Patent Publication Number: US-6658930-B2

Title: Metal pad for downhole formation testing

Description:
FIELD OF THE INVENTION 
     This invention relates to downhole tools used to acquire and test a sample of fluid from a formation. More particularly, this invention relates to a sealing arrangement that creates a seal between a sample probe and a formation in order to isolate the probe from wellbore fluids. 
     BACKGROUND OF THE INVENTION 
     Formation testing tools are used to acquire a sample of fluid from a subterranean formation. This sample of fluid can then be analyzed to determine important information regarding the formation and the formation fluid contained within, such as pressure, permeability, and composition. The acquisition of accurate data from the wellbore is critical to the optimization of hydrocarbon wells. This wellbore data can be used to determine the location and quality of hydrocarbon reserves, whether the reserves can be produced through the wellbore, and for well control during drilling operations. 
     Formation testing tools may be used in conjunction with wireline logging operations or as a component of a logging-while-drilling (LWD) or measurement-while-drilling (MWD) package. In wireline logging operations, the drill string is removed from the wellbore and measurement tools are lowered into the wellbore using a heavy cable (wireline) that includes wires for providing power and control from the surface. In LWD and MWD operations, the measurement tools are integrated into the drill string and are ordinarily powered by batteries and controlled by either on-board or remote control systems. 
     To understand the mechanics of formation testing, it is important to first understand how hydrocarbons are stored in subterranean formations. Hydrocarbons are not typically located in large underground pools, but are instead found within very small holes, or pores, within certain types of rock. The ability of a formation to allow hydrocarbons to move between the pores, and consequently into a wellbore, is known as permeability. Similarly, the hydrocarbons contained within these formations are usually under pressure and it is important to determine the magnitude of that pressure in order to safely and efficiently produce the well. 
     During drilling operations, a wellbore is typically filled with a drilling fluid (“mud”), such as water, or a water-based or oil-based mud. The density of the drilling fluid can be increased by adding special solids that are suspended in the mud. Increasing the density of the drilling fluid increases the hydrostatic pressure that helps maintain the integrity of the wellbore a and prevents unwanted formation fluids from entering the wellbore. The drilling fluid is continuously circulated during drilling operations. Over time, as some of the liquid portion of the mud flows into the formation, solids in the mud are deposited on the inner wall of the wellbore to form a mudcake. 
     The mudcake acts as a membrane between the wellbore, which is filled with drilling fluid, and the hydrocarbon formation. The mudcake also limits the migration of drilling fluids from the area of high hydrostatic pressure in the wellbore to the relatively low-pressure formation. Mudcakes typically range from about 0.25 to 0.5 inch thick, and polymeric mudcakes are often about 0.1 inch thick. The thickness of a mudcake is generally dependent on the time the borehole is exposed to drilling fluid. Thus, in MWD and LWD applications, where a section of the borehole may be very recently drilled, the mudcake may be thinner than in wireline applications. 
     The structure and operation of a generic formation tester are best explained by referring to FIG.  1 . In a typical formation testing operation, a formation tester  100  is lowered to a desired depth within a wellbore  102 . The wellbore  102  is filled with mud  104 , and the wall of wellbore  102  is coated with a mudcake  106 . Once formation tester  100  is at the desired depth, it is set in place by extending a pair of feet  108  and an isolation pad  110  to engage the mudcake  106 . Isolation pad  110  seals against mudcake  106  and around hollow probe  112 , which places internal cavity  119  in fluid communication with formation  122 . This creates a fluid pathway that allows formation fluid to flow between formation  122  and formation tester  100  while isolated from wellbore fluid  104 . 
     In order to acquire a useful sample, probe  112  must stay isolated from the relative high pressure of wellbore fluid  104 . Therefore, the integrity of the seal that is formed by isolation pad  110  is critical to the performance of the tool. If wellbore fluid  104  is allowed to leak into the collected formation fluids, an non-representative sample will be obtained and the test will have to be repeated. 
     Isolation pads that are used with wireline formation testers are generally simple rubber pads affixed to the end of the extending sample probe. The rubber is normally affixed to a metallic plate that provides support to the rubber as well as a connection to the probe. These rubber pads are often molded to fit with the specific diameter hole in which they will be operating. These types of isolator pads are commonly molded to have a contacting surface that is cylindrical or spherical. 
