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TECHNICAL FIELD 
       [0001]    The present invention relates to the field of formation testing and formation fluid sampling, and in particular to the determination, within the borehole, of various physical properties of the formation or the reservoir and of the fluids contained therein using a downhole instrument or “tool” comprising a snorkel interface. 
       BACKGROUND ART 
       [0002]    A variety of systems are used in borehole geophysical exploration and production operations to determine chemical and physical parameters of materials in the borehole environs. The borehole environs include materials, such as fluids or formations, near a borehole as well as materials, such as fluids, within the borehole. The various systems include, but are not limited to, formation testers and borehole fluid analysis systems conveyed within the borehole. In all of these systems, it is preferred to make all measurements in real-time and within instrumentation in the borehole. However, methods that collect data and fluids for later retrieval and processing are not precluded. 
         [0003]    Formation tester systems are used in the oil and gas industry primarily to measure pressure and other reservoir parameters of a formation penetrated by a borehole, and to collect and analyze fluids from the borehole environs to determine major constituents within the fluid. Formation testing systems are also used to determine a variety of properties of the formation or reservoir near the borehole. These formation or reservoir properties, combined with in situ or uphole analyses of physical and chemical properties of the formation fluid, can be used to predict and evaluate production prospects of reservoirs penetrated by the borehole. By definition, formation fluid refers to any and all fluid including any mixture of fluids. 
         [0004]    Formation tester tools can be conveyed along the borehole by variety of means including, but not limited to, a single or multi-conductor wireline, a “slick” line, a drill string, a permanent completion string, or a string of coiled tubing. Formation tester tools may be designed for wireline usage or as part of a drill string. Tool response data and information as well as tool operational data can be transferred to and from the surface of the earth using wireline, coiled tubing and drill string telemetry systems. Alternately, tool response data and information can be stored in memory within the tool for subsequent retrieval at the surface of the earth. 
         [0005]    Formation tester tools typically comprise a fluid flow line cooperating with a pump to draw fluid into the formation tester tool for analysis, sampling, and optionally for subsequent exhausting the fluid into the borehole. Typically, a sampling pad is pressed against the wall of the borehole. A probe port or “snorkel” is extended from the center of the pad and through any mudcake to make contact with formation material. The snorkel and pad are designed to isolate the pressure and fluid movement to and from the formation and the wellbore. The best sample to be analyzed and/or taken should be from an undisturbed formation without any wellbore contamination. 
         [0006]    Fluid is drawn into the formation tester tool via a flow line cooperating with the snorkel. Fluid is sampled for subsequent retrieval at the surface of the earth, or alternately exhausted to the borehole via the flow lines and pump systems. 
         [0007]    When performing formation tester probe operations in a wellbore, it is critical to maintain a proper seal against the formation while performing a drawdown/build-up sequence. As significant differential pressures (1,000&#39;s of psi) can be created during this operation, the sampling pad, typically made of an elastomeric material, may extrude between the surface of the wellbore and the interface of the snorkel. Generally, soft pliable rubber is wanted for the pad seal, however, this is more likely to extrude. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0008]    The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate an implementation of apparatus and methods consistent with the present invention and, together with the detailed description, serve to explain advantages and principles consistent with the invention. In the drawings, 
           [0009]      FIG. 1  is a cross-sectional view of a snorkel according to one embodiment. 
           [0010]      FIG. 2  is a detail view of the snorkel of  FIG. 1 . 
           [0011]      FIG. 3  is a cross-sectional view of a snorkel according to the prior art. 
           [0012]      FIG. 4  is a detail view of the snorkel of  FIG. 2 . 
           [0013]      FIG. 5  is another cross-sectional view of the snorkel of  FIG. 1 , orthogonal to the cross-sectional view of  FIG. 1   
           [0014]      FIG. 6  is a detail view of the snorkel of  FIG. 1  in the view of  FIG. 5 . 
           [0015]      FIG. 7  is a cross-sectional view of the prior art snorkel of  FIG. 3 , orthogonal to the cross-sectional view of  FIG. 3 . 
           [0016]      FIG. 8  is a detail view of the snorkel of  FIG. 3  in the view of  FIG. 7 . 
