Patent Publication Number: US-6216782-B1

Title: Apparatus and method for verification of monophasic samples

Description:
TECHNICAL FIELD OF THE INVENTION 
     This invention relates in general to testing and evaluation of subterranean formation fluids and, in particular to, a fluid sampling tool and method for monitoring the temperature of the sample to determine whether the sample has undergone phase change degradation during collection or retrieval from the wellbore. 
     BACKGROUND OF THE INVENTION 
     Without limiting the scope of the present invention, its background is described with reference to testing hydrocarbon formations, as an example. 
     It is well known in the subterranean well drilling and completion art to perform tests on formations intersected by a wellbore. Such tests are typically performed in order to determine geological or other physical properties of the formation and the chemical and physical properties of the fluids contained therein. For example, parameters such as permeability, porosity, fluid resistivity, temperature, pressure and bubble point may be determined. These and other characteristics of the formation and fluid contained therein may be determined by performing tests on the formation before the well is completed. 
     One type of testing procedure is to obtain a fluid sample from the formation to, among other things, determine the composition of the formation fluids. In this procedure, it is important to obtain a sample of the formation fluid that is representative of the fluids as they exist in the formation. For example, the sample is used to determine the economic value of fluids within the formation. In addition, the composition of the formation fluids is used to determine the type and capacity of the processing equipment required to process fluids extracted from the formation. 
     In the past, sampling of formation fluids was accomplished by collecting a large volume of fluid through the drill string which may be on the order of thousands of gallons of formation fluids. This type of large scale sampling is, however, timely and expensive. In an alternative sampling procedure, formation fluids may be sampled on a smaller scale by lowering a sampling tool into the wellbore on a wireline, slick line or tubing string. In this case, when the sampling tool reaches the desired depth, one or more ports are actuated from the closed position to the opened position to allow collection of the formation fluids. The ports may be actuated in variety of ways such as by electrical, hydraulic or mechanical methods. Once the ports are opened, formation fluids travel through the ports and a sample of the formation fluids is collected within a chamber of the sampling tool. After the sample has been collected, the sampling tool may be withdrawn from the wellbore so that the formation fluid sample may be analyzed. 
     It has been found, however, that with the use of conventional formation sampling tools, the fluid sample is obtained relatively quickly which can cause phase change degradation of the formation fluid due to flashing as the fluid flows into the sampling chamber. This phase change degradation may result in irreversible chemical and physical changes in the formation fluid. For example, in a typical sampling procedure, the formation fluids flow through one or more valves or passageways to enter the sampling chamber. The inherent pressure drop across the valves or passageways creates the possibility that lighter fractions present in the sample will flash, or come out of solution, during collection. Once flashing has occurred, the resulting sample may no longer be representative of the fluids present in the formation. 
     It has also been found that as conventional formation sampling tools are retrieved from the wellbore, the reduction in hydrostatic pressure acting on the sampling tool may result in a reduction of the fluid pressure within the sampling chamber. This drop in pressure may similarly cause phase change degradation of the sample as the sampling tool is removed from the wellbore. In the past, it has been difficult to know whether the sample has undergone phase change degradation either during collection or retrieval from the wellbore. As such, it has been difficult to determine whether the sample is representative of the fluids present in the formation. 
     Therefore, a need has arisen for an apparatus and method for obtaining a fluid sample from a formation without phase change degradation of the sample during collection or retrieval of the sampling tool from the wellbore. A need has also arisen for such an apparatus and method that is capable of verifying whether the sample has undergone phase change degradation. 
     SUMMARY OF THE INVENTION 
     The present invention disclosed herein provides a downhole sampling apparatus and a method for obtaining a fluid sample from a formation without the occurrence of phase change degradation of the sample during collection or retrieval of the sampling tool from the wellbore. The downhole sampling apparatus and method of the present invention is capable of verifying whether the sample has undergone phase change degradation by monitoring the temperature of the sample during collection and retrieval of the downhole sampling apparatus from the wellbore. 
