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FIELD OF THE DISCLOSURE 
       [0001]    This disclosure relates generally to geological formations and, more particularly, to methods and apparatus to perform pressure testing of geological formations. 
       BACKGROUND 
       [0002]    Wells are generally drilled into the ground to recover natural deposits of hydrocarbons and/or other desirable materials trapped in geological formations in the Earth&#39;s crust. A well is drilled into the ground and/or directed to a targeted geological location and/or geological formation by a drilling rig at the Earth&#39;s surface. Data collected from pressure testing a geological formation can be used to determine one or more properties of the geological formation and/or a formation fluid present in the geological formation. 
       SUMMARY 
       [0003]    Example methods and apparatus to perform pressure testing of geological formations are disclosed. A disclosed example method includes positioning a testing tool in a wellbore formed in the geological formation, sealing a sample interval around the testing tool, sealing a first guard interval around the testing tool and adjacent to the sample interval, reducing a first pressure in the sample interval, reducing a second pressure in the first guard interval, maintaining a volume of a first chamber fluidly coupled to the sample interval during a time interval, and measuring a plurality of pressure data for a fluid captured in the first chamber during the time interval. 
         [0004]    A disclosed example downhole tool for pressure testing a geological formation includes first and second packers to form an inner interval around the testing tool, a third packer to seal a first outer interval around the testing tool adjacent to the inner interval, a first pump to reduce a first pressure in the inner interval, a second pump to reduce a second pressure in the first outer interval, and a pressure gauge to measure a plurality of pressure data for a fluid captured in the inner interval while the second pressure is reduced and a volume of the inner interval is maintained. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  illustrates an example wellsite drilling system within which the example methods and apparatus described herein may be implemented. 
           [0006]      FIG. 2  illustrates an example manner of implementing a logging while drilling (LWD) module for the example wellsite drilling system of  FIG. 1 . 
           [0007]      FIG. 3  illustrates an example manner of implementing the pressure testing system of  FIG. 2 . 
           [0008]      FIG. 4  is a graph characterizing an example operation of the example pumping system of  FIG. 2 . 
           [0009]      FIG. 5  illustrates another example manner of implementing the pressure testing system of  FIG. 2 . 
           [0010]      FIG. 6  is a flowchart of an example process that may be executed by, for example, a processor to perform pressure testing of a geological formation. 
           [0011]      FIG. 7  is a schematic illustration of an example processor platform that may be used and/or programmed to carry out the example process of  FIG. 6  to implement any of all of the example methods and apparatus described herein. 
       
    
    
       [0012]    Certain examples are shown in the above-identified figures and described in detail below. In describing these examples, like or identical reference numbers may be used to identify common or similar elements. The figures are not necessarily to scale and certain features and certain views of the figures may be shown exaggerated in scale or in schematic for clarity and/or conciseness. 
       DETAILED DESCRIPTION 
       [0013]    The example methods and apparatus disclosed herein use multiple packers to mechanically stabilize a packed and/or sealed-off section of the wellbore (i.e., an inner interval, a sampling interval, etc.) in which pressure testing and/or fluid sampling operations may be performed. By mechanically stabilizing the sampling interval, the pressure buildup characteristics of a geological formation can be more accurately measured, computed and/or otherwise determined. To stabilize the sampling interval, guard intervals are formed on opposite sides of the sampling interval by the use of additional outer packers. The hydraulic pressure in the guard intervals may be controlled and/or maintained to reduce the differential pressure(s) across the inner packer elements that form the sampling interval during, for example, a pressure drawdown and a subsequent pressure buildup test. For example, a low pressure-differential may be maintained across the inner packers. Additionally or alternatively, the difference between the wellbore pressure (i.e., hydrostatic pressure) and the drawdown pressure may be distributed across the guard intervals and the sampling interval to facilitate pressure testing in wellbores having high hydrostatic pressures. 
         [0014]    While example methods and apparatus are described herein with reference to so-called “sampling-while-drilling,” “logging-while-drilling,” and/or “measuring-while drilling” operations, the example methods and apparatus may, additionally or alternatively, be used to perform pressure testing of geological formations during a wireline sampling operation. 
         [0015]      FIG. 1  illustrates an example wellsite drilling system that can be employed onshore and/or offshore. In the example wellsite system of  FIG. 1 , a borehole  11  is formed in one or more subsurface formations F by rotary and/or directional drilling. 
