Abstract:
A micro-CT sample holder for placing a sample under hydrostatic pressure during scanning, a micro-CT scanning system using the sample holder, and method of rock sample inspection under hydrostatic pressure. The sample holder includes a pressure vessel having a thinned-walled region and an interior chamber fixed reference stops inside the interior chamber. A position-locating anvil holds the sample and rests on the one or more fixed reference stops so the position of the sample is fixed under pressure. The thin-walled region surrounds the sample to minimize radiation interactions yet sufficient for maintaining a design pressure within the pressure vessel. The pressure vessel includes threads near the top opening to receive a compression screw assembly for closing the top opening and applying a variable pressure on the pressure vessel with a piston acting on a fluid inside the interior chamber.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority to U.S. Pat. App. 62/106,712 titled “Methods and Systems of Testing Formation Samples Using a Rock Compression Chamber”, filed Jan. 22, 2015 by inventor Abraham Grader, which is incorporated by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    Oilfield operators spend a great deal of time and resources when drilling and developing fields for petroleum products. It is essential for the operators to obtain detailed rock properties in order to optimize the production process. Some existing techniques for determining rock properties are not effective for many types of rocks. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0003]    Accordingly, there are disclosed in the drawings and the following description methods and systems of testing formation samples using a hydrostatic rock compression chamber: 
           [0004]      FIG. 1  is a cross-sectional diagram of an illustrative rock compression chamber in a micro-CT configuration; and 
           [0005]      FIG. 2  is a line drawing of a preferred embodiment of the sample positioning in the illustrative rock compression chamber. 
       
    
    
       [0006]    It should be understood, however, that the specific embodiments given in the drawings and detailed description thereto do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and modifications that are encompassed together with one or more of the given embodiments in the scope of the appended claims. 
       DETAILED DESCRIPTION 
       [0007]    Disclosed herein are methods and systems for testing formation samples using a hydrostatic rock compression chamber (sample holder) to inspect a rock sample from an underground formation suspected of containing oil and gas products but not limited to oil and gas. In this disclosure, the sample may be (but not limited to) a cylinder of formation rock that is suspected to contain fluids in the form of water, oil, or gas in unknown concentrations. The sample may be imaged using X-ray CT prior to compression to determine its interior solid and pore structure from which various properties are determined. Applying pressure to the rock sample is essential to place the sample under simulated conditions downhole and to witness and record the elastic and plastic recovery of the rock under simulated downhole stress conditions. The images of the rock under stress yield, through analysis, the rock properties and their dependency on stress. 
         [0008]    Accordingly,  FIG. 1  shows an illustrative rock compression chamber test assembly  100  (test assembly including the sample holder) which is comprised of a relief valve  102 , a pressure gauge  104 , a compression screw assembly  106 , a vessel body assembly  108 , an optional strain gauge  110  with a set of strain gauge wires  112 , a rock sample assembly  114 , a bottom plug  116 , and an optional plastic enforcer tube  118 . The strain gauge  110  may be connected to a monitoring device (not shown) by the strain gauge wires  112  to monitor and record pressures felt by the test assembly  100 . A test measurement device, such as a micro-CT scanner, comprised of a radiation source  180 A, a detector  180 B, along with a controller and measurement recording device  190  is used to irradiate the vessel body assembly  108  (and the sample enclosed within) with radiation and record the resultant signals. The optional plastic enforcer tube  118  may be used to add structural strength to the vessel body assembly  108 , specifically around a thin-walled region  170 . The plastic enforcer tube  118  is optional, but when used may prevent bending when the thin-walled region  170  is of such length that the thinness results in a low structural strength despite being sufficient to maintain the design pressure levels internally. 
         [0009]    The vessel body assembly  108  is comprised of a vessel body  160 , a set of threads  162 , and an interior chamber  168  including an air chamber  164  and a hydraulic fluid chamber  166 . The interior chamber  168  extends most of the length of the interior of the vessel body  160  and includes the air chamber  164 . The vessel body assembly  108  is made of titanium alloy or other similar material that has high strength while having as little mass as possible. The use of titanium or similar materials is preferred to minimize the losses associated with interactions with the radiation(s) emitted by the source  180 A. In one embodiment, the interior chamber  168  is cylindrical with an inner diameter of 7 mm, with the air chamber  164  cylindrical with an inner diameter of 6 mm. Other embodiments have interior diameters of 1-5 mm. Typical thickness of the wall in the thin-walled region is 1 mm of titanium alloy but may be as thin as 0.5 mm in other embodiments, based on the design pressure. A typical design pressure is 4000 psig with operational pressures up to the 2500 psig range. All embodiments include appropriate safety factors for any given component. 
