Patent Publication Number: US-7913554-B2

Title: Method and apparatus for balanced pressure sampling

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 11/274,707, filed on Nov. 15, 2005, which claims the benefit of and priority to U.S. Provisional Application No. 60/552,882, filed on Nov. 17, 2004. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The invention relates to methods and apparatus for recovering samples of reservoir fluid. 
     2. Background of the Related Art 
     A reservoir is a rock formation in which fluids such as hydrocarbons, e.g., oil and natural gas, and water have accumulated. Due to gravitational forces, the fluids in the reservoir are segregated according to their densities, with the lighter fluid towards the top of the reservoir and the heavier fluid towards the bottom of the reservoir. One of the main objectives of formation testing is to obtain representative samples of the reservoir fluid. Commonly, reservoir fluid is sampled using a formation tester, such as the Modular Formation Dynamics Tester™ (MDT™), available from Schlumberger Technology Corporation, Houston, Tex. In practice, the formation tester is conveyed, generally on the end of a wireline, to a desired depth in a borehole drilled through the formation. The formation tester includes an inlet device, which may be a probe or packer, that can be set against the borehole wall and through which reservoir fluid can be drawn into a flow line in the formation tester. The formation tester also typically includes a pump and one or more sample chambers. Typically, fluid monitoring devices, such as optical fluid analyzers, are also inserted into the flow line to monitor the type and quality of fluid flowing at various points in the flow line. 
     The inlet device or probe is inserted into the formation through mudcake lining on the borehole wall. Thus, the fluid initially drawn into the flow line through the probe is a mixture of reservoir fluid and mud filtrate. To obtain a sufficiently quality fluid sample, a cleanup step in which mud filtrate is purged from the flow line is performed. This step typically involves pumping the fluid drawn into the flow line back into the borehole. However, the fluid discharged into the borehole contains reservoir fluid, which can contaminate the drilling mud in the borehole and change the properties of the drilling mud, possibly necessitating additional steps to clean or stabilize the drilling mud. As pumping continues, more and more of the reservoir fluid is consumed around the inlet of the probe. Eventually, a fluid mixture that is more representative of the reservoir fluid starts to enter the flow line. Fluid monitoring devices, such as optical fluid analyzers, are used to monitor the content of the fluid entering the flow line and how the fluid proceeds through the tool and can assist in determining when the fluid entering the flow line is of sufficient quality to be sampled. 
     When the mud filtrate content of the fluid entering the flow line is reduced to an acceptable level, the sample chamber is opened and fluid in the flow line is pumped into the sample chamber. Typically, the sample chamber includes a cylinder in which a piston is disposed. The sample is collected on top of the piston while the backside of the piston is exposed to either borehole pressure or atmospheric pressure. Typically, the backside of the piston is exposed to borehole pressure, which means that fluid is pumped into the sample chamber against borehole pressure. Borehole pressure is normally deliberately maintained above formation pressure to keep the well safe. Thus, pumping fluid into the sample against borehole pressure often results in the sample collected in the sample chamber being over-pressured, creating an unstable pressure-volume-temperature (PVT) environment. Moreover, in cases where a higher pressure differential is provided, additional power is typically required to pump the sample into the downhole tool. 
     Despite such advances in sampling technology, there remains a need to provide techniques that are capable of efficiently obtaining samples representative of the formation. It is desirable that such techniques provide pressure sufficient to prevent samples from deteriorating or becoming biphasic. It is further desirable that such techniques provide a pressure that is at a reduced pressure differential from the sample to facilitate pumping or drawing fluid into the downhole tool. Such techniques preferably provide one or more of the following, among others: maintaining sample pressure above the bubble point, reducing sampling time, reducing power requirements for sampling and balancing pressures to the formation. 
