Patent Publication Number: US-6339886-B1

Title: Remotely measured caliper for wellbore fluid sample taking instrument

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention is generally related to the field of wellbore logging instruments. More specifically, the invention is related to devices for measuring the internal diameter of a wellbore to enable more reliable determination of whether a wellbore fluid sample taking instrument is likely to be properly placed in hydraulic communication with earth formations from within the wellbore. 
     2. Description of the Related Art 
     Wireline formation fluid sample taking instruments are used to extract samples of connate fluid from earth formations penetrated by a wellbore. Generally, these instruments include a tubular probe which is extended from the housing of the instrument and is hydraulically sealed against the wall of the wellbore. The probe is then selectively placed in hydraulic communication with a pump or a sample chamber, or some similar combination of elements used to withdraw fluid from within the pore spaces of the earth formation. See for example, U.S. Pat. No. 4,507,957 issued to Montgomery et al which describes one type of fluid sample taking instrument. 
     Wireline formation fluid sample taking instruments are susceptible to failure of a sealing element (“packer”) surrounding the probe if the wellbore wall is not smooth, or if the wellbore is substantially enlarged beyond the diameter of a drilling bit used to drill through the earth formations. The packer may also fail to seal properly if the instrument is not properly centered in the wellbore and is put into skewed contact with the wellbore wall. In these cases, when the system operator causes the instrument to withdraw fluid, fluid disposed in the wellbore itself can be drawn across the face of the packer and enter the probe, thereby making the sample unrepresentative of the connate fluid in the earth formation. The system operator can generally determine whether his type of seal failure has occurred by observing measurements of the fluid pressure in the probe. Rapid increase in pressure to the same pressure as the hydrostatic pressure of the fluid in the wellbore typically indicates packer failure. If the cause of the packer failure is roughness of the wellbore wall, slight movement of the instrument along the wellbore may result in a successful reattempt at withdrawing a formation fluid sample. 
     However, the system operator may not be able to determine whether the instrument is disposed in a part of the wellbore in which the wellbore diameter is substantially enlarged past the drilling bit diameter, or even enlarged past the operating diameter range of the fluid sample taking instrument. The system operator also may not be able to determine if the instrument is not well centered in the wellbore where the packer is placed into skewed contact with the wellbore wall. It is known in the art to use “caliper” logs to estimate whether the instrument is disposed in such an enlarged part of the wellbore. Caliper logging instruments which can be included with other types of wellbore logging instruments are well known in the art. See for example, U.S. Pat. No. 4,432,143 issued to Moriarty et al or U.S. Pat. No. 4,559,709 issued to Beseme et al. Generally speaking, the caliper logging instruments known in the art include an “arm” or other member which is placed in continuous contact with the wellbore wall. The arm is coupled to some type of position sensor to determine the amount of lateral extension of the arm from the instrument housing. 
     Using caliper logs which have been measured by instruments other than the formation sample taking instrument is not always conclusive as to whether the wellbore diameter at the selected formation sample depth is greater than the extension range of the formation fluid sample taking instrument. As is known in the art, the wellbore may “wash” or otherwise become enlarged past the diameters recorded by the earlier-run caliper log by the time the fluid sample taking instrument is to be run in the wellbore. It is therefore desirable to include a caliper instrument along with the formation sample taking instrument. 
     The typical wellbore caliper logging instrument is designed, however, for measurement while the instrument is being moved along the wellbore. Further, the typical wellbore caliper logging instrument includes complex linkages to couple the arm to the position sensor, because the sensor itself must generally be located inside the instrument housing to avoid destruction by the fluid in the wellbore. See U.S. Pat. No. 4,559,709 issued to Beseme et al for example. It has proven impracticable to include an arm-type caliper, or any other type of caliper, at the location of the probe in a formation fluid sample taking instrument, principally because the sample taking instrument includes a “back up” shoe or similar device which is hydraulically extended from the instrument housing at a circumferential location opposite to the probe, to force the probe into contact with the wellbore wall under very high lateral force. The complex hydraulic components necessary to actuate the typical back up shoe have made including a position sensor at the location of the back up shoe and probe very difficult. 