         [0016]    As illustrated in  FIG. 1 , a drill string  12  is suspended within the borehole  11  and has a bottom hole assembly (BHA)  100  having an optional drill bit  105  at its lower end. A surface system includes a platform and derrick assembly  10  positioned over the borehole  11 . The example derrick assembly  10  of  FIG. 1  includes a rotary table  16 , a kelly  17 , a hook  18  and a rotary swivel  19 . The drill string  12  is rotated by the rotary table  16 , energized by means not shown, which engages the kelly  17  at the upper end of the drill string  12 . The example drill string  12  is suspended from the hook  18 , which is attached to a traveling block (not shown), and through the kelly  17  and the rotary swivel  19 , which permits rotation of the drill string  12  relative to the hook  18 . Additionally or alternatively, a top drive system could be used. 
         [0017]    In the example of  FIG. 1 , the surface system further includes drilling fluid or mud  26  stored in a pit  27  formed at the well site. A pump  29  delivers the drilling fluid  26  to the interior of the drill string  12  via a port in the swivel  19 , causing the drilling fluid to flow downwardly through the drill string  12  as indicated by the directional arrow  8 . The drilling fluid  26  exits the drill string  12  via ports in the drill bit  105 , and then circulates upwardly through the annulus region between the outside of the drill string  12  and the wall of the borehole  11 , as indicated by the directional arrows  9 . The drilling fluid  26  lubricates the drill bit  105 , carries formation cuttings up to the surface as it is returned to the pit  27  for recirculation, and creates a mudcake layer on the walls of the borehole  11 . 
         [0018]    The example BHA  100  of  FIG. 1  includes, among other things, any number and/or type(s) of logging-while-drilling (LWD) modules (two of which are designated at reference numerals  120  and  120 A) and/or measuring-while-drilling (MWD) modules (one of which is designated at reference numeral  130 ), a roto-steerable system or mud motor  150 , and the optional drill bit  105 . 
         [0019]    The example LWD modules  120  and  120 A of  FIG. 1  are each housed in a special type of drill collar, as it is known in the art, and each contain any number of logging tools and/or fluid sampling devices. The example LWD modules  120 ,  120 A include capabilities for measuring, processing, and/or storing information, as well as for communicating with surface equipment, such as a logging and control computer  160  via, for example, the MWD module  130 . 
         [0020]    An example LWD module  200  having four packers to improve the accuracy and/or conditions in which pressure testing of the geological formation F may be performed is described below in connection with  FIG. 2 . Example manners of implementing a pressure testing system  220  ( FIG. 2 ) for any of the LWD modules  120 ,  120 A,  200  are described below in connection with  FIGS. 3 and 5 . 
         [0021]    Another example manner of implementing an LWD module  120 ,  120 A is described in U. S. Publication No. 2008/0066535, entitled “Adjustable Testing Tool and Method of Use,” published on Mar. 20, 2008, and which is hereby incorporated by reference in its entirety. 
         [0022]    The example MWD module  130  of  FIG. 1  is also housed in a special type of drill collar and contains one or more devices for measuring characteristics of the drill string  12  and/or the drill bit  105 . The example MWD tool  130  further includes an apparatus (not shown) for generating electrical power for use by the downhole system. Example devices to generate electrical power include, but are not limited to, a mud turbine generator powered by the flow of the drilling fluid, and a battery system. Example measuring devices include, but are not limited to, a weight-on-bit measuring device, a torque measuring device, a vibration measuring device, a shock measuring device, a stick slip measuring device, a direction measuring device, and an inclination measuring device. 
         [0023]      FIG. 2  is a schematic illustration of an example manner of implementing either or both of the example LWD modules  120  and  120 A of  FIG. 1 . While either of the example LWD modules  120  and  120 A of  FIG. 1  may be implemented by the example device of  FIG. 2 , for ease of discussion, the example device of  FIG. 2  will be referred to as LWD module  200 . The example LWD module  200  of  FIG. 2  may be used to perform, among other things, pressure testing of a geological formation F. The example LWD module  200  is attached to the drill string  12  ( FIG. 1 ) driven by the rig  10  to form the wellbore or borehole  11 . When the LWD module  200  is part of a drill string, the LWD module  200  includes a passage (not shown) to permit drilling mud to be pumped through the LWD module  200  to remove cuttings away from a drill bit. 
