Abstract:
A method for sampling fluid from a subsurface formation includes retrieving fluids from the formation using a plurality of pumps. The method also includes the steps of controlling a flow of the retrieved fluids using at least a first valve and a second valve and estimating an operating parameter of at least one pump of the plurality of pumps. The method further includes the step of controlling the first valve and the second valve using the estimated operating parameter to initiate a fluid sampling event.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This applications claims priority from U.S. Provisional Application Ser. No. 61/450,906, filed Mar. 9, 2011 and from U.S. Provisional Application Ser. No. 61/452,492, filed Mar. 14, 2011, the disclosure of which is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE DISCLOSURE 
     This disclosure pertains generally to investigations of underground formations and more particularly to systems and methods for controlling devices for formation testing and fluid sampling within a borehole. 
     BACKGROUND OF THE DISCLOSURE 
     Commercial development of hydrocarbon fields requires significant amounts of capital. Before field development begins, operators desire to have as much data as possible in order to evaluate the reservoir for commercial viability. While data acquisition during drilling provides useful information, it is often also desirable to conduct further testing of the hydrocarbon reservoirs in order to obtain additional data. Therefore, after a borehole for a well has been drilled, the hydrocarbon zones are usually tested with tools that acquire fluid samples, e.g., liquids from the formation. These boreholes typically have well fluids at relatively high hydrostatic pressure. Because fluid sampling tools often also have one or more openings that allow fluid communication between the tool interior and the borehole environment (or ‘borehole exits’), it is desirable to control flow across these openings to prevent undesirable invasion of a sampling tool by well fluids. 
     In one aspect, the present disclosure addresses the need to enhance control of borehole exits. 
     SUMMARY OF THE DISCLOSURE 
     In aspects, the present disclosure provides methods for sampling fluid from a subsurface formation. The method may include retrieving fluids from the formation using a plurality of pumps; controlling a flow of the retrieved fluids using at least a first valve and a second valve; estimating an operating parameter of at least one pump of the plurality of pumps; and controlling the first valve and the second valve using the estimated operating parameter to initiate a fluid sampling event. 
     In aspects, the present disclosure includes an apparatus for sampling fluid from a subsurface formation. The apparatus may include a plurality of pumps configured to retrieve fluids from the formation; at least a first valve and a second valve configured to control a flow of the retrieved fluids; and a controller configured to control the first valve and the second valve using an estimated operating parameter of at least one pump of the plurality of pumps to initiate a fluid sampling event. 
     Examples of certain features of the disclosure have been summarized rather broadly in order that the detailed description thereof that follows may be better understood and in order that the contributions they represent to the art may be appreciated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a detailed understanding of the present disclosure, reference should be made to the following detailed description of the embodiments, taken in conjunction with the accompanying drawings, in which like elements have been given like numerals, wherein: 
         FIG. 1  shows a schematic of a control apparatus for a fluid sampling tool according to one embodiment of the present disclosure; 
         FIG. 2  shows illustrative stroke rates for pumps used in fluid sampling tools according to the present disclosure; and 
         FIG. 3  shows a schematic of an apparatus for implementing one embodiment of the method according to the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     In aspects, the present disclosure relates to devices and methods for providing enhanced control of flow control devices used to retrieve fluids. In particular, embodiments of the present disclosure minimize, if not eliminate, backflow through borehole exits. Illustrative control schemes according to this disclosure employ timing techniques that coordinate valve actuation with pump operation to ensure that sample retrieval occurs at desired times and/or at specified conditions. The teachings may be advantageously applied to a variety of systems in the oil and gas industry, water wells, geothermal wells, surface applications and elsewhere. Merely for clarity, certain non-limiting embodiments will be discussed in the context of tools configured for wellbore uses. 
     Referring initially to  FIG. 1 , there is schematically illustrated one embodiment of a fluid retrieval tool  100  that may be used to retrieve fluids from a desired location e.g., a hydrocarbon bearing reservoir. The tool  100  may include a sampling probe  102  that has a pad  104  in which is formed a sampling passage  106  and a perimeter passage  108 . The sampling probe  102  may be a concentric pad type wherein the passage  106  is encircled by the perimeter passage  108 . Thus, formation fluid is drawn from two separate and distinct regions  101 A,  101 B, on a borehole wall  24 . In one embodiment, fluid retrieved via the sampling passage  106  may be conveyed to and stored in one or more sampling tanks  110 . Fluid retrieved via the perimeter passage  108  may be conveyed to a location outside of the tool  100 . For convenience, the area outside of the tool  100  will be referred to as the “borehole.” It should be understood that this area includes the annular space between the tool  100  and a borehole wall  24 . Fluid retrieval may be performed by pump systems discussed in greater detail below. 
