Non-invasive compressibility and in situ density testing of a fluid sample in a sealed chamber

In situ density and compressibility of a fluid sample are determined for a fluid sample collected downhole. The density and compressibility of the fluid sampled is determined by measuring a distance to a piston contained within the sample chamber using an external magnetic field sensor that senses a magnetic field emanating from a magnet provided on the piston internal to the sample chamber. The testing is performed quickly and at the surface in a noninvasive fashion (e.g., without opening the sample chamber).

BACKGROUND

An application of formation fluid testing is to confirm the mobile fluid phase in the reservoir. This determination can be important in reservoirs in which there is significant uncertainty about the formation water salinity. This situation is further complicated in poor permeability reservoirs where there can be a long oil-water transition zone. Defining the mobile fluid phase down the transition zone can be achieved by sampling with, for example, a pump-out wireline formation tester (PWFT). This tool incorporates downhole sensors to analyze the fluid while pumping, the results of which are used to determine when and how to sample the formation fluid. The fluid samples are received into sample chambers.

After the sample chambers are retrieved to the surface, the chambers typically are sent to a lab for transfer of the sampled fluid and detailed analysis. Often, there is a long delay between retrieving the sample chambers and obtaining the analysis results; at times the delay can be on the order of weeks. Such delays are undesirable given the high costs associated with drilling operations.

DETAILED DESCRIPTION

The embodiments disclosed herein are directed to surface testing of a sealed sample chamber containing a fluid sample obtained downhole from the formation. The fluid sample is received into the sample chamber and held at in situ pressure inside the sample chamber (i.e., pressure of the fluid while in the formation). The surface testing is relatively quick, noninvasive (i.e., testing is performed without opening the sample chamber) and includes a determination of either or both of the fluid sample's in situ density and compressibility. The testing is performed in an automated fashion (i.e., with little or no human involvement) by a computer-operated testing system. The testing is performed without opening the sample chambers. Once the testing is complete at, for example, the rig site, the sample chambers can be sent to a lab for further testing.

The sample chambers for which the surface testing is performed generally comprise a cylindrical container containing one or more pistons that seal against the inner wall of the container and can be moved from one end of the container to the other. Some sample chambers have only a single piston while other sample chambers have two pistons. Some sample chambers include a buffer fluid (air, water, nitrogen, etc.). The operation of the sample chambers varies with the various types of chambers and the embodiments disclosed herein for determining density and compressibility are effected by the various chamber designs. Accordingly, the following discussion includes an overview of various sample chamber designs, followed by an explanation of the preferred embodiments of a testing system.

Four illustrative sample chambers are shown and discussed below with regard toFIGS. 1-4.FIG. 1illustrates a downhole fluid sample chamber10including a sleeve-shaped cylinder12which forms an interior fluid compartment18therein. Cylinder12is substantially closed by an upper end cap14and lower end cap16, with compartment18between the end caps and initially containing low pressure air that fills compartment18through valve34which is then closed to trap the air in the compartment18. A separator piston20(also referred to herein as a “filling piston”) movably sealed to the cylinder12initially is positioned in the upper end of the cylinder adjacent the end cap14. When the sample chamber10is used in a well to collect a sample, and in response to the fluid sample pressure being greater than the air pressure in chamber18, the separator piston20moves downward, thereby compressing the air which causes the pressure of the air to increase. The separator piston20movies downward until the air pressure on the lower side of the piston in compartment18is substantially equal to the pressure of the fluid sample on the upper side of the piston (between the piston and upper end cap14). With the pressures substantially equal, the piston20will reside between the upper end cap14and lower end cap16, and generally slightly above the lower end cap16.

