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BACKGROUND OF INVENTION  
       [0001]     The invention relates to methods and apparatus for sampling reservoir fluid.  
         [0002]     A reservoir is a rock formation in which fluids, e.g., hydrocarbons such as 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 a probe that can be set against the borehole wall, allowing reservoir fluid to be drawn into a flow line in the formation tester. The formation tester also includes a pump and one or more sample chambers. Typically, optical fluid analyzers are inserted into the flow line of the formation tester to monitor the fluid(s) flowing in various locations of interest. For example, an optical analyzer is often run directly above the probe to monitor the type of fluid entering the flow line.  
         [0003]     Initially, the fluid drawn into the flow line is a mixture of reservoir fluid and mud filtrate. To obtain a sufficiently high quality fluid sample, a cleanup step in which mud filtrate is purged from the flow line is performed. This step involves pumping fluid through the flow line and out into the well. 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. Optical fluid analyzers are used to monitor the content of the fluid entering the flow line and how the fluid proceeds through the tool. 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. Usually, the following ancillary objectives are set for this step: (1) that a certain minimum volume of reservoir fluid be captured, and (2) that the reservoir fluid captured be single hydrocarbon phase, e.g., oil phase or gas phase, but not both. Finally, the sample chamber is closed and returned to the surface.  
         [0004]     In practice, there is only a certain maximum time allowed before the cleanup step must be terminated. Therefore, there is no guarantee that the fluid mixture in the flow line is adequately decontaminated prior to being captured in the sample chamber. Further, the sample chamber may be returned to the surface without a sample, e.g., because the sample chamber was not opened successfully. Further, the sample chamber may be returned to the surface unclosed, e.g., because the sample chamber was not successfully closed after the fluid sample was collected. In this case, the content of the sample chamber may be lost or exposed to contaminants or undergo a phase change as it is returned to the surface. Prior to the present invention, the inventors are not aware of methods for verifying in real-time that the three steps described above, i.e., cleanup, sample capture, and sample chamber closing, are successfully accomplished before the sample chamber is retrieved to the surface.  
         [0005]     From the foregoing, there is desired a method of assuring quality fluid sample capture from a reservoir.  
       SUMMARY OF INVENTION  
       [0006]     In one aspect, the invention relates to a method of sampling reservoir fluid which comprises establishing communication between a reservoir and an entry port of a flow line disposed in a borehole penetrating the reservoir, separating fluid received in the entry port into individual fluid components and sequentially flowing slugs of each individual fluid component along the flow line, observing the slugs as they move along the flow line in order to determine the composition of the slugs, estimating when a desired slug containing a sufficient volume of desired fluid component would be in the vicinity of a sample chamber in the flow line, and opening the sample chamber to capture the desired slug when the desired slug is in the vicinity of the sample chamber.  
         [0007]     In another aspect, the invention relates to a system for sampling reservoir fluid which comprises a tool body having a flow line with an entry port and an exit port and being adapted to be suspended in a borehole penetrating a reservoir. The system includes a fluid separator installed in the flow line for separating fluid received from the entry port into individual fluid components and sequentially outputting slugs of each individual fluid into the flow line.  
         [0008]     The system includes a fluid analyzer installed in the flow line downstream of the fluid separator for determining the composition of the slugs as they move along the flow line.  
         [0009]     The system includes at least one sample chamber in the flow line downstream of the fluid analyzer for capturing a desired slug containing a desired fluid component.  
         [0010]     Other features and advantages of the invention will be apparent from the following description and the appended claims. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0011]      FIG. 1  is a simplified diagram of a downhole tool for sampling reservoir fluid in accordance with one embodiment of the invention.  
         [0012]      FIGS. 2A and 2B  are schematic illustrations of a dual-displacement pump.  
         [0013]      FIG. 3  shows an example of an output of the dual-displacement pump of  FIGS. 2A and 2B .  
         [0014]      FIG. 4  shows flow line behavior before sample capture.  
         [0015]      FIG. 5  shows flow line behavior after sample capture.  
         [0016]      FIG. 6  shows flow line behavior during verification of sample chamber closure. 
     
    
     DETAILED DESCRIPTION  
       [0017]     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.  
