to the discharge outlet. 
     The volume of the spent power fluid directed from the engine to the pump&#39;s upper volume during the upstroke is greater than the pump&#39;s upper volume. Because the spent power fluid is typically water, oil, or some other incompressible liquid, the fluid in the pump&#39;s upper volume during the upstroke will have enough liquid to be discharged from the upper pump volume to the annulus regardless of the amount of produced gas contained in the upper volume. With the decreasing of the upper pump volume, the pump piston can also compress any compressible portion of the fluid in this upper volume. Eventually during the upstroke, the bias of the second check valve opens at a discharge pressure in response to the decreasing upper pump volume, and the entire volume of fluid in the upper pump volume (except of course for remnants in some spaces) is expelled out of the upper volume when discharging fluid out of the pump. These operations of the pump all combine together to prevent gas lock. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a pump according to the prior art disposed in production tubing in a wellbore. 
         FIG. 2A  shows a cross-section of the prior art pump during an upstroke. 
         FIG. 2B  shows a cross-section of the prior art pump during a downstroke. 
         FIGS. 3A-3E  illustrate a cross-sectional view of a hydraulically actuated pump according to the present disclosure during an upstroke. 
         FIGS. 4A-4B  show the pump section of the disclosed pump in additional detail. 
         FIGS. 5A-5B  show portions of the disclosed pump during a downstroke. 
         FIG. 6A  shows a schematic view of the disclosed pump during an upstroke. 
         FIG. 6B  shows a schematic view of the disclosed pump during a downstroke. 
     
    
    
     DETAILED DESCRIPTION 
     A hydraulically actuated pump  100  shown in  FIGS. 3A-3E  has an engine section  110  (shown primarily in  FIGS. 3A-3C ) and a pump section  115  (shown primarily in  FIGS. 3C-3E  and also shown in isolated detail in  FIGS. 4A-4B ). As shown in  FIG. 3B , the engine section  110  has an engine piston  130  movably disposed within an engine barrel  120 . As shown in  FIG. 3D , the pump section  115  has a pump piston  150  movably disposed within a pump barrel  140 , which is separate from the engine barrel  120 . A rod  160  shown in  FIGS. 3C-3D  interconnects these two pistons  130 / 150  so that the two pistons  130 / 150  move in tandem in their respective barrels  120 / 140 . The rod  160  has an internal passage  162  and passes through seal elements  164  ( FIG. 3C ) where the engine and pump barrels  120 / 140  are divided from one another. These seal elements  164  isolate fluid from passing on the outside of the rod  160  between the barrels  120 / 140 . However, as discussed later, the rod&#39;s passage  162  does allow fluid to communicate between the barrels  120 / 140  during operation of the pump  100 . 
     Briefly, the engine piston  130  is hydraulically actuated between upward and downward strokes by power fluid communicated from the surface to the pump  100  via tubing  16 . As the engine piston  130  strokes, the pump piston  150  is moved in tandem with the engine piston  130  by the rod  160 . The pump piston  150  varies two volumes  142 / 144  of its barrel  140 , sucks in production fluid into volume  144 , and discharges produced fluid and spent power fluid out of volume  142  in the process. To actuate the engine section  110 , a reversing valve  180  ( FIG. 3B ) is disposed in the engine piston  130 . This reversing valve  180  controls the flow of the power fluid within separate volumes  122 / 124  of the engine barrel  120  and controls the flow of the spent power fluid from the engine barrel  120  to the pump barrel  140 . 
     With a basic understanding of the pump  100 , discussion now turns to further details of the pump  100  and its operation. As noted previously, power fluid communicated to the pump  100  via the tubing  16  actuates the pump  100 . Turning first to the engine section  110  (shown primarily in  FIGS. 3A-3C ), the power fluid enters the top of the pump  100  via a head  200  ( FIG. 3A ) having ports at  201  and having a check valve  202 . Entering the ports at  201  and passing through a passage  204 , the power fluid travels out cross ports  206  and into an annulus  17   a  between the tubing  16  and the pump&#39;s housing  102 . Seating cups  208  ( FIG. 3A) and 210  ( FIG. 3C ) isolate this portion of the annulus  17   a  from the rest of the tubing  16 . Eventually, the power fluid in the annulus  17   a  enters the engine barrel  120  through cross ports  125  ( FIG. 3C ). (Passage of the power fluid from the tubing  16  to the engine barrel  120  is also shown in the schematic illustration of the pump  100  in  FIG. 6A ). 
