Abstract:
A method for de-watering a gas well the water is pumped by an inverted API pump acting as a reciprocating pump from a position in the well casing below the gas formation and the gas escapes through the well casing around the transport tubes. The reciprocating pump is driven by a downhole engine in the form of a cylinder and piston which is moved by a hydraulic pump at the surface acting to generate a flow in a hydraulic fluid to drive the piston from a start position to an end position and causing fluid to be transferred from the cylinder on the other side of the piston to a counterbalance chamber against a back pressure provided by a charge of nitrogen gas. At the end of a pumping stroke of the reciprocating pump, pressure in the hydraulic fluid from the hydraulic pump through is released to cause the back pressure of the counterbalance chamber to drive the piston back to the start position.

Description:
[0001]     This application claims benefit of the date of filing under 35 U.S.C. 119 of Provisional Application 60/635,608 filed Dec. 14 th  2004. 
     
    
       [0002]     The invention relates in general to the field of artificial lift, well pumping systems and relates more specifically to designs meeting a unique set of economic criteria for equipment which can be deployed to de-watering shallow marginal natural gas wells.  
       BACKGROUND OF THE INVENTION  
       [0003]     The unique design requirement of lifting water from economically marginal gas wells, flows from a user group planning meeting on Mar. 4, 2004. PTAC (Petroleum Technology Alliance of Canada) which issued a call for technical papers on the topic of de-watering marginal shallow gas wells, with the southeast Alberta, Brooks area shallow gas pools in mind. The PTAC forum was subsequently held in Calgary, Alberta, May 12, 2004. The transcripts and reaction comments arising from the forum are available to the public. Proposals submitted for funding assistance did not meet the collective needs of the planning group at a June 2004 deadline. To date producer needs are still being met using labour intensive frequent swabbing and/or endless tubing cleanouts.  
         [0004]     Down-hole hydraulic pumps with the valving, piston and pump (and its variations) were originally developed under the trade names “Kobe” and “Oilmaster”. Both have been available to the industry for more than five decades. The product enjoys worldwide acceptance under the current direction of Weatherford Oil Tool. These pumps find special application lifting large volumes of light oil in deep wells.  
         [0005]     More recently Canadian application 2,258,237 by Cunningham suggested bringing the valving to the surface, and proposed using a downhole double acting hydraulic piston, three (3) strings of tube and a conventional oil well pump for placement in a horizontally drilled heavy oil well. The double acting feature of the hydraulic piston would be particularly useful as a pump pull-down in the highly viscous heavy oil applications for which the system was conceived.  
         [0006]     Canadian application 2,260,518 proposes using a down-hole rotary hydraulic drive, coupled to a progressing cavity pump rather than the reciprocating version suggested by the Cunningham application. Both address the task of pumping heavy oil in deviated well-bores.  
         [0007]     While not detracting from the general applicability of this invention and claim(s) therein, it is useful to focus on a specific pumping system designed for de-watering marginal gas wells.  
         [0008]     Data has not yet been made public, quantifying the increased cash value of recoverable gas reserves expected under pumped off de-watering conditions, as cited in Canadian application 2,341,129 by Nicholson.  
       SUMMARY OF THE INVENTION  
       [0009]     According to one aspect of the invention there is provided a method of pumping material from a well comprising: 
        providing a hydraulic pump at the surface;     providing a downhole single acting cylinder having a piston therein;     providing a downhole hydraulic counterbalance chamber having back pressure;     providing a first transport tube from the hydraulic pump at the surface to the downhole cylinder;     providing a downhole pump for receiving and pumping the material;     providing a second transport tube from the downhole pump to the surface;     causing the hydraulic pump at the surface to transmit hydraulic fluid from the surface to the downhole cylinder on one side of the piston to drive the piston through a stroke from a start position to an end position and causing fluid to be transferred from the cylinder on the other side of the piston to be transferred to the counterbalance chamber against the back pressure;     causing the movement of the piston to drive a piston rod to actuate the downhole pump to pump the material;     and at the end of a pumping stroke of the downhole pump, causing the back pressure of the counterbalance chamber to drive the piston back to the start position.        