The upper end cap14includes a fluid passageway22therein for transmitting formation fluid into the cylinder12and to the top side of the separator piston20. An isolation valve36is located along the flow path22in the upper end cap14. Valve36is closed once the fluid sample is obtained. A fluid line24extends from the upper end cap14to the formation of interest30, and an electronic flow line control valve26is positioned along the flow line24for controlling the fluid flow from the formation to the sampling cylinder.FIG. 1further illustrates an annular packer element28for sealing engagement with the face of formation30, so that formation fluid passes through the center of packer element28and to the flow line24, and then to the cylinder12. The lower end cap16also has a flow line32therein which communicates between the compartment18and exterior of the cylinder12, with a normally closed valve34controlling the release of fluid along the flow line32.

A valve25extends from line24. The valve25remains closed when the tool is downhole. A pressure gauge (not shown) may be fluidly connected to the outlet of the valve25at the surface, and the valve25briefly opened to determine the pressure of the test fluid in the cylinder12.

The fluid compartment18within the cylinder12thus initially serves as an air chamber for atmospheric air. To collect a formation fluid sample, the flow line control valve26is open to introduce formation fluid into the interior of the cylinder12, thereby forcing the piston20downward. As the piston20moves downward toward the lower end cap16, the air between piston20and the lower end cap16becomes increasingly compressed. Formation fluid at in situ (formation) pressure, fills the compartment18between piston18and upper end cap14. Once the pressure of the compressed air below the piston20and the fluid sample above the piston20are at substantially the same pressure, the piston10stops moving and the flow line control valve26may be closed, thereby trapping the collected fluid sample within the cylinder12.

FIG. 2illustrates an alternate sample chamber10bwherein water, ethylene glycol, oil, or another selected incompressible fluid may be used as a buffer fluid. The flow line24may be as discussed above inFIG. 1to provide fluid communication with the interior of the cylinder12. In the embodiment ofFIG. 2, an upper packer27and a lower packer29are used to isolate fluid within formation30from the remainder of the wellbore. Any of the embodiments shown inFIGS. 1-4may be used with the packer element28as shown inFIG. 1or the straddle packers27and29shown inFIG. 2. Central member or choke sub15is fixed within the cylinder12. The separator piston20initially may be positioned substantially adjacent the upper end cap14, with the space between the piston20and the central member15being filled with water. The space below the member15and the lower end cap16may initially be filled with air at atmospheric pressure. A vent tube46may pass water from above the central member15to an area below the central member15and through a choke44positioned along the flow path45within the member15. A bypass valve48may be used at the surface to recycle the chamber and for piston management. Flow line50fluidly connects the chamber below the central member15to the exterior of cylinder12. Both the flow line32in lower end cap16and the outlet from flow line50in the central member15may each be closed by a plug33.

When the flow line control valve26is open, formation pressure acts on the separator piston20and forces the buffer fluid, which, as noted above, may be water, ethylene glycol, oil or another selected incompressible liquid, through the restriction or choke44, thereby establishing a threshold flowing pressure at which the formation fluid enters the chamber. The chamber56below the central member15and above the lower end cap16may be referred to as a choke chamber. Formation fluid forces the water through the choke and into the air filled (or gas filled) choke chamber56, thereby compressing the air. Space52below central member15may thus be compressed air, with the interface54shown between the compressed air and the liquid. The separator piston20continues to move downward until the pressure of the compressed air is approximately equal to the pressure of the sample fluid from the formation. The flow line control valve26then may be closed to trap the collected fluid sample within the cylinder12.

Referring now toFIG. 3, an embodiment of yet another sample chamber10cis shown. In this embodiment, a downhole pump60is included, with the inlet of the pump60connected to the downhole formation of interest30and the outlet of the pump60connected to the cylinder12. In such embodiments, a tool string includes the pump60and multiple (e.g.,15) sample chambers10c. Each sample chamber10chas an associated electronically controlled valve26that, when opened, connects the pump60to that particular sample chamber. Initially, the piston20may be provided in the upper portion of compartment18adjacent the upper end cap14. Valve34within the lower end cap16is open. Accordingly, the lower side of the piston20is exposed through open valve34to wellbore fluid at hydrostatic pressure. Downward motion of the piston20continues until piston20reaches its full extent of travel and rests against the upper surface17of the lower end cap16. At that point, valves34and36are closed.

