Patent Publication Number: US-7584655-B2

Title: Formation tester tool seal pad

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not applicable. 
   BACKGROUND 
   During the drilling and completion of oil and gas wells, it may be necessary to engage in ancillary operations, such as monitoring the operability of equipment used during the drilling process or evaluating the production capabilities of formations intersected by the wellbore. For example, after a well or well interval has been drilled, zones of interest are often tested to determine various formation properties such as permeability, fluid type, fluid quality, formation temperature, formation pressure, bubblepoint and formation pressure gradient. These tests are performed in order to determine whether commercial exploitation of the intersected formations is viable and how to optimize production. 
   Wireline formation testers (WFT) and drill stem testing (DST) have been commonly used to perform these tests. The basic DST test tool consists of a packer or packers, valves or ports that may be opened and closed from the surface, and two or more pressure-recording devices. The tool is lowered on a work string to the zone to be tested. The packer or packers are set, and drilling fluid is evacuated to isolate the zone from the drilling fluid column. The valves or ports are then opened to allow flow from the formation to the tool for testing while the recorders chart static pressures. A sampling chamber traps clean formation fluids at the end of the test. WFTs generally employ the same testing techniques but use a wireline to lower the test tool into the well bore after the drill string has been retrieved from the well bore, although WFT technology is sometimes deployed on a pipe string. The wireline tool typically uses one or more packers also, although the packer/packers are placed closer together, compared to drill pipe conveyed testers, for more efficient formation testing. In some cases, packers are not used. In those instances, the testing tool is brought into contact with the intersected formation and testing is done without zonal isolation. 
   WFTs may also include a probe assembly for engaging the borehole wall and acquiring formation fluid samples. The probe assembly may include an isolation pad to engage the borehole wall. The isolation pad seals against the formation and around a hollow probe, which places an internal cavity in fluid communication with the formation. This creates a fluid pathway that allows formation fluid to flow between the formation and the formation tester while isolated from the borehole fluid. 
   Another testing apparatus is a measurement while drilling (MWD) or logging while drilling (LWD) tester. Typical LWD/MWD formation testing equipment is suitable for integration with a drill string during drilling operations. Various devices or systems are provided for isolating a formation from the remainder of the wellbore, drawing fluid from the formation, and measuring physical properties of the fluid and the formation. With LWD/MWD testers, the testing equipment is subject to harsh conditions in the wellbore during the drilling process that can damage and degrade the formation testing equipment before and during the testing process. These harsh conditions include vibration and torque from the drill bit, exposure to drilling mud, drilled cuttings, and formation fluids, hydraulic forces of the circulating drilling mud, and scraping of the formation testing equipment against the sides of the wellbore. Sensitive electronics and sensors must be robust enough to withstand the pressures and temperatures, and especially the extreme vibration and shock conditions of the drilling environment, yet maintain accuracy, repeatability, and reliability. 
   In order to acquire a useful sample, the probe must stay isolated from the relative high pressure of the borehole fluid. Therefore, the integrity of the seal that is formed by the seal pad is important to the performance of the tool. If the borehole fluid is allowed to leak into the collected formation fluids, a non-representative sample will be obtained and the test will have to be repeated. The reliability and ability for seal pads or isolation probes to seal and isolate becomes increasingly more difficult when the borehole temperature rises due to the materials used in the seal pad to form or maintain a seal between the pad and formation or borehole. 