     SUMMARY OF THE INVENTION 
     The invention is an apparatus and method for measuring the internal diameter of a wellbore used with a wireline formation sample taking instrument. The apparatus includes an hydraulically actuated probe and back up shoe for selective engagement with the wall of a wellbore, an hydraulic pump and selectively controllable valves for selectively controlling the extension and retraction of the probe and back up shoe, and an hydraulic fluid reservoir to supply hydraulic fluid to the pump for extending and retracting the probe and back up shoe. The reservoir includes a pressure compensator for balancing the pressure in the reservoir to an hydrostatic pressure in the wellbore. A position sensor is coupled to the compensator for determining a position of the compensator in the reservoir, so that a measurement corresponding to a fluid volume in the reservoir can be made. The measurement corresponding to the fluid volume also corresponds to an amount by which the back up shoe and probe are extended outward from the instrument. The amount of extension corresponds to the internal diameter of the wellbore at the location of the probe and back up shoe. 
     In the method of the invention, the volume of fluid in the reservoir is measured, at the selected depth at which the instrument is to be “set” for taking a fluid sample, when the shoe and probe on the instrument are fully retracted. The shoe and probe are then extended, and the volume of fluid in the reservoir is measured again. The fluid volume corresponds to the amount of extension of the probe and shoe. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a wireline formation fluid sample taking instrument disposed in a wellbore drilled through earth formations. 
     FIG. 2 shows hydraulic circuits for extending and retracting the back up shoe and probe for the instrument in FIG. 1, including a sensor for measuring the volume of hydraulic fluid in a reservoir in the instrument. 
     FIG. 3 shows an alternative embodiment of the sensor in FIG.  2 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A wireline formation sample taking instrument which is suitable for use with this invention is described in U.S. Pat. No. 5,635,631 issued to Yesudas et al, for example. It should be noted that the instrument described in the Yesudas et al &#39;631 patent is not the only wireline formation sample taking instrument to which the invention can be adapted. Other wireline formation fluid sample taking instruments, such as the one described in the Moriarty et al &#39;143 patent referred to earlier, can also be used with this invention. The elements of the formation sample taking instrument which are necessary for the invention will be further explained. 
     The sample taking instrument is shown in FIG. 1 generally at  13 . The instrument  13  is attached to one end of an armored electrical cable  12  and can be lowered, using the cable  12 , into a wellbore  10  drilled through the earth. The cable  12  can be extended into and withdrawn from the wellbore  10  by means of a winch  19  or similar device known in the art which is located at the earth&#39;s surface. 
     The instrument  13  includes a back-up shoe and an hydraulically actuated mechanism (not shown separately in FIG. 1) for extending the shoe, shown generally at  17 , which are disposed within a housing  16 . The mechanism will be further explained. The housing  16  also includes a tubular probe  18  which can be selectively extended and put into contact with the wall of the wellbore  10 . A sample tank  15  can be attached to the lower end of the housing  16  and can be selectively hydraulically connected to the probe  18  in order to store samples of fluids withdrawn from the earth. The probe  18 , the back-up shoe  17  and selective valves (not shown) disposed within the housing  16  for operating the probe  18  and the shoe  17  can be of types familiar to those skilled in the art, and can receive hydraulic operating power from an hydraulic power unit  9  attached to the upper end of the housing  16 . 
     The various operating functions of the tool  13 , including extension of the back up shoe  17  and extension of the probe  18 , can be controlled by the system operator entering command signals into control circuits  23  which are located at the earth&#39;s surface and are electrically connected to the cable  12 , as is understood by those skilled in the art. The command signals can be decoded in an electronics unit  14  disposed within the housing  16 . The tool  13  can include sensors (not shown) for measuring pressure and volume within hydraulic lines (not shown in FIG. 1) connected to a sample chamber (not shown in FIG.  1 ). Measurements made by the sensors (not shown) can be transmitted to the earth&#39;s surface as electrical signals generated by the electronics unit  14 . At the earth&#39;s surface the signals can be decoded by a signal processor  21  which is also electrically connected to the cable  12 . The decoded signals are reformatted into measurements which can be both observed by the system operator and can be recorded by a recorder  22  connected to the signal processor  21 . 
     As the tool  13  is lowered into the wellbore  10 , the depth at which the tool is located is indicated by a depth indicator  20  which is in contact with the cable  12  and measures the amount of cable  12  extended into the wellbore  10 . When the tool  13  is determined to be positioned adjacent to a formation of interest, shown generally at  11 , the system operator enters commands into the control circuits  23  to lock the tool  13  in position by extending the back-up shoe  17 . The probe  18  is then extended, and withdrawal of a fluid sample can be initiated. 