     SUMMARY OF THE INVENTION 
     In one aspect, the invention relates to a method of sampling reservoir fluid from a rock formation penetrated by a borehole. The method comprises positioning a downhole tool having a flow line in the borehole, establishing an inlet port through which fluid passes from a first point in the formation into the flow line, establishing an outlet port through which fluid passes from the flow line into a second point in the formation, and passing fluid between the formation and the flow line through the inlet and outlet ports. 
     In another aspect, the invention relates to a tool for sampling reservoir fluid from a rock formation penetrated by a borehole. The tool comprises a tool body for positioning in the borehole, the tool body having at least one flow line, a plurality of fluid communicating devices coupled to the tool body, the fluid communicating devices comprising an inlet device which provides an inlet port through which fluid passes from the formation into the flow line and an outlet device which provides an outlet port through which fluid passes from the flow line into the formation, and a fluid chamber disposed in the tool body for collecting fluid from the flow line. 
     Other features and advantages of the invention will be apparent from the following description and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a schematic representation of a tool for sampling reservoir fluid. 
         FIGS. 1B and 1C  show alternate arrangements for the inlet and outlet probes shown in  FIG. 1A . 
         FIG. 1D  is a schematic view of the tool of  FIG. 1A  in an example environment in which the invention can be practiced. 
         FIG. 1E  is a detailed view of an alternate configuration of the tool of  FIG. 1A . 
         FIGS. 2A-2E  show various modular tool configurations for sampling reservoir fluid. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention will now be described in detail with reference to a few preferred embodiments, as illustrated in accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that the invention may be practiced without some or all of these specific details. In other instances, well-known features and/or process steps have not been described in detail in order to not unnecessarily obscure the invention. The features and advantages of the invention may be better understood with reference to the drawings and discussions that follow. 
     Embodiments of the invention provide a method and an apparatus for sampling reservoir fluid. The apparatus includes a flow line and two ports that can be set against a wall of a borehole traversing a rock formation. When the ports are set against the borehole wall, reservoir fluid can be circulated from the formation into the flow line and back into the formation, avoiding discharge of fluid in the flow line into the borehole. Since the reservoir fluid is not discharged into the borehole, contamination of the drilling mud in the borehole is also avoided. 
     The apparatus for sampling reservoir fluid includes at least one sample chamber for collecting a sample of the reservoir fluid. The method for sampling reservoir fluid includes filling the sample chamber with fluid in the flow line against formation pressure. The method and apparatus of the invention advantageously minimize the differential pressure across the fluid collected in the sample chamber. The apparatus can be used to create a flow circuit in the rock formation, which can allow in-situ core flood test. Such test can be used to obtain a direct measurement of the near-borehole permeability. 
       FIG. 1A  is a schematic representation of a tool  100  for sampling reservoir fluid in a formation  102  traversed by a borehole  104  according to an embodiment of the invention. The borehole  104  may be an open hole or a cased hole. The tool  100  includes a flow line  106  defined in a tool body  108 . Various devices such as valves and pumps may be disposed in the flow line  106  to control flow of fluid through the flow line  106 . 
     The tool body  108  may be a unitary housing or may be made of multiple housings coupled together. The tool  100  includes a sample chamber  110  normally disposed in the tool body  108  for collecting reservoir fluid from the formation  102 . In practice, the tool  100  may include one or more sample chambers. Examples of sample chambers suitable for use in the invention include, but are not limited to, the Modular Sample Chamber, Multi-Sample module, or Single-Phase Multi-Sample Chamber included in the Schlumberger MDT™. 
     A typical sample chamber  110  includes a cylinder  112  and a piston  114  disposed in the cylinder  112 . The piston  114  defines compartments  112   a ,  112   b  inside the cylinder  112 . The compartment  112   a  is for collecting a sample of the reservoir fluid. The compartment  112   b  may be filled (preferably) with water or other types of fluids, such as hydraulic fluid, and maintained at a desired pressure. The fluid in the compartment  112   b  will be displaced into the flow line  106  as reservoir fluid is collected in the compartment  112   a.    