         [0010]    Continuing with the vessel body assembly  108 , the bottom plug  116  is placed into the interior chamber  168 . The bottom plug  116  uses at least one O-ring  150  to create a fluid-tight seal to define and isolate the air chamber  164 . The vessel body  160  also includes the thin-walled region  170  to minimize the amount of matter the radiation has to traverse when emitted from the source  180 A to the detector  180 B. 
         [0011]    The compression screw assembly  106  is comprised of a compression screw body  140  (stainless steel  316  or similar alloy for material compatibility with the vessel body  160 ), a conduit connector  142 , a set of threads  144  for mechanical coupling to the threads  162  present on the vessel body  160 , a piston arm  146 , a fluid conduit  148  located within the piston arm  146 , and at least one O-ring  150  to maintain pressure and establish fluid isolation when the test assembly  100  is in use. The compression screw assembly  106  may be made of steel or other material that can withstand high pressures without deforming. Reference marks (not shown) may be placed on the outside of the compression screw assembly  106  to allow an operator to monitor the position of the compression screw assembly  106  in reference to the vessel body assembly  108 . The conduit connector  142  may either be used to seal off the fluid conduit  148  or to attach additional test devices such as the relief valve  102  or the pressure gauge  104  as required. Once screwed into the vessel body assembly  108 , the compression screw assembly  106  may be tightened using the threads  144  against the vessel body  160  threads  162 . In this way, the piston arm  146  of the compression screw assembly  106  reduces the volume of the interior chamber  168  of the vessel body assembly  108 . As a consequence, as the volume of the interior chamber  168  is reduced, the pressure in the interior chamber  168  is increased. Pressure in the interior chamber  168  is thus controlled by tightening or loosening the compression screw assembly  106  in relation to the vessel body assembly  108 . Pressure felt at the end of the piston arm  146  may be monitored by attaching the pressure gauge  104  to the fluid conduit  148 . 
         [0012]    The hydraulic fluid is preferably only slightly compressible (less than 1% at 1000-4000 psig at room temperature is standard) so that the compression of the air chamber allows for ease of setting the pressure inside the interior chamber  168 . In an embodiment without the air chamber  164 , the hydraulic fluid is preferably more compressible than standard as standard hydraulic fluid acted on by a piston makes setting a precise pressure level difficult. 
         [0013]    To conduct an analysis, the sample is prepared. A sample of formation rock of interest (not shown) may be prepared by cutting a sample of rock into a cylinder shape approximately 5 mm long and 5 mm in diameter. The rock sample is then preferably encapsulated by a covering that is impermeable to fluids such as water and petroleum components. The covering may be one of heat shrink material, waterproof paint, plastic wrap, or any of a number of other materials. The purpose of the covering separate the rock from the compressing fluid in the chamber  166 , so that net confining stress is transmitted to the rock sample assembly  114 . The rock sample assembly  114  is thus comprised of a portion of formation rock cut into a cylinder shape and covered in a fluid-tight covering. In other embodiments, the rock sample assembly includes one or two anvils (see  FIG. 2 ) either below or both above and below the rock sample itself. Preferably, the anvils are made of aluminum or other material similar in composition to the rock sample so as not to interfere with the sample testing. 
         [0014]    The threads  162  are shown internal, but other embodiments may be external, so long as the compression screw assembly  106  has matching threads and the piston engages the hydraulic fluid in the hydraulic fluid chamber  116 . Note that the base  172  may include screws or dowels (not shown) or holes to accept screws or dowels to aid in placement and in securing the sample holder in the micro-CT scanner. 