     In one embodiment, the downhole sampling apparatus of the present invention comprises a housing having a sampling chamber and a sampling port defined therein. The sampling port is in communication with the sampling chamber and the formation traversed by the wellbore. A temperature monitoring device is at least partially disposed within the sampling chamber. The temperature monitoring device monitors the temperature of formation fluid collected in the sampling chamber to determine whether the formation fluid undergoes phase change degradation. The temperature monitoring device is operatively connected to a temperature recorder so that temperature fluctuations in the formation fluid may be recorded. 
     In another embodiment, the downhole sampling apparatus of the present invention comprises a housing having a fluid passageway that is in communication with the formation. A sampling device is disposed within the housing. The sampling device has a sampling chamber and a sampling port defined therein. The sampling port is in communication with the sampling chamber and the fluid passageway. A temperature recorder is also disposed within the housing. The temperature recorder includes a temperature monitoring device that is in communication with the fluid passageway for monitoring the temperature of formation fluid entering the sampling port. 
     In either embodiment, a check valve is disposed within the sampling port for allowing formation fluid flow through the sampling port into the sampling chamber while preventing reverse flow from the sampling chamber out through the sampling port. The sampling device may also include first and second operating fluid chambers. A control valve is disposed between the first and second operating fluid chambers for initially isolating the first operating fluid chamber from the second operating fluid chamber. When the control valve is actuated, the first operating fluid chamber is in communication with the second operating fluid chamber such that operating fluid flows from the first operating fluid chamber to the second operating fluid chamber. Once this has occurred, formation fluid may flow through the sampling port into the sampling chamber. A flow restrictor may be use to impede the rate of fluid flow from the first operating fluid chamber to the second operating fluid chamber. A floating piston may be disposed between the sampling chamber and the first operating fluid chamber. 
     The sampling device may also have an isolation valve that allows outside hydrostatic pressure into the sampling device after a predetermined volume of operating fluid has flowed from the first operating fluid chamber to the second operating fluid chamber. A check valve may be used to trap the hydrostatic pressure within the sampling device. 
     In one the method of the present invention, the sampling device is run into the wellbore to a depth at which the formation fluids are to be sampled. The sampling tool then collects formation fluids from the formation in the sampling chamber through the sampling port. The temperature of formation fluids collected in the sampling chamber is monitored to determine whether the formation fluids undergo phase change degradation. The temperature of the formation fluids may be recorded with a temperature recorder. 
     In another method of the present invention, a housing having the sampling device and a temperature recorder disposed therein is run into the wellbore to a depth at which the formation fluids are to be sampled. The formation fluids are allowed to pass through a fluid passageway within the housing. Formation fluids are collected in a sampling chamber of the sampling device. The temperature of the formation fluids is measure by a temperature monitoring device as the fluids pass through the fluid passageway of the housing. The temperature recorder records the temperature measurement to determine of whether the formation fluids have undergone phase change degradation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, including its features and advantages, reference is now made to the detailed description of the invention, taken in conjunction with the accompanying drawings in which like numerals identify like parts and in which: 
     FIG. 1 is a schematic illustration of an offshore oil or gas drilling platform utilizing an apparatus for verification of monophasic samples of the of the present invention positioned adjacent to a formation to be tested; 
     FIG. 2 is a schematic illustration of one embodiment of an apparatus for verification of monophasic samples of the present invention; 
     FIG. 3 is a schematic illustration of another embodiment of an apparatus for verification of monophasic samples of the present invention; 
     FIGS. 4A-4C are schematic illustrations of a sampling device in its various positions for use with an apparatus for verification of monophasic samples of the present invention; and 
     FIG. 5 is a schematic illustration of another embodiment of an apparatus for verification of monophasic samples of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While the making and using of various embodiments of the present invention are discussed in detail below, it should be appreciated that the present invention provides many applicable inventive concepts which can be embodied in a wide variety of specific contexts. The specific embodiments discussed herein are merely illustrative of specific ways to make and use the invention, and do not delimit the scope of the invention. 
     Referring now to FIG. 1, an offshore oil and gas drilling platform operating an apparatus for verification of monophasic samples is schematically illustrated and generally designated  10 . Semisubmersible platform  12  is positioned over a submerged oil or gas formation  14  located below sea floor  16 . Conduit  18  extends from deck  20  of platform  12  to a well head installation apparatus  22  located adjacent to sea floor  16 . The wellhead installation apparatus  22  typically includes blowout prevention devices  24 . The platform  12  is equipped with derrick  26  and a hoisting apparatus  28  for raising and lowering tools drill string  30  and testing tools including an apparatus for verification of monophasic samples or sampling assembly  32 . 