         [0024]    To seal off intervals and/or portions  205 ,  206  and  207  of the example wellbore  11 , the example LWD module  200  of  FIG. 2  includes packers  210 ,  211 ,  212  and  213 . The example packers  210 - 213  of  FIG. 2  are inflatable elements that encircle the generally circularly shaped LWD  200 . The example intervals  205 - 207  of  FIG. 2  likewise encircle the LWD  200 . When inflated to form a seal with a wall  215  of the wellbore  11 , as shown in  FIG. 2 , the example inner pair of packers  210  and  211  form the inner and/or sampling interval  205  in which pressure testing of the geological formation F is performed. Other formation and/or formation fluid tests and/or measurements may also be performed in the inner interval  205 . When inflated to form a seal with the wall  215  of the wellbore  11 , as shown in  FIG. 2 , the example outer pair of packers  212  and  213  form respective guard intervals  206  and  207  on respective and/or opposite sides of the inner interval  205 . The example packers  210 - 213  of  FIG. 2  have a height of 1.5 feet and a spacing of 3 feet. However, other size packers and/or packer spacing(s) may be used depending on an expected mud filtrate invasion depth, and/or a desired formation fluid cleanup and/or production performance. 
         [0025]    To allow the example pressure testing system  220  to be fluidly coupled to the intervals  205 - 207 , the example LWD module  200  of  FIG. 2  includes ports  225 ,  226  and  227  for respective ones of the intervals  205 - 207 . As described below in connection with  FIGS. 3-5 , the example pressure testing system  220  of  FIG. 2  is able to pump fluid from the sample and/or inner interval  205  via the port  225  to perform a cleanup or sampling operation of the sample interval  205  (e.g., lift and/or remove mudcake), and/or to drawdown the pressure in the sample interval  205  and measure subsequent pressure buildup data. The example pressure testing system  220  is also able to draw fluid out of and/or push fluid into the guard intervals  206  and  207  to adjust, control and/or maintain pressure(s) in the guard intervals  206  and  207 . In some examples, the pressure testing system  220  reduces the pressure in the guard intervals  206  and  207  to approximately the formation pressure (or a pressure between the formation pressure and the wellbore pressure) while the sample interval  205  is being drawn down to perform a pressure buildup test. In such an example, the pressure differential experienced by the inner packers  210  and  211  (see  FIG. 3 ) is reduced to less than the pressure differential that would be experienced by the packers  210  and  211  were the outer packers  212  and  213  not present, inflated and/or implemented. In other examples, the pressure testing system  220  of  FIG. 2  maintains the pressures in the guard intervals  206  and  207  to be substantially equal to (or having a fixed offset from) the pressure in the inner interval  205 . By reducing and/or controlling the pressure differentials experienced by the inner packers  210  and  211 , the inner packers  210  and  211  are less susceptible to mechanical instability (e.g., creeping, sliding and/or deformation), thereby improving the accuracy of the subsequent pressure buildup data. Moreover, because the example inner packers  210  and  211  of  FIG. 2  are subjected to lower differential pressures they may be implemented using simpler packer structures (e.g. shorter packers, packers having less or none reinforcement structures such as cables, etc.). The use of shorter and/or simpler packer structures may be advantageous to reduce the overall length of the LWD module  200 . Example manners of implementing the example pressure testing system  220  of  FIG. 2  is described below in connection with  FIGS. 3 and 5 . 
         [0026]    The example pressure testing system  220  of  FIG. 2  is also fluidly coupled to a port  228  located below the example outer packer  213 . The example port  228  of  FIG. 2  is directly exposed to the fluid(s) present in the wellbore  11 . The example port  228  may, alternatively, be located above the example outer packer  212 . Moreover, the port  228  may be fluidly coupled to an additional port (not shown) located above the packer  212  via a bypass flowline of the LWD module  200  (not shown). Among other things, the example port  228  of  FIG. 2  can be used to balance the pressure of the portion of the wellbore  11  located above the packer  212  with the pressure of the portion of the wellbore  11  located below the packer  213 , and/or to allow fluid to be moved between any of the intervals  206 - 207  and the wellbore  11  via a bypass flowline of the LWD module  200  (not shown). 