     Fluid can flow from the flow line  106  into the compartment  112   a  through a flow line  116   a . A valve  116  may be used to control communication between the flow lines  106 ,  116   a . As described, the valve  116  is a surface-controlled valve, but may also be controlled at the surface or downhole by manual or automatic means. Fluid can flow from the compartment  112   b  into the flow line  106  through a flow line  116   b . A valve  116   c , which may be surface-controlled, may also be used to control communication between the flow lines  106  and  116   b . A valve  117  (or other suitable device) may be disposed in the flow line  106  to prevent communication between the flow lines  116   a ,  116   b  when the surface-controlled valve  116  in the flow line  116   a  is open. 
     The tool  100  includes probes (or ports)  118 ,  120  that can be set against the borehole  104  wall to establish fluid communication between the flow line  106  and the formation  102 . Examples of probes suitable for use in the invention include the Single-Probe Module or Dual-Probe Module included in the Schlumberger MDT™ or described in U.S. Pat. Nos. 4,860,581 and 6,058,773. Typically, the probe modules include a probe coupled to a frame. The frame and the probe can be extended and retracted relative to the tool body. In one embodiment, the probe  118  is an inlet probe providing a channel through which fluid can flow from the formation  102  into the flow line  106 , and the probe  120  is an outlet probe providing a channel through which fluid can flow from the flow line  106  into the formation  102 . When the probes  118 ,  120  are set against the borehole  104  wall, fluid can be circulated from the formation  102  into the flow line  106  and back into the formation  102 . This allows discharge of fluid from the flow line  106  into the borehole  104  to be avoided, thus eliminating or minimizing contamination of drilling mud in the borehole  104 . 
     A method for sampling reservoir fluid includes a cleanup phase in which fluid is circulated from the formation  102  into the flow line  106  and back into the formation  102 . This circulation continues until the fluid in the flow line  106  is sufficiently clean to be captured in the sample chamber  110 . When the fluid in the flow line  106  is sufficiently clean, the valve  116  may be opened and the valve  117  may be closed to allow fluid to be transferred from the flow line  106  into the compartment  112   a  of the sample chamber  110 . At this point, the backside  114   b  of the piston  114  is exposed to formation pressure through the flow line  116   b , which is hydraulically connected to the probe  120 . Thus, the sample chamber  110  is filled with fluid against formation pressure. This minimizes the change in pressure of the sample collected in the sample chamber  110  since the pressure differential between the flow lines  116   a ,  116   b  need only be large enough to displace the piston  114 . 
     Additional valves, such as valves  115   a, b  may also be provided to selectively divert fluid through the flow lines. These valves are shown near inlets to selectively isolate the inlets. In this manner, fluid may be selectively permitted to enter and/or exit the inlets/outlets. Gauges, such as pressure gauges  119   a, b  may also be provided to measure parameters of fluid in the flow lines. 
     The flow rate and pressure of reservoir fluid from the flow line  106  into the compartment  112   a  may be controlled by metering the fluid flowing out of the compartment  112   b  using, for example, choke valves. Alternately, throttle valves at the inlet of the compartment  112   a  may be used to regulate flow rate and pressure of the reservoir fluid into the compartment  112   a  as taught by, for example, Zimmerman et al. in U.S. Pat. No. 4,860,581. A throttle valve  116   c  at the outlet of compartment  112   b  may also be used to regulate the flow rate and pressure of the reservoir fluid into the compartment  112   a . In addition, flow rate and pressure of reservoir fluid into the compartment  112   a  may be controlled by the rate and/or duty cycle of a pump in the flow line  106  (e.g., pump  122 ). Pumps may be positioned at various locations in the flow line(s), for example, on either side of valve  117 . 