   What is needed is a seal pad designed for the hostile conditions that is able to maintain a seal or isolation in these conditions, and that provides reliable sealing performance with an increased durability and resistance to damage. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more detailed description of preferred embodiments of the present invention, reference will now be made to the accompanying drawings, wherein: 
       FIG. 1  is a schematic view of an embodiment of a system including a formation tester tool disposed in a subterranean well; 
       FIG. 2  is a schematic view of a formation tester tool disposed in a well; 
       FIG. 3  is section view of a probe assembly in a retracted position, in accordance with one embodiment; 
       FIG. 4  is section view of a probe assembly in an extended position, in accordance with one embodiment; 
       FIG. 5A  shows a top cross-section view of a seal pad, in accordance with one embodiment; 
       FIG. 5B  shows a front view of the seal pad of  FIG. 5A ; 
       FIG. 6  shows a front view of a seal pad, in accordance with one embodiment; 
       FIG. 7  shows a front view of a seal pad, in accordance with one embodiment; 
       FIG. 8A  shows a top cross-section view of a seal pad, in accordance with one embodiment; 
       FIG. 8B  shows a front view of the seal pad of  FIG. 8A ; 
       FIG. 9  shows a front view of a seal pad, in accordance with one embodiment; 
       FIG. 10  shows a front view of a seal pad, in accordance with one embodiment; 
       FIG. 11  shows a top cross-section view of a seal pad, in accordance with one embodiment; 
       FIG. 12  shows a front view of the seal pad of  FIG. 11 ; 
       FIG. 13  shows a front view of a seal pad, in accordance with one embodiment; 
       FIG. 14  is a perspective view of a seal pad, in accordance with one embodiment; 
       FIG. 15  is a front view of the seal pad of  FIG. 14 ; 
       FIG. 16  is a top, section view of the seal pad of  FIG. 15 ; 
       FIG. 17  is a side, section view of the seal pad of  FIG. 15 ; 
       FIG. 18  shows a front view of a seal pad, in accordance with one embodiment; 
       FIG. 19  shows a front view of a seal pad, in accordance with one embodiment; 
       FIG. 20  shows a front view of a seal pad, in accordance with one embodiment; 
       FIG. 21  shows a cross-section view of a seal pad, in accordance with one embodiment; 
       FIG. 22  shows a cross-section view of a seal pad, in accordance with one embodiment; and 
       FIG. 23  shows a front view of a seal pad, in accordance with one embodiment; 
   

   DETAILED DESCRIPTION 
   In the following detailed description, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that structural changes may be made without departing from the scope of the present invention. Therefore, the following detailed description is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims and their equivalents. 
   Certain terms are used throughout the following description and claims to refer to particular system components. This document does not intend to distinguish between components that differ in name but not function. 
   In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . ”. Also, the terms “couple,” “couples”, and “coupled” used to describe any mechanical or electrical connections are each intended to mean and refer to either an indirect or a direct mechanical or electrical connection. Thus, for example, if a first device “couples” or is “coupled” to a second device, that interconnection may be through an electrical conductor directly interconnecting the two devices, or through an indirect electrical connection via other devices, conductors and connections. Further, reference to “up” or “down” are made for purposes of ease of description with “up” meaning towards the surface of the borehole and “down” meaning towards the bottom or distal end of the borehole. In addition, in the discussion and claims that follow, it may be sometimes stated that certain components or elements are in fluid communication. By this it is meant that the components are constructed and interrelated such that a fluid could be communicated between them, as via a passageway, tube, or conduit. Also, the designation “MWD” or “LWD” are used to mean all generic measurement while drilling or logging while drilling apparatus and systems. 
   In the drawings and 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. The present invention is susceptible to embodiments of different forms. Specific embodiments are described in detail and are shown in the drawings, 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. 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. The various characteristics mentioned above, as well as other features and characteristics described in more detail below, will be readily apparent to those skilled in the art upon reading the following detailed description of the embodiments, and by referring to the accompanying drawings. 
     FIG. 1  illustrates a system  100  for drilling operations. The system  100  includes a drilling rig  102  located at a surface  104  of a well. The drilling rig  102  provides support for a drill string  105 . The drill string  105  penetrates a rotary table for drilling a borehole  8  through subsurface formations  109 . A downhole tool  113  may be any of a number of different types of tools including measurement-while-drilling (“MWD”) tools, logging-while-drilling (“LWD”) tools, etc. It should be noted the system  100  can be used with a wireline tool as well. 
   The downhole tool  113  includes, in various embodiments, one or a number of different downhole sensors, which monitor different downhole parameters and generate data that is stored within one or more different storage mediums within the downhole tool  113 . The downhole tool  113  can include a power source, such as a battery or generator. A generator could be powered either hydraulically or by the rotary power of the drill string. The generator could also be on the surface and the power supplied through conductor or conductors in a wireline or drillpipe. 
   The downhole tool  113  includes a downhole sampling device such as a formation tester tool  10 , which can be powered by the power source. In one embodiment, the formation tester tool  10  may be mounted on a drill collar or wireline deployed. Thus, even though formation tester  10  is shown as part of drill string  105 , the embodiments of the invention described below may be conveyed down borehole  8  via any drill string or wireline technology, as is partially described above and is well known to one skilled in the art. 
     FIG. 2  schematically illustrates the formation tester tool  10  in position to retrieve subterranean formation fluid from the borehole  8 , in accordance with one embodiment. The formation tester tool  10  includes a probe  162  and a seal pad  163  that contacts the wall  112  of the borehole  8  through mud cake  24  isolating the borehole and seals out mud flowing in the bore. In one option, the probe  162  includes a snorkel that extends into the formation to obtain formation fluid. The snorkel is, in an embodiment, is fluidly connected to a main sampling flowline  164 . The formation tester tool  10  optionally further includes one or more extendible backup pistons  130 . 
