Abstract:
Apparatus and methods to perform focused sampling of reservoir fluid are described. An example method couples a sampling probe to a subterranean formation and, while the sampling probe is coupled to the subterranean formation, varies a pumping ratio of at least two displacement units to reduce a contamination level of a formation flu id extracted via the sampling probe from the subterranean formation.

Description:
RELATED APPLICATION 
       [0001]    This patent claims the benefit of the filing date of U.S. Provisional Patent Application No. 60/882,364 filed on Dec. 28, 2006. 
     
    
     FIELD OF THE DISCLOSURE 
       [0002]    The present disclosure relates generally to reservoir evaluation and, more particularly, to apparatus and methods to perform focused sampling of reservoir fluid. 
       BACKGROUND 
       [0003]    Drilling, completion, and production of reservoir wells involve monitoring of various subsurface formation parameters. For example, parameters such as reservoir pressure and permeability of the reservoir rock formation are often measured to evaluate a subsurface formation. Fluid may be drawn from the formation and captured to measure and analyze various fluid properties of a fluid sample. Monitoring of such subsurface formation parameters can be used, for example, to determine formation pressure changes along the well trajectory or to predict the production capacity and lifetime of a subsurface formation. 
         [0004]    Some known downhole measurement systems may obtain these parameters through wireline logging via a formation tester or sampling tool. Alternatively, a formation tester or sampling tool may be coupled to a drill string in-line with a drill bit (e.g., as part of a bottom hole assembly) and a directional drilling subassembly. Such formation testing or sampling tools may be implemented using fluid sampling probes, each of which has a one or more nozzles, inlets, or openings into which formation fluid may be drawn. A variety of types of sampling tools or probes are currently used to extract formation fluid. For example, some sampling tools use an extendable probe, which is sometimes generally referred to as a packer, having a single nozzle or inlet to draw formation fluid. The probe (e.g., the nozzle or inlet), is typically surrounded by a circular or ring-shaped rubber interface or packer that is extended toward and forced against a borehole wall to sealingly engage the nozzle or inlet with a subterranean formation. In some cases, the seal provided by a packer may be implemented using an inflatable packer device such as, for example that described in U.S. Pat. No. 6,301,959. Some sampling probes or packers provide multiple inlets (e.g., two inlets) where at least one inlet is a sample inlet and at least one other inlet is a guard inlet. However, in the case of a multi-inlet configuration, multiple packers may be used such that at least one packer includes a sample inlet and another separate packer or packers include the guard inlet or inlets. 
         [0005]    In operation, a sampling probe or packer may be extended via hydraulics from the downhole tool to drive its nozzle or inlet against the borehole wall adjacent a portion of the formation to be evaluated. A pumpout assembly is then activated to draw fluid from the formation into the probe and to convey the formation fluid to a downhole testing device and/or a sample collection vessel that can be retrieved to the surface to enable laboratory analysis of the sample fluid contained therein. Additionally, as noted above, the sampling probe inlet is typically surrounded by a packer that facilitates the sealing of the sampling probe inlet against the borehole wall and, thus, facilitates the application of a pressure to the formation to efficiently draw fluid from the formation. 
         [0006]    When drawing fluid from a formation, a certain amount of filtrate can also be drawn into the probe along with the formation fluid, thereby contaminating the sample fluid. The degree of contamination (e.g., the percent contamination) in the sample fluid is initially relatively large, but typically decreases over time as the sampling probe continues to draw formation fluid from the formation. Thus, fluid extracted from the formation by the sampling probe is usually discarded until, at some time during the sampling process, the level of contamination is sufficiently low to permit capture of a sample having an acceptable purity for testing or evaluation purposes. 
         [0007]    With single inlet sampling probes (i.e., a sampling probe providing only a sample inlet and no guard inlet), a relatively large amount of fluid may have to be drawn from the formation before an acceptable purity or contamination level is achieved. However, to draw such a large amount of fluid may require a significant amount of time, which can be costly, particularly if the job is delayed by the sampling process. Additionally, while the level of contamination can be reduced significantly by first drawing a large amount of fluid from the formation, the minimum level or degree of contamination achievable with a single inlet probe may remain high enough to affect the accuracy of the test results. 
         [0008]    While single inlet sampling probes have proven to be relatively effective, dual inlet or guard probes can provide improved, focused sampling of formation fluids. Such dual inlet or guard probes typically include concentric nozzles or inlets, where a central nozzle or inlet is configured to act as the sampling inlet and an outer nozzle or inlet is configured to act as a guard inlet. More specifically, the guard inlet, which forms a perimeter or ring around the central or sampling inlet, is configured to draw substantially all of the filtrate away from the central part of the probe and, thus, the central inlet, thereby enabling the central or sampling inlet to draw in formation fluid that is relatively free of contamination (e.g., filtrate). Dual inlet or guard probes also utilize two packers to seal the probe against the formation to be evaluated. An outer packer surrounds the guard nozzle or inlet and an inner packer surrounds the central sample nozzle or inlet in the area between an outer wall of the sample inlet and an inner wall of the guard inlet. 
         [0009]    In contrast to single inlet probes, dual inlet of guard probes can significantly reduce the time required to achieve a sufficiently low level of sample contamination (i.e., a reduced sample cleanup time), which can significantly decrease costs associated with evaluation of a formation (e.g., reduced station times). Additionally, dual inlet or guard probes can also provide significantly improved sample purity (i.e., a lower level of contamination) than possible with conventional single inlet probes. Such an increased level of sample purity can provide more accurate information for optimizing completion and production decisions. 
         [0010]    Although dual inlet or guard probes have enabled significantly reduced sample cleanup times and improved sample purity levels, such dual inlet probes can introduce certain operational complexities or difficulties. In particular, each nozzle or inlet typically has its own independently controlled pumpout and flowlines (e.g., guard and sample flowlines), which makes it difficult to control precisely the relative pumping rates (i.e., the pumping distribution) of the sample and guard nozzles or inlets and flowlines. An inability to control precisely the relative pumping rates of the guard and sample inlets and flowlines can lead to higher levels of contamination in the sample fluid, compromising of the inner packer seal or breakage of the inner packer, longer sample cleanup times, etc. Further, the use of an independent pumpout for each inlet and flowline results in less available power for each pumpout and can also result in a lower overall power efficiency. 
         [0011]    With some known dual inlet or guard probe systems, the differential pressure developed across the pumpouts is relatively fixed based primarily on the configuration of the displacement units within the pumpouts and the mobility of the fluid to be sampled. Thus, for a particular fluid mobility, a particular displacement unit may be selected to provide a desired pumping rate for each of the guard and sample inlets and flowlines as well as a relative pumping rate or pumping distribution between the guard and sample systems. However, fluid mobility may not be known precisely prior to sampling and, thus, a selected displacement unit may develop a differential pressure that results in poor fluid sampling (e.g., flow between the sample and guard inlets and, thus, increased sample contamination) and/or compromise of or damage to the inner packer. Additionally, further adjustments of the pumping rate and differential pressure developed by the pumpout(s) typically requires replacement of the displacement unit(s) at the surface, which is time consuming and costly. 