     In one arrangement, using vacuum pressure, a sample pump  120  draws fluid from the sampling passage  106  and a perimeter pump  140  draws fluid from the perimeter passage  108 . The sample pump  120  pumps the fluid via a line  122  to either the borehole or the tank  110 . For example, the line  122  may be in fluid communication with a borehole valve  124  that provides fluid communication with the borehole and a valve  126  that provides communication with the sampling tank  110 . Likewise, the perimeter pump  140  conveys or pumps the fluid via a line  142  to a borehole valve  144  that provides fluid communication with a borehole. The valves  124 ,  126 ,  144  may be actuated between an open position and a closed position using actuators (not shown) that are responsive to control signals. The valves  124 ,  126 ,  144  may be bi-directional valves that allow fluid flow in both directions. Valves  124 ,  144  are borehole exits because they control fluid communication with the borehole. The pumps  120 ,  140  may be energized by the same power source  160  or independent power sources. The power source  160  may be electric, hydraulic, pneumatic, etc. 
     In embodiments, the pumps  120 ,  140  may be a single-action or dual action piston pumps. For example, the pump  120  may include a cylinder  128  in which a piston  130  reciprocates. Similarly, the pump  140  may include a cylinder  148  in which a piston  150  reciprocates. During the piston stroke, i.e., as the pistons  130 ,  150  travel from one end of the cylinders  128 ,  148  to the other, respectively, pressurized fluid is ejected into the lines  122 ,  142 , respectively. It should be noted that at the end of a piston stroke, fluid pressure may drop in the line  122  due to the cessation of piston movement. If both valves  124 ,  126  are open and if the pressure in the line  122  is less than borehole pressure, then borehole fluids may enter via the borehole valve  124  and invade the sample tank  110  via the sample valve  126 . This condition is sometimes referred to as “backflow.” 
     To minimize or eliminate backflow, embodiments of the present disclosure control one or more aspects of the operation of tool  100  to ensure that sample retrieval activity is initiated only when the pressure in the line  122  is greater than the pressure in the borehole. 
     An illustrative method to prevent backflow involves timing the closing of the valve  124  and the opening of the valve  126  with the operation of the pumps  120 ,  140 . Referring to  FIG. 2 , there is shown a line  180  illustrating the stroke rate of the pump  120  ( FIG. 1 ) and a line  182  showing the stroke rate of the pump  140  ( FIG. 1 ). The stroke rates of the pumps  120 ,  140  may be different; e.g., the stroke rate of the pump  140  may be about three times greater than the stroke rate of the pump  120 . A transient state of the pistons  130 ,  150  is shown with numeral  184 . The transient state  184  may be when the pistons  130 ,  150  are decelerating, accelerating, or stationary. All of these states indicate of an end or beginning of a piston stroke. The time between transients states  184  will be referred to as stroke period or stroke duration. The maximum pressure in the line  122  ( FIG. 1 ) may occur during the stroke period of either or both pumps  120 ,  140 . The pressure may drop when one, or more likely both, of the pumps  120 ,  140  are at the transient state  184 . Therefore, the positions (i.e., opening and closing) of the valves  124 ,  126  are changed during the stroke period to give sufficient time for the valves  124 ,  126  to fully close and open, respectively. 
     In some arrangements, the stroke period may be minutes whereas the time to change positions of the valves  124 ,  126  may be seconds. As shown, the valves  124 ,  126  may be actuated on or after the relatively faster pump  140  initiates a stroke. This point in time is shown with numeral  186 . By coordinating the change in valve positions with the pump strokes of the pumps  120 ,  140 , the pressure in the line  122  may be maintained at a value higher than the pressure in the borehole (or ‘positive pressure differential’). Thus, backflow may be minimized, if not eliminated. 