     To avoid or reduce contamination of the fluid captured in the sample chamber  110 , the point at which the probe  118  engages the formation  102  should be sufficiently distanced from the point at which the probe  120  engages the formation  102 . This can be achieved by maintaining a minimum vertical distance between the probes  118 ,  120  and/or by locating the probes  118 ,  120  such that they are diametrically opposed ( FIGS. 1B and 1C ). The tool  100  should also be placed in the borehole  104  such that when the outlet probe  120  is extended it engages a porous (and/or permeable) section of the formation  102 . Otherwise, it may be difficult to discharge the fluid in the flow line  106  into the formation  102 . 
     The tool  100  may include a pump  122  in the flow line  106 . The pump  122  may be any type of pump, e.g., reciprocating piston, retractable piston, or hydraulic powered pump. The pump  122  may be positioned to be operable in a pump-in mode, pump-out mode, or internal mode. For example, the pump  122  can pump fluid from the borehole  104  into the flow line  106  for distribution to various points in the tool  100  as needed. In another example, the pump  122  can draw fluid from the formation  102  into the flow line  106  and pump the fluid in the flow line  106  back into the formation  102 . The pump  122  can also pump from one point in the flow line  106  to any other point in it. For example, the pump  122  can pump fluid from the flow line  106  into the sample chamber  110 . However, the invention is not limited to use of the pump  122  to pump fluid from the formation  102  into the sample chamber  110  and/or out into the formation  102 . In an alternate embodiment, the tool  100  may rely on pressure differential between the probes  118 ,  120  to create flow of fluid from the formation  102  into the flow line  106  and sample chamber  110  and/or from the flow line  106  into the formation  102 . For the pump-in mode, pump-out mode, or internal mode, the backside  114   b  of piston  114  may be exposed to formation pressure. 
     In some cases, a pressure differential sufficient to drive fluid through the flow lines may be provided a pump, hydrostatic pressure and/or pressure differentials across different formations. For example, where an inlet is positioned at a first formation having a first pressure, and an outlet is positioned at a second formation having a second pressure, a sufficient pressure differential between the first and second pressures may be used to facilitate movement of fluid. 
       FIG. 1D  is a schematic of an example environment within which the present invention may be used. In the illustrated example, the present invention is carried by the tool  100 . The tool  100  is deployable into the borehole  104  penetrating the subterranean formation  102  and suspended therein with a conventional wireline  103 , or conductor or conventional tubing or coiled tubing (not shown), below a rig  107  at the surface  109 , as will be appreciated by one of skill in the art. The borehole  104  may be an open hole or a cased hole. A mudcake lining  111  is formed on the borehole  104  wall by drilling mud. 
     While the tool  100  is depicted as a modular downhole tool, it will be appreciated by one of skill in the art that the tool  100  may be used in any downhole tool. For example, the tool  100  may be used in a drilling tool including a drill string and a drill bit. The drilling tool may be of a variety of drilling tools, such as measurement-while-drilling (MWD), logging-while-drilling (LWD), or other drilling system. The tool  100  may have a variety of configurations, such as modular, unitary, wireline, coiled tubing, autonomous, drilling, and other variations of downhole tools. 
       FIG. 1E  shows another configuration of the tool  100  that includes multiple inlet ports, outlet ports, and sample chambers for multiple sampling of reservoir fluid. The tool  100  is provided with a plurality of fluid communicating devices, e.g., inlet devices  130 ,  132  and outlet devices  134 ,  136 . While a specific arrangement of inlet and outlet devices is provided, it will be appreciated that one or more inlet and one or more outlet devices may be used. The illustrated example shows a variety of types of inlet and outlet devices. Such devices may be functional as inlet and/or outlet devices as desired. Examples of probes and/or packers used in downhole tools are described in U.S. Pat. Nos. 6,301,959; 4,860,581; 4,936,139; 6,585,045; 6,609,568; and 6,719,049 and U.S. Patent Application Publication No. 2004/0000433. 