     FIGS. 3 and 4  show a schematic representation of a probe assembly  50  for formation tester tool  10 , in accordance with one embodiment.  FIG. 3  is side, section view of the probe assembly  50  in a retracted position, and  FIG. 4  is top, section view of the probe assembly  50  in an extended position. Also, in  FIG. 4  formation tester tool  10  is shown disposed in a borehole  8  drilled into a formation. The wall  112  of borehole  8  is coated with mud cake  24  that is formed by the circulation of wellbore fluid through the wellbore. 
   Formation tester tool  10  has a substantially cylindrical body that is typical of tools used in downhole environments. Formation tester tool  10  includes hydraulic conduits and sample conduits therethrough. For example, a sample conduit can be in fluid communication with a drawdown chamber whose volume can be varied by actuating one or more draw-down pistons, such as are known in the art. 
   Formation probe assembly  50  generally includes stem a  92 , a piston chamber  94 , a piston  96  adapted to reciprocate within piston chamber  94 , a snorkel  98  adapted for reciprocal movement within piston  96 , and a seal pad  180  located at an end of piston  96 . Snorkel  98  includes a central passageway  127 . Formation probe assembly  50  is configured such that piston  96  extends and retracts through aperture  52  of the formation tester tool  10 . Stem  92  includes a tubular extension  107  having central passageway  108 . Central passageway  108  is in fluid connection with fluid passageways leading to other portions of tester tool  10 , including a drawn down assembly, for example. Thus, a fluid passageway is formed from the formation through snorkel passageway  127  and central passageway  108  to the other parts of the tool. 
   Formation probe assembly  50  is assembled such that piston  96  includes shoulders  97  to allow hydraulic pressure to be used to extend and retract the piston. In use, snorkel  98  further extends into the formation wall to communicate with the formation fluid. Probe assembly  50  is extended by applying fluid pressure through hydraulic conduits so that hydraulic pressure is applied to shoulder  97 . The pressure advances piston  96  and seal pad  180  toward the wall of the wellbore. 
   Seal pad  180  seals and prevents drilling fluid or other contaminants from entering the probe assembly  50  during formation testing. Typically, the pressure of the formation fluid is less than the pressure of the drilling fluids that are injected into the borehole. A layer of residue from the drilling fluid forms mud cake  24  on the borehole wall and separates the two pressure areas. Pad  180 , when extended, contacts the borehole wall and, together with the mud cake, forms a seal. 
   In order to acquire a useful sample, probe assembly  50  should stay isolated from the relative high pressure of wellbore fluid. Therefore, the integrity of the seal that is formed by seal pad  180  is important to the performance of the tool. If wellbore fluid is allowed to leak into the collected formation fluids, a non-representative sample will be obtained and the test will have to be repeated. 
     FIGS. 5A and 5B  show a seal pad  230 , in accordance with one embodiment. Seal pad  230  includes a plate or fixture  233  suitable to be attached to the testing tools discussed above and represented by pad  163  in  FIG. 2  or seal pad  180  of  FIGS. 3 and 4 . Seal pad  230  generally includes a first outer sealing element  234  and a second inner sealing element  236  arranged in concentric manner on plate  233  such that a space  235  is formed therebetween. Seal pad  230  includes a port  240  for formation fluid to enter the testing tool assembly. When the seal pad  230  is set against the formation wall  112  ( FIG. 2 ) so that the elements  234  and  236  come into contact with the mud cake  24  and/or the formation wall  112  or a close proximity to it depending on the amount of trapped mud cake  24 . Additional force may be applied to the plate  233  with hydraulic and/or mechanical force backing up tool  10  with back up pistons  130 ; the amount of force will vary depending on the downhole conditions but will be greater than 1 psi. 
   The elements  234  and  236  may include but are not limited to rubber products, HNBR, Teflon, peak, metal, alloys and/or combination thereof. The elements  234  and  236  may be supported and/or energized by additional materials behind the elements so to enable them to adjust the shape of the borehole and/or retract into the pad  230  depending on the force applied. In most cases mud cake  24  is present and the mud cake  24  and/or borehole fluid may be captured and trapped in the slot or space  235  as the pad is deployed from tool  10  and the mud cake is fully or partially sealed in place by elements  234  and  236  so to form a compressed liquid barrier between elements  234  and  236 . The compression and compaction of the trapped mud cake  24  and borehole fluid in the slot or space  235  will depend on the thickness and compressibility of the mud cake between the pad and the formation wall  112  and the size and shape of the elements  234  and  236 .  FIG. 5  shows a single slot  235  but pad may consist of more that one slot  235  between elements  234  and  236  and/or any number of elements or slots to form a seal and/or isolation using trapped mud cake  24  and/or the formation fluid. 