         [0027]    In some examples, one or more probes (not shown) having pretest capabilities may be implemented to perform formation pressure and/or mobility measurements in one or more of the intervals  206  and  207 , below the example outer packer  213  and/or above the example outer packer  212 . Such probes may be used to obtain values representative of formation parameters in a substantially shorter time period than when using a packer interval. Formation parameter values obtained with the probe(s) may be used by example pressure testing system  220  for example to maintain the pressures in the guard intervals  206  and  207  to be substantially equal to (or having a fixed offset from) the formation pressure. Example probes and methods to use the same are described in U.S. Pat. No. 7,031,841, entitled “Method for Determining Pressure of Earth Formations,” and issued on Apr. 18, 2006; and in U.S. Pat. No. 6,986,282, entitled “Method and Apparatus for Determining Downhole Pressures during a Drilling Operation,” and issued on Jan. 17, 2006. U.S. Pat. No. 7,031,841, and U.S. Pat. No. 6,986,282 are hereby incorporated by reference in their entireties. 
         [0028]    Additionally or alternatively, pressure values obtained with the probe(s) may be used to determine propagation properties of pressure pulses in the formation. Example manners of determining propagation properties of pressure pulses in the formation are taught for example in U.S. Pat. No. 4,936,139, entitled “Downhole Method for Determination of Formation Properties,” and issued on Jun. 26, 1990. 
         [0029]      FIG. 3  illustrates an example manner of implementing the example pressure testing system  220  of  FIG. 2 . To pump fluid from the inner interval  205  via the port  225 , the example pressure testing system  220  of  FIG. 2  includes any type of pump  305 . When activated, the example pump  305  of  FIG. 3  pumps fluid from the port  225  into, for example, a sample container and/or bottle, the wellbore  11  (e.g., via a bypass flowline (not shown)), and/or a fluid analysis module. As shown in  FIG. 4 , the example pump  305  may be used to pump fluid from the inner interval  205  to drawdown the pressure P S  of the inner interval  205  to initiate a pressure buildup test. In the example of  FIG. 4 , the inner interval pressure P S  is reduced by the pump  305  to a pressure that is less than the formation pressure P F . In some examples, the pump  305  operates until a specified amount of reservoir fluid has been pumped. Additionally or alternatively, the pump  305  operates until the drawdown pressure is reached, the pump  305  is stopped, and the inner interval pressure P S  is measured while it builds backup towards the formation pressure P F , and while the volume(s) of any flowlines and/or chambers fluidly coupled to the port  225  are held constant. To measure the inner interval pressure P S , the example pressure testing system  220  of  FIG. 2  includes any type of pressure gauge  310 . 
         [0030]    To adjust the pressure in the guard intervals  206  and  207 , the example pressure testing system  220  of  FIG. 3  includes any type of pump  315 . The example pump  315  of  FIG. 3  is controllable to pump fluid into and/or out of the guard intervals  206  and  207  to increase and/or decrease the pressure in the guard intervals  206  and  207 , respectively. An example pump  315  includes a hydraulic piston  320  to adjust the volume in a chamber  325  fluidly coupled to the ports  226  and  227 . To measure the pressure P G  of the guard intervals  206  and  207 , the example pressure testing system  220  of  FIG. 2  includes any type of pressure gauge  330 . To measure the pressure P W  of the wellbore  11 , the example pressure testing system  220  of  FIG. 2  includes any type of pressure gauge  335 . In some examples, a single pump is used to implement the pump  305  and the pump  315 . 
         [0031]    To perform a pressure buildup test, the example pressure testing system  220  of  FIG. 3  includes a controller  340 . The example controller  340  of  FIG. 3  controls the example pump  305  and piston  320  to initiate a pressure buildup test, and measures the pressure in the inner interval  205  during the subsequent pressure buildup phase via the example pressure gauge  310 . The example controller  340  also controls the inflation and deflation of the example packers  210 - 213 . The example controller  340  of  FIG. 3  is implemented by any type of general-purpose processor, processor core, and/or microcontroller. Alternatively, the example controller  340  may be implemented by one or more circuit(s), programmable processor(s), application specific integrated circuit(s) (ASIC(s)), programmable logic device(s) (PLD(s)) and/or field programmable logic device(s) (FPLD(s)), etc., and/or any combination of hardware, firmware and/or software. 