     In the illustrated example, the inlet device  130  is a probe having two channels or ports  130   a ,  130   b . One or more such inlets may be provided in any of the inlet/outlet devices. The use of an additional inlet  130   b  is typically used to draw contamination away from the formation fluid as it is drawn into inlet  130   a  as described more fully in U.S. Patent Application Publication No. 2004/0000433. Such inlets/outlets may be used across the same or different formations along the wellbore. 
     The inlet device  132  includes dual packers  142  mounted on the tool body  108 . The dual packers  142  sealingly engage the borehole  104  wall. Inlets  150   a ,  150   b  are provided on the portion of the tool body  108  between the dual packers  142 . The inlets  150   a ,  150   b  are in fluid communication with the fluid in the borehole  104  between the packers  142 . As shown with respect to inlet device  132 , one or more inlets may also be provided between packers. Multiple sets of dual packers with inlets positioned therebetween may be provided. The use of one or more inlets for probes and/or packers may also be used to provide an optional release of fluid into the wellbore and/or formation as desired. 
     While inlet device  132  is described as being used for drawing fluid into the downhole tool, the inlet device  132  may also be used as an outlet device. This may particularly be useful in cases where a large surface area along the borehole is needed to find a flowing zone. 
     The outlet devices  134 ,  136  are probes having single flow lines or ports  134   a ,  136   a , respectively. The outlet devices  134 ,  136  are positioned at various depths in the wellbore. The position of the inlets may be selected to provide inlets and outlets at desired locations about the wellbore. 
     The tool  100  is provided with flow line  152 , which is selectively and fluidly connected to flow line  134   a  of the outlet device  134  and to flow line  130   a  of the inlet device  130 . In this configuration formation fluid may be drawn in through inlet device  130  and discharged through outlet device  134 . Flowline  166  may also be used to selectively and fluidly connect  130   b  and  150   b . Flow line  166  may also be used to selectively and fluidly connect  130   b  and  136   a . With such configurations, formation fluid may be drawn in through inlet device  130  and discharged through inlet device  132  and/or  136  (functioning as an outlet device). Flow lines may be positioned in the tool to fluidly connect a variety of inlet and outlet devices to perform the sampling operation. Valves, such as valves  115   c ,  115   d  and  125 , may be provided in the flow lines to permit selective fluid communication of the input and output devices. In this manner, a variety of configurations may be used. 
     Sample chamber  154  is positioned along the flow line  152 . Sample chamber  154  may be any suitable fluid chamber capable of collecting fluid from the formation, such as previously listed. Other examples of sample chambers are taught in, for example, U.S. Pat. Nos. 4,936,139; 4,860,581; 6,467,544 and 6,659,177. In the illustrated example, the sample chamber  154  has compartments  154   a ,  154   b  defined by a piston  156  movably disposed in the chamber. The compartment  154   a  is typically for collecting formation fluid from the flow line  152 . The compartment  154   b  may be filled with water or other type of fluid, e.g., hydraulic fluid, and may be maintained at any desired pressure. 
     The compartment  154   a  is selectively and fluidly connected to the flow line  152  through flow line  158  and valve  158   a . The compartment  154   b  is selectively and fluidly connected to the flow line  152  through flow line  160  and valve  160   a . The compartment  154   b  may also be provided with additional pressure sources. As shown, compartment  154   b  is fluidly connected to a pressure tank  162  and may be selectively exposed to the borehole  104  through the port  164  and valve  164   a . The pressure tank  162  can receive fluid displaced from compartment  154   b.    
     Pump  165  is provided in the flow line  152 . Pump  165  may be operated in pump-in/out, pump-up/down, or internal mode as previously explained. One or more pumps may be provided at various locations to draw fluid into or eject fluid from the tool. The pump may be operated at a desired speed to manipulate pressures in the flow lines. 