 
         [0019]     Preferably the use of a dual tubing hydraulic well pumping system achieves a low energy consumption transfer of power down a well bore.  
         [0020]     Preferably the hydraulically balanced “U” tube configuration requires that only a limited amount mass be moved to complete a pumping cycle, and it does so at low fluid friction loss compared to that required of conventional mechanical pumping.  
         [0021]     Preferably the use of a downhole, gas over hydraulic oil counterbalance chamber eliminates the capital cost and space requirement of a third hydraulic tubular string.  
         [0022]     Preferably the pump comprises an inverted compression stroke tubing rod pump directly connected to the piston rod of the downhole hydraulic piston.  
         [0023]     Preferably the method is used for de-watering shallow, marginally economic gas wells wherein the material is water collected at the downhole pump and the gas escapes through the well casing around the first and second tubes.  
         [0024]     An economically viable pumping system should meet or exceed these criteria: 
        A capital cost target of $C30,000 (high flow wells $C130,000)     Incremental operating cost ranging $C200-$C300/month     A low maintenance pump system, likely coil tubing conveyed     A new pumping power source, solar, wind, batteries or combinations of     Simple, low cost solution to deal with the water     Pump types which can handle abrasive solids in waters which range from those with some solids, all the way up to muddy colloidal suspensions (gummy).        
 
         [0031]     The claims and equipment modifications embodied in this invention are considered capable of meeting and/or exceeding all except the last of the given criteria. Pumping well liquids containing abrasives has long been a problem for the industry. In this set of field operation conditions, solutions are required both in pump metallurgy and getting the sand to surface because of the low velocities associated with small volumes of water being lifted.  
         [0032]     High energy consumption progressive cavity pumps are commercially available to pump high solids liquids, but in this case would defy most of the other given criteria. Operators will continue to rely on frequent swabbing and coil tubing foam cleanouts for the worst of the well cases. The equipment embodied in this invention partially addresses the problem, but is limited in application to wells producing clear to mildly abrasive waters.  
         [0033]     The arrangement thus provides in general terms a novel means of transferring power down a well tube to a hydraulic/mechanical pump device, and lift produced water up a second tube in the well.  
         [0034]     It does so at an unprecedented low level of system energy consumption. Solar/wind energy drive combinations become economically viable at current cost regimes.  
         [0035]     It is useful to focus on a specific field pumping application to better convey the ideas embodied in the invention. To this purpose, we choose to describe applying the device to the task of de-watering shallow, marginally economic gas wells. The concepts do, however, have application in pumping other below ground liquids such as potable water and low viscosity crude oil.  
         [0036]     The arrangement thus provides a dual tubing or concentric hydraulic well pumping system to achieve a low energy consumption transfer of power down a well bore. The hydraulically balanced “U” tube configuration requires that only a limited amount mass be moved to complete a pumping cycle, and it does so at low fluid friction loss compared to that required of conventional mechanical pumping.  
         [0037]     The arrangement thus provides a downhole, free piston oil over gas counterbalance chamber, thus eliminating the capital cost and space requirement of a third hydraulic tubular string.  
         [0038]     The arrangement thus provides an inverted compression stroke tubing rod pump directly connected to a downhole hydraulic piston engine apparatus.  
         [0039]     Preferably the two tubes are arranged with an inner one inside an outer one. Although a parallel arrangement can also be used.  
         [0040]     This concentric arrangement allows the two tubes to be run into the gas well with the outer tube passing through a lubricator while the well is under pressure, thus avoiding the requirement to “kill” the well using water back flow.  
         [0041]     In a particularly advantageous arrangement the reciprocating pump is an inverted API pump since this can accommodate the influx of some gas with the liquid without gas lock up of the pump.  
         [0042]     According to a second aspect of the invention there is provided a method of pumping liquid from a first location to a second location comprising: 
        providing a hydraulic pump at the second location for generating a flow of hydraulic fluid under pressure;     providing a first transport tube extending from the hydraulic pump at the second location to the first location;     providing at the first location a reciprocating pump operable in response to the supply of hydraulic fluid for receiving and pumping the liquid;     providing a second transport tube from the reciprocating pump at the first location to the second location;     wherein there is provided a sensor for measuring pressure in the first transport tube and wherein changes in the pressure are used to detect when there is insufficient liquid in the reciprocating pump during a pumping stroke to avoid pumping when no liquid is present.        