       SUMMARY 
       [0012]    In accordance with one exemplary embodiment, an apparatus for use with a downhole tool is disclosed. The apparatus includes a displacement device and a valve. The displacement device has a first plurality of chambers that are fluidly coupled to a flowline associated with the downhole tool, and the valve is fluidly coupled between the first plurality of chambers to vary a fluid pumping rate through the flowline. 
         [0013]    In accordance with another exemplary embodiment, an apparatus for use with a downhole tool is disclosed. The tool includes a first displacement unit to vary a first fluid characteristic associated with a first flowline, a second displacement unit to vary a second fluid characteristic associated with a second flowline, wherein the first and second displacement units are operatively coupled to operate synchronously, and a motor operatively coupled to the first and second displacement units. 
         [0014]    In accordance with another exemplary embodiment, a pump for use with a downhole tool is disclosed. The pump includes a plurality of chambers, a plurality of pistons and at least one valve. Bach of the plurality of pistons corresponds to at least one of the chambers, and are operatively coupled to move synchronously. The at least one valve is fluidly coupled to at least one of the chambers to selectively change a flowrate provided by the pump. 
         [0015]    In accordance with another exemplary embodiment, a method including: coupling a sampling probe to a subterranean formation, and varying a pumping ratio of at least two displacement units that are mechanically coupled to reduce a contamination level of a formation fluid extracted via the sampling probe from the subterranean formation, while the sampling probe is coupled to the subterranean formation is disclosed. 
         [0016]    In accordance with another exemplary embodiment, an apparatus for use in a borehole is disclosed. The apparatus for use in a borehole includes a first displacement unit fluidly coupled to a first flowline, a second displacement unit fluidly coupled to a second flowline, and a motor operatively coupled to the displacement units to cause the displacement units to reciprocate synchronously. 
         [0017]    In accordance with another exemplary embodiment, a method of controlling flowrate in a downhole tool is disclosed. The method includes lowering the downhole tool into a wellbore, fluidly coupling a first flowline associated with a first displacement unit to a subterranean formation in the wellbore, fluidly coupling a second flowline associated with a second displacement unit to the subterranean formation and synchronously reciprocating the first and second displacement units with a motor to extract fluid from the subterranean formation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a schematic diagram of a known pumpout configuration for a guard sampling probe assembly. 
           [0019]      FIG. 2A  is a schematic diagram of an example pumpout configuration having a dual displacement unit assembly where the differential pressure across each displacement unit can be controlled independently. 
           [0020]      FIG. 2B  is a schematic diagram of an alternative pumpout configuration having a dual displacement unit assembly where the pumped fluid can be routed independently to one or both displacement unit. 
           [0021]      FIG. 3  is a schematic diagram of an example focused sampling system that may be implemented using a pumpout configuration having a dual displacement unit assembly. 
           [0022]      FIG. 4  is an alternative dual displacement unit configuration that may be used to implement the example focused sampling system of  FIG. 3 . 
           [0023]      FIGS. 5   a ,  5   b , and  5   c  depict various tool topologies employing the example methods and apparatus described herein. 
           [0024]      FIG. 6  illustrates an example variable displacement unit comprising a dual displacement unit. 
           [0025]      FIG. 7  is a table illustrating the various operational modes that can be provided by the example variable displacement unit of  FIG. 6 . 
           [0026]      FIG. 8  depicts another variable displacement unit configuration. 
           [0027]      FIG. 9  schematically depicts a variable displacement unit configuration that incorporates more than four chambers. 
           [0028]      FIG. 10  depicts yet another example variable displacement unit. 
           [0029]      FIG. 11  is a schematic diagram of an example processor platform that may be used and/or programmed to implement any or ail example apparatus and methods described herein. 
       
    
    
     DETAILED DESCRIPTION  
       [0030]    The example pumpout configurations described in greater detail below may be used with dual or guard probe sampling tools to provide improved, focused sampling of formation fluids. More specifically, the example pumpout configurations may be used to mechanically synchronize the displacement units associated with the guard and sample flowlines. However, it should be understood that while the example pumpout configurations described herein are discussed in connection with dual or guard probe sampling tools, the example pumpout configurations are more generally applicable and, thus, may be used with, for example, one or more single inlet probes if desired. 
         [0031]    In contrast to conventional pumpout configurations used with dual or guard sampling probes, the example pumpout configurations described herein include controls to vary individually the differential pressure across each of the displacement units and, thus, the pumping rate distribution between or pumping ratio of the sample and guard flowlines. Such variations in differential pressure and pumping rate distribution can be automatically controlled to provide more rapid, focused formation fluid sampling while the tool remains in a downhole position. Thus, in contrast to some known systems, the example focused formation fluid sampling systems described herein eliminate the need to vary the pumping mode and/or the power provided to the hydraulic system, and/or removal and replacement of one or both displacement units (i.e., at the surface) to achieve a desired pumping rate distribution, for example. Further, the example focused formation fluid sampling systems described herein can be controlled in an adaptive manner to automatically control the differential pressure across the displacement units and the pumping rate of the guard and sample flowlines in response to variations in the formation characteristics and/or the formation fluid characteristics (e.g., fluid mobility), thereby enabling more rapid and accurate sampling, eliminating or minimizing the risk of inner packer failure, etc. 
         [0032]    Before providing a detailed description of the example pumpout configurations noted above, a brief description of a known pumpout configuration is first provided in connection with  FIG. 1 .  FIG. 1  is a schematic diagram of a known pumpout configuration or system  100  for use with a guard sampling probe assembly. In many oil extraction applications, positive displacement pumps are often used to extract fluid from a formation. A displacement pump is configured to displace a particular amount of fluid per stroke or per revolution. The fluid extracted from a formation is often thick and gritty making it impractical to use hydraulic pumps in a direct-pumping configuration. Instead, a hydraulic pump or a linear motor is typically connected to a displacement unit configured to generate a pumping force sufficient to extract the fluid from the formation. Traditional displacement units can generate a pumping pressure generally based on the volume of its piston chamber(s) and the characteristics of the attached pump or motor. In general, the known pumpout system  100  can be used with a dual or guard sampling probe to provide focused sampling of formation fluids. As depicted in  FIG. 1 , the known system  100  includes displacement units  102  and  104 , each of which is driven independently in a conventional manner by a respective motor and/or hydraulic system (neither of which are shown). The displacement unit  102  is fluidly coupled to a guard flowline  106  via cheek valves  108 ,  110 ,  112 , and  114  to enable fluid to be drawn from a guard nozzle, inlet, or portion of a dual or guard sampling probe (not shown) and conveyed or pumped in the direction of the arrow to, for example, a borehole annulus. Similarly, the displacement unit  104  is fluidly coupled to a sample flowline  116  via check valves  118 ,  120 ,  122 , and  124  to enable fluid to be drawn from a sample nozzle, inlet or portion of the dual or guard sampling probe and conveyed or pumped in the direction of the arrow to, for example, a sample collection vessel. Alternatively, the flow line  116  may be coupled to the back side of a sliding piston positioned in a sample collection vessel, as known in the art as a reverse low shock sampling technique. 