     The tool  100  is provided with flow line  166 , which is fluidly connected to flow line  136   a  of the outlet device  136 , to flow line  130   b  of the inlet device  130 , and to inlet  150   b  of the inlet device  132 . Sample chamber  168  is positioned along the flow line  166 . The sample chamber  168  may be any suitable fluid chamber as previously described. The sample chamber  168  has compartments  168   a ,  168   b  defined by a piston  170  movably disposed in the chamber. 
     The compartment  168   a  may be used for collecting formation fluid from the flow line  166 . The compartment  168   b  may be filled with water or other type of fluid, e.g., hydraulic fluid, and may be maintained at any desired pressure. The compartment  168   a  is selectively and fluidly connected to the flow line  166  through flow line  172  and valve  172   a . The compartment  168   b  is selectively and fluidly connected to the flow line  166  through flow line  174  and valve  174   a . The compartment  168   b  may also be provided with a pressure source, such as a pressure tank  162 , and may be selectively exposed to the borehole  104  through the port  176  and valve  176   a . The pressure tank  162  can receive fluid displaced from the compartment  168   b . Pump  177  is provided in the flow line  166 . Pump  177  may be provided to pump fluid through the flowline. As with pump  165 , pump  177  may be operated in pump-in/out, pump-up/down, or internal mode as previously explained. 
     The flow lines  130   a ,  130   b  of the inlet device  130  may include pretest pistons  180 , sensors  182  and fluid analyzers  184 . The sensors  182  may measure parameters, such as pressure differential, between the flow lines  130   a ,  130   b . The pretest pistons  180  may be provided to draw fluid into the tool and perform a pretest operation. Pretests are typically performed to generate a pressure trace of the drawdown and buildup pressure in the flowline as fluid is drawn into the downhole tool through the probe. 
     Pretest pistons, sensors, fluid analyzers and other devices may be positioned along various flow lines to measure various parameters of the fluid and/or perform tests. For example, the pretest piston may be positioned along each flow line at each inlet to create pressure variations. Data from the pretest piston may be used to generate pressure curves of the formation. These curves may be compared and analyzed. Additionally, the pretest pistons may be used to draw fluid into the tool to break up the mudcake lining on the borehole wall. The pistons may be cycled synchronously, or at disparate rates, to align and/or create pressure differentials across the respective flow lines. The pretest pistons, sensors and analyzers may also be used to diagnose and/or detect problems, such as improper seal, contamination or other problems encountered during operation. 
     The tool  100  may be provided with a variety of additional devices, such as restrictors, diverters, processors, and other devices for manipulating flow and/or performing various formation evaluation operations. The tool  100  may also be provided with a variety of sensors or other monitoring devices, which may be used to monitor, for example, temperature, pressure, and fluid properties. Examples of sensors include, but are not limited to, pressure gauges, optical fluid analyzers, and viscometers. The sensors may be positioned in a variety of locations depending on the desired measurement. The sensors may be part of a module designed to manipulate and/or monitor fluids to determine fluid properties. The configuration of the fluid measuring and/or manipulating devices is preferably flexible and permits various testing and manipulation. 
     The tool  100  described in  FIG. 1E  may be used to sample reservoir fluid from the formation  102  as previously described. The tool  100  allows fluid to be sampled at multiple depths in the formation synchronously or asynchronously, e.g., through the inlet devices  130 ,  132 . The tool  100  also allows samples of fluids having different qualities to be collected from the same depth in the formation, e.g., using the inlet device  130  which has two inlet flow lines or ports. For balanced pressure sampling, the sample chambers  154 ,  168  can be filled against formation pressure as previously described, i.e., by exposing the compartments  154   b ,  168   b  to the ports or channels in outlet devices  134 ,  136 , respectively. For low shock sampling, the sample chambers  154 ,  168  may be filled against borehole pressure, i.e., by exposing the compartments  154   b ,  168   b  to the borehole  104  through the ports  164 ,  176 , respectively. Fluid flow into the sample chambers or out of the sample chambers can be controlled as previously described to ensure that formation fluid is collected and maintained above its bubble point pressure. 