     Power fluid from the cross ports  125  enters the lower engine volume  124 . Filling this lower volume  124 , the power fluid interacts with the surfaces of the reversing valve  180  ( FIG. 3B ) and moves the valve  180  to either an upper or lower position on the piston  130 . Depending on pressure levels and the current stroke of the pump  100 , the power fluid shifts the valve  180  from one position to the other, thereby controlling the flow of the power fluid in the engine section  110  and controlling the strokes of the pump  100 . 
     In  FIG. 3B , the reversing valve  180  is shown in its lower position during the pump&#39;s downstroke. In  FIG. 5A , the valve  180  is shown in its upper position in  FIG. 5A  during the pump&#39;s upstroke. Looking at this upper position in  FIG. 5A , the reversing valve  180  closes off a side passage  182  and restricts the flow of power fluid from the engine&#39;s lower volume  124  into the upper volume  122 . Yet, the reversing valve  180  moved from its seat  186  permits the spent power fluid in the engine&#39;s upper volume  122  to pass through side passages  188   a  and  188   b  and into the rod&#39;s passage  162 . Thus, during the upstroke with the valve  180  in its upward position, power fluid entering the engine section  110  only acts upon the engine piston&#39;s lower end, thereby urging the engine piston  130  upward in the housing  102 . In addition, the reversing valve  180  in its upward position routes the spent power fluid above the engine piston  130  to the pump&#39;s upper volume  142  where it can mix with produced fluid. 
     In the upstroke, the engine piston  130  draws the pump piston  150  ( FIG. 3D ) upward via the interconnecting rod  160 . Focusing now on the pump section  110  (shown primarily in  FIGS. 3C-3E  and shown in isolated detail in  FIGS. 4A-4B ), the upward drawn pump piston  150  decreases its barrel&#39;s upper volume  142  while increasing the lower volume  144 . The suction induced in the lower volume  144  draws in production fluid as one or more standing valves  170  ( FIG. 3E ) open and allow the fluid to enter the production fluid inlet  145 . (Drawing of production fluid into the pump&#39;s lower volume  142  during the upstroke is shown in  FIG. 6A ). 
       FIG. 3E  shows one standing valve  170 , while  FIG. 4B  shows two standing valves  170 . The standing valves  170  can be ball valves each having a ball movable relative to a seat, although other types of valves can be used. In addition to standing valves, a production fluid valve  272  may also be used at the bottom of the assembly as shown in  FIG. 3E . 
     At the pinnacle of the upstroke, the pump  100  starts its downstroke with the reversing valve  180  shifting to its lower position shown in  FIG. 3B . Looking again at the pump&#39;s engine section  110  (shown primarily in  FIGS. 3A-3C ), an actuating pin  185  ( FIG. 3B ) abuts upper volume&#39;s top bumper  187  ( FIG. 3A ), mechanically initiating the shifting of the reversing valve  180  and allowing fluid pressure to motivate the valve  180  downward. Shifted to its lower position in  FIG. 3B , the reversing valve  180  permits the power fluid to flow from the engine&#39;s lower volume  124  into the upper volume  122  via the side passage  182  and a conduit passage  184 , which passes through the actuating pin  185 . At the same time, the reversing valve  180  engages its seat  186  and restricts the power fluid in the upper volume  122  from flowing into the rod&#39;s passage  162 . As a result, a volume of spent power fluid remains in the rod  160 , but power fluid is allowed to fill the engine&#39;s upper volume  122 . (Travel of power fluid in the engine section  110  during the downstroke is shown in  FIG. 6B ). 
     Because the engine piston  130 &#39;s area in the upper volume  122  is greater than its area in the lower volume  124 , the power fluid exerting pressure in the upper volume  122  urges the engine piston  130  downward, moving the pump piston  150  ( FIG. 3D ) downward as well. Focusing again on the pump section  110  (shown primarily in  FIGS. 3C-3E  and shown in isolated detail in  FIGS. 4A-4B ), the lower pump volume  144  decreases, while the upper volume  142  increases as the pump piston  150  urges downward in the piston barrel  140 . In addition, the one or more standing valves  170  close and prevent the produced fluid in the lower volume  144  from being expelled. Instead, the produced fluid in the lower volume  144  is forced out through the cross ports  146  ( FIG. 3E ) into an annulus  103  between the pump&#39;s barrel  140  and the housing  102 . Traveling up this annulus  103 , the produced fluid being sufficiently pressurized passes through a first internal valve  230  ( FIG. 3C ) and is drawn into the pump&#39;s increasing upper volume  142 . (Travel of produced fluid in the pump section  115  during the downstroke is best shown in  FIG. 6B ). 