           [0017]      FIG. 9  is an elevation view of a formation tester according to one embodiment. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0018]    In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the invention. It will be apparent, however, to one skilled in the art that the invention may be practiced without these specific details. In other instances, structure and devices are shown in block diagram form in order to avoid obscuring the invention. References to numbers without subscripts or suffixes are understood to reference all instance of subscripts and suffixes corresponding to the referenced number. Moreover, the language used in this disclosure has been principally selected for readability and instructional purposes, and may not have been selected to delineate or circumscribe the inventive subject matter, resort to the claims being necessary to determine such inventive subject matter. Reference in the specification to “one embodiment” or to “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiments is included in at least one embodiment of the invention, and multiple references to “one embodiment” or “an embodiment” should not be understood as necessarily all referring to the same embodiment. 
         [0019]    Wellbores are effectively circular. However, this is not required. The more advanced formation testers have pad and snorkel assemblies that will pivot and tilt so that the tester will provide a better seal to the formation. Conventional (prior art) snorkel designs have a flat surface, so that the edges of the snorkel rest on the curved surface of the wellbore. This leaves a gap between the snorkel and the wellbore that is at a maximum in a plane orthogonal to the initial contact between the snorkel and the wellbore. In various embodiments described below, the interface surface of the snorkel is formed with a cylindrical geometry to minimize the extrusion gap between the snorkel and the wellbore. The snorkel may be configured to prevent rotation of the snorkel, to ensure that the cylindrical geometry is correctly oriented with the wellbore surface. 
         [0020]      FIGS. 1 and 5  are orthogonal cross-section views of a snorkel  110  according to one embodiment.  FIG. 1  is a cross section view along line B-B of  FIG. 5 , while  FIG. 5  is a cross-sectional view along line A-A of  FIG. 1 . For purposes of clarity, only the snorkel  110  of the formation tester tool is illustrated in  FIGS. 1 and 5 . 
         [0021]    As illustrated in  FIG. 1 , the snorkel  110  has been extended from piston cylinder  120  through the pad  116  (shown in phantom) to make contact with surface  102  of the borehole formed in formation  100 . A screen  114  is preferably threaded into the body  118  of the snorkel, to screen cuttings or other solid matter from entering the snorkel  110 . Other common elements of a snorkel, such as a mud plug, are omitted for clarity. Instead of a flat interface surface as in a conventional snorkel, radially outward surface  112  of the snorkel body  118  has been machined or otherwise formed to a cylindrical geometry, with the cylinder oriented parallel to the longitudinal axis of the borehole. The radius of the cylindrical geometry is sized to correspond to the radius of the borehole, so that the curved edge of the snorkel body  118  at the surface  112  matches the curvature of the surface  102  of the borehole. 
         [0022]    As is best illustrated in  FIG. 2 , which is a detail view of the interface surface  112  of the snorkel  110  of  FIG. 1 , the curved interface surface  112  eliminates or minimizes the gap between the borehole surface  102  and the interface surface  112 . By minimizing the gap, the potential for the pad  116  to extrude through that gap into the interior of the snorkel  110  is also minimized. The wellbore is not required to be perfectly circular, nor the snorkel&#39;s cylindrical diameter to be exactly the same as the wellbore. If the snorkel  110 &#39;s cylindrical diameter does not exactly match that of the wellbore, even though a gap would exist between the curved interface surface  112  and the borehole surface  102 , the gap would be smaller than that produced by a flat interface surface. 
         [0023]    The pad  116  is typically designed with a cylindrical surface made of an elastomeric material such as a rubber. In one embodiment, the pad  116  includes a structural support element  210  to reduce the rubber extrusion. The support element  210  may also have a cylindrical geometry similar to that of the snorkel  110 . 
         [0024]    The snorkel  110  is configured to make contact with the surface  102  in a desired rotational orientation. Conventional snorkels are allowed to rotate. If the snorkel  110  were to rotate so that the cylindrical geometry of the interface surface  112  was oriented orthogonal to the longitudinal axis of the borehole, instead of parallel to the longitudinal axis of the borehole, rather than minimizing the gap between the snorkel  110  and the borehole surface  102 , the cylindrical geometry would increase the gap over that caused by the flat interface surface of a conventional snorkel. Therefore, in one embodiment, the body  118  of the snorkel may be keyed, allowing insertion of an anti-rotation pin  130  to prevent rotation of the snorkel body  118  relative to the piston cylinder  120  as the snorkel  110  extends or retracts, thus ensuring the desired orientation of the snorkel  110  relative to the borehole. The configuration and placement of the anti-rotation pin  130  of  FIG. 1  is illustrative and by way of example only. The anti-rotation pin  130  may be placed in any desired location. Other techniques for preventing rotation of the snorkel  110  relative to the borehole may be used as desired. 