 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0048]     One embodiment of the invention will now be described in conjunction with the accompanying drawings in which:  
         [0049]      FIG. 1  is an overview of a system according to the invention which is used for pumping waste water from a gas well, the system including dual parallel string arrangement.  
         [0050]      FIG. 2  is a theoretical graph of pump pressure.  
         [0051]      FIG. 3  is a longitudinal cross-sectional view of the downhole components of the system of  FIG. 1 , the system being modified to include a dual concentric string arrangement.  
         [0052]      FIG. 4  is a transverse cross-sectional view of the downhole components of the system of  FIG. 1 . 
     
    
     DETAILED DESCRIPTION  
       [0053]     As shown in  FIGS. 1, 3  and  4  there is provided an apparatus for pumping water from a gas well.  
         [0054]     The gas well is generally indicated at  24  and includes a gas formation  24 A and a well casing  24 B for transporting that gas to the surface for collection in conventional manner. The structure of the well casing and the gas formation are shown only schematically as these are well known to a person skilled in the art.  
         [0055]     As is well known, water tends to collect at a lower end  24 C of the well casing which can increase in depth to a situation where the water interferes with the production of gas from the formation  24 A. The intention is that the water level be maintained below the gas formation at a water level  24 D.  
         [0056]     A pumping system for removing the water at low energy consumption includes a downhole section  28  which communicates through first and second transport tubes  27  and  30  to the surface. First transport tube  27  connects to a hydraulic pump  23 . The hydraulic pump  23  is controlled by a control unit  25  which includes inputs  25 A from a timer and  25 B from a pressure sensor connected to the first transfer tube  27  and therefore responsive to the pressure within that tube. The second transfer tube  30  transports the pumped water to a water storage system  30 A. Electric power for the hydraulic pump is supplied from a battery storage system  21  which is powered by a solar array  20  and/or by other power systems  20 A. The other power systems generally are of a nature which uses relatively low level of energy and particularly a low level of purchased energy so that recycling system such as wind energy can be used.  
         [0057]     The downhole system  28  includes a hydraulic piston  28 A, a pump  28 B driven by the cylinder and a counter balance chamber  28 C connected to the cylinder. These components are formed by cylindrical housings connected end to end in a row so that the cylinder  28 A is located between the pump  28 B and the counter balance chamber  28 C. The cylindrical components are connected together by connectors thus providing a first connector sub  28 D between the cylinder  28 A and the counter balance chamber  28 C, a second connecting sub  28 E between the cylinder and the pump and a third connecting sub  28 F connecting between the top of the pump and the transfer tubes  27  and  30 .  
         [0058]     In general the system operates by the hydraulic pump  23  generating pressure in a hydraulic fluid which is supplied through the transport tube  27  to the cylinder  28 A. This cylinder  28 A contains a piston  6 , as shown in  FIG. 3 , so that the supply of fluid to the underside of the piston  6  through the tube  5  acts to drive the piston upwardly. A piston rod  7  communicates the upward movement to a pump piston  9  within a cylinder  10  of the pump  28 B. Thus supply of the fluid through the tube  27  drives the pump upwardly to push collected water from the cylinder  10  into the tube  30  for transfer to the surface.  
         [0059]     Meanwhile fluid from the upper side of the cylinder  28 A is transferred to the counter balance chamber  28 C through a pipe  2 . Within the counter balance chamber  28 C is provided a gas chamber so that supply of the hydraulic fluid from the cylinder  28 A into the counter balance chamber  28 C compresses the gas to form a back pressure which increases as the piston  6  moves along the cylinder  28 A.  
         [0060]     In general when the piston  6  reaches the upper end of its stroke thus completing the stroke of movement of the pump piston  9 , the hydraulic pump  23  is closed off and the valve  25 C actuated to release the pressure in the transfer tube  27 . This release of pressure allows the back pressure in the counter balance chamber  28 C to return the fluid to the upper part of the cylinder  28 A thus returning the piston  6  and the pump piston to the initial position for a further subsequent stroke.  