         [0033]    Each of the displacement units  102  and  104  is selected to provide a desired differential pressure and/or pumping rate to extract sample fluid from a particular formation. For example, a formation yielding a relatively low mobility fluid may require the use of displacement units that are configured to provide relatively high differential pumping pressures. Thus, with the known system  100 , several different displacement unit configurations providing different differential pressures are typically available. In this manner, appropriate displacement units can be selected and installed in a downhole tool to suit the needs of a particular formation, fluid, and/or sampling application. 
         [0034]    Further, as depicted in  FIG. 1 , the displacement units  102  and  104  may be differently sized or configured to provide a desired pumping rate distribution or pumping ratio and/or pressure across an inner packer of the sampling probe. Typically, the displacement unit  102  used in connection with the guard flowline  106  is sized to provide a pumping rate that is two to four times the pumping rate that the displacement unit  104  provides to the sample flowline  116 . While it is possible to select displacement units that generally suit the needs of a particular sampling application, such a selection may be complicated by the uncertainties associated with formation characteristics, formation fluid characteristics, changes that occur to the formation and/or the fluid being sampled therefrom, etc. As a result, an initial selection of displacement units may fail to perform as anticipated or desired. To improve sampling performance, the downhole tool can be removed from the borehole and one or both of the displacement units  102  and  104  can be replaced with differently configured units that may provide the desired sampling performance. However, such an empirical process of determining the best or substantially optimal displacement unit configurations may require several time consuming and expensive replacement and test cycles to ensure that a desired or acceptable sampling is performed. 
         [0035]    The mechanical operational independence of the displacement units  102  and  104  used in the known system  100  also results in certain operational inefficiencies and/or difficulties. For example, because the pressures developed across each of the displacement units  102  and  104  can vary significantly about an average value throughout the strokes of respective pistons  126  and  128 , pressure spikes developed by the displacement units  102  and  104  can induce significant transient perturbations of the local flow pattern near the inlets of the sampling probe, thereby adversely affecting the ability of the sampling probe to effectively separate formation fluid and filtrate. To alleviate the effects of such pressure variations, the known system  100  typically utilizes a relatively complex synchronization operation via which the pumping through the sample flowline  116  is interrupted when the piston  126  of the displacement unit  102  (i.e., for the guard flowline  106 ) is near the end of its stroke. 
         [0036]    As noted above, the known system  100  utilizes a separate motor (e.g., electric and/or hydraulic) for each of the displacement units  102  and  104 , which typically results in a lower overall power efficiency and reduces the power available to operate each of the displacement units  102  and  104 . As a result, the known system  100  typically does not operate both of the displacement units  102  and  104  during a cleanup phase of the sampling process. For example, to perform the cleanup (i.e., a procedure by which the sampled fluid is drawn and discarded until a desired level of sample purity is achieved to enable the subsequent collection of a sample to be analyzed), only the displacement unit  102  may be operated and the system  100  may be configured in a commingle mode in which the displacement unit  102  pumps or draws formation fluid through both the guard and sample flowlines  106  and  116 . When the formation fluid being drawn by the displacement unit  102  reaches the desired level of purity (i.e., reaches a sufficiently, low level of contamination), the system  100  switches to a split mode of operation in which both of the displacement units  102  and  104  operate independently and in which fluid is drawn from the guard portion of the sampling probe by the displacement unit  102  and from the sample portion of the sampling probe by the displacement unit  104 . 
         [0037]    Another difficulty associated with the known system  100  depicted in  FIG. 1  relates to the minimum pumping rate and differential pressure achievable with the displacement unit  104  that is used to pump fluid from the sample portion of the dual probe. In particular, although several displacement units may be available to provide a desired differential pressure and pumping rate, in some applications such as those involving relatively low mobility formation fluids, it may not be possible to reduce the differential pressure below a level that is potentially destructive to the inner packer of the sampling probe. 
         [0038]      FIG. 2A  is a schematic diagram of an example pumpout configuration  200  having a dual displacement unit assembly  202  where the differential pressure across each displacement unit can be controlled independently. Also, in contrast to the known system  100  of  FIG. 1 , the displacement unit assembly  202  includes displacement units  204  and  206  that are mechanically linked or coupled to operate in unison or in a synchronized manner. The example dual displacement unit assembly  202  may be implemented as a single body or housing having four chambers (i.e., two chambers for each of the displacement units  204  and  206 ) and respective pistons  208  and  210  attached to a common shaft  212  and motor (not shown). Alternatively, the dual displacement unit assembly  202  may be implemented as multiple bodies or housings (e.g., two or more housings), each of which contains one or portions of the displacement units  204  and  206 . In the case where multiple bodies or housings are used, each of the pistons  208  and  210  may have respective shafts (not shown) that are mechanically coupled, joined, linked, or otherwise operatively coupled to enable synchronized operation (e.g., pumping) of the displacement units  204  and  206 . In any case, the mechanical coupling and, thus, synchronization of the operation of the displacement units  204  and  206  may eliminate the need to employ the relatively complex synchronization technique (i.e., momentary interruption of the displacement unit drawing fluid from the sample portion of the sampling probe) used in connection with the known system  100  of  FIG. 1 . In other words, the mechanical coupling and synchronization of the displacement units  204  and  206  in the example displacement unit assembly  202  serves to eliminate or substantially minimize pressure and flow pattern transients near the interface between the formation and the guard and sample inlets of a dual sampling probe, thereby eliminating or substantially minimizing the adverse affect of such transients on fluid separation (i.e., separation of filtrate from formation fluid) at the sampling probe/formation interface. 
         [0039]    In the example system  200  of  FIG. 2A , the displacement unit  204  is fluidly coupled to a guard flowline  214  via check valves  216 , 218 ,  220 , and  222  to draw fluid from a guard portion of a sampling probe (not shown) and to convey the drawn fluid to a borehole annulus (not shown) in the direction of the arrow. Similarly, the displacement unit  206  is fluidly coupled to a sample flowline  224  via check valves  226 ,  228 ,  230 , and  232  to draw fluid, for example, from a sample portion of the sampling probe and to convey the drawn fluid to, for example, a sample chamber or vessel (not shown) in the direction of the arrow. In contrast to the known system  100  of  FIG. 1 , the example pumpout system  200  includes a displacement unit control  234  that can measure the pressures in the guard and sample flowlines  214  and  224  via respective pressure sensors  236  and  238  and modulate respective flow control valves  240  and  242  to automatically and adaptively control the differential pressures and pumping rates provided by the displacement units  204  and  206 . More specifically, at least partially opening the valve  240  provides a fluid path (e.g., a shunt having an optional flow restriction) between chambers  244  and  246  of the displacement unit  204 , thereby reducing the differential pressure developed by the displacement unit  204  and reducing the effective pumping rate of the displacement unit  204  for the guard flowline  214 . Similarly, at least partially opening the valve  242  provides a fluid path between chambers  248  and  250  of the displacement unit  206 , thereby reducing the differential pressure developed by the displacement unit  206  and reducing the effective pumping rate of the displacement unit  206  for the sample flowline  224 . A flow rate sensor may be added to advantage for monitoring the flow rate in the sample flowline  224  and/or the guard flowline  214  while any of the valves  240  and  242  are controllably operated. 