         [0018]      FIG. 1  shows a simplified diagram of a downhole tool  100  for sampling reservoir fluid in accordance with one embodiment of the invention. The downhole tool  100  is conveyed to a selected depth in a borehole  102  drilled through a rock formation  104  using, for example, a wire-line  106 . The downhole tool  100  includes a flow line  108  having an entry port  110  and an exit port  112 . A probe assembly  114 , such as 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, both assigned to Schlumberger Technology Corporation, may be mounted at the entry port  110 . The probe assembly  114  can be extended to and set against the borehole wall  116  so that reservoir fluid in the formation  104  can be drawn into the entry port  110 . Although not shown, a packer assembly, such as the Dual-Packer Module included in the Schlumberger MDT or described in U.S. Pat. No. 4,860,581, may be installed below the probe assembly  114 . The packer assembly may be inflated against the borehole wall  116  to isolate an interval of the formation  104  to be tested.  
         [0019]     The downhole tool  100  includes a pump  200 , such as the Pump-Out module included in the Schlumberger MDT or described in U.S. Pat. Nos. 4,860,581 and 6,058,773, installed above the probe assembly  114 . In the following discussion, the portion of the flow line  108  between the entry port  110  and the pump  200  will be referred to as the inlet flow line  108   a , and the portion of the flow line  108  between the pump  200  and the exit port  112  will be referred to as the outlet flow line  108   b . The downhole tool  100  further includes a fluid type analyzer  120 , such as the Live Fluid Analyzer (LFA) included in the Schlumberger MDT, installed above the pump  200 . The fluid type analyzer  120  analyzes the output of the pump  200  as opposed to the input to the pump  200  as done in conventional formation testing. The downhole tool  100  further includes at least one sample chamber  122 , such as the Modular Sample Chamber, Multi-Sample module, or Single-Phase Multi-Sample Chamber included in the Schlumberger MDT. Other components necessary to make the downhole tool  100  fully functional, such as a power cartridge, which are not shown are within the purview of one skilled in the art. The wireline  106  may be used to transmit data to the surface.  
         [0020]     In one embodiment, the pump  200  is a dual-displacement pump. The function of the dual-displacement pump  200  is two-fold. One function is to pump fluid into or out of the flow line  108  or into or out of the sample chamber  122 . The second function is to separate a fluid mixture received in the inlet flow line  108   a  into individual fluids according to the density of the individual fluids and then sequentially output slugs of these individual fluids into the outlet flow line  108   b , where the slugs move like a train along the outlet flow  108   b . In this way, a slug containing a desired fluid (such as a hydrocarbon) can be captured into the sample chamber  122  while slugs containing unwanted fluid (such as mud filtrate) can be allowed to exit the outlet flow line  108   b  through the exit port  112 . The dual-displacement pump  200  can create trains of slugs of individual fluids in a predictable pattern because fluid separation occurs in each cycle of the pump. However, the invention is not limited to use of a dual-displacement pump as a fluid separator. In general, any device that can separate a fluid mixture into individual fluids and create slugs of these individual fluids in a predictable pattern can be used in the invention. An example of a down hole fluid separator is any chamber that has a significantly larger diameter than the diameter of the flow line  108 . The chamber would be placed somewhere in the flow line  108  (before the fluid analyzer  120 ). The fluid mixture would enter the chamber from the flow line which is attached to the bottom of the chamber. Fluid phases of different density would segregate in the chamber, and the separated mixture would leave from the top of the chamber into the flow line. The segregation causes the fluids to leave as a train of slugs. It should be noted that if the fluid separator is not a pump, a pump would still be needed to pump fluid into and out of the flow line  108  or into and out of the sample chamber  122 .  
         [0021]      FIG. 2A  shows the pump  200  configured as a dual-displacement pump. In this embodiment, the pump  200  includes hydraulic cylinders  202 ,  204 . The pistons  206 ,  208  of the hydraulic cylinders  202 ,  204 , respectively, are coupled together so that they move concurrently. The hydraulic cylinders  202 ,  204  have pump chambers  202   a ,  204   a , which can be selectively connected to the inlet flow line  108   a  to receive fluid or the outlet flow line  108   b  to dispense fluid. Inlet check valves  210   a  and outlet check valves  210   b  control which of the flow lines  108   a ,  108   b  is in communication with the pump chambers  202   a ,  204   a  at any time. The hydraulic cylinders  202 ,  204  also have hydraulic fluid chambers  202   b ,  204   b , which are in communication with a hydraulic fluid source  212 . Hydraulic fluid is alternately supplied to the hydraulic fluid chambers  202   b ,  204   b  to either move the pistons  206 ,  208  downwardly (down-stroke) or upwardly (up-stroke). The dual-displacement pump arrangement eliminates refill dead time by refilling one of the hydraulic cylinders  202 ,  204  with fluid while the other hydraulic cylinder is dispensing fluid.  