     Even though FIG. 1 depicts sampling assembly  32  of the present invention connected to drill string  30 , it should be understood by those skilled in the art that sampling assembly  32  may alternatively be run downhole on a wireline, slick line or the like. It will also be apparent to one skilled in the art that sampling assembly  32  of the present invention is not limited to use with platform  12 . Sampling assembly  32  is also well-suited for use with other offshore platforms or during onshore production operations. 
     Referring now to FIG. 2, therein is depicted one embodiment of a sampling assembly of the present invention that is generally designated  40 . Sampling assembly  40  may be lowered into place within the wellbore on a wireline (not pictured). Sampling assembly  40  has a housing  42  that surrounds sampling device  44  and temperature recorder  46 . Sampling device  44  includes a sampling chamber  48 . A sampling port  50  communicates sampling chamber  48  with the exterior of housing  42  such that fluids from formation  14  of FIG. 1 may be collected by sampling device  44 , as will be more fully described below. A temperature sensor  52  is at least partially disposed within sampling chamber  48 . Temperature sensor  52  is operably connected to temperature recorder  46  via coupling  54 . 
     The temperature sensor  52  may be a thermocouple, an RTD or other temperature measuring device. Temperature sensor  52  monitors temperature changes occurring during collection and retrieval of fluid samples from formation  14 . Temperature recorder  46  is used for recording variations in the temperature of the sample as a function of time. Temperature variations during the collection and retrieval operation can provide the operator with information regarding the sampling process including information indicating whether the fluid has undergone phase change degradation resulting in chemical and/or physical changes in the formation fluid making the sample less representative of the formation fluids as they exist in formation  14 . 
     For example, if the temperature profile measured during the collection and retrieval process remains relatively constant, this tends to indicate that no significant portion of the sample has flashed. This type of constant temperature profile, therefore, indicates that no significant phase changes have occurred, thereby indicating that a representative sample of the formation fluids as they exist in the formation has been obtained. 
     Alternatively, if a significant temperature fluctuation is recorded by temperature recorder  46 , this tends to indicate that flashing has occurred and that the fluid has undergone phase change degradation. Since flashing is an endothermic process, the flashing of a low molecular fraction of the sample will cause a decrease in the temperature of the sample. Such a decrease in temperature may indicate that the sample is not monophasic and is now less representative of the formation fluids as they existed in the formation. 
     In operation, either the magnitude of the observed temperature change or the rate of temperature change, may be indicative of phase change degradation of the sample. Thus, when the sample is retrieved, the operator may review the temperature history of the sample recorded by recorder  46  to determine whether resampling of the formation fluids is necessary to obtain a more representative sample of formation fluid. The decision whether or not to resample may be based either upon the magnitude of the observed temperature change, (Δtemp), the rate of temperature change, (Δtemp/Δt), or a combination of both. 
     Referring now to FIG. 3, therein is depicted another embodiment of a sampling assembly that is generally designated  60 . Sampling assembly  60  may typically be lowered into the wellbore as part of a pipe string such as drill string  30 . Sampling assembly  60  has a housing  62  and defines a fluid passageway  64  that allows formation fluids to travel therethrough as indicated by arrow  66 . Disposed within housing  62  is a sampling device  68  that includes a sampling chamber  70 . A sampling port  72  is in communication with sampling chamber  70  and fluid passageway  64 . Also disposed within housing  62  is temperature recorder  74 . Temperature recorder  74  includes a temperature sensor  76  that is in fluid communication with fluid passageway  64 . In this embodiment, temperature recorder  74  records fluctuations in the temperature of the formation fluids flowing through fluid passageway  64 . As explained above, when a sample is collected in sampling chamber  70 , if the temperature profile remains relatively constant, this indicates that no significant phase change has occurred. If, on the other hand, a significant temperature fluctuation is recorded by temperature recorder  46 , this indicates that flashing has occurred and that the fluid in the sample may have undergone phase change degradation. 