         [0032]    As shown in  FIG. 4 , at a time  405  the example controller  340  ( FIG. 3 ) activates the pump  305  to reduce the inner interval pressure P S  from the wellbore pressure P W  to a pressure less than the formation pressure P F . While the inner interval pressure P S  is being reduced, the example controller  340  adjusts the position of the piston  320  to adjust the guard interval pressure P G  to a desired and/or target pressure. The guard interval pressure P G  may be adjusted in accordance with any number of pressure management strategies. For example, the guard interval pressure P G  may be reduced to the formation pressure P F  (e.g. estimated from a measurement performed by a probe). In such an example, the pressure differentials experienced by each of the inner packers  210  and  211  is substantially zero at the end of the pressure buildup test, while the pressure differentials experienced by the outer packers  212  and  213  are substantially the difference between the wellbore pressure P W  and the formation pressure P F  at the end of the pressure buildup test. In another example, the guard interval pressure P G  is adjusted to a pressure between the wellbore pressure P W  and the formation pressure P F  to distribute the pressure difference across the inner packers  210  and  211  and the outer packers  212  and  213 . In such an example, the example LWD module  200  can operate in a wellbore having a higher hydrostatic pressure to drawdown pressure difference than can be withstood by a single pair of inner packers  210  and  211  and/or the pump  305 . The example controller  340  can determine how much to reduce the pressure P G  of the guard intervals  206  and  207  based on the wellbore pressure P W  measured by the pressure gauge  335  and a desired drawdown pressure. For example, for a large wellbore to drawdown pressure difference, the example controller  340  distributes the pressure difference across the outer packers  212  and  213  and the inner packers  210  and  211 . Otherwise, the example controller  340  adjusts the guard interval pressure P G  to be substantially equal to the formation pressure P F . 
         [0033]    When, at time  410 , the drawdown pressure has been reached and the guard interval pressure P G  adjusted, the controller  340  starts measuring pressure buildup data in the inner interval  205  using the pressure gauge  310 . 
         [0034]      FIG. 5  illustrates another example manner of implementing the example pressure testing system  220  of  FIG. 2 . Because elements of the example pressure testing system  220  of  FIG. 5  are similar or identical to those discussed above in connection with  FIG. 3 , the descriptions of those similar or identical elements are not repeated here. Instead, similar or identical elements are illustrated with identical reference numerals in  FIGS. 3 and 5 , and the interested reader is referred back to the descriptions presented above in connection with  FIG. 3  for a complete description of those like numbered elements. 
         [0035]    In contrast to the example pressure testing system  220  of  FIG. 3 , the example pressure testing system  220  of  FIG. 5  includes pressure controllers  505  and  510  for respective ones of the guard intervals  206  and  207 . The example pressure controller  505  of  FIG. 5  actively controls the pump  315  to maintain the guard interval pressure P G1  of the guard interval  206  based on the inner interval pressure P S  and the wellbore pressure P W . For example, the pressure controller  505  adapts and/or maintains the guard interval pressure P G1  to be substantially equal to the inner interval pressure P S  to reduce the mechanical stresses experienced by the inner packer  210 . When the wellbore to drawdown pressure difference is large, the example controller  505  adapts the guard interval pressure P G1  to distribute the pressure difference between the outer packer  212  and the inner packer  210 . The pressure P G1  of the guard interval  206  is measured by the example pressure gauge  330 . 
         [0036]    Likewise, the example pressure controller  510  of  FIG. 5  actively controls a pump  315 B, which is substantially identical to the example pump  315 , to maintain the guard interval pressure P G2  of the second guard interval  207  based on the inner interval pressure P S  and the wellbore pressure P W . The pressure P G2  of the guard interval  207  is measured by a pressure gauge  330 B, which is substantially identical to the pressure gauge  330 . While in some examples, the pressures P G1  and P G1  are maintained at substantially the same pressure, the pressures P G1  and P G1  may be maintained at different pressures. For example, independent control of the pressure P G1  in the first guard interval  206  and the pressure P G2  in the second guard interval  207  may be beneficial when one of the outer packers  212 ,  213  experiences mechanical instability (e.g., creeping, sliding and/or deformation). In such circumstances, the pressure in the corresponding guard intervals  206  or  207  may require adjustment to minimize the impact of the mechanical instability of the outer packer  212 ,  213  on the pressure P G  in the testing interval  205 . 
         [0037]    The example pressure controllers  505  and  510  of  FIG. 5  are implemented by any type of general-purpose processor, processor core, and/or microcontroller. Alternatively, the example pressure controllers  505  and  510  may be implemented by one or more circuit(s), programmable processor(s), ASIC(s), PLD(s) and/or FPLD(s), etc., and/or any combination of hardware, firmware and/or software. 