         [0025]    In another embodiment, the snorkel  110  may be formed with an elliptical or other non-circular body  118  to prevent undesired rotation of the snorkel  110  relative to the piston cylinder  120 , and thus to the borehole. 
         [0026]      FIG. 3  is a view of a snorkel  300  according to the prior art that has been extended to make contact with the surface  102  of the borehole formed in formation  100 .  FIG. 3  is oriented in the same orientation as  FIG. 1 . As illustrated in  FIG. 3 , the flat interface surface  320  of the body  310  of the snorkel  300  does not match the curvature of the borehole surface  102 . Thus, as best illustrated in the detail view of  FIG. 4 , the flat surface  320  creates a gap between the flat interface surface  320  and the surface  102  of the borehole, leaving room for extrusion of the surface pad  116  through that opening. The extrusion may damage the pad  116 , the snorkel  300 , or both. 
         [0027]      FIG. 5 , oriented orthogonally to  FIG. 1 , is a cross-sectional view along line A-A of  FIG. 1  that illustrates that the cylindrical geometry machined into the surface  112  of the snorkel  110  avoids a gap between the interface surface  112  and the surface  102  of the borehole. As best illustrated in the detail view of  FIG. 6 , the cylindrical geometry of the interface surface  112  of the snorkel  110  allows the snorkel surface  112  to rest on the surface  102  along the line A-A, preventing extrusion of the pad  116  into the snorkel  110 . 
         [0028]    In contrast, prior art snorkel  300  when viewed along line A-A, as illustrated in  FIG. 7  and in detail view  FIG. 8 , does not contact the surface  102  at any point along line A-A, presenting a gap  800  and into which the pad  116  may extrude. 
         [0029]    By using a cylindrical geometry at the interface surface  112  of a snorkel  110 , a properly oriented snorkel  110  that is configured for the size of the borehole, extrusion of the sample pad between the borehole surface  102  and the snorkel interface surface  112  can be minimized or eliminated. Using an internal support element  210  that also has a cylindrical geometry may further reduce extrusion of the pad  116 . 
         [0030]      FIG. 9  illustrates conceptually the major elements of an embodiment of a formation tester system  900  that employs one or more snorkel&#39;s  110  as described above, operating in a well borehole  928  that penetrates earth formation  100 . 
         [0031]    The formation tester borehole instrument or tool  910  comprises a plurality of operationally connected sections including a packer section  911 , a probe or port section  912 , an auxiliary measurement section  914 , a fluid analysis section  916 , a sample carrier section  918 , a pump section  920 , a hydraulics section  924 , an electronics section  922 , and a downhole telemetry section  925 . Two fluid flow lines  950  and  952  are illustrated conceptually with broken lines and extend contiguously through the packer, probe or port tool, auxiliary measurement, fluid analysis, sample carrier, and pump sections  911 ,  912 ,  914 ,  916 ,  918  and  920 , respectively. Although two fluid flow lines  950  and  952  are illustrated in  FIG. 9 , embodiments of the tool  910  may use one fluid flow line or more than  2  fluid flow lines as desired. 
         [0032]    Fluid is drawn into the tester tool  910  through a snorkel  110  of a probe or port tool section  912 . The probe or port section  912  can comprise one or more snorkels  110  as input ports. Fluid flow into the probe or port section  912  is illustrated conceptually with the arrows  936 . During the borehole drilling operation, the borehole fluid and fluid within or near the borehole formation  100  may be contaminated with drilling fluid typically comprising solids, fluids, and other materials. Drilling fluid contamination of fluid drawn from the formation  100  is typically minimized using one or more probes cooperating with a borehole isolation element such as the pad  116  and the snorkel  110 . One or more snorkels  110  extend from the pad onto the formation  100  as described above. The formation  100  may further be isolated from the borehole  928  by one or more packers controlled by the packer section  911 . A plurality of packers can be configured axially as straddle packers. 
         [0033]    Fluid passes from the probe or port section  912  through one or more flow lines  950  and  952  under the action of the pump section  920 . The pump section  920  cooperating with other elements of the tool  910  allows fluid to be transported within the flow lines  950  and  952  upward or downward through various tool sections. 
         [0034]    An auxiliary fluid measurement may be made using auxiliary measurement section  914 . The auxiliary measurement section  914  typically comprises one or more sensors that measure various physical parameters of the fluid flowing within one or more of the flow lines  950  and  952 . 