FIG. 4illustrates yet another configuration for a sample chamber10dwhich utilizes a compressible gas (e.g., nitrogen) that is pressurized to downhole conditions to compensate for sample contraction upon cooling. A pump60and valves26and25are provided as with the prior embodiment. The separator piston20thus is initially positioned adjacent the upper end cap14, and nitrogen gas is contained in the space66between the separator piston20and a separate charging piston64, which is sealed to cylinder12and does not pass fluid through piston64. Wellbore fluid at hydrostatic pressure is exposed to the lower side of the charging piston64as valve34is open to provide fluid flow along the flow line32in the lower end cap16. The outlet from the downhole pump60is directed to the cylinder12, and as the compartment62(between the separator piston20and upper end cap14) is filled with the fluid sample from the formation, the separator piston20, the nitrogen gas and the charging piston64are pushed downward until the charging piston64reaches its full extent of travel, as shown inFIG. 4. Additional pumping moves the separator piston20further downward, compressing the nitrogen charge. Once overpressurized to the desired level, the flow line control valve26may be closed thereby trapping the collected sample within the sample chamber.

In each of the embodiments ofFIGS. 1-4, the separator (filling) piston includes a magnet76. The magnet76is supported on the separator piston20, and preferably positioned within the separator piston. Due in part to the corrosive nature of the fluids contained within the cylinder12, the cylinder12and the piston20preferably are fabricated from a high nickel alloy, such as inconel718or a titanium alloy. Besides being corrosion resistant, these materials are relatively non-magnetic. The magnet76within the piston20may be disk shaped, and typically may be an Al—Ni—Co or Sm—Co material. The magnet76is fixed coaxially within the separator piston20and is magnetized along its axis, which is substantially coaxial with the axis of the cylinder12.

The embodiments ofFIGS. 3 and 4include the use of downhole pump60with the sample chambers. The downhole pump60preferably includes a filling sensor61such as a potentiometer that produces a signal indicative of the volume of fluid pumped by the pump60into the sample chamber. Once pumping ceases, the filling sensor's value is read and transmitted to the surface via, for example, wireline or mudpulse techniques for recording by a test system (discussed below). The embodiments ofFIGS. 1 and 2do not use a pump and a different technique (discussed below) is used to determine the volume of the captured fluid sample. This latter technique can be used as well even in the embodiments in which a pump60, and associated filling sensor, is available.

FIG. 5shows a preferred embodiment of a test system100.FIG. 5also shows a sample chamber10(which can be any suitable sample chamber such as any of the chambers10a-10dofFIGS. 1-4) loaded into the test system100. Test system100may be located, for example, at the surface. The sample chamber10may reside, for example, on a support structure. As shown, the test system100comprises a control unit110coupled to a hydraulic unit120and to a linear position device130. The control unit110may be, for example, a computer. The control unit110comprises a processor112coupled to a computer-readable storage medium114that contains executable software118. The computer-readable storage medium114may comprise volatile memory (e.g., random access memory) or non-volatile storage (e.g., hard disk drive, flash storage, CD ROM, etc.). The software114is executed by processor112and, as such, causes the processor to perform, or at least initiate, some or all of the functionality described herein attributed to the test system100and/or control unit110. One or more input/output (I/O) devices116are also included and coupled to the processor112. Such I/O devices may include, for example, a keyboard, a mouse, a touchpad, a display, etc.

The hydraulic unit120comprises a hydraulic pump that is connectable to the sample chamber10via a hydraulic line122. The hydraulic unit120can vary the pressure inside the hydraulic line in accordance with a signal119from the control unit110. The control unit110thus can cause the hydraulic unit120to increase or decrease the pressure in the hydraulic line. The content of the hydraulic line may be a gas such as nitrogen, but other suitable hydraulic gasses or fluids may be used as well.