     In some arrangements, the sampling event may be human initiated. For example, sensors may transmit signals representative of one or more selected operating parameters to the surface. Illustrative operating parameters may include aspects of a piston stroke, such as position, duration, direction, speed, etc. Based on these measurements, a human operator may initiate a sampling event while a positive pressure differential between the line  122  and the borehole is present. 
     In other arrangements, a controller  162  may be used to control the operation of tool  100  to ensure that sample retrieval occurs at desired times and/or at specified conditions. For example, the controller  162  may estimate one or more operating parameters of the pumps  120 ,  140  and use the estimated control parameter(s) to control the valves  124 ,  126  and/or the pumps  120 ,  140 . 
     In arrangements where the pumps  120 ,  140  may have different stroke times (i.e., stroke duration), a sensor  158  may be used to directly or indirectly estimate the positions of the pistons  130 ,  150 . Illustrative direct measurements may be made by a position sensor that estimates the piston position using physical contact, magnetic signals, acoustic signals, electrical signals, etc. Illustrative indirect measurements may be made by a pressure sensor that detects changes in pressure or flow sensors that detect a change in flow rate. Other indirect measurement may include parameters associated with the motor or power source driving the pumps  120 ,  140  (e.g., torque, current, voltage). The changes or the rate of changes may be indicative of an end of a piston stroke. While the sensor  158  is shown adjacent to the pumps  120 ,  140 , it should be understood that the sensors may be positioned wherever needed to acquire information regarding a given operating parameter; e.g., at the power source  160 , in the lines  122 ,  142 , etc. 
     In an illustrative control scheme, the controller  162  may first monitor sensor signals to identify when the slower pump  120  has reached the end of the stroke. Next, the controller  162  monitors sensor signals to identify when the faster pump  140  has reached the end of the stroke. At or immediately after that time, the controller  162  opens the sample valve  126  and closes the borehole valve  124  to initiate the sampling event. Because both pumps  120 ,  140  are at or near the initial period of their stroke, it is improbable, if not impossible, for either pump  120 ,  140  to stop pumping while both the borehole valve  124  and the sample valve  126  are open and in fluid communication with one another. 
     In another illustrative control scheme, the controller  162  may monitor sensor signals to identify the positions of the pistons  130 ,  150 . The controller  162  may be preprogrammed with an operating parameter such as piston cycle. The controller  162  may include instructions for estimating the time remaining for when the pistons  130 ,  150  reach the end of their stroke. If the remaining time is greater than the time needed to close the borehole valve  124 , then the controller  162  may initiate a sampling activity by opening the sample valve  126  and closing the borehole valve  124 . If the remaining time is not sufficient, then the controller  162  continues to monitor sensor signals until it determines that the time for completing the strokes of the pistons  130 ,  150  is sufficient to initiate sampling. 
     In still another embodiment, the controller  162  may control the pump  120  and/or the pump  140  to cause a desired time period for initiating a sampling event. For example, the controller  162  may transmit control signals that instruct one or both pumps  120 ,  140  to de-energize or otherwise return to a known operating state; e.g., the pistons  130 ,  150  move to a known position. Thereafter, the controller  162  may re-energize the pumps  120 ,  140  and initiate the sampling event by actuating the valves  124 ,  126 . 
     Embodiments of the present disclosure may initiate a sampling activity without use of information relating to the pump  120 ,  140 . For example, the pumps  120 ,  140  may be configured to operate at a known rate. The valves  124 ,  126  may be configured to open/close using this preprogrammed information. 
     Moreover, the control methodologies of the present disclosure may be utilized during any phase of the sampling event (e.g., from initiation of the sampling event to termination of the sampling event). Referring to  FIG. 1 , it should be noted that the vacuum applied by the perimeter pump  140  at region  101 B may draw contaminated fluid away from the inlet to the sample line  106  of the sample pump  120 . By drawing away contaminated fluids, there is a greater likelihood that the sample pump  120  draws “pristine” formation fluid from the region  101 A. However, if operation of the perimeter pump  140  is interrupted while the sample pump  120  is operating, then contaminated fluid from region  101 B may be drawn into the sample line  106  by the sample pump  120 . During a sampling event, i.e., when valve  126  is open, this contaminated fluid may flow into the tank  110  and compromise the quality of the fluid sample. To ensure that primarily pristine fluid is received into the sample tank  110  during sampling, the valve  126  and/or pump  140  may be controlled to maintain a sufficient vacuum pressure in region  101 B while valve  126  is open and permitting communication between line  122  and the sample tank  110 . For example, the valve  126  may be actuated to the closed position before the perimeter pump  140  reaches the end of its stroke (or transient state  184 ). Alternatively or additionally, the operation of the perimeter pump  140  may be controlled such that the end of its stroke is reached only after the valve  126  is closed. 