   After being set, formation fluid can be drawn into one or more flowlines  164  ( FIG. 2 ) through port or ports  240  which may contain a probe or snorkel. During the flow of formation fluid into flowline/flowlines  164  through port/ports  240  a drawdown of the pressure may take place. During this drawdown there may be a pressure differential between space  235  and inlet port  240 , this differential may cause the trapped mud to release filtrate from the fluid in slot  235  across element  236 , this may the form additional mud cake across the face of the element  236 . (Mud is generally made up from liquid and solid and when the liquid is separated from the solids we call the liquid filtrate and the solids left behind mud cake.) Additionally any loss of volume from slot  235  may cause a flow of filtrate across element  234  casing a barrier of mud cake to form on the end of element  234 . Build up of mud cake in addition to the trapped mud cake may supply additional seal to the pad between the borehole  8  and the formation flow port  240 . 
     FIG. 6  shows a front view of a seal pad  231 , in accordance with one embodiment. Seal pad  231  includes a similar configuration as seal pad  230  discussed above. In this embodiment, seal pad  231  includes an oblong or oval shape, with sealing elements  234 ,  236  and space  235  all having a generally oblong or oval shape. 
     FIG. 7  shows a front view of a seal pad  232 , in accordance with one embodiment. Seal pad  231  includes a similar configuration as seal pad  230  discussed above. In this embodiment, seal pad  231  includes three ports  240 . Other embodiments can include fewer or more ports. 
     FIGS. 8A and 8B  show a seal pad  250 , in accordance with one embodiment. Seal pad  250  includes a plate  253  and a flexible metallic pad  242  with one or more sealing rings  241  arranged to form sealing elements. Seal pad  250  can be used on the testing tools discussed above and represented by pad  163  in  FIG. 2  or seal pad  180  of  FIGS. 3 and 4 . The surface of the pad  242  in the area of the rings  241  may be flexible and of a radius greater than the borehole so to promote the outer edge  254  of pad  242  to come into contact first when the plate  253  is deployed from assembly  50  ( FIG. 3 ). 
   As the plate  253  is deployed and compressed into the formation wall  112  ( FIG. 2 ), the metallic surface of pad  242  flexes and conforms to the shape of the borehole wall  112  trapping or compressing mud cake  24  between sealing members  241 , this may provide the initial seal against the borehole fluid once the pad  232  makes contact with the formation wall  112 . 
   When extended, the metallic pad  242  pushes into the mudcake  24  and/or formation wall  112  it may form a primary seal and it may also trap the mud cake  24  between the sealing elements  241  for a secondary sealing system. 
   The raised rings  241  of material on the surface of the metal pad  242  may also be embedded into the formation wall  112  forming a seal or isolation. With the primary and secondary seals energized, a fluid sample can be collected from the formation wall  112 ; formation fluid may now be drawn into the flowline  164  through port  240  which may contain a probe or snorkel. 
   In one embodiment, the metallic pad  242  includes a smooth surface. The pad  242  in the outer edge  254  may be flexible and of a radius greater than the borehole so to promote the outer edge  254  of pad  242  to come into contact first when the plate  233  is deployed from assembly  50  ( FIG. 3 ). The flexible pad  242  may form to the shape of the borehole as it is pushed into the mudcake  24  and/or formation wall  112  it may form a primary seal, the seal may be formed by a combination of the surface of the pad  242  and the formation wall  112  and/or the compaction of the mud cake  24  into and voids between the mud cake  24  and the formation wall  112 . The smooth surface may allow for creation of suction and hence better sealing against the borehole wall  112 . 
   In one embodiment the metallic pad  242  has a coated surface, and the coating may consist but not limited to rubber products, HNBR, Teflon, peak, metal, alloys or and combination and be bonded, glued or attached in any manner to allow for the metallic pad to flex. The pad  242  in the outer edge may be flexible and of a radius greater than the borehole so to promote the outer edge of pad  242  to come into contact first when the plate  233  is deployed from assembly  50  ( FIG. 3 ). The flexible pad  242  may form to the shape of the borehole as it is pushes into the mudcake  24  and/or formation wall  112  it may form a primary seal, the seal may be formed by a combination of the coated surface of the pad  242  and the formation wall  112  and/or the compaction of the mud cake  24  into and voids between the mud cake  24  and the formation wall  112 . The coated surface and the flexible nature of the pad  242  may allow for creation of sealing against the borehole wall  112 . 