A pressure sensor124is provided on the hydraulic line122. The pressure sensor124produces an electrical signal125that is proportional to the pressure in the hydraulic line122. Signal125is provided to the control unit110which can monitor the pressure in the hydraulic line via the pressure sensor124.

The linear position device130determines the location of the piston20within the sealed sample chamber. The linear position device130comprises a sensor locating device131and a magnetic field sensor132which can move along or near the exterior surface of the sample chamber10in the x-direction between one end127of the sample chamber and the other end129. The magnetic field sensor132is sensitive to the magnetic field emanating from the piston's magnet76. The magnetic field sensor132preferably comprises a Hall sensor, magnetoresistive sensor, fluxgate field sensor, induction coil sensor, induction coil gradiometer, or other suitable type of sensor. The sensor132may have single axis or multi-axis sensitivity. Further, the electrical signal123from the magnetic field sensor132is provided to the control unit110.

The sensor locating device131is able to determine the position of the magnetic field sensor132and produce a signal121that encodes the sensor's position. The signal121is referred to as the position signal. The sensor locating device131determines the position of the sensor132via any of a variety of techniques. For example, the sensor locating device131may comprises a linear potentiometer, a laser distance sensor, an ultrasonic distance meter, a digital ruler, a draw wire sensor, etc.

In some embodiments, the voltage level of the position signal121from the sensor locating device131may vary from a lower voltage (e.g., 0V) to a higher voltage (e.g., 5V). The lower voltage corresponds to the sensor132being at one end of its travel path (i.e., at one end of the sample chamber10), while the upper voltage corresponds to the sensor being at the opposing end of its travel path (i.e., at the other end of the sample chamber). A voltage halfway between the lower and higher voltages corresponds to the mid-point of the sample chamber. Thus, in such embodiments, the voltage level from sensor132correlates to location/distance along the length of the sample chamber.

In accordance with a preferred embodiment, the magnet76is installed in or on the piston20such that the magnet's north pole is pointed in the x-direction. The strength of the magnetic field emanating from magnet76varies with respect to location along the line of travel in the x-direction of the magnetic field sensor132. The x- and y-components of the magnetic field from magnet76are depicted inFIGS. 6aand 6b, respectively. The x-component of the magnetic field depicted inFIG. 6ashows that the x-component of the magnetic field has an absolute value that is a maximum at x=0, which corresponds to the location of the magnet76and thus the piston20. That is, as the magnetic field sensor132sweeps from one end of the sample chamber to the other, the detected magnetic field (x-component) is a maximum (in absolute value) when the sensor132is adjacent the magnet76. The magnetic field sensor132produces an electrical signal123that is provided to the control unit110which thus is able to monitor the magnitude of the sensor's signal123to detect the peak in the detected magnetic field. Once the magnetic field peak is detected, the control unit110reads the position signal from the sensor locating device131to determine the sensor's position corresponding to the peak of the magnetic field. From that position, the control unit110is able to determine the distance D1the piston20is within the sealed chamber from end127.

FIG. 6bdepicts the y-component of the magnetic field from magnet76. In some embodiments, the magnetic field sensor132has sensitivity in the y-direction instead of, or in addition to, the x-direction In such embodiments, the control unit110can determine when the magnetic field sensor132is adjacent the magnetic76, and thus piston20, by determining when the y-component of the magnetic field crosses through 0 at point139. As explained above, when the control unit110determines that the sensor132is adjacent the magnet76, the control unit110reads the position signal from the sensor locating device131to determine the sensor's position corresponding to the peak magnetic field. From that position the control unit110is able to determine the distance D1the piston20is within the sealed chamber from end127.