         [0040]    Thus, in one example, the chambers  244  and  246  may have the same lengths as the chambers  248  and  250 , but may have different cross-sectional areas to provide a desired intrinsic or base pumping distribution rate or pumping ratio between the guard and sample flowlines  214  and  224 . In operation, the displacement unit control  234  can be then used (e.g., as a feedback controller) to control the degree to which the valves  240  and  242  are open/closed to vary the differential pressures and pumping rates of the displacement units  204  and  206  to achieve a desired pumping rate distribution or pumping ratio and/or to control (e.g., to minimize) the pressure across the inner packer (not shown) of the sampling probe. In contrast to the known system  100  of  FIG. 1 , the differential pressures developed by the displacement units  204  and  206  as well the pumping rates and pumping rate distribution provided thereby can be varied without having to change (e.g., replace) either of the displacement units  204  and  206  and/or the power supply (e.g., the power distribution) by, for example, removing and replacing the displacement units at the surface. 
         [0041]    Further, the example system  200  also eliminates the minimum differential pressure and pumping rate limitations associated with the known system  100  of  FIG. 1 . In particular, the minimum differential pressure and/or pumping rates of the displacement units  204  and  206  are not based solely on the mechanical configurations of the displacement units  204  and  206  and/or the characteristics of the motor driving the units  204  and  206 . Instead, the minimum differential pressures and/or pumping rates can be determined by the flow paths provided by the valves  240  and  242 . For example, the greater the degree to which the valves  240  and  242  are open, the lower the flow restriction between the chambers  244  and  246  and the chambers  248  and  250 . As the flow restriction between chambers is reduced, the differential pressures developed across the displacement units  204  and  206  are reduced. As a result, the range of differential pressures and pumping rates achievable with the example system  200  of  FIG. 2A  may be significantly greater than possible with the known system  100  of  FIG. 1 . 
         [0042]    As noted/above, the pumpout system  200  is described herein in a configuration enabling for example a low shock sampling technique. However, the pumpout systems described herein may also be used for reverse low shock sampling techniques as well. In the example of  FIG. 2A , the guard flowline  224  may be selectively fluidly connected to the back side of a sliding piston positioned in a sample collection vessel (not shown). 
         [0043]    The example system  200  depicted in  FIG. 2A  can be implemented in various manners to achieve the same or similar results. For example, while two pressure sensors (i.e., the sensors  236  and  238  are shown as providing feedback information associated with the guard and sample flowlines  214  and  224  to the displacement unit control  234 , more or fewer such sensors could be used instead. Additionally or alternatively, pressure sensors could be used to measure fluid pressures at different and/or additional points within the flowlines  214  and  224 . Still further, different types of sensors such as, for example, fluid flow sensors could be used in addition to or instead of the pressure sensors  236  and  238 . 
         [0044]    The valves  240  and  242  may be implemented using any fluid valve suitable to vary the flow paths between the chambers  244  and  246  and the chambers  248  and  250 . For example, a metering type valve (e.g., a sliding stem plug valve, a rotary valve such as a ball valve, etc.), a pressure relief valve, or any other suitable valve or combination of valves could be used to implement the valves  240  and  242 . 
         [0045]    The displacement unit control  234  may be implemented using a processor-based system (e.g., the processor-based system  1100  of  FIG. 11 ) having a memory or other storage device or computer accessible medium or media to store software or other executable instructions or code, which can be executed by a processor to perform the methods or operations described herein. Alternatively or additionally, the displacement unit control  234  may include analog circuitry, digital circuitry, signal conditioning circuitry, power conditioning circuitry, etc. Still further, although the displacement unit control  234  is depicted in the example system  200  of  FIG. 2A  as being implemented as single block or device, some or all of the operations performed by the displacement unit control  234  may be performed by one or more devices or units located entirely downhole, entirely at the surface, or downhole and at the surface. 
         [0046]    The mechanical synchronization and ability to adaptively vary the differential pressure and pumping rates of the displacement units  204  and  206  within the displacement unit assembly  202  in the example system  200  of  FIG. 2A  enables the example system  200  to be more flexibly adaptive to different, changing, and/or unpredictable formation characteristics, fluid types, drilling environments, etc. More specifically, conditions or properties such as uncertainty in the local flow pattern of a formation, contamination transport, depth of mud filtrate invasion, permeability anisotropy and viscosity, etc. can affect the displacement unit differential pressures and pumping rates at which a dual or guard probe provides its most effective fluid separation. 
         [0047]    In one example, the system  200  can be configured (e.g., the displacement unit control  234  may be programmed) to pump out during a sample cleanup phase of operation in which the pumping rate(s) of the displacement unit assembly  202  is doubled relative to the pumping rate(s) used to collect the sample to be analyzed. Such a doubled pumping rate may be used in conjunction with a commingled pumpout mode (i.e., where fluid drawn in the from the sample and guard inlets is mixed or not separated). When the fluid drawn, from the formation reaches a desired purity level (i.e., the contamination level is acceptably low) after, for example, a predetermined time period or when a desired purity level is otherwise detected (e.g., using optical analysis), the displacement unit control  234  can automatically adjust (e.g., via the valves  240  and  242 ) the differential pressures and pumping rates of the displacement units  204  and  206  to achieve a desired pumping rate distribution (e.g., a pumping rate distribution that achieves a desired fluid separation at the interface between the sampling probe inlets and the formation). Additionally, during both the sample cleanup phase (during which the pumping rate is relatively high) and the sample production mode (during which an acceptably pure sample is taken for subsequent analysis), the displacement unit control  234  can monitor pressures in the flowlines  214  and  224  and provide appropriate responsive control signals to the valves  240  and  242  to ensure that the pressure developed across the inner packer (not shown) (i.e., a differential pressure across the inner packer) does not exceed a level that could compromise the integrity of the inner packer. 
         [0048]      FIG. 2B  is a schematic diagram of an alternative pumpout configuration  200 ′ having a dual displacement unit assembly  202 , where the pumped fluid can be routed independently to one or both displacement unit. For brevity, the components of the pumpout configuration  200 ′ that are similar to the pumpout configuration  200  have the referred with the same numeral. Also, some optional elements, such as valves  240  and  242  have not been repeated. In the configuration  200 ′, the flowline  214  is not connected to a guard portion of a sampling probe, and the flowline  224  is not connected to a sample portion of a sampling probe. Instead, the flowlines  214  and  224  are fluidly connected to a fluid connector  260 . Similarly, the fluid connector  260  is fluidly connected to flowlines  214 ′ and  224 ′. The flow line  214 ′ and  224 ′ may be in turn fluidly connected to a guard portion and a sample portion of a sampling probe, respectively. The fluid connector  260  may comprise one or more valves or restrictors that may be used to vary the flow rate in flow lines  214 ′ and/or  224 ′, as further detailed below. 