     FIG. 9  shows a front view of a seal pad  256 , in accordance with one embodiment. Seal pad  256  includes a similar configuration as seal pad  250  discussed above. In this embodiment, seal pad  256  includes a more oblong or oval shape. 
     FIG. 10  shows a front view of a seal pad  257 , in accordance with one embodiment. Seal pad  257  includes a similar configuration as seal pad  250  discussed above. In this embodiment, seal pad  257  includes two ports  240 . Other embodiments utilize different numbers of ports. 
     FIGS. 11 and 12  show a seal pad  260 , in accordance with one embodiment. Seal pad  260  includes a piston pad that includes a plate or fixture  263  suitable to be attached to the testing tool and represented by pad  163  in  FIG. 2  or pad  180  in  FIGS. 3 and 4 . The seal pad  260  generally includes a first sealing element such as pad edge  262  and a second sealing element, such as pad edge  263 . Pad  260  and edges  263  and  262  can be formed of metal. The pad  260  also includes a movable piston  267  having at least one seal  269  between plate  263  which may have sealing element  264  attached. Sealing element  264  can include but not limited to rubber products, HNBR, Teflon, peak, metal, alloys or and combination. Piston  267  may also have a retainer  268  to limit the extent at with the piston  267  can move forward or to keep it attached to plate  263 . Movable sealing element  264  is located in the space  265  between edges  263  and  262 . 
   The pad  260  is set against the formation wall  112  ( FIG. 2 ) so that the pad edge  262  and/or  263  come into contact with the mud cake  24  and/or the formation wall  112  or a close proximity to it depending on the amount of trapped mud cake  24 . Additional force may be applied to the plate  263  with hydraulic and/or mechanical force backing up tool  10  ( FIG. 2 ) with back up pistons  130 ; the amount of force will vary depending on the downhole conditions but will be greater than 1 psi. 
   Pad edge  262  and/or  263  may be coated with materials and/or shaped to promote a seal between the formation wall  112  and the borehole fluid. Pad edges  262  and/or  263  may employ other embodiments discussed in this disclosure to form a seal. 
   Formation fluid may now be drawn into the flowline through port  240  which may contain a probe or snorkel. During the flow of formation fluid into the tool flowline through port  240 , a drawdown of the pressure may take place. During the drawdown there may be a pressure differential between the borehole fluid representing the fluid behind plate  263  and inlet port  240  which may be maintained by the seal formed by pad edges  263  and/or  262 . 
   There may be a differential pressure across piston  267  if there is any fluid communication between flow path port  240  and the slot or space  265  containing sealing element  264  between pad edge  262  and  263 . This differential pressure may cause the piston  267  to move forward due to the pressure isolation provided by seal  269  which may exert force equal to the differential pressure across the area of piston  267  between the sealing element  264  and formation wall  112  and/or the mud cake  24 . The greater the differential pressure across piston  267  the greater the force is applied to sealing element  264  improving the seal between the borehole and the desired flow of fluid into the flowline  164  through flowpath  240 . The inner edge of surface of edge  263  adjacent to sealing element  264  may be shaped to support the sealing element  264  to reduce extrusion damage. 
     FIG. 13  shows another embodiment of a seal pad similar to seal pad  260 , but including an oblong or oval shape and having three ports  240 . 
     FIGS. 14-17  show further details of a seal pad  178 , in accordance with one embodiment.  FIG. 14  is a perspective view of seal pad  178 ,  FIG. 15  is a front view of the seal pad  178 ,  FIG. 16  is a top, section view of the seal pad  178 , and  FIG. 17  is a side, section view of the seal pad  178 . Seal pad  178  is suitable to be attached to a testing tool and is represented by pad  163  in  FIG. 2  or pad  180  in  FIGS. 3 and 4 . Seal pad  178  generally includes a base  181  with a first sealing element, such as a first inner metallic ring  182 , and a second sealing element, such as a second outer metallic ring  184  extending outward from the base  181 . In one embodiment, the base  181  and the first inner metallic ring  182  and the second outer metallic ring  184  are machined from a solid metallic unit, such as stainless steel. The first metallic ring  182  and the second metallic ring  184  are positioned such that there is a space  195  defined between the first metallic ring  182  and the second metallic ring  184 . In one embodiment, the first inner metallic ring  182  and the second outer metallic ring  184  are substantially concentric, such that space  195  is dimensioned substantially equal all around the seal pad  178 . Space  195  is configured such that when the seal pad  178  is pressed against a well bore wall, mud cake is trapped within the space  195  between first metallic ring  182  and second metallic ring  184 . The mud cake then acts as an o-ring seal to help seal pad  178  provide a seal for the formation tester probe assembly probe. 