Depending on whether a single-axis or multi-axis magnetic field sensor132is used, the control unit110determines when the sensor132is adjacent the magnet76using the x-component of the magnetic field, the y-component of the magnetic field, or a combination of both. If both the x- and y-components are used, the magnetic field sensor132provides two signals to the control unit110—one signal corresponding to each magnetic field component. The control unit110may, for example, use one signal as confirmation that the other signal is accurately indicating magnet76location. Alternatively, the control unit110may average the times at which the control unit110determines the magnet location from both signals and determine the piston location using the position signal121from the sensor locating device131corresponding to the computed average time value.

In accordance with various embodiments, the sample chamber is cylindrical. The volume of a cylinder is computed as Dπr2where D is the length of the cylinder and r is its cross-sectional radius. Referring toFIG. 5, the collected fluid sample is in the portion77of the sample chamber between the piston20and sample chamber end127. The volume of that portion and thus the volume of the fluid sample is (D1)(πr12) where r1is the cross-section radius of sample chamber10. The radius r1is known ahead of time and distance D1is determined by the control unit110by reading the magnetic field sensor132and sensor locating device131.

FIGS. 7-10illustrate various method embodiments. The order of the actions depicted in these methods may be as shown in the figures or may be different from that shown. Further, not all of the actions are necessarily performed sequentially. Instead, two or more actions may be performed in parallel.

InFIG. 7, well drilling begins at202. The drilling operation may comprise any type of drilling such as vertical, deviated or horizontal drilling, multi-lateral drilling, or conventional drilling or under-balanced drilling. During the drilling phase, various types of tests may be performed using wireline, measurement while drilling (MWD), logging while drilling (LWD), etc. The testing described herein can be with any such type of testing paradigm.

At204, a sample chamber10is weighed at the surface and thus before a fluid sample is collected. The chamber's weight is recorded into the test unit100(e.g., the control unit110). At206, the sample chamber10is placed into the test system100and the piston's position is determined and also recorded into the test system100(e.g., in storage114). This “initial” piston position thus is the position before a fluid sample is taken. At208, the sample chamber10is lowered down the well bore and a fluid sample is collected at210.

If a downhole pump60is used (e.g., as with the sample chamber embodiments ofFIGS. 3 and 4) and such a pump includes a filling sensor as explained above, at212the pump measures or estimates the sample volume which may be transmitted to the surface via wireline or mudpulse communication techniques as noted above. The sample chamber10is brought up to the surface at214and, at216, the sample chamber10is again weighed. The post-sample weight of the sample chamber10is recorded into the test system. The difference in the before and after weights of the sample chamber corresponds to the weight/mass of the fluid sample itself.

At218, the sample chamber10is loaded into the test system and, at220, the hydraulic line122is connected to the sample chamber. At222, the control unit110determines the mass of the fluid sample by, for example, subtracting the initial (no sample) weight of the sample chamber from the weight of the chamber containing the fluid sample. The mass may be stored in storage114. At224, the control unit110determines the in situ density of the fluid sample in the sealed sample chamber.

FIGS. 8 and 9illustrate two embodiments for determining the in situ density of the fluid sample. At226, the control unit110determines the compressibility of the fluid sample andFIG. 10illustrates a technique for determining compressibility.

FIG. 8illustrates an embodiment224for determining in situ density. This embodiment is particularly useful if a downhole pump was used to fill the sample chamber. At242, the control unit110retrieves the sample volume as measured or estimated by the downhole pump60and transmitted to surface as explained above. At244, the control unit retrieves the sample mass as well. At246, the control unit divides the mass by the volume to compute in situ density.