     FIG. 2 shows in very simplified form the mechanism for extending and retracting the probe  18  and the back up shoe  17 , as well as a sensor  37  which forms part of the invention. The hydraulic power unit ( 9  in FIG. 1) can include an electric motor-operated hydraulic pump  32  which draws hydraulic oil from a reservoir  34  disposed in the instrument housing. The pump  32  discharge pressure can be controlled by a regulator or bypass valve or the like, shown generally at  33 . Hydraulic oil under pressure from the pump  32  can be selectively directed to an extension line  31 , or a retraction line  31  by a crossover valve  39 , which can be solenoid operated. When the pressurized oil is directed into the extend line  31 , the oil is conducted to the extend side of hydraulic cylinder  18 A which extends the probe  18 , and cylinders  17 A and  17 B which extend the shoe  17 . Low pressure oil from the other (retract) side of these hydraulic cylinders  17 A,  17 B,  18 A can be returned to the reservoir  34  through the retract line  30  and crossover valve  39  as the cylinders extend. To retract the probe  18  and shoe  17 , the crossover valve  39  is operated to direct the pressurized and returned oil to the opposite lines  31 ,  30 . 
     In wellbore logging and sample taking instruments which include hydraulic actuation mechanisms for the probe and back up shoe such as those just described, the reservoir  34  is typically compensated for external hydrostatic fluid pressure in the wellbore ( 10  in FIG.  1 ). As is well known in the art, pressure compensation enables reliable extension and retraction of hydraulic cylinders such as those at  17 A,  17 B and  18 A in FIG. 2 because the hydraulic oil prior to pressurization by the pump  32  is at substantially the same pressure as the hydrostatic fluid pressure in the wellbore  10 . Therefore the hydraulic pressure applied by the pump  32  need not overcome the hydrostatic pressure to extend the cylinders. The compensation system shown in FIG. 2 is typical and includes a compensator piston  35  which is exposed to wellbore fluid pressure on one side and the reservoir oil on the other side. The compensation system typically includes a spring  36  or similar biasing means to keep the compensator piston  35  in the correct position irrespective of reservoir or external hydrostatic pressure. 
     In the invention, a position sensor, shown generally at  37 , can be coupled to the compensator piston  35 . The position sensor  37  can be a linear potentiometer or a linear variable differential transformer, both sensor types being known in the art. Typically, the sensor  37  can be exposed to the hydraulic oil without damage, while exposure to fluid under pressure in the wellbore would be destructive to the sensor  37 . 
     Alternatively, as shown in FIG. 3, the sensor  37  can be disposed in a blind, pressure sealed tube  41 . The sensor in this case is a linear variable differential transformed (“LVDT”). A magnetic core  39  is disposed inside the tube  41  and is coupled to the compensator piston  35  by a rod  38  or similar linkage. The output of the LVDT changes corresponding to the amount of the core  39  disposed within the cross sectional area enclosed by coils  40 . A caliper using a type of LVDT motion sensor is described, for example in U.S. Pat. No. 5,299,359 issued to Estes at al. 
     Referring back to FIG. 2, the position of the compensator piston  35  in the reservoir  34  will be directly related to the volume of hydraulic oil in the reservoir  34 . The hydraulic oil volume in the reservoir  34  will depend on, among other things, the ambient pressure and temperature in the reservoir  34 , but primarily will depend on the amount by which the cylinders  17 A,  17 B,  18 A are extended from the instrument housing ( 16  in FIG.  1 ). Hydraulic cylinders such as  17 A,  17 B,  18 A have larger internal volume when they extended than when they are retracted primarily because of the volume of the piston and rod which are displaced from the cylinder itself when the cylinder is extended. It has been determined that the amount by which the cylinders  17 A,  17 B,  18 A are extended is directly related to the oil volume in the reservoir  34  at any particular ambient pressure and temperature. Measurement of the position of the compensator piston  35  will therefore directly correspond to the volume of oil in the reservoir  34 . Therefore the measurement of the position of the compensator piston  35  will correspond to the amount by which the cylinders  17 A,  17 B,  18 A are extended. 
     The correspondence between the position of the compensator piston  35  and the amount of cylinder extension is also related, as previously explained, to the ambient pressure and temperature in the reservoir  34 . It has been determined, however, that the relationship between the position of the compensator piston  35  and the amount by which the cylinders  17 A,  17 B,  18 A are extended only requires recalibration for one known value of extension and compensator piston  35  position in order to enable determining the correct amount of extension of the cylinders  17 A,  17 B,  18 A. One known value of extension is when the cylinders  17 A,  17 B,  18 A are fully retracted (zero extension). 