         [0049]    In the shown example, the fluid connector  260  comprises four valves  261 ,  262 ,  263 , and  264 , controlling the flow between flowlines  224 ′ and  214 ,  214 ′ and  214 ,  214 ′ and  224 , and  224 ′ and  224 , respectively. In a first exemplary operational mode, the valves  262  and  263  of the fluid connector  260  are closed, and the valves  261  and  264  of the fluid connector  260  are open. In this operational mode, fluid is drawn from the flowline  224 ′ by both displacement units  204  and  206 , and no fluid is drawn from the flowline  214 ′. This operational mode may be used to advantage for forcing a high flow rate at the sample inlet or portion of a guarded probe. In a second exemplary operational mode, the valves  262  and  263  of the fluid connector  260  are open, and the valves  261  and  264  of the fluid connector  260  are closed. In this operational mode, fluid is drawn from the flowline  214 ′ by both displacement units  204  and  206 , and no fluid is drawn from the flowline  224 ′. This operational mode may be used to advantage for forcing a high flow rate at the guard inlet or portion of a guarded probe. In a third exemplary operational mode, the valves  261 ,  262 ,  263  and  264  of the fluid connector  260  are open. In this operational mode, fluid is drawn from the flowline  214 ′ and  224 ′ simultaneously by both displacement units  204  and  206 . This operational mode may be used to advantage for achieving a flow rate regime at the guard inlet and the sample inlet of a guarded probe that minimize the pressure differential across the guard inlet and the sample inlet. In a forth operational mode, the valves  262  and  264  of the fluid connector  260  are open, and the valves  261  and  263  of the fluid connector  260  are closed. In this operational mode, fluid is drawn from the flowline  214 ′ by the displacement unit  204  and fluid is drawn from the flowline  224 ′ by the displacement unit  206 . This operational mode may be used to advantage for achieving a flow rate regime at the guard inlet and the sample inlet of a guarded probe that corresponds to the characteristics of the displacement units  204  and  206  respectively. It should be understood that these operational modes are given for illustration purposes, and that other operational modes may be achieved by manipulating the valves of the fluid connector  260  and/or modifying the layout and the number of valves included in the fluid connector  260 , as desired. 
         [0050]    During a sampling operation, it may be useful to switch from one operational mode to another, thereby varying the flow rate in flow lines  214 ′ and/of  224 ′. The switch may be piloted under control of the displacement unit control  234 , in a predetermined manner, or based on measurement collected by sensors in the tool, such as sensors  236  and  238 , or other sensors. The displacement unit control may initiate the switch automatically or under commands received by a surface operator. Further, it should be noted that the displacement unit control may be capable of partially opening or closing valves in the fluid connector  260 , to achieve a plurality of operational modes. For example, in another operational mode, the valves  261 , and  264  of the fluid connector  260  are open, and the valves  262  and  263  are partially closed, causing a pressure drop between the flowline  214 ′ and the flowline  224 ′. 
         [0051]      FIG. 3  is a schematic diagram of an example focused sampling system  300  that may be implemented using a pumpout configuration having a dual displacement unit system. As depicted in  FIG. 3 , a dual or guard sampling probe  302  having a guard nozzle, inlet, or portion  304  and a sample nozzle, inlet, or portion  306  is disposed adjacent to a formation  308  from which a fluid sample is to be drawn and analyzed. The sampling probe  302  includes concentric inner and outer packers  310  and  312 , which may be implemented in any conventional or known manner. 
         [0052]    A guard flowline  314  and sample flowline  316  associated with the guard and sample inlets  304  and  306 , respectively, are fluidly coupled to a fluid hydraulics block  318 . The fluid hydraulics block  318  is configured to manage the distribution of the flowlines  314  and  316  to chambers (e.g.,  320  and  322 ) within displacement units  324  and  326  Of a displacement unit assembly  328 . The fluid hydraulics block  318  may be implemented using check valves (e.g., mud check valves) such as the arrangement of the check valves  216 ,  218 ,  220 ,  222 ,  226 ,  228 .  230 , and  232  shown in  FIG. 2A . Also, generally, the displacement unit assembly  328  corresponds to the displacement unit assembly  202  and the displacement units  324  and  326  correspond to the displacement units  204  and  206 , respectively, shown in  FIG. 2A . However, as described in greater detail below, the example displacement unit assembly  328  represents one particular implementation of the displacement unit assembly  202  of  FIG. 2A . 
         [0053]    In addition to routing the flowlines  314  and  316  to the displacement units  324  and  326 , the fluid hydraulics block  318  also conveys outputs  330  and  332  from the displacement units  324  and  326 , and a bypass line  334  to a fluid routing block  336  which, in turn, can selectively route fluid to the borehole annulus and/or a sample capture system (not shown). To control the operations of the example system  300 , a displacement unit control  338  is provided. The displacement unit control  338  may be similar or identical to the displacement unit control  234  described in connection with  FIG. 2A-2B . Thus, the displacement unit control  338  may be eon figured to monitor or measure the pressures (e.g., via pressure sensors (not shown)), within the flowlines  314  and  316  and adaptively control the operations of the displacement unit assembly  328  to vary or control the differential pressures, pumping rates, and/or pumping rate distribution provided by the displacement unit assembly  328 . Additionally, the displacement unit control  338  may control the fluid routing block  336  to, for example, route all fluid drawn via the sampling probe  302  to the borehole annulus during a sample cleanup mode or phase and to the borehole annulus and the sample capture system during a sample collection mode or phase. 
         [0054]    Turning in more detail to the displacement unit assembly  328 , the displacement unit  324  is depicted as a roller screw type pump. Although not depicted in  FIG. 3 , the displacement unit  326  may be configured identically or similarly to the displacement unit  324  and, thus, may also be a roller screw type pump. Alternatively, the displacement unit  326  may use a different pump configuration than the displacement unit  324 . As can been seen in  FIG. 3 , the displacement unit  324  includes pistons  340  and  342  having respective sliding seals  344  and  346 . The pistons  340  and  342  are also mechanically or operatively coupled via a shaft  348  and, thus, reciprocate in unison or synchronously in response to rotation of a roller screw  350 . A shaft  352  extending from the roller screw  350  is supported by bearings  354  and  356  and driven via a motor  358  through a gearbox  360 . As shown in  FIG. 3 , the displacement unit  326  may be coupled to the motor  358  through another gearbox  362 . Optionally, a clutch may be used between the motor  358  and the gearbox  362 , and/or between the motor  358  and the gearbox  360 . 