         [0061]     The transfer of fluid from the hydraulic pump to the cylinder  28 A requires little movement of hydraulic flow and the return stroke of the cylinder merely acts to return the same level of fluid back to the surface. The amount of fluid therefore pumped is very low in order to achieve each single stroke of the pump.  
         [0062]     With reference to  FIGS. 1 and 2 , the solar array  20  may be used to recharge the storage batteries  21  during daylight hours. Power demand by the hydraulic pump  23  from a solar source is limit to 3 hours per day in winter operation, but is extended by using other choices of power (electrical grid, wind, engine drive) and/or daily on-time setting. The concept of downhole pumping using a hydrostatically balanced “U” tube system, and tube sizing for low friction loss laminar flow, typically leads to less than a 2 horsepower energy draw during each 1.5 minute stroke of the downhole cylinder  28 A and direct coupled to the one (1.3) liter plunger pump  28 B. Many marginal gas wells load up with less than 1 cu. meter of water in two weeks of production.  
         [0063]     The control unit  25  groups the adjustable instrument systems and data gathering in an explosion proof well site enclosure. Pump motor start/stop and controlled pressure bleed back of the hydraulic oil each pumping cycle is located in this enclosure. The control unit includes a micro controller which additionally stores data for retrieval.  
         [0064]     On signal, the hydraulic power pack  23  pumps hydraulic fluid down the primary coil tubing string  27  which may be for example a 1″ (25.4 mm) tube to initiate an up-stroke of the downhole hydraulic piston  28 A. Hydraulic fluid trapped above the engine piston is stored at increasing pressure in the nitrogen gas filled counterbalance chamber  28 C. At the same time, the direct coupled plunger piston and traveling valve above the engine, forces produced water to the surface through a secondary coil tubing string  30  which may be for example a 1¼″ (31.8 mm) tube. The standing and traveling valve arrangement in the plunger pump  28 B accommodates the pumping action.  
         [0065]     The pump is shut down and a controlled pressure bleed-back at the surface is initiated when the surface control system senses the “pressure spike” of the downhole engine piston reaching the top of its stroke. Depending on pre-selected cylinder area ratios and stroke length, a given pump system may typically produce 1.5 liters of formation water per cycle. The low energy requirement and engineered well system supports the investment and operating cost criteria of dewatering marginal gas wells.  
         [0066]     Referring now to  FIGS. 3 and 4 , the sub-surface pumping system consists of the three cylindrical chambers  28 B,  28 A and  28 C stacked one on top another, sized to fit a given well casing internal dimension. Each is connected to the other by threaded subs  28 D,  28 E and  28 F. Each sub is internally ported to accommodate various fluid passages. Hydraulic fluid moves to and from these ports through external high-pressure tubes. Looking down the well casing on a plan view,  FIG. 4  shows how one such pumping assembly is arranged eccentrically to fit in a slim-hole 4½″ (114.3 mm) casing  24 B.  
         [0067]     The entire sub-surface pump assembly is pre-charged with pressurized nitrogen, purged of air pockets trapped in the cylinders and lowered to well setting depth attached to the outer coil steel tubing string  27 .  
         [0068]     The lowest of the three cylinders, namely the counter balance chamber  28 C is constructed with an external tube  2  so as to carry pressurized hydraulic oil from top of the chamber  28 C past Sub  28 D, and thence externally to the top of the piston area shown in the cylinder  28 A. The sub  28 G seals the bottom of the counter balance chamber  28 B  
         [0069]     In the arrangement shown in  FIG. 3 , the fluids are conveyed into and from the gas well using concentric, not parallel coiled tubes  27  and  30 . Thus the tube  30  surrounds the tube  27  and includes an expanded portion  30 A which surrounds also the pump in the sub  28 B and connects to the sub  28 E. This communicates the hydraulic fluid under pressure to the sub  28 E which then conveys it through the tube  5  to the bottom of the sub  28 A underneath the piston  6 . A barrier cylinder free piston  6 A is provided on top of the fluid supplied through the pipe  5  so as to separate the fluid from a charge of oil between the free piston  6 A and the piston  6 . A stop  6 B in a further sub  6 C acts to limit the movement of the piston  6 A. Thus a “water back to oil” barrier cylinder and free piston section is provided below the engine, that is the cylinder  28 A and the piston  6  in that cylinder.  