         [0038]    In addition to controlling the example pump  305  and measuring the pressure buildup data via the example pressure gauge  310 , as described above in connection with  FIGS. 3 and 4 , the example controller  340  of  FIG. 5  activates and/or deactivates the pressure controllers  505  and  510 . 
         [0039]    While example manners of implementing the example pressure testing system  220  of  FIG. 2  have been illustrated in  FIGS. 3 and 5 , one or more of the elements, controllers and/or devices illustrated in  FIGS. 3  and/or  5  may be combined, divided, re-arranged, omitted, eliminated, and/or implemented in any other way. For example, the example pressure controller  505  could be implemented in the example pressure control system  220  of  FIG. 2  to adapt, control and/or maintain the pressure in both of the guard intervals  206  and  207  via the pump  315 . Further, a pressure testing system and/or LWD module may include elements, controllers and/or devices instead of, or in addition to, those illustrated in  FIGS. 3  and/or  5 , and/or may include more than one of any or all of the illustrated elements, controllers and/or devices. 
         [0040]      FIG. 6  illustrates an example process that may be carried out to perform pressure testing of a geological formation. The example process of  FIG. 6  may be carried out by a processor, a controller and/or any other suitable processing device. For example, the process of  FIG. 6  may be embodied in coded instructions stored on a tangible machine and/or computer-readable medium such as a flash memory, a CD, a DVD, a floppy disk, a read-only memory (ROM), a random-access memory (RAM), a programmable ROM (PROM), an electronically-programmable ROM (EPROM), and/or an electronically-erasable PROM (EEPROM), an optical storage disk, an optical storage device, a magnetic storage disk, a magnetic storage device, and/or any other tangible medium, which can be accessed, read and/or executed by a processor, a general purpose or special purpose computer or other machine with a processor (e.g., the example processor platform P 100  discussed below in connection with  FIG. 7 ). Alternatively, some or all of the example process of  FIG. 6  may be implemented using any combination(s) of circuit(s), ASIC(s), PLD(s), FPLD(s), discrete logic, hardware, firmware, etc. Also, some or all of the example process of  FIG. 6  may be implemented manually or as any combination of any of the foregoing techniques, for example, any combination of firmware, software, discrete logic and/or hardware. Further, although the example operations of  FIG. 6  are described with reference to the flowchart of  FIG. 6 , many other methods of implementing the operations of  FIG. 6  may be employed. For example, the order of execution of the blocks may be changed, and/or one or more of the blocks described may be changed, eliminated, sub-divided, or combined. Additionally, any or all of the example process of  FIG. 6  may be carried out sequentially and/or carried out in parallel by, for example, separate processing threads, processors, devices, discrete logic, circuits, etc. 
         [0041]    The example process of  FIG. 6  begins with the example LWD module  200  of  FIG. 2  being positioned in a wellbore (block  605 ). The example controller  340  ( FIGS. 3 and 5 ) inflates the packers  210 - 213  to seal and/or form the intervals  205 - 207  (block  610 ). In some examples, the inner packers  210  and  211  are inflated prior to the outer packers  212  and  213 , however, all of the packers  210 - 213  may alternatively be inflated essentially simultaneously. 
         [0042]    In some examples, the controller  340  collects pressure data to estimate the wellbore pressure P W  and the formation pressure P F . For example, the wellbore pressure P W  may be obtained via the pressure sensor  335  ( FIGS. 3 and 5 ), and the controller  340  may initiate a pretest using a probe (not shown) to estimate the formation pressure P F . In other examples, prior knowledge of the formation F (e.g. from a remotely performed pressure test, a pressure gradient, etc.) are used estimate the formation pressure P F . 
         [0043]    The controller  340  activates the pump  305  to, for example, perform initial cleanup, and/or mudcake removal in the inner interval  205  (block  615 ). In some example implementations, such as when no formation pressure estimate has been obtained otherwise, a formation pressure estimation may also be obtained at block  615  by detecting a mudcake breach and/or by permitting the pressure P S  in the interval  205  to stabilize after mudcake removal. 