     Looking again at the pump&#39;s engine section  110  (shown primarily in  FIGS. 3A-3C ), a shifter  132  on the engine piston  130  engages the lower end of the barrel  120  at or near the low point of the downstroke and mechanically initiates movement of the reversing valve  180  upward so that the power fluid in the engine section  110  can motivate the reversing valve  180  to its upward position as shown in  FIGS. 3C and 5A . The shifted valve  180  in this upward position blocks passage of the power fluid to the engine&#39;s upper volume  122 . The build-up of power fluid in the lower volume  124  causes the engine piston  130  to urge upward in an upstroke, while the spent power fluid in the upper volume  122  passes through the shifting valve  180  and the rod&#39;s passage  162  to the pump&#39;s upper volume  142 . (Travel of spent power fluid from the engine section  110  to the upper pump volume  142  during the upstroke is shown in  FIG. 6A ). 
     Focusing again on the pump section  110  (shown primarily in  FIGS. 3C-3E  and shown in isolated detail in  FIGS. 4A-4B ), the pump piston  150  ( FIG. 3D ) moves upward with the engine piston&#39;s movement upward. This increases the pump section&#39;s lower volume  144  to draw in new production fluid though the one or more open standing valves  170 . However, the upward moving pump piston  150  also decreases the pump&#39;s upper volume  142 , which already contains the previously produced fluid and now fills with the spent power fluid conveyed by the rod&#39;s passage  162  from the engine section  110 . (Flow of spent power fluid and previously produced fluid in the pump&#39;s upper volume  142  during the upstroke is shown in  FIG. 6A ). 
     During the upstroke and as shown in  FIG. 3C , the fluid in the pump&#39;s upper volume  142  is discharged at sufficient discharge pressure through a second internal valve  250 , out a discharge outlet  148 , and into an annulus  17   b  between the pump&#39;s housing  102  and the surrounding tubing  16 . As shown in  FIG. 3E , the discharged fluid in the annulus  17   b  eventually travels through a passage  282  in an assembly  280  connecting the tubing  16  to a parallel string  284  that carries the discharged fluid uphole. (Passage of discharged fluid to the parallel string  284  during the upstroke is shown in  FIG. 6A ). Although depicted in a free parallel arrangement, the pump  100  can be deployed using other arrangements known in the art, such as a fixed insert or a concentric fixed arrangement. 
     If the fluid in the pump&#39;s upper volume  142  is not entirely incompressible fluid, the second internal valve  250  permits compressible fluid in this volume  142  to be compressed during the upstroke before discharging the fluid through the outlet  148 . Thus, the fluid in the upper volume  142  can be part liquid and part gas (i.e., the spent power fluid being liquid, while the produced fluid diverted to the upper volume  142  being entirely or partially gas). In either case, the volume of the spent power fluid conveyed by the rod&#39;s passage  162  from the engine&#39;s upper volume  122  during the upstroke will be greater than the produced fluid (gas and/or liquid) diverted to the pump&#39;s upper volume  142 . Thus, any gas in the upper pump volume  142  can be compressed by the upward moving pump piston  150  to discharge pressure, and all of the fluid in upper pump volume  142  can be discharged through internal valve  250 , out the outlet  148 , and into the annulus  17   b . By compressing any gas in the pump&#39;s upper volume  142  and discharging all the fluid above the pump piston  150  (except for a small remnant in various spaces), the pump  100  does not reach a situation where the pump piston  150  merely compresses gas in its upper volume  142  but fails to discharge any fluid out of the pump  100 . In this way, the pump  100  can avoid issues with gas lock found in conventional assemblies. 