         [0070]     The accumulator section  28 C has a free piston  3  separating the fluid from the engine supplies through the tube  2  from the nitrogen gas. Thus the piston  3  defines a chamber which contains the N 2  gas when the tool is in the horizontal transport position.  
         [0071]     A produced water inlet  10 A is provided at the bottom of the cylinder  10  and is covered by an engineered sand screen  10 B of known technology at the intake ports.  
         [0072]     A seal mandrel  27 A of known technology for the produced formation water coil tube  27  is provided at the connection between the tube  27  and the top of the cylinder  10  or the reciprocating pump.  
         [0073]     The nitrogen gas cushion is added at the surface. The pressure used in the cushion  4  is a technical calculation based on the hydrostatic pressure head in the hydraulic power oil tube  5  plus 15% for over-pressure to bottom out the engine piston after each power stroke.  
         [0074]     The cylinder  28 A includes the double acting hydraulic cylinder piston  6  and piston rod  7  which are constructed to seal under high internal pressure 2,000 psi (14,000 kpa) both inside the tube and at the sub  28 E through which the rod passes.  
         [0075]     When the time delay relay in the surface control system signals the start of a new pumping cycle, hydraulic flow down the primary coil tubing string  27  transfers pressure energy to the engine piston  6  in the lower section of the cylinder  28 A. At the instant this applied flow pressure overcomes the forces of fluid flow friction, pressure ballooning in the tube and compression of the 15% over-pressure preload in the nitrogen cushion  4 , the engine piston  6  will travel upward. Hydraulic liquid in the area above the engine piston  6  will be returned to storage through the pipe  2  under increasing pressure in the counter balance chamber  4 . The piston rod  7  carried by the piston  6  moves the plunger  9  in the pump chamber tube  10  in a vertical compression stroke so as to force accumulated well fluids collecting in the chamber  10  into the outer coil tubing string  30  and up to surface.  
         [0076]     An elastomer rod seal is positioned at sub  28 E to wipe abrasive solids from the exposed portion of the piston rod  7 . The power stroke is ended when both the engine piston  6  and the pump plunger  9  “top out” in their respective tubes. The pressure spike in the liquid system is sensed back at the surface by sensor  25 B. The pressure switch instrument device shuts the hydraulic pump off, and at the same time opens the valve  25 C which forms a pressure bleed-back solenoid valve loop. The controlled pressure bleed-back part of the pumping cycle begins as shown in the data chart displayed in the  FIG. 2 .  
         [0077]     The pump  28 B is a modified traveling barrel API (American Petroleum Institute) sucker rod pump, common to the oil industry. Arranging the pump for a compression type up-stroke is unique in an arrangement of this type. The plunger pump is, in itself, a precision hardened and honed tool, capable lifting well liquids to surface at high hydrostatic pressure. The hollow plunger  9  is an elongated version of the shorter hydraulic piston  6  situated below. The “soft pack” seals on the pump plunger serve to prolong run life in a somewhat un-lubricated and abrasive well fluid pumping environment. During the controlled pressure bleed back at the surface, a standing valve  13  in the oil well pump chamber closes so as to prevent a back-flow of the water from the coil tube string  30 . At the instant when pressure, both above and below the engine piston  6  is “balanced”, the plunger moves slowly downward. When a void space is created above the pump plunger valve  11 , well fluids (both liquids and some gas) flood into pump intake ports  15 , up through the hollow plunger tube interior, and into the void.  
         [0078]     While conventional top stroking rod pumps often “gas lock”, given any liquid entry at all, the inverted API pump is inherently superior at compressing gas. Gas lock is routinely cleared by this construction.  