         [0044]    The controller  340  activates the pump  305  to drawdown the pressure P S  of the inner interval  205  (block  620 ). At substantially the same time, the controller  340  of  FIG. 35  controls the pump  315  or activates the pressure controllers  505  and  510  ( FIG. 5 ) to adjust, set and/or otherwise reduce the pressures P G1  and/or P G2  of the guard intervals  206  and  207  (block  625 ). Alternatively, if the pressure testing system  220  of  FIG. 3  is being used, at block  625  the example controller  340  controls the pump  315  to adjust, set and/or otherwise reduce the pressure P G  of the guard intervals  206  and  207 . In some cases, the pressures P G1  and/or P G2  (or the pressure P G ) are controlled based on an estimate of the formation pressure P F , as well as the wellbore pressure P W . In particular, the pressures P G1  and/or P G2  (or the pressure P G ) are preferably maintained above the formation pressure P F  in order to minimize the risk of establishing a hydraulic communication between one of the outer intervals  206  or  207  and the formation F ( FIG. 2 ), which could have negative effect on the quality of the pressure buildup data and their interpretation. The drawdown and the guard interval pressure reductions may be performed in parallel to maintain the mechanical stability of the inner packers  210 - 211 . The controller  340  then freezes and/or fixes the volume of any flowlines and/or chambers fluidly coupled to the sample interval  205  (block  630 ). 
         [0045]    If the pressure controllers  505 ,  510  are not available for the guard intervals  206  and  207  (block  635 ), the controller  340  measures the pressure buildup data using the pressure gauge  310 , see  FIG. 3  (block  640 ). If there are pressure controllers  505 ,  510  available for the guard intervals  206  and  207  (block  635 ), the controller  340  measures the pressure buildup data using the pressure gauge  310  while the pressure controllers  505 ,  510  maintain the guard interval pressures P G1  and P G2 , see  FIG. 5  (block  645 ). 
         [0046]    When the pressure buildup test is complete, the controller  340  stores the measured pressure buildup data (block  650 ), and de-activates the pressure controllers  505  and  510  (if present) and deflates the packers (block  655 ). Control then exits from the example process of  FIG. 6 . Alternatively, at block  610  only the inner packers  210  and  211  are inflated. After the initial cleanup is performed at block  615 , the outer packers  212  and  213  are inflated. 
         [0047]      FIG. 7  is a schematic diagram of an example processor platform P 100  that may be used and/or programmed to implement any or all of the example methods and apparatus disclosed herein. For example, the processor platform P 100  can be implemented by one or more general-purpose processors, processor cores, microcontrollers, etc. 
         [0048]    The processor platform P 100  of the example of  FIG. 7  includes at least one general-purpose programmable processor P 105 . The processor P 105  executes coded instructions P 110  and/or P 112  present in main memory of the processor P 105  (e.g., within a RAM P 115  and/or a ROM P 120 ). The processor P 105  may be any type of processing unit, such as a processor core, a processor and/or a microcontroller. The processor P 105  may execute, among other things, the example process of  FIG. 6  to perform pressure testing of a geological formation. 
         [0049]    The processor P 105  is in communication with the main memory (including a ROM P 120  and/or the RAM P 115 ) via a bus P 125 . The RAM P 115  may be implemented by dynamic random-access memory (DRAM), synchronous dynamic random-access memory (SDRAM), and/or any other type of RAM device(s), and ROM may be implemented by flash memory, EPROM, EEPROM, a CD, a DVD and/or any other desired type of memory device(s). Access to the memory P 115  and the memory P 120  may be controlled by a memory controller (not shown). The memory P 115  may be used to store pressure buildup data. 
         [0050]    The processor platform P 100  also includes an interface circuit P 130 . The interface circuit P 130  may be implemented by any type of interface standard, such as an external memory interface, serial port, general-purpose input/output, etc. One or more input devices P 135  and one or more output devices P 140  are connected to the interface circuit P 130 . The input devices P 135  may be used to collect and/or receive pressure data from a pressure gauge. The output devices P 140  may be use to control and/or activate a pump. 
         [0051]    Although certain example methods, apparatus and articles of manufacture have been described herein, the scope of coverage of this patent is not limited thereto. On the contrary, this patent covers all methods, apparatus and articles of manufacture fairly falling within the scope of the appended claims either literally or under the doctrine of equivalents.

Summary:
Example methods and apparatus to perform pressure testing of geological formations are disclosed. A disclosed example method comprises positioning a testing tool in a wellbore formed in the geological formation, sealing a sample interval around the testing tool, sealing a first guard interval around the testing tool and adjacent to the sample interval, reducing a first pressure in the sample interval, reducing a second pressure in the first guard interval, maintaining a volume of a first chamber fluidly coupled to the sample interval during a time interval, and measuring a plurality of pressure data for a fluid captured in the first chamber during the time interval.