     The internal valves  230 / 250  are shown in more detail in  FIG. 5B . As noted previously, the first internal valve  230  controls fluid communication from the pump&#39;s lower volume  144  to its upper volume  142  ( FIG. 3D ). As shown in  FIG. 5B , the internal valve  230  is a check valve that allows fluid flow in one direction when a sufficient fluid pressure is reached to open the valve. The check valve  230  has an inlet  240  in fluid communication with the pump&#39;s lower volume  144  ( FIG. 3D ) via the annulus  103  and has an outlet  245  in fluid communication with the pump&#39;s upper volume  142 . A spring  236  or other biasing element disposed in a pocket biases a ring  234  toward the inlet  240 . Disposed between this ring  234  and the inlet  240 , at least one ball  232  seats against the inlet  240  to restrict fluid flow therethrough. Sufficient pressure exerted by produced fluid on the check valve  230  opens the valve  230  and allows the produced fluid to pass therethrough to the pump&#39;s upper volume  142 . 
     The second internal valve  250  is similar to the first valve  230  and has at least one ball  252 , a ring  254 , and a spring  256 . However, this second valve  250  has a reverse arrangement to control fluid flow from the upper pump volume  142  via inlet  260  to the pump&#39;s discharge outlet  148  via outlet  265 . Thus, sufficient pressure exerted by fluid in the pump&#39;s upper volume  144  on this second valve  250  opens the valve  250  and allows the fluid to pass therethrough to the discharge outlet  148 . 
     In addition to handling gas lock issues, the disclosed pump  100  also has features for handling any debris that may be present during operation. Fundamentally, the pump  100 &#39;s low speed operation helps to keep the velocity of produced fluid low enough so that debris is not motivated or otherwise mobilized to enter the pump&#39;s inlet  145 . Produced water from the reservoir (i.e., connate water) does not have a high debris carrying potential as long as its velocity remains low. Because the pump  100  can be operated at low speeds and keep the velocity of the produced fluid low, debris borne by the produced fluid may not be able to enter the pump&#39;s inlet  145  and may instead tend to collect and dune in the bottom of the casing. 
     To further handle debris that may attempt to enter the pump  100 , a sand screen  290  shown in  FIG. 3E  can be connected near the intake  274  of the bottom hole assembly downhole from the pump&#39;s inlet  145 . Although only a top portion is shown, the sand screen  290  has a mesh or the like (not shown) with passages that can prevent solid particulates in produced fluid from passing through the screen  290 . In this way, the sand screen  290  can prevent debris from entering the intake  274 , thereby preventing debris from disturbing the pump&#39;s operation. 
     If any very fine particles smaller than the passages in the sand screen  290  do enter the pump  100 , however, a sump or volume  286  can be provided in the bottom hole assembly  280  of the free parallel arrangement in  FIG. 3E . This sump  286  is downstream of the connecting passage  282  and can collect any produced debris that has passed through the pump  100 . Although shown with a particular size, it will be appreciated that the sump  286  can be larger than shown and can also include a tubing member coupled to the assembly  280  downstream from the passage  282 . 
     In addition to the above features, the pump  100  in some implementations may be fixed in the bottom hole assembly and may not be retrievable. In such a situation, the various flow passages inside the fixed pump  100  can be intentionally opened during operation to bypass solids through the pump  100 . The need to perform such a bypass operation will most likely be needed when the pump  100  is being used to pump a mixture of water and coal fines. 
     The foregoing description of preferred and other embodiments is not intended to limit or restrict the scope or applicability of the inventive concepts conceived of by the Applicants. In exchange for disclosing the inventive concepts contained herein, the Applicants desire all patent rights afforded by the appended claims. Therefore, it is intended that the appended claims include all modifications and alterations to the full extent that they come within the scope of the following claims or the equivalents thereof.

Summary:
A hydraulic pump avoids problems with gas lock found in conventional pumps. The pump draws in production fluid in a lower pump volume during the pump&#39;s upstroke and diverts the produced fluid to an upper pump volume during the downstroke. Spent power fluid is communicated to the upper pump volume during the pump&#39;s upstroke. The pump piston in the upstroke expels the entire volume via a check valve that communicates the upper pump volume with a discharge outlet. The check valve increasing the discharge pressure of the upper pump volume, the upper pump volume of the spent power fluid being greater than the upper pump volume, and the upper pump piston compressing produced gas in the upper pump volume all combine to prevent or reduce the chances that the pump will gas lock during operation.