         [0079]     The system disclosed herein thus provides a technique for pumping water to the surface where the power requirements are sufficiently low to allow in some cases the use of solar energy and in other cases to make economically viable what might otherwise be wells which are uneconomical. One technique to yet further reduce the power consumption is to tailor the pumping action to expected requirements by timing the pumping strokes to what is in effect the minimum allowable to maintain the water levels at the required position below the gas formation. Another technique is to halt the pumping action when dry strokes are encountered. A dry stroke, that is where the pump chamber is filled wholly with gas without any water, can be detected by sensing the pressure profile during the pumping stroke. Thus in the presence of liquid, the pressure will rise rapidly when the hydraulic pump is turned on due to the presence of the incompressible liquid. In the absence of liquid the pressure profile will rise but more slowly as the gas in the pump cylinder is compressed. The dry stroke can be dealt with in different ways.  
         [0080]     First setting is called the “fixed timeout”. During the normal strokes, the controller is able to sense the dry stroke (by comparing the downhole pressure). Then, the controller will perform a “fixed” timeout period. This timeout period will be much longer than the normal stroke period. For example, if the normal period is 4 minutes, the timeout period will be 2 hours; if the normal period is 1 hour, the timeout period will be 6 hours. This timeout period needs to be preset.  
         [0081]     Second setting is called the “dynamic timeout”. Again, the controller will be sensing the downhole pressure, yet, for this setting, there will be a couple of different timeout periods stored in the controller. Based on different downhole pressure and the characteristics of the well, the controller will select the best timeout period. For example, if the normal period is 4 minutes, the timeout period could be 1 hour, 2 hours or 3 hours. The controller will select the best timeout period.  
         [0082]     The control unit can also be arranged to carry out the following actions:  
         [0000]     Stage 1, Sense the “Cut-Off” Pressure.  
         [0083]     The controller just acts as an On/Off timer switch. When the pressure is higher than the cut-off pressure, stop the pump, and count down for a “wait period”, then starts the pump again.  
         [0000]     Stage 2, Real Time Monitoring  
         [0084]     The controller is connected to a computer. Real time pressures are display at the computer. All data are stored into the computer.  
         [0000]     Stage 3, 24 Hours Timer (or 1 Year Timer)  
         [0085]     A 24 hours timer is added. We will be able to setup the system pumps at day time. (When the sun is shinning) For example, the pump starts at 7 am and stops at 6 pm during summer time.  
         [0086]     It can be easily programmed to have 1 year timer into the controller. In this case, the controller will change the start time according to the month and the day.  
         [0087]     Note: the system does not necessarily run only in the day time. It can be operated 24 hours and keep pumping water out.  
         [0000]     Stage 4, Low Voltage Cut-Off  
         [0088]     To protect the system from running with low battery charge levels, the controller will stop the pump. The timer and controller are still running, but it will not send the “ON” signal to the pump. The system will run normally when the battery is 80% charged.  
         [0000]     Stage 5, Dry Stroke Prevention  
         [0089]     To protect the pump, there will be 2 ways to prevent pumping a dry stroke. With the fixed timeout, the timeout period is set, the pump will stop for a fixed period of time. With the dynamic timeout, the controller selects a timeout period based on the pressure and the character of that well.  
         [0000]     Stage 6, Record Gas Production and Set Data Remotely  
         [0090]     Another sensor can be provided as indicated at  24 C to record the gas production on the well. And adding a function to send the gas production and pumping pressure back to the office remotely. In this case, the system can monitor the performance of the pump.  
         [0091]     The concentric two tube configuration, will be run into the gas well through a lubricator (not shown) under pressure. Thus the well will not have to be “killed” with load water. Possible formation damage will be averted. Other systems are not able to do this.  
         [0092]     As shown in  FIG. 1  there is provided at the surface a choke  40  for the produced gas which is supplied to a compression stage  41 . Also a well isolation cylinder  42  serves as a pressure safety device, a stroke indicator and a surface “oil to water” power fluid interface divide.  
         [0093]     Since various modifications can be made in my invention as herein above described, and many apparently widely different embodiments of same made within the spirit and scope of the claims without department from such spirit and scope, it is intended that all matter contained in the accompanying specification shall be interpreted as illustrative only and not in a limiting sense.