         [0055]    The gearboxes  360  and  362  may be selected to provide a desired torque/speed characteristic and may be implemented using a fixed gear ratio (e.g., a reduction or n:1 ratio) or a continuously variable type of configuration. The motor  358  may be directly coupled to the gearboxes  360  and  362  or, alternatively, may be coupled to the gearboxes  360  and  362  via clutches. In configuration shown in  FIG. 3 , the motor  358  may have dual shafts, which extend from opposite ends of the motor  358  and, thus, in ease where there is no interposing clutch between the motor  358  and the gearboxes  360  and  362 , the displacement units  324  and  326  always operate in a mechanically synchronous manner. In other words, when the motor  358  is operational, the shafts of the motor  358  cause the displacement units  324  and  326  to pump in a synchronized manner. However, other configurations using a clutch that interposes between the motor  358  and the gearboxes  360  and/or  362 , allow fully independent control of the pumping rate for the guard and sample flowlines  314  and  316 . Alternatively, although not depicted in  FIG. 3 , each of the displacement units  324  and  326  may be driven by a respective, separate motor (e.g., similar or identical to the motor  358 ). 
         [0056]    The example system  300  depicted in  FIG. 3  may, for example, be used to provide a sampling while drilling system. In particular, the example system  300  may be implemented within a tool string as part of, for example, a bottom hole assembly. Also, the example system  300  may utilize its ability to adaptively vary the differential pressures and/or pumping rates of the displacement units  324  and  326  to provide a substantially pure or contamination free sample in a relatively short sample time, thereby reducing the possibility of sticking during drilling operations. In one example implementation, the displacement unit control  338  may control the pumping rates of the displacement units  324  and  326  to be at their maximum levels during the beginning of a sampling procedure and then adaptively adjust the pumping rates to achieve a lowest possible contamination level (i.e., highest purity) sample fluid in the shortest possible time. In some examples, the contamination history of the formation fluid (e.g., as provided by an optical fluid analyzer) may be used to adaptively adjust the pumping rates and pumping distribution of the displacement units  324  and  326  to achieve a pumping rate or ratio that provides a sampling probe focus that achieves a desirably or sufficiently low sample contamination level. 
         [0057]    In the example shown in  FIG. 3 , the base or intrinsic, pumping rate of the displacement units  324  and  326  can be configured by adjusting certain mechanical parameters such as, for example, the ratios of the gearboxes  360  and  362 , adjusting the pitch of the roller screws (e.g., the roller screw  350 ), configuring the effective cross-sectional areas of the chambers (e.g., the chambers  320  and  322 ). With the example in  FIG. 3 , the foregoing displacement unit mechanical parameters can be set independently and, thus, differently for each of the displacement units  324  and  326  to achieve a desired base pumping rate distribution or ratio. In the case where clutches are used between the gearboxes  360  and  362  and the displacement units  324  and  326 , the clutches may be engaged/disengaged to vary the duty cycle (i.e., the clutches may be used to vary the duty cycle of the displacement units  324  and/or  326 ). Further adaptive variations to the pumping rates and pumping rate distribution can then be implemented by controlling the fluid hydraulics block  318  to vary the differential pressure across the displacement units  324  and  326  as previously discussed. 
         [0058]      FIG. 4  is an alternative displacement unit configuration  400  that may be used to implement the example displacement unit assembly  328  of  FIG. 3 . In contrast to the example displacement unit assembly  328  of  FIG. 3 , the example system  400  includes two displacement units  402  and  404  that are driven via a motor  406  by a common gearbox  408  and shaft  410 . In the example system  400 , the displacement units  402  and  404 , the gearbox  408 , and the motor  406  may be implemented using devices similar or identical to those described in connection with  FIG. 3  above. However, because the displacement units  402  and  404  share a common shaft, a single roller screw assembly and gearbox can be used instead of having to provide two roller screw assemblies and two gearboxes. Thus, while the flow provided to guard and sample flowlines by the example system  400  is synchronous with the reciprocating motion of the single roller screw, the base or intrinsic flow rate or pumping rates and pumping rate distribution is adjusted by varying the effective areas of the chambers within the displacement units  402  and  404 . Of course, as with the example system  300  of  FIG. 3 , further adaptive adjustments to the pumping rates and pumping rate distribution can be performed by the fluid hydraulics block  318  and the displacement unit control  338  as described above. 
         [0059]    In yet another example, the example pumpout system described herein may be implemented using a mixed variety of actuator types for driving them. In particular, one of the displacement units may be driven using, for example, a motor driven gearbox and a roller screw such as that described in connection with  FIG. 3  above. The other displacement unit may be hydraulically driven in a manner similar to the displacement units used in the Schlumberger Modular Formation Dynamics Tester (MDT). In this example, a single electric motor may be used to drive the gearbox and its associated displacement unit and, a hydraulic oil pump (e.g. a fixed displacement hydraulic oil pump), which generates a high pressure oil to drive its associated displacement unit. In addition, the displacement units disclosed herein are not limited to the disclosed reciprocating piston, but may include any type of displacement unit able to accomplish the intended purpose, including but not limited to centrifugal type pumps or Moineau type pumps. If desired, the pumpout system may be controlled using feedback from an optical fluid analyzer and/or a flow meter. 
         [0060]      FIGS. 5   a ,  5   b , and  5   c  depict various tool topologies employing the example methods and apparatus described herein. In the  FIGS. 5   a - 5   e , the guard probe tool would be preferentially, but not necessarily, as close as possible to the bottom of the well.  FIG. 5   a  depicts a relatively compact configuration  500  that includes a single power module or section  502  that powers two displacement units  504  and  506 , which may be installed in one collar  508 , and which may be similar to the examples shown in  FIGS. 3 and 4 . In  FIG. 5   b , a second power module  510  is provided and the displacement units  506  and  504  are mounted with their respective power modules  510  and  502  in separate collars  512  and  514 . In  FIG. 5   c , the displacement units  504  and  506  are contained in separate collars  516  and  518 , where the collar  516  also contains a guard probe tool  520 . In the illustration of  FIGS. 5   a - 5   c , a sample flowline (not shown) fluidly connects a sample inlet of a guarded probe extendable from the guard probe tool extends to a sample capture sub. The fluid in this flowline may be drawn with the displacement unit  506 . Still in the illustration of  FIGS. 5   a - 5   b , a guard flowline (not shown), fluidly connects the guard inlet of a guarded probe extendable from the guard probe tool to an exit port (e.g. to the wellbore) in the module  504 . The fluid in this flowline may be drawn with the displacement unit  504 . 
         [0061]    The tools topologies illustrated in  FIGS. 5   a - 5   c  are equally applicable for any means of conveyance known by those skilled in the art. However, it should be noted that the power module may differ according to the power source available with any particular conveyance mean. For example, if power is provided to the tool through a wireline cable, the power module may include a current or voltage transformer, and/or voltage surcharge protection. In other examples, power may be provided through fluid circulation through a conduit (e.g., a drill string bore) via a turbine and an alternator. 