     As noted previously, embodiments of the present disclosure may be used in numerous situations. Merely to better describe the better disclosure, an embodiment suited for subsurface operations is shown in  FIG. 3 .  FIG. 3  schematically illustrates a wellbore system  10  deployed from a rig  12  into a borehole  14 . While a land-based rig  12  is shown, it should be understood that the present disclosure may be applicable to offshore rigs and subsea formations. The wellbore system  10  may include a carrier  16  and a fluid retrieval tool  100 . As described previously, the tool  100  may include a probe  102  that contacts the borehole wall  24  for extracting formation fluid from a formation  26 . 
     In some embodiments, the wellbore system  10  may be a drilling system configured to form the borehole  14 . In such embodiments, the carrier  16  may be a coiled tube, casing, liners, drill pipe, etc. In other embodiments, the wellbore system  10  may convey the tool  100  with a non-rigid carrier. In such arrangements, the carrier  16  may be wirelines, wireline sondes, slickline sondes, e-lines, etc. The tool  100  may be controlled by a surface controller  30  and/or a downhole controller  32 . The surface controller  30  and/or the downhole controller  32  may operate as the controller  162  ( FIG. 1 ). Signals indicative of the parameter may be transmitted to a surface controller  30  via a suitable communication link. Illustrative communication links include, but are not limited to, data carrying conductors (e.g., wires, optical fibers, wired pipe), mud pulses, EM signals, RF signals, acoustical signals, etc. 
     Referring now to  FIGS. 1 and 3 , during one exemplary use, the fluid retrieval tool  100  is positioned adjacent a formation of interest and the probe  102  is pressed into sealing engagement with the borehole wall  24 . The pumps  120 ,  140  may be operated to retrieve formation fluids. Often, fluid is pumped from the formation and ejected into the borehole via valves  124 ,  144  until it is determined that the retrieved fluid is sufficiently free of contaminants. Thereafter, it may be desired to direct the formation fluid into one or more sample tanks  110 . Prior to initiating a sampling event, one or more operating parameters of the pumps  120 ,  140  may be monitored as discussed previously. For example, the positions of the pistons  130 ,  150  may be determined directly or indirectly. Upon determining that the pistons  130 ,  150  are positioned such that adequate time is available to complete a change in valve positions (e.g., open to close and close to open), the controller  162  may transmit appropriate control signals to cause the valve  124  to close and the valve  126  to open. The change in positions of the valves  124 ,  126  may occur in any order (e.g., valve  124  closes before valve  126  opens or valve  124  closes after valve  126  opens) or simultaneously. 
     In some embodiments, the controller  162  may include mechanical, electromechanical, and/or electrical circuitry configured to control one or more components of the tool  100 . In other embodiments, the controller  162  may use algorithms and programming to receive information and control operation of the tool  100 . Therefore, the controller  162  may include an information processor that is data communication with a data storage medium and a processor memory. The data storage medium may be any standard computer data storage device, such as a USB drive, memory stick, hard disk, removable RAM, EPROMs, EAROMs, flash memories and optical disks or other commonly used memory storage system known to one of ordinary skill in the art including Internet based storage. The data storage medium may store one or more programs that when executed causes information processor to execute the disclosed method(s). ‘Information’ may be data in any form and may be “raw” and/or “processed,” e.g., direct measurements, indirect measurements, analog signal, digital signals, etc. 
     The term “carrier” as used in this disclosure means any device, device component, combination of devices, media and/or member that may be used to convey, house, support or otherwise facilitate the use of another device, device component, combination of devices, media and/or member. As used herein, the term “fluid” and “fluids” refers to one or more gasses, one or more liquids, and mixtures thereof. 
     While the foregoing disclosure is directed to the one mode embodiments of the disclosure, various modifications will be apparent to those skilled in the art. It is intended that all variations be embraced by the foregoing disclosure.