     Preferably, the fluid is pumped at a pressure to maintain the sample quality. In particular, it is preferred that the sample is pumped at a pressure above its bubble point to prevent the sample from becoming bi-phasic. In some configurations, the buffer cavity of the sample chambers (i.e.  154   b ) may be positioned in fluid communication with the wellbore to provide pressure to the sample cavity (i.e.  154   a ) during sampling. However, the present configurations may also be used to apply formation pressure to the buffer cavity to apply pressure to the sample cavity. The formation is typically lower than the wellbore pressure, thereby providing a lower pressure differential in the sample chamber. It may be desirable to use this lower pressure differential to reduce the amount of pumping power required during sampling. 
     The tool  100  may be physically implemented in a variety of ways. The tool  100  may be conveniently constructed from modules such as those described in U.S. Pat. Nos. 4,860,581 and 6,058,773, both assigned to the assignee of the present invention. The following are descriptions of modular tool configurations. 
       FIG. 2A  shows a tool configuration  200  including a power cartridge  202 , hydraulic power modules  204 ,  205 , single probe modules  206 ,  212 , pump module  208 , and sample modules  210 . The power cartridge  202  supplies electrical power to the modules in the tool  200 . The tool  200  has a bussed flow line (not shown) that runs through each module. In some cases, the bussed flow line runs through each module except for the power cartridge  202 . In one embodiment, the tool  200  also includes hydraulic busses (not shown) that run through the hydraulic power modules  204 ,  205  and the probe modules  206 ,  212 , respectively. The hydraulic power modules  204 ,  205  supply the hydraulic power needed to extend/retract the probes  206   a ,  212   a  of the probe modules  206 ,  212 , respectively. Alternately, a single hydraulic power module may provide hydraulic power to both probe modules  206 ,  212 .  FIG. 2B  shows the probes  206   a ,  212   a  in an extended position. 
       FIG. 2C  shows the single probe modules ( 206 ,  212  in  FIG. 2A ) replaced with a dual probe module  214 . One of the probes of the dual probe module  214 , e.g., probe  214   a , can serve as the inlet probe while the other, e.g., probe  214   b , serves as the outlet probe. 
       FIG. 2D  shows the tool  200  incorporating a flow control module  216 . The flow control module  216  measures and controls flow rate and pressure into the sample module(s)  210 . 
       FIG. 2E  shows the tool  200  incorporating a fluid type analyzer  218 , such as the Live Fluid Analyzer (LFA) included in the Schlumberger MDT™. The fluid type analyzer  218  can be installed below the pump  208  as shown or above the pump  208 . Depending on the location of the fluid type analyzer  218  relative to the pump  208 , the fluid type analyzer either analyzes the input to the pump  208  or the output of the pump  208 . The output of the fluid type analyzer  218  can be used to determine when to open the sample chamber in the sample module(s)  210  to capture fluid. As previously discussed, it is not mandatory that a pump is included in the tool. However, when the pump is not included the modules in the tool  200  should be arranged such that pressure differential can be used advantageously to drive flow from the formation into the flow line of the tool  200  and back into the formation or chamber in the sample module(s)  210 . 
     The invention typically provides the following advantages. During the cleanup phase, fluid from the flow line of the tool is discharged into the formation. This avoids contamination of the drilling mud in the borehole. Further, fluid can be pumped or flowed into the sample chamber against formation pressure (as opposed to against borehole pressure). This creates a stable PVT environment as the pressure differential across the sample chamber is minimized. Another advantage is that when taking the sample a flow circuit is created between the inlet probe and outlet probe. The invaded zone in the formation will act as a barrier to the flow into the borehole along this circuit, creating a flow channel through the rock formation. By varying the flow rates/differential pressure of sampling, an in-situ flow test of the formation can be performed so that a direct measurement of near-borehole permeability can be made. 
     While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.