     While conventional rubber pads are reasonably effective in some wireline operations, when a formation tester is used in a MWD or LWD application, they have not performed as desired. Failure of conventional rubber pads has also been a concern in wireline applications that may require the performance of a large number of formation pressure tests during a single run into the wellbore, especially in wells having particularly harsh operating conditions. In a MWD or LWD environment, the formation tester is integrated into the drill string and is thus subjected to the harsh downhole environment for a much longer period than in a wireline testing application. In addition, during drilling, the formation tester is constantly rotated with the drill string and may contact the side of the wellbore and damage any exposed isolator pads. The pads may also be damaged during drilling by the drill cuttings that are being circulated through the wellbore by the drilling fluid. 
     Therefore, there remains a need in the art to develop an isolation pad that provides reliable sealing performance with an increased durability and resistance to damage. Therefore, the present invention is directed to methods and apparatus for isolator pad assemblies that effectively seal against a wellbore and are resistant to damage typically incurred during drilling operations. It is also an object of the present invention to provide an isolator pad assembly that has an extended life so as to enhance the number of tests that can be performed without replacing the pad. 
     SUMMARY OF THE PREFERRED EMBODIMENTS 
     Accordingly, there are provided herein methods and apparatus for isolator pad assemblies that comprise a primarily metallic pad member and a retractable resilient sealing member. The resilient sealing member is maintained in a retracted, protected position until extended to seal against the wellbore. Once extended to a sealing position, the resilient sealing member acts as a primary seal while the metallic pad member acts as a secondary seal. 
     One embodiment of a preferred isolator pad comprises a cylindrical outer sleeve that is sealingly engaged with a tool body and is capable of lateral translation in respect to the tool body. Affixed to the extending end of the outer sleeve is a metallic pad that has a contacting surface that is curved and preferably has a raised lip surrounding a penetration through the pad. An inner sleeve is slidingly engaged within the penetration through the pad and has a resilient ring molded to one end. The inner sleeve has an extended position wherein the resilient ring extends past the outer surface of the pad and a retracted position where the resilient ring does not extend past the surface of the pad. 
     Once the formation testing tool reaches the desired location in the wellbore, the tool is activated and the outer sleeve extended. The metallic pad engages the mudcake on the wellbore and compresses the mudcake until the raised lip contacts the formation. Once the outer sleeve and pad are extended, the inner sleeve extends so that the resilient ring contacts the mudcake. The contact between the resilient ring and the mudcake forms a primary seal to prevent wellbore fluids from entering the inner sleeve during a formation test. A secondary seal is formed by the metallic pad compressing the mudcake. 
    
    
     Thus, the present invention comprises a combination of features and advantages that enable it to reliably isolate a formation testing probe from wellbore fluids and protect the sealing arrangement from damage during the drilling process. These and various other characteristics and advantages of the present invention will be readily apparent to those skilled in the art upon reading the following detailed description of the preferred embodiments of the invention and by referring to the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more detailed understanding of the preferred embodiments, reference is made to the accompanying Figures, wherein: 
     FIG. 1 is a schematic representation of a prior art formation testing tool; 
     FIG. 2 is section view of one embodiment of an isolator probe assembly in a retracted position; and 
     FIG. 3 is a section view of the embodiment of FIG. 2 shown in an extended position. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the description that follows, like parts are marked throughout the specification and drawings with the same reference numerals, respectively. The drawing figures are not necessarily to scale. Certain features of the invention may be shown exaggerated in scale or in somewhat schematic form and some details of conventional elements may not be shown in the interest of clarity and conciseness. In the following description, an extended position is taken to mean toward the wall of the wellbore and a retracted position is toward the center of the wellbore. Likewise, in some instances, the terms “proximal” and “proximally” refer to relative positioning toward the center of the wellbore, and the terms “distal” and “distally” refer to relative positioning toward the wall of the wellbore. 
     The present invention relates to methods and apparatus for seals that isolate a sample probe of a formation testing tool from wellbore fluids. The present invention is susceptible to embodiments of different forms. There are shown in the drawings, and herein will be described in detail, specific embodiments of the present invention with the understanding that the present disclosure is to be considered an exemplification of the principles of the invention, and is not intended to limit the invention to that illustrated and described herein. In particular, various embodiments of the present invention provide for isolator pad assemblies especially suited for use in MWD or LWD applications but these assemblies may also be used in wireline logging or other applications. Reference is made to using the embodiments of the present invention with a formation testing tool, but the concepts of the invention may also find use in any tool that seeks to acquire a sample of formation fluid that is substantially free of wellbore fluid. It is to be fully recognized that the different teachings of the embodiments discussed below may be employed separately or in any suitable combination to produce desired results. 