         [0015]    Turning now to  FIG. 2 , the line drawing of a preferred embodiment of the sample positioning in the illustrative rock compression chamber is shown. In this embodiment, the rock sample assembly  114  includes the rock sample  210  between a position-locating anvil  220  below the rock sample  210  and an upper anvil  230  above the rock sample. The position-locating anvil  220  rests on shoulders  250  that act as fixed reference stops  250 . The user knows that the position-locating anvil  220  will rest on the fixed reference stops  250  and that the bottom of the sample  210  meets the top of the position-locating anvil  220 . So the location of the bottom of the sample  210  is always known and fixed. The upper anvil  230  is optional. 
         [0016]    Using the upper anvil  230  allows for encapsulation to occur part of all of the upper anvil  230 , the sample  210 , and all or part of the position-locating anvil  220 . The position-locating anvil  220  preferably has passages  225  for the hydraulic fluid to flow below the position-locating anvil  220  and contact the bottom plug  116 . The bottom plug  116  is sized to fit snugly against the inner wall so that a gas bubble of varying size is maintained in the air chamber  164 . In practice, the bottom plug  116  may use the O-ring  150  shown in  FIG. 1  or be properly sized for an inside wall sufficiently uniform in geometry that capillary forces prevent the hydraulic fluid from invading the air chamber  164 . The shoulder  250  may be completely perpendicular to the inner wall or have a slope. In another embodiment, the stop or stops  250  may be part of an insert that bottom plug  116  moves along or inside instead of being integral with the vessel body  160 , so long as the air chamber  164  is not invaded by the hydraulic fluid to prevent control of the pressure within the interior chamber  168  within the hydraulic fluid chamber  166 . 
         [0017]    To prepare the test assembly  100  for testing, the vessel body assembly  108  is cleaned of all contaminants. The bottom plug  116  is inserted into the interior chamber  168 . It is important to not allow the bottom plug  116  to travel all of the way to the bottom of the vessel body assembly  108  as the air chamber  164  plays an important part in the test assembly  100 . Then, the rock sample assembly  114  is placed in the interior chamber  168 , resting above the bottom plug  116 . Hydraulic fluid within hydraulic fluid chamber  166 , surrounding the encapsulated sample assembly  114 , is then injected into the interior chamber  168  above the rock sample assembly  114 , bottom plug  116 , and the air chamber  164  filling the remainder of the interior chamber  168  with fluid to form the hydraulic fluid chamber  166  around the rock sample assembly  114 . Thus assembled, the interior chamber  168  contains the rock sample assembly  114  above the air chamber  164  and surrounded by hydraulic fluid in the hydraulic fluid chamber  166 . Finally, the compression screw assembly  106  is threaded into the vessel body assembly  108 . 
         [0018]    To conduct an analysis of the rock sample assembly  114 , the compression screw assembly  106  is tightened in relation to the vessel body assembly  108  while the pressure in the interior chamber  168  is monitored by the pressure gauge  104  or the strain gauge  110 . It is desirable to place the rock sample assembly  114  under pressure while conducting the test as it is desirable to simulate actual downhole pressures to witness the characteristics of the rock sample assembly  114  under estimated downhole conditions. Once the desired pressure is reached, the analysis may begin using a CT scan or other measurement scanning techniques using the test measurement device. For a CT scan, the radiation are typically X rays, while the detector is typically a scintillator array or even a single crystal. The controller and measurement recording device  190  is typically a computer system with motor controllers and switches. In an illustrative embodiment, the X rays are created from electrons accelerated with voltages ranging from 20-100 kV with power output about 10 W. 
         [0019]    In another embodiment, there may be a bottom threaded hole with a screw plug (not shown) at the base  172  of the vessel body  160 . Note that the preferred placement of radiation source  180 A and detector  180 B is as close as practical to the sample  210 . In practice, those locations, when the plastic enforcer tube  118  is not present, are where the plastic enforcer tube  118  is shown in  FIG. 1 . When the plastic enforcer tube  118  is present, those locations are approximately 1 mm from the plastic enforcer tube  118 . Those of skill in the art will appreciate the location being close to but distant enough to avoid complications with being right at the surface of the plastic enforcer tube  118 . Note that substantially incompressible herein means anything less than 0.2% at operating pressure. All embodiments or illustrative examples not given as alternatives to each other are combined with each other as additional disclosed embodiments.