     Referring next to FIGS. 4A-4C, therein is depicted a sampling device  78  suitable for use with sampling assembly  40  of FIG. 2 or sampling assembly  60  of FIG.  3 . Sampling device  78  has a housing  80  that defines a flow passageway  82  and a passage  84 . Passage  84  includes a transverse portion  86 . A check valve  88  such as a ball check valve, is disposed in fluid passageway  82 . Housing  80  defines an off-center longitudinal bore  90  therein which intersects transverse passage portion  86  and thus is in communication with passageway  82 . An isolation valve  92 , such as a sliding isolation valve, is disposed in bore  90 . An enlarged upper portion  94  of isolation valve  92  carries a seal  96  thereon. Seal  96  seals on opposite sides of horizontal portion  86  of passage  84  when isolation valve  92  is in the initial position shown in FIG. 4A. A smaller diameter lower portion  98  of isolation valve  92  extends downwardly from upper portion  94 . 
     Housing  80  defines a first bore  100 , a smaller second bore  102  and a third bore  104  therein which is larger than second bore  102 . A plunger  106  is disposed in housing  80  and has an enlarged upper end  108  slidably disposed within first bore  100  of housing  80  and a smaller lower end  110  slidably disposed in second bore  102 . It will be seen that an annular area differential is defined between enlarged upper end  108  and smaller lower end  110  of plunger  106 . Plunger  106  defines a longitudinally extending opening  112  therethrough. A seal  114  provides sealing engagement between upper end  108  of plunger  106  and first bore  100 , and similarly, another seal  116  provides sealing engagement between lower end  110  and second bore  102 . A floating piston  118  is disposed in third bore  104  of housing  80  and is initially spaced below plunger  106 . Sealing is provided between floating piston  118  and third bore  104  by seal  120 . 
     Disposed below third bore  104  is a flow restrictor  122  having a flow restriction port  124  that is sized sufficiently small to restrict fluid flow therethrough. Flow restriction port  124  may also be referred to as orifice  124 . Other flow restriction devices, such as removable orifices may also be used. Flow restrictor  122  is used for impeding fluid flow therethrough, as will be further described herein. 
     A control valve  126  is disposed in housing  80  for initially isolating the lower portion of housing  80  from the upper portion of housing  80  and for placing the lower portion of housing  80  in communication with the upper portion of housing  80  when activated. Control valve  126  may be actuated with an annulus pressure responsive activator. Other types of activators, however, such as an electronically controlled solenoid valve, or other means for opening a port known in the art may be used. 
     Below control valve  126 , housing  80  defines fourth bore  128  and fifth bore  130 . Fifth bore  130  may also be referred to as a sampling port  130 . A floating piston  132  is disposed within fourth bore  128 . A seal is provided therebetween by seal  134 . A check valve  136  is disposed in sampling port  130  for allowing fluid flow therethrough into housing  80  while preventing fluid flow from housing  80  outwardly through sampling port  130 . 
     An air cavity  138  is defined within first bore  100 , second bore  102  and third bore  104  above floating piston  118 . Air cavity  138  is initially filled with atmospheric air. Opening  112  through plunger  106  insures that pressure is equalized within air cavity  138 . 
     An upper hydraulic fluid chamber  140  is defined in housing  80  between floating piston  118  and control valve  126 . Thus, floating piston  118  is in communication with upper hydraulic fluid chamber  140  and air chamber  138 , and floating piston  118  separates upper hydraulic fluid chamber  140  from air chamber  138 . 
     A lower hydraulic fluid chamber  142  is defined in housing  80  below control valve  126  and above floating piston  132 . Upper and lower hydraulic fluid chambers  140  and  142  are filled with low pressure hydraulic fluid when sample device  78  is assembled. A sampling chamber  144  is defined between floating piston  132  and check valve  136 . Sampling chamber  144  enlarges to receive a fluid sample by movement of floating piston  132 . Extending partially into sampling chamber  144  is temperature sensor  146  that is used to monitor the temperature of the fluid sample within sampling chamber  144  during collection of formation fluids and the retrieval of sampling device  78  from the wellbore as explained above. 