     Referring now to FIG. 2, a cross-sectional view of one embodiment of an isolator probe assembly  10  is shown in a retracted position and housed a tool body  12 . Assembly  10  generally comprises an outer sleeve  14 , a pad member  16 , an inner sleeve  18 , and a bridging tube  19 . Inner sleeve  18  is also known as a snorkel and includes filter  17 . Assembly  10  and tool body  12  are shown disposed in a wellbore  20  drilled into a formation  22 . The wall of wellbore  20  is coated with a mudcake  24  that is formed by the circulation of wellbore fluid  26  through the wellbore. 
     Tool body  12  has a substantially cylindrical body that is typical of tools used in downhole environments. Body  12  includes a hydraulic conduit  28  and a sample conduit  30  therethrough. Sample conduit  30  is in fluid communication with a drawdown chamber (not shown) whose volume can be varied by actuating one or more draw-down pistons (not shown), such as are known in the art. In this manner, the pressure in sample conduit  30  can be selectively controlled. Likewise, hydraulic conduit  28  is in fluid communication with a hydraulic power supply (not shown) that supplies hydraulic fluid to conduit  28 . 
     Outer sleeve  14  of assembly  10  is a generally cylindrical and is disposed within a corresponding cavity in body  12 . The outer surface of outer sleeve  14  includes a reduced diameter portion  13  extending toward the tool axis from a main portion  15 . A shoulder  17  is defined between reduced diameter portion  13  and main portion  15 . The outer surfaces of reduced diameter portion  13  and main portion  15  are in sealing engagement with the inner surface of the cavity in the tool body. Outer sleeve  14  is sealed to and slidable relative to tool body  12 . 
     Outer sleeve  14  includes an axial central bore  32  therethrough. Central bore  32  includes a reduced diameter portion  33  within reduced diameter portion  13 , an intermediate diameter portion  35 , and a large diameter portion  37 . Intermediate diameter portion  35  and large diameter portion  37  of bore  32  are within main portion  15  of outer sleeve  14 . A proximal shoulder  31  is defined between reduced diameter portion  13  and intermediate diameter portion  35  and an intermediate shoulder  39  is defined between intermediate diameter portion  35  and large diameter portion  37 . Central bore  32  is in fluid communication with sample conduit  30 . A conduit  54  provides fluid communication between shoulder  17  on the outer surface of sleeve  14  and intermediate shoulder  39  in bore  32 . 
     Pad  16  is preferably generally disc-shaped, with a substantially flat trailing side  42  and a cylindrically or spherically curved contact surface  44 . The diameter of pad  16  is preferably greater than the largest diameter of outer sleeve  14 . If desired, a recess  11  in tool body  12  is sized and configured to receive pad  16  so that no portion of assembly  10  extends beyond the outer surface of the tool body  12  when the assembly  10  is in its retracted position. 
     An annular stop member  36  extends from trailing side  42 , away from the borehole wall. Annular stop member  36  defines a central bore  40 , which has a uniform diameter along its length and which extends through pad  16 . Stop member  36  is preferably affixed to the inner surface of large diameter portion  37  of bore  32  in outer sleeve  14  by means of threads  34  or other suitable device. A seal  65  is provided between stop member  36  and the inner surface of bore  32 . 
     Pad  16  preferably includes a raised lip or boss  48  that extends outward from contact surface around the circumference of bore  40 . Lip  48  preferably has a curved leading edge. Pad  16  is preferably constructed of a stainless steel or other corrosion resistant metal. 
     Inner sleeve  18  is a generally cylindrical body having a bore  21  therethrough. Near the proximal end of sleeve  18 , the outer surface of sleeve  18  includes an enlarged diameter portion  23  forming a shoulder  25  and the inner surface of bore  21  includes a reduced diameter portion  27  forming a shoulder  29 . Inner sleeve  18  also preferably includes filter  17  that serves to prevent large pieces of mudcake from entering bridging tube  19 . 
     A resilient ring  46  is molded to the distal end of inner sleeve  18 . Resilient ring  46  preferably has a radiused leading edge and is preferably molded to sleeve  18  such that only the base  47  of ring  46  is affixed to inner sleeve  18 . Resilient ring  46  is preferably constructed from a resilient material such as rubber or a resilient polymer. 