         [0022]     During the down-stroke cycle, hydraulic fluid is supplied to the hydraulic fluid chamber  202   b , forcing the pistons  206 ,  208  to move downwardly. As the pistons  206 ,  208  move downwardly, slugs of fluids are dispensed from the pump chamber  202   a  into the outlet flow line  108   b  while the pump chamber  204   a  is filled with fluid from the inlet flow line  108   a . The fluid admitted into the pump chamber  204   a  is usually a mixture of fluids, such as hydrocarbons, water, and mud filtrate. Due to gravity, the fluid in the pump chamber  204   a  separates into individual fluids, with the lighter fluid towards the top of the chamber and the heavier fluid towards the bottom of the chamber. To allow mud filtrate to form a distinct layer from hydrocarbons, the mud filtrate is preferably water-based as opposed to oil-based. During the up-stroke cycle, as shown in  FIG. 2B , hydraulic fluid is supplied to the hydraulic fluid chamber  204   b , forcing the pistons  206 ,  208  to move upwardly. As the pistons move upwardly, the separated fluids in the pump chamber  204   a  are dispensed into the outlet flow line  108   b , one individual fluid at a time. The individual fluids are dispensed as slugs. While the pump chamber  204   a  is dispensing, the pump chamber  202   a  is filled with fluid from the inlet flow line  108   a.    
         [0023]     Each cycle of the dual-displacement pump  200  results in fluid separation. Thus, slugs of individual fluids are continuously dispensed from the pump  200  into the outlet flow line  108   b  in a predictable manner. For illustrative purposes,  FIG. 3  shows slugs, generally indicated at  300 , leaving the pump. The pump stroke direction when the slugs  300  were produced is indicated in the figure. Two main volume slugs  302   a ,  302   b , each main volume corresponding to an individual fluid, are shown for each pump stroke. At the beginning of each down-stroke and up-stroke of the pump is a dead volume slug  304   a ,  304   b , respectively. The dead volume is the section of outlet flow line ( 108   b  in  FIGS. 2A, 2B ) between the port of each pump chamber ( 202   a ,  204   a  in  FIGS. 2A, 2B ) and the out-put check valve ( 210   b  in  FIGS. 2A, 2B ). The dead volume  304   a  at the beginning of each down-stroke contains the last fluid pumped out from the previous up-stroke of the pump. If the fluid in the pump chamber  202   a  contains gas, the dead volume  304   a  would be gas. The dead volume  304   a  can be a good indicator of whether any gas flows out of the pump, especially when there is very little gas flow.  
         [0024]     Returning to  FIG. 1 , since fluid leaves the pump  200  as slugs of individual fluids  300 , an operator at the surface can advantageously time opening of the sample chamber  122  such that as much as possible of one of the slugs  300  containing a desired fluid can be captured. In order to do this, the operator needs to know when the slug containing the desired fluid is leaving the pump  200 . This is accomplished by observing over a time period the pattern in which slugs are outputted from the pump. To do this, the slugs  300  leaving the pump  200  are passed through the fluid type analyzer  120 . The fluid type analyzer  120  may include a gas detector, e.g., a gas refractometer, to distinguish between a liquid phase and a gas phase and/or a liquid detector, e.g., an optical absorption spectrometer, to distinguish between liquid and/or gas phases. As the slugs  300  pass through the fluid type analyzer  120 , the fluid type analyzer  120  generates an output, such as an absorption spectra, that can be used to determine the composition of the slugs passing through it.  FIG. 4  shows an example of flow (indicated at  400 ) detected by the fluid type analyzer  120  prior to sample capture. Oil slugs  402  and water slugs  404  are shown. Oil and water are reliably detected by their different absorption spectra. The fluid (slug) volume is shown as a function of time. By observing this output, the operator can predict when to expect a slug having a sufficient volume of a desired fluid component, e.g., oil.  
         [0025]     Returning to  FIG. 1 , an operator at the surface monitors the output of the fluid type analyzer  120  and the pressure at the entry port  110  of the flow line  108 , the hydraulic pressure of the pump  200 , and the stroke direction of the pump  200 . The operator uses this information to determine when to open the sample chamber  122  to capture the desired slug. The volumes of most types of sample chambers are significantly smaller than the volume of one pump stroke. Thus, it should be possible to capture enough low-contamination hydrocarbon slug into a sample chamber even when there is still significant amount of mud filtrate contamination. This capability will significantly shorten sampling times. The operator can time opening of the sample chamber  122  to avoid capturing as much as possible of any unwanted fluids. For example, if a single-phase oil sample is desired, and free gas is also slugging out of the pump  200 , the operator can time opening and closing of the sample chamber  122  to avoid capturing the gas. The main and dead volumes in the fluid analyzer  120  output are observed to see if the slug leaving the pump is single phase or two phase. The dead volumes are observed to see if the slug leaving the pump contains free gas.  