     In operation, once sampling device  78  is positioned within the wellbore proximate formation  14  of FIG. 1, control valve  126  may be activated. Tubing pressure may be communicated through open check valve  136  and sampling port  130  to sampling chamber  144 . This pressure is communicated through floating piston  132  and thereby communicated to the hydraulic fluid in lower hydraulic fluid chamber  142 . 
     As previously stated, orifice  124  acts as a flow restrictor for impeding fluid flow from lower hydraulic fluid chamber  142  into upper hydraulic fluid chamber  140 . That is, this flow restrictor allows higher pressure hydraulic fluid in lower hydraulic fluid chamber  142  to bleed slowly across the fluid restriction into upper hydraulic fluid chamber  140 . 
     As floating piston  132  moves inside fourth bore  128 , sampling chamber  144  is enlarged. As floating piston  132  moves upwardly, the hydraulic fluid in lower hydraulic fluid chamber  142  above floating piston  132  will continue to flow into upper hydraulic fluid chamber  140 . This causes floating piston  118  in third bore  104  to be moved upwardly until it engages lower end  110  of plunger  106 , as seen in FIG.  4 B. As plunger  106  moves upwardly, plunger  106  engages isolation valve  92  placing isolation valve  92  in the open position shown in FIG.  4 C. 
     When isolation valve  92  is in this open position, outside hydrostatic pressure is allowed to flow into air chamber  138  through passageway  82 . This hydrostatic fluid pressure acts against the area differential defined between enlarged upper end  108  and lower end  110  of plunger  106  and forces plunger  106 , and thus floating piston  118 , downwardly. The downward movement causes some reverse fluid flow and increased pressure in upper and lower hydraulic fluid chambers  142  and  144  and therefore in sampling chamber  144 . This causes check valve  136  to be moved to the closed position. 
     It will be seen by those skilled in the art that the hydraulic fluid and the fluid sample are thus pressurized to a pressure above the well hydrostatic pressure. Check valve  88  in passageway  82  will close and trap the hydrostatic pressure inside housing  80  which continues to act downwardly on plunger  106 . Sampling device  78  may then be retrieved with the fluid sample contained in sampling chamber  144  at a pressure above the well hydrostatic pressure. 
     The slow movement of fluid from lower hydraulic fluid chamber  142  to upper hydraulic fluid chamber  140  through orifice  124  allows the fluid sample to flow slowly into sampling chamber  144 , thereby preventing fluid flashing. Keeping the fluid sample at a pressure above hydrostatic pressure greatly reduces or eliminates phase change degradation of the sample as sampling device  78  is removed from the wellbore. 
     During the entire collection and retrieval process, temperature sensor  146  monitors the temperature of the sample in sampling chamber  144 . As explained above, these temperature measurements may be recorded with a temperature recorded such as temperature recorder  46  of FIG.  2 . After sampling device  78  is removed from the wellbore, the temperature profile from the temperature recorder may be analyzed to verify that the sample is monophasic. If significant temperature variations have occurred in the sample, resampling may be required to obtain a sample that is more representative of the fluids as they exist in formation  14 . 
     Referring now to FIG. 5, therein is depicted another embodiment of a sampling assembly that is generally designated  150 . Sampling assembly  150  may typically be lowered into the wellbore as part of a pipe string such as drill string  30 . Sampling assembly  150  has a housing  152  and defines a fluid passageway  154  that allows formation fluids to travel therethrough. Disposed within housing  152  is a sampling device  156 . Sampling device  156  includes a sampling chamber  158  and a temperature recorder  160 . A sampling port  162  is in communication with sampling chamber  158  and fluid passageway  154 . Temperature recorder  160  is operably coupled to a temperature sensor  164  that monitors the temperature of fluids within sampling chamber  158 . As explained above, when a sample is collected in sampling chamber  158 , if the temperature profile remains relatively constant, this indicates that no significant phase change has occurred. If, on the other hand, a significant temperature fluctuation is recorded by temperature recorder  160 , this indicates that flashing has occurred and that the fluid in the sample may have undergone phase change degradation. 
     While this invention has been described with a reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is, therefore, intended that the appended claims encompass any such modifications or embodiments.