     Inner sleeve  18  is received in bore  32  of outer sleeve  14  and is slidable therein. When the assembly  10  is in its retracted position, the proximal end of inner sleeve  18  bears on intermediate shoulder  39 . The distal end of sleeve  18  extends into annular stop member  36  of pad  16  and is in slidable, sealing engagement with the inner surface of bore  40 . Seal  67  prevents fluid flow along the interface between sleeve  18  and the inner surface of bore  40 . 
     Bore  21  of inner sleeve  18  receives bridging tube  19 . Bridging tube  19  is preferably cylindrical, with its outer diameter corresponding to the inner diameter of reduced diameter portion  27  of bore  21 . Bridging tube  19  is in slidable, sealing engagement with bore  21  of inner sleeve  18  and intermediate diameter portion  35  of bore  32  in outer sleeve  14 . Bridging tube  19  includes a fluid conduit  41  that provides fluid communication between bore  32  and bore  21 . Conduit  41  preferably communicates with bore  32  via an axial opening  43  and with bore  21  via one or more lateral openings  45  at the distal end of tube  19 . When assembly  10  is in its retracted position, as shown in FIG. 2, bridging tube  19  preferably extends almost to the distal edge of probe assembly  10  and filter  19  in order to prevent debris from collecting in the assembly. Bridging tube  19  may also be keyed to prevent rotation relative to inner sleeve  18  or outer sleeve  14 . 
     Referring now to FIG. 3, probe assembly  10  is extended by applying fluid pressure through hydraulic conduit  28  so that hydraulic pressure is applied between outer sleeve  14  and body  12 . The pressure advances outer sleeve  14  pad  16  toward the wall of the wellbore. A hydraulic chamber  52  is defined between tool body  12  and outer sleeve  14  and between seals  62  and  64 . Outer sleeve  14  and inner sleeve  18  are preferably arranged so that outer sleeve  14  extends before inner sleeve  18  extends. This may be achieved by arranged the respective pressure areas and adjusting the sliding friction relationships of sleeves  14 ,  18  so that it takes a greater fluid pressure to move inner sleeve  18  than the pressure required to move outer sleeve  14 . 
     Thus, pad  16  is advanced through the mudcake  24  until raised lip  48  contacts the formation  22 . Contact surface  44  of pad  16  compresses mudcake  24  against formation  22 , forming a region  58  of mudcake that has very low permeability, thus forming a secondary seal. It is preferred that mudcake  24  be present on the wellbore wall to provide a compressible material that can form a seal with pad  16 . Contact surface  44  of pad  16  may be smooth or rough. 
     As additional hydraulic fluid is pumped into hydraulic chamber  52  and through port  54  into large diameter portion  37  of bore  32 , pressure increases behind inner sleeve  18 , advancing it toward formation  22 . A second hydraulic chamber  56  is defined between outer sleeve  14 , inner sleeve  18 , and bridging tube  19 , and between seals  61 ,  63 ,  65  and  67 . Inner sleeve  18  advances until resilient ring  46  is compressed against formation  22  and forms a primary seal. Bridging tube  19  preferably maintains a position that does not allow fluid flow into assembly  10  but is retracted to allow fluid to flow through filter  17  as the pressure within conduit  30  decreases. 
     In this manner, the combination of the primary seal created by resilient ring  46  and the secondary seal created by pad  16  hydraulically isolates the interior  60  of probe assembly  10  from wellbore fluid  26 . Once the assembly  10  is in its extended position, a sample of formation fluid can be acquired by decreasing the pressure within sample conduit  30 , which will allow fluid from formation  22  to flow through mudcake  24 , into bore  21 , through filter  17 , into bridging tube  14 , and thus into sample conduit  30 . Once a suitable sample has been collected, probe assembly  10  can be returned to the retracted position by reducing the pressure within hydraulic conduit  28 . Assembly  10  is preferably retractable by applying positive fluid pressure but may also be retracted using only hydrostatic pressure from the well. 
     Therefore, the above described extendable probe assembly provides a sealing pad that is protected from damage during the drilling process and can to take a plurality of samples during a single trip into the wellbore. The use of both primary and secondary sealing mechanisms also increases the reliability of the sealing system. 
     The embodiments set forth herein are merely illustrative and do not limit the scope of the invention or the details therein. It will be appreciated that many other modifications and improvements to the disclosure herein may be made without departing from the scope of the invention or the inventive concepts herein disclosed. Because many varying and different embodiments may be made within the scope of the inventive concept herein taught, including equivalent structures or materials hereafter thought of, and because many modifications may be made in the embodiments herein detailed in accordance with the descriptive requirements of the law, it is to be understood that the details herein are to be interpreted as illustrative and not in a limiting sense.