FIG. 9illustrates an alternative embodiment224for determining in situ density, particularly if downhole pump60is not used or, if a pump is used without the ability to measure or estimate sample volume. At252, the control unit110causes the magnetic field sensor232to begin sweeping across the outside of the sample chamber10to determine the position of the piston20. The position of the piston20informs the control unit110as to the distance D1. Using distance D1, at254the control unit110computes the in situ sample volume. In some embodiments, in situ volume is based on both the initial piston position (FIG. 7, 206) before the fluid sample is collected and the final piston position (252). Specifically, the distance D1is computed to be the difference between the initial and final piston positions. Computing the difference in initial and final piston positions is useful if a gas was included in the compartment in which the sample fluid is subsequently collected—the total volume of the compartment containing the sample fluid is the volume of both the sample fluid and the volume of the initial gas and thus should be compensated for the volume of the gas for greater volume accuracy.

At256, the control unit110computes the sample volume as explained above. At258, the control unit110retrieves the sample mass from storage114and, at260, the in situ density is computed by, for example, dividing the sample mass by the sample volume.

FIG. 10provides a method embodiment226in which compressibility of the fluid sample is determined. In general, the piston20is moved within the sample chamber10to various positions thereby applying different pressures on the fluid sample. The volume of the fluid sample is determined at each pressure setting. At272, the control unit110preferably causes the magnetic field sensor to repeatedly sweep back and forth across the outside of the sample chamber10along the x-direction. At274, the control unit110asserts a signal to the hydraulic unit120to cause the hydraulic unit thereby to incrementally increase the pressure in the hydraulic line122. Referring briefly toFIG. 5, the hydraulic fluid in the hydraulic line122is in fluid communication with space79behind the piston20. Thus, an increase in pressure in the hydraulic line122is also asserted against the piston20. Until the pressure of the hydraulic fluid in space79exceeds the pressure of the fluid sample in space77, the piston20will not move toward end127of the chamber10. Thus, at276, the control unit110monitors the signals121and123from the linear position device130to detect movement of the piston20. The pressure at which the piston20begins to move is referred to as the “opening pressure.” Once the opening pressure is reached, increasing the hydraulic pressure further causes the piston20to move toward end127of the chamber10. Further, for each such hydraulic pressure in excess of the opening pressure, the piston will move to and stop at a certain point within the chamber10; that point is the location at which the hydraulic pressure substantially equals the pressure of the fluid sample (which itself experiences an increases in pressure as the piston20increasingly compresses the fluid sample).

If piston movement is not detected at276, then control loops back to274at which the hydraulic pressure is again incremented (e.g., in increments of 100 psi). Once piston movement is detected, however, control passes to278at which the control unit110records the pressure of the hydraulic line as measured by pressure sensor124. Because hydraulic pressure of line122is substantially equal to the fluid sample pressure, the pressure measured by pressure sensor124is also the pressure of the fluid sample.

At280, the control unit also measures the position of filling piston20thereby to determine distance D1associated with piston20. Preferably, distance D1is computed as the difference between the newly measured piston distance and its initial distance before the fluid sample was collected (FIG. 7, 206). At282, using newly determined distance D1and the known radius of the cylindrical sample chamber10(or, in general, known cross-sectional area of chamber if the chamber has a shape other than cylindrical), the sample volume is computed by control unit110as the cross-sectional area times D1.

At284, the control unit110computes the fluid sample compressibility for the current sample pressure. Compressibility is defined as the fractional change of volume due to changes in pressure for a constant temperature and is estimated using the following equation:

Comp=-1V⁢(∂V∂p)T
where V is the total volume of the sample chamber10and T is a constant temperature. The control unit110calculates and stores and/or displays a compressibility value for each pressure measured at278.

At286, the control unit110determines whether a stopping condition is met. In some embodiments, the stopping condition may comprise a threshold pressure level (e.g., 10,000 psi). If the stopping condition has not been reached, then control passes to288in which the control unit110causes the hydraulic unit120to incrementally change (e.g., increase) the hydraulic pressure in hydraulic line122to further move piston20to thereby further compress the fluid sample in space77. The increment in pressure may be in increments of, for example, 500 psi. The process loops back to278for another pressure measurement, and so on. If, at284, it is determined that the stopping condition has been met, then the process stops at290.