     To calibrate the cylinder extension with respect to the position of the compensator piston  35 , the position of the piston  35  can be measured first when the cylinders  17 A,  17 B,  18 A are fully retracted. The calibration is preferably performed at the earth&#39;s surface with the instrument ( 13  in FIG. 1) in a test rack or the like. The cylinders  17 A,  17 B,  18 A can then be extended inside a caliper “ring” (not shown), generally a hoop-shaped piece of steel having a known internal diameter. Caliper rings are well known in the art. After extending the cylinders  17 A,  17 B,  18 A fully inside the ring (not shown), the position of the compensator piston  35  is again measured. Since the internal diameter of the ring is known, the new position of the compensator piston  35  will then be calibrated with respect to a known amount of extension of the cylinders  17 A,  17 B,  18 A. This process can be repeated for a number of other diameter caliper rings, until substantially all of the extension range of the cylinders  17 A,  17 B,  18 A has been covered by this calibration process. As a practical matter, it has been determined that it is only necessary to calibrate the cylinder extension/compensator piston position at one known diameter larger than the fully retracted position to obtain quantitative measurements of the amount of extension of the cylinders  17 A,  17 B,  18 A, since the relationship is substantially linear. This calibration results in a characterization of the position of the compensator piston  35  with respect to the amount by which the cylinders  17 A,  17 B,  18 A are extended. The amount by which the cylinders  17 A,  17 B,  18 A are extended generally corresponds to the internal diameter of the wellbore at the particular test depth for which a fluid sample is to be taken. 
     When the instrument ( 13  in FIG. 1) is used in an actual wellbore, the effects of ambient pressure and temperature at any particular sample test depth on the position of the compensator piston  35  can be accounted for by measuring the position of the compensator piston  35  at “zero” extension of the cylinders  17 A,  17 B,  18 A to determine an offset (a value added to each measurement) by which to adjust the values of the relationship determined in the calibration done at the earth&#39;s surface. The offset preferably is determined at each test depth to maintain accuracy of the measurements. 
     When a sample it to be taken at any particular depth in the wellbore ( 10  in FIG.  1 ), the amount of extension of the cylinders  17 A,  17 B,  18 A can be measured while the instrument is “set”. If it is determined that the internal diameter of the wellbore  10  likely exceeds the extension range of the cylinders (usually by the measurement showing a wellbore diameter equal to the maximum possible amount of extension of the cylinders), the system operator can retract the cylinders  17 A,  17 B,  18 A and move the instrument  13  to a different depth where the wellbore diameter may be within the extension range of the cylinders  17 A,  17 B,  18 A. This represents a substantial improvement over the operation of prior art formation sample taking instruments, where the only indication that the cylinders were fully extended was obtainable by the system operator observing the pressure in the hydraulic lines. The hydraulic pressure would show a similar indication whether the shoe  17  and probe  18  were in contact with the wellbore wall or the cylinders were fully extended where the wellbore was too big to enable the shoe  17  and probe  18  to “seat” on the wellbore wall. The invention enables the system operator to determine whether failure to properly seal the probe  18  against the wellbore wall is a result of enlarged wellbore diameter. 
     In cases where the instrument  13  is not well centered in the wellbore so that the probe  18  would be placed into skewed contact with the wellbore wall, the measurement of wellbore diameter obtained by the invention may actually be less than the diameter of the drill bit used to drill the wellbore, because the orientation of the instrument in the wellbore may prevent the probe  18  and shoe  17  from extending to the full diameter of the wellbore. Measurements made by the invention which indicate that the probe  18  and shoe  17  are extended to less than the diameter of the drill bit can therefore be used to indicate probable skewed contact of the probe  18  with the wellbore wall. In this case the system operator may elect to move the instrument  13  slightly in the wellbore to relieve the skewed contact condition. 
     While the particular embodiment of the invention described herein forms part of a wireline formation sample taking instrument, those skilled in the art of well logging will recognize that the invention can also be used with any instrument having hydraulically extensible and retractable arms, shoes, pads or the like to be selectively placed in contact the wall of a wellbore or casing and to measure the internal diameter of the wellbore or casing. U.S. Pat. No. 5,680,049 issued to Gissler et al, for example, describes an instrument for measuring resistivity of a wellbore having a conductive metal casing inserted therein. The invention can be readily adapted to measure the amount of extension of extensible “arms” on the instrument described in the Gissler et al patent, since they are hydraulically actuated, and the instrument includes an hydraulic fluid reservoir to supply fluid for extending and retracting the arms. In particular, hydraulically actuated arms, shoes or the like can have the power necessary to lift a well logging instrument away from the lower wall of a wellbore which is drilled at very high inclinations from vertical. In conjunction with the invention, such devices may have the capacity to measure the diameter of such highly inclined wellbores whereas caliper instruments known in the art may lack such ability. 
     Those skilled in the art will devise other embodiments of this invention which do not depart from the spirit of the invention as described herein. Accordingly, the invention should be limited in scope only by the attached claims.