         [0062]    The foregoing example adaptive focused formation fluid sampling apparatus and methods utilize displacement units or displacement unit assemblies for which the differential pressures, pumping rates, and/or pumping ratios or distribution can be adaptively varied to provide more rapid sample cleanup and increased sample purity (or reduced contamination) in comparison to known sampling apparatus and methods. In general, the foregoing example apparatus and methods utilize valves (e.g., acting as shunts) coupled between the chambers of displacement units to enable the flow of fluid between the chambers (e.g., a recirculation path) and thereby vary the differential pressures across the chambers as well as the pumping rates of the displacement units. A displacement unit control may be used to provide feedback control (e.g., by measuring flowline pressures) to adaptively control the degree to which the valves are open/closed to vary the differential pressures and pumping rates to achieve a desired fluid separation, to minimize the differential pressure across the inner packer, etc. 
         [0063]    However, the effective displacements provided by the foregoing example displacement units is substantially fixed (i.e., cannot be adaptively varied) given the mechanical configurations of those units. Additionally, in a case where a displacement unit (e.g., known displacement units and/or the example displacement units described herein) is driven by a hydraulic motor, the hydraulic motor also typically provides an effective displacement that is substantially fixed given its mechanical configuration. Thus, whether a displacement unit is configured for use as a pump (e.g., to extract formation fluid as discussed in connection with  FIGS. 1-5  above) or a motor (e.g., to drive another displacement unit that is acting as a pump), these displacement units typically have a substantially fixed displacement. Thus, traditionally, when selecting a displacement unit for use as a pump (e.g., to extract formation fluid) or motor, a displacement unit having a particular mechanical configuration that provides a desired basic or intrinsic pumping force, displacement, pumping rate, etc, is selected. As a result, if it is later determined (e.g., after attempting to use the displacement unit in its intended application) that the displacement unit fails to provide sufficient (or provides an excessive) pumping force, displacement, pumping rate, etc., it may be necessary to remove the tool from the borehole and replace the displacement unit with one having a different mechanical configuration that provides an acceptable performance. 
         [0064]    The methods and apparatus described below in connection with  FIGS. 6-9  may be used to vary the effective fluid displacement of a displacement unit being driven by a hydraulic pump and/or a linear motor. In contrast to known (i.e. fixed displacement) displacement units, the displacement units described in connection with  FIGS. 6-9  below provide a plurality of selectable piston chambers haying different volumes that enable the effective displacement of the displacement units to be varied to suit the needs of a particular application. In this manner, a single variable displacement unit can be configured to have a plurality of different effective displacements to satisfy the needs of a relatively wide range of applications. Additionally, the example variable displacement units described in connection with  FIGS. 6-9  can be driven or fed via a fixed displacement pump or a linear motor to provide a selectably variable displacement and flow rate that could not otherwise be provided directly by the fixed displacement motor or pump. In light of the above and the brevity of the description, the embodiments shown in  FIGS. 6-9  will be described herein as single displacement units  600 ,  900  driven by a shaft  603 ,  903  coupled to a linear motor  601  and  901 , respectively. The single displacement units  600 ,  900  may also be coupled to a second or complimentary displacement unit via the same or similar shaft coupled to the motor, thereby achieving synchronized displacement units. 
         [0065]      FIG. 6  illustrates an example variable (i.e., variable displacement and flow rate) displacement unit  600  that is fluidly coupled to the linear motor  601  via the shaft  603 . The linear motor  601  may be implemented with a rotation motor, a gearbox, and a roller screw as mentioned above. When used as a pump, a flowline  602  may be fluidly coupled to the formation and the flowline  604  may be fluidly coupled to an interior of the tool, including for example a sample chamber, a exit port to the wellbore, etc. (not shown). As such, the displacement unit  600  may be used to pump formation fluid, such as guard or sample fluid from the formation, whereas a complimentary displacement unit (not shown) may pump the other of the guard or sample fluid from the formation. The variable displacement unit  600  includes a plurality of independently controllable three-way two-position valves V 1 -V 4 . The variable displacement unit  600  also includes a piston rod  606  and pistons  608 ,  610 , and  612 , which are slidably engaged with a body or housing  613  to form chambers  614 ,  616 ,  618 , and  620 . As described in more detail below, the chambers  614 ,  616 ,  618 , and  620  may be selectively filled via the valves V 1 , V 2 , V 3 , and V 4  with formation fluid from the flowline  602  as the pistons  608 ,  610 , and  612  move in a reciprocating motion in directions generally indicated by arrows  622 . In operation, the motor  601  provides the forces or motion needed to reciprocate the shaft  603  and piston rod  606  to perform a pumping application. The chambers M 1  and M 2  may be filled with hydraulic fluid maintained at or slightly above wellbore pressure via a compensator (not shown). 
         [0066]    In the illustrated example, the piston rod  606  has a first portion having a diameter d 1  and a second relatively larger portion having a diameter d 2 . As can be seen in  FIG. 6 , the difference in the diameters d 1  and d 2  results in the displacements of the chambers  614  and  616  being different (e.g., greater) than the displacement of the chambers  618  and  620 . Further, with the example configuration shown in  FIG. 6 , the difference in displacements that results from the differing piston rod diameters enables the variable displacement unit  600  to be configured (by controlling the valves V 1 -V 4 ) to provide two different effective displacements (or flowrates) in a reciprocating action. More specifically, the valves V 1 -V 4  can be controlled to route hydraulic fluid from the flowline  602  so that the effective displacement of the variable displacement unit  600  equals the sum of the displacements of the chambers  616  and  620  (when the piston rod  606  moves toward M 1 ) and the sum of the displacements of the chambers  614  and  618  (when the piston rod  606  moves toward M 2 ). Alternatively, the valves V 1 -V 4  may be controlled so that the effective displacement of the variable displacement unit  600  equals the difference of the displacements of the chambers  616  and  618  (when the piston rod  606  moves toward M 1 ) and the difference of the displacements of the chambers  614  and  620  (when the piston rod  606  moves toward M 2 ). Still further, the valves V 1 -V 4  may be controlled to provide the greater effective displacement (i.e., a sum of displacements) in one direction of motion of the piston rod  606  and the relatively lower effective displacement (i.e., a difference of displacements) in the other direction of motion. 
         [0067]    In the illustrated example of  FIG. 6 , the variable displacement unit  600  is a reciprocating unit. However, in other example implementations, the variable displacement unit  600  may be a rotary unit. Additionally, although the displacement unit  600  is depicted as being coupled to the motor  601  and the shaft  603 , in other example implementations, the displacement unit  600  may instead be coupled to a hydraulic (e.g. fixed displacement) pump (not shown). For example, the chambers M 1  and M 2  may be used to provide the forces or pressures needed to extract fluid from a formation, thereby eliminating the need for the motor  601  and shaft  603 . 