         [0026]     Fluid sampling involves establishing communication between the formation  104  and the entry port  110  of the flow line  108 . Once communication is established between the formation  104  and the entry port  110 , the pump  200  draws fluid into the inlet flow line  108   a  through the entry port  110  and sequentially dispenses slugs of individual fluids into the outlet flow line  108   b . The slugs move in trains through the fluid analyzer  120 . In general, the slugs move at the speed of the most viscous phase. As the slugs move through the fluid type analyzer  120 , they are detected and analyzed by the fluid type analyzer  120 . The operator monitors the output of the fluid type analyzer  120  to determine the fluid type of the slugs produced by the pump  200 . From the output, the operator can predict when a slug having a desired fluid is leaving the pump  200 . This also allows the operator to determine when to open the sample chamber  122  to capture the desired slug.  
         [0027]     To capture the desired slug, the operator opens the sample chamber  122  and closes the exit port  112  of the flow line  108  at approximately the same time. Then, the operator starts pumping fluid through the flow line  108 . If the exit port  112  is closed and the sample chamber  122  is successfully opened, the fluid in the flow line  108  will be diverted into the sample chamber  122 . As soon as the sample chamber  122  becomes full, the pump  200  would experience increasing resistance and the hydraulic pressure of the pump  200  would increase noticeably. At the same time, flow from the formation would basically stop and the pressure at the entry port  110  would build up towards formation pressure. By observing the relationship between volume pumped into the sample chamber  122 , hydraulic pressure of the pump  200 , and the pressure of the fluid entering the flow line  108 , it may be verified that the sample chamber  122  opened successfully and captured fluid pumped from the formation. For a good sample capture, there should be more than one pump stroke between opening of the sample chamber  122  and pressure up, i.e., when the hydraulic pressure of the pump  200  increases noticeably, and drawdown should remain constant. When trying to capture a single hydrocarbon phase, the sample chamber  122  should be opened somewhere during the pump down-stroke.  
         [0028]      FIG. 5  shows an example of a sample capture log. On the right (indicated at  500 ) is the output of the fluid type analyzer. On the left (indicated at  502 ) is the entry port pressure, pump hydraulic pressure, and pump stroke direction information.  FIG. 5  shows that the operator opened the sample chamber correctly ( 504 ), caught the desired slug ( 506 ), and over-pressured the sample in the sample chamber ( 508 ). Note how the pressure ramps up when the sample chamber becomes full ( 510 ). Even after the sample chamber is full, pumping continues until the pump reaches maximum compression capability. Once the operator determines that the sample chamber is full, the operator issues a command to close the sample chamber. The operator then verifies that the sample chamber is actually closed. This verification involves closing the exit port ( 112  in  FIG. 1 ) and then attempting to pump fluids from above the pump ( 200  in  FIG. 1 ) to below the pump, i.e., from the side of the pump closest to the sample chamber to the other side of the pump. If the sample chamber ( 122  in  FIG. 1 ) is closed, high resistance is experienced, and the hydraulic pressure of the pump rises rapidly to a maximum allowable value (see pressure curve  600  in  FIG. 6 ). If the sample chamber is open, the pump will be able to push a volume of fluid equal to the volume of the sample chamber before reaching this maximum value.  
         [0029]     The invention provides one or more advantages. Using the method and system described above, slugs of individual fluids can be created, and a slug containing a desired fluid can be captured in a sample chamber while slugs containing unwanted fluids can be avoided. This can reduce sampling times in that it is not necessary to wait for the fluid in the flow line to have an acceptable level of contamination before the sample is captured. Whether the sample chamber is successfully opened, sample is captured, and sample chamber is successfully closed can be verified in real time. A record of the capture can be stored for later use. The tool can be constructed using existing formation tester components, such as included in the Schlumberger MDT.  
         [0030]     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.

Summary:
A method of sampling reservoir fluid includes establishing communication between a reservoir and an entry port of a flow line disposed in a borehole penetrating the reservoir. The method includes separating fluid received in the entry port into individual fluid components and sequentially flowing slugs of each individual fluid component along the flow line, observing the slugs as they move along the flow line in order to determine the composition of the slugs, estimating when a desired slug containing a desired fluid component would be in the vicinity of a sample chamber in the flow line, and opening the sample chamber to capture the desired slug when the desired slug is in the vicinity of the sample chamber. The method also includes checking that the sample chambers open and close successfully. Finally, the method further includes creating an accurate record of events, which can then be used to audit the sampling process.