         [0035]    The fluid analysis section  916  is typically used to perform fluid analyses on the fluid while the tool  910  is disposed within the borehole  928 . As an example, fluid analyses can comprise the determination of physical and chemical properties of oil, water, and gas constituents of the fluid. 
         [0036]    Fluid is directed via one or more of the flow lines  950  and  952  to the sample carrier section  918 . Fluid samples can be retained within one or more sample containers within the sample carrier section  918  for return to the surface  942  of the earth for additional analysis. The surface  942  is typically the surface of earth formation  100  or the surface of any water covering the earth formation  100 . 
         [0037]    The hydraulic section  924  provides hydraulic power for operating numerous valves and other elements within the tool  910 . The electronics section  922  comprises necessary tool control to operate elements of the tool  910 , motor control to operate motor elements in the tool  910 , power supplies for the various section electronic elements of the tool  910 , power electronics, an optional telemetry for communication over a wireline to the surface, an optional memory for data storage downhole, and a tool processor for control, measurement, and communication to and from the motor control and other tool sections. The individual tool sections may also contain electronics (not shown) for section control and measurement. 
         [0038]    The tool  910  may have a downhole telemetry section  925  for transmitting various data measured within the tool  910  and for receiving commands from surface  942  of the earth. The downhole telemetry section  925  can also receive commands transmitted from the surface  942  of the earth. The upper end of the tool  910  is terminated by a connector  927 . The tool  910  is operationally connected to a conveyance apparatus  930  disposed at the surface  942  by means of a connecting structure  926  that is typically a tubular or a cable. More specifically, the lower or downhole end of the connecting structure  926  is operationally connected to the tool  910  through the connector  927 . The upper or uphole end of the connecting structure  926  is operationally connected to the conveyance apparatus  930 . The connecting structure  926  can function as a data conduit between the tool  910  and equipment disposed at the surface  942 . 
         [0039]    If the tool  910  is a logging tool element of a wireline formation tester system, the connecting structure  926  may comprise a multi-conductor wireline logging cable and the conveyance apparatus  930  may be a wireline draw works assembly comprising a winch. If the tool  910  is a component of a measurement-while-drilling or logging-while-drilling system, the connecting structure  926  may be a drill string and the conveyance apparatus  930  may be a rotary drilling rig. If the tool  910  is an element of a coiled tubing logging system, the connecting structure  926  may be coiled tubing and the conveyance apparatus  930  may be a coiled tubing injector. If the tool  910  is an element of a drill string tester system, the connecting structure  926  may be a drill string and the conveyance apparatus  930  may be a rotary drilling rig. 
         [0040]    Surface equipment  932  is operationally connected to the tool  910  through the conveyance apparatus  930  and the connecting structure  926 . The surface equipment  932  comprises a surface telemetry element (not shown), which communicates with the downhole telemetry section  925 . The connecting structure  926  functions as a data conduit between the downhole and surface telemetry elements. The surface unit  932  typically comprises a surface processor that optionally performs additional processing of data measured by sensors and gauges in the tool  910 . The surface processor also cooperates with a depth measure device (not shown) to track data measured by the tool  910  as a function of depth within the borehole  928  at which it is measured. The surface equipment  932  typically comprises recording means for recording logs of one or more parameters of interest as a function of time and/or depth. 
         [0041]      FIG. 9  is illustrative and by way of example only, and illustrates basic concepts of an embodiment of the system  900  that employs the snorkel  110 . The system  900  may be incorporated in a more general downhole fluid analysis device. The various sections of the tool  910  may be arranged in different axial configurations, and multiple sections of the same type may be added or removed as desired for specific borehole operations. Some tools  910  may omit one or more of the sections described above as desired. 
         [0042]    It is to be understood that the above description is intended to be illustrative, and not restrictive. For example, the above-described embodiments may be used in combination with each other. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention therefore should be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. In the appended claims, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.”

Summary:
A snorkel and pad for use with a formation testing tool is formed with a cylindrical geometry at the interface where the snorkel and pad contacts the inner surface of a borehole. The cylindrical geometry reduces or eliminates gaps that a flat interface surface would leave between the snorkel and the inner surface of the borehole, reducing the possibility that a surrounding pad could extrude through the gap. The snorkel is prevented from rotating during operation, ensuring the correct orientation of the cylindrical geometry interface surface relative to the inner surface of the borehole. The snorkel may be used as part of a formation testing system.