         [0068]      FIG. 7  is a table illustrating the various operational modes that can be provided by the example variable displacement unit  600  of  FIG. 6 . As shown in  FIG. 7  there are four distinct operational modes, each of which is defined by a unique configuration of the valves V 1 -V 4 . In MODE  1 , for example, the valve V 1  is set so that fluid can flow from port C to port  1  and the chamber  614 , the valve V 2  is set so that fluid can flow from port C to port  2  and the chamber  616 , V 3  is set so that fluid can flow from port C to port  1  and the chamber  618 , and V 4  is set so that fluid can flow from port C to port  2  and the chamber  620 . In this example, the chambers  614  and  616  are assumed to provide a displacement of “L” and the chambers  618  and  620  are assumed to provide a displacement of “S,” where S is less than L. Thus, in MODE  1 , formation fluid from the flowline  602  flows into the chambers  616  and  620 , urges the piston rod  606  displacement toward the chamber M 1 . Additionally, in MODE  1 , the effective displacement of the variable displacement unit  600  equals the sum of the displacements of the chambers  616  and  620  (i.e., L+S). Additionally, MODE  2  provides an effective displacement of L−S for piston rod travel in the direction of M 1 , MODE  3  provides an effective displacement of L+S for piston rod travel in the direction of M 2 , and MODE  4  provides an effective displacement of L−S for piston rod travel in the direction of M 2 . 
         [0069]      FIG. 8  depicts another variable displacement unit configuration  800  that provides two additional (for a total of four) effective displacements. In general, the configuration  800  includes the variable displacement unit configuration  600  of  FIG. 6  and four additional three-way valves V 5 , V 6 , V 7 , and V 8 . The valves V 5  and V 6  can be set to enable fluid from the flowline  602  to bypass the chambers  614  and  616  to provide an effective flowrate of S and, alternatively, the valves V 7  and V 8  can be set to enable the chambers  618  and  620  to be bypassed to provide an effective flowrate of L. Thus, with the example configuration  800  of  FIG. 8 , the valves V 1 -V 8  can be set to provide effective flowrates of L, S, L−S, and L+S in both directions of travel of the piston rod  606  (i.e., in a reciprocating motion). While the example configuration  800  of  FIG. 8  depicts four additional three-way valves, if desired, only two additional three-way valves (i.e., V 5  and V 6  or V 7  and V 8 ) could be used to provide just one additional (for a total of three) effective flowrates. Further, it will be appreciated by those versed in the art that some or all the three-way valves V 1 -V 8  may be implemented with combinations of two way valves and check valves, or other kind of valves providing a similar functionality. 
         [0070]      FIG. 9  schematically depicts a variable displacement unit configuration  900  that incorporates more than four chambers. As shown in  FIG. 9 , the example configuration  900  can include any desired number of chambers and associated fluid routing and bypass valves to achieve any desired number of different effective displacements. 
         [0071]      FIG. 10  depicts yet another variable displacement unit configuration  1000   a . In particular,  FIG. 10  depicts a first portion  1000   a  that may be used in combination with a second portion  1000   b  to create a first displacement unit  1000 . With the addition of the second portion  1000   b , such as through a shaft  1003  or through direct affixation, the displacement unit  1000  will operate, with some additional valves as depicted in  FIG. 2A , to provide a continuous flow. 
         [0072]    In addition, the displacement unit  1000  may be coupled to a second or complimentary displacement unit, via the shaft  1003  for example, thereby achieving synchronized displacement units. As such, the displacement unit  1000  may be used to pump formation fluid, such as guard or sample fluid from the formation, whereas a complimentary displacement unit (not shown) may pump the other of the guard or sample fluid from the formation. The example displacement unit  1000  shown in  FIG. 10  may, for example, be used to implement the displacement units described in connection with  FIGS. 2-5 . In general, the example portion  1000   a  is configured adjust its effective displacement or flowrate of sample fluid that is being drawn from a formation. 
         [0073]    Turning in detail to  FIG. 10 , the example portion  1000   a  includes a plurality of piston displacement units  1002 ,  1004 ,  1006 , and  1008 , each of which provides a different flowrate. As depicted in  FIG. 10 , the pistons displacement units  1002 ,  1004 ,  1006 , and  1008  are mechanically coupled (e.g., chained) to each other and the common shaft  1003 . In unison or a mechanically synchronized manner, each of the piston displacement units  1002 ,  1004 ,  1006 , and  1008  draws fluid from an inlet flowline  1012  via respective check valves  1014 ,  1016 ,  1018 , and  1020  when the shaft  1003  is moved to the left in the illustrated example. As the shaft  1003  is moved back to the right in the illustrated example, the fluid previously drawn in by the displacement units  1002 ,  1004 ,  1006 , and  1008  is forced under pressure into an outlet flowline  1022  via respective check valves  1024 ,  1026 ,  1028 , and  1030 . In operation, one of the displacement units  1002 ,  1004 ,  1006 , and  1008  provides a best (e.g., a substantially optimal) displacement for the pressure and/or flowrate of the sample fluid. However, those of the units  1002 ,  1004 ,  1006 , and  1008  that do not provide the best displacement (e.g., all but one) can continue to pump fluid between their respective counterpart units in portion  1000   b  to avoid any unnecessary pressure build-ups in the unused units. Similarly, any of the units  1002 - 1008  may be used in combination to obtain a variety of flow rates and/or pressures. 
         [0074]      FIG. 11  is a schematic diagram of an example processor platform  1100  that may be used and/or programmed to implement any or all example apparatus and methods described herein. In particular, the example processor platform  1100  may be used to implement the example displacement unit control  234  of  FIG. 2A-2B  and/or the example displacement unit control  338  of  FIG. 3 . Further, the processor platform  1100  can be implemented by one or more general purpose processors, processor cores, microcontrollers, etc. 
         [0075]    The processor platform  1100  of the example of  FIG. 11  includes at least one general purpose programmable processor  1105 . The processor  1105  executes coded instructions  1110  and/or  1112  present in main memory of the processor  1105  (e.g., within a RAM  1115  and/or a ROM  1120 ). The processor  1105  may be any type of processing unit, such as a processor core, a processor and/or a microcontroller. The processor  1105  may execute, among other things, the example processes described herein such as, for example, adaptively controlling one or more displacement units to extract a formation fluid sample, and/or to more quickly reduce the contamination level of a formation fluid sample. The processor  1105  is in communication with the main memory (including a ROM  1120  and/or the RAM  1115 ) via a bus  1125 . The RAM  1115  may be implemented by DRAM, SDRAM, and/or any other type of RAM device, and ROM may be implemented by flash memory and/or any other desired type of memory device. Access to the memory  1115  and  1120  may be controlled by a memory controller (not shown). 
         [0076]    The processor platform  1100  also includes ah interface circuit  1130 . The interface circuit  1130  may be implemented by any type of interface standard, such as a USB interface, a Bluetooth interface, CAN interface, an external memory interface, serial port, general purpose input/output, etc. One or more input devices  1135  and one or more output devices  1140  are connected to the interface circuit  1130 . The input devices  1135  and/or output devices  1140  may be used to receive sensor signals (e.g., from one or more pressure or flow sensors) and/or to control one or more valves. 
         [0077]    Certain examples are shown in the above-identified figures and described in detail below. In describing these examples, like or identical reference numbers are 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. Although certain methods, apparatus, and articles of manufacture have been described herein, the scope of coverage of this patent is not limited thereto. To 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.