   In one embodiment, the seal pad  178  further includes an elastomer o-ring  186  encircling the first inner metallic ring  182 . The elastomer o-ring  186  can be mounted by mounting a metal retaining member  188  over the o-ring  186  and attaching retaining member  188  using fasteners  190 , such as screws. In one embodiment, o-ring  186  can be configured so as to extend slightly beyond the outer surface of inner metallic ring  182 . O-ring  186  helps provide sealing against the well bore wall. In this example, the metallic outer surfaces of first metallic ring  182  and second metallic ring  184  limit the compression of o-ring  186  when the seal pad  178  is pressed against a well bore wall. This allows for more used of the seal pad  178  without having to replace o-ring  186  since compression of an elastomer o-ring at high temperatures breaks down the elastomer o-ring. 
   The outer surface of seal pad  178  is generally congruent to the inner surface of a cylindrical wall  112  ( FIG. 2 ) of the borehole. Thus, the outer surfaces of inner metallic ring  182 , outer metallic ring  184 , and o-ring  186  can define a partial cylindrical surface. This means the pad  178  exerts generally equal pressure against the wall  112  at all parts of it surface. This provides for a better seal. 
     FIG. 18  shows a seal pad  179  similar to seal pad  178  but having an oblong or oval shape. 
     FIG. 19  shows a front view of a seal pad  300 , in accordance with one embodiment. In this example, seal pad  300  includes a first flow path port  240  and a second flow path port  24 A. Seal pad  300  includes one or more sealing elements  341  arranged on a flexible metal pad  302  that form a seal between flow area ports  240  and  240 A. In one embodiment, flow area ports  240  and  240 A are directed to independent pumps as described in U.S. Pat. No. 6,301,959, entitled Focused Formation Fluid Sampling Probe, which is incorporated herein by reference. Although shown as an oval shape, in other embodiments, seal pad  300  can have any other shape, as discussed above. 
     FIG. 20  shows a front view of a seal pad  310 , in accordance with one embodiment, and  FIG. 21  shows a cross-section view of seal pad  310 , in accordance with one embodiment. Seal pad  310  includes one or more series of sealing elements  344  and  344 A, which are contained between pad edges  342  and  343 , similar to what discussed above in an earlier embodiment. Sealing elements  344  and  344 A can include a movable piston  237  having at least one seal  239  between plate  233 . The combination of one or more sealing elements  344  and  344 A forms a seal between flow area ports  240  and  240 A, which may be directed to independent pumps as described in U.S. Pat. No. 6,301,959. This embodiment shows the pad as an oval shape but the pad can be any shape that may enable forming a seal with the borehole. 
     FIG. 22  shows a cross-section view of a seal pad  310 A, in accordance with one embodiment. Seal pad  310 A is similar to seal pad  310  but while seal pad  310  shown in  FIG. 21  shows the back side of piston  237  exposed to the pressure of the borehole, the embodiment of  FIG. 22  includes a configuration where the back of sealing element  244 A is connected to flow path port  240 A, and in this case the force applied to the sealing element  244 A depends on the pressure difference between flow path ports  240  and  240 A. The piston pad may be ported to apply force using differential pressure to sealing elements  344  by connecting the back side of piston to a flow path such as port  240  or  240 A. 
     FIG. 23  shows a front view of a seal pad  350 , in accordance with one embodiment. Seal pad  350  includes one or more series of outer and inner sealing elements  354  and  356  forming one or more slots  355 . These combinations of slots  355  form an isolation seal between flow area  240  and  240 A, which may be directed to independent pumps as described in U.S. Pat. No. 6,301,959. Although this embodiment shows a pad as an oval shape the pad can be any shape that may enable forming a seal with the borehole. 
   Referring to  FIGS. 3-4  and  14 - 17 , the operation of formation probe assembly  50  will now be described, in accordance with one embodiment. Probe assembly  50  is normally in the retracted position ( FIG. 3 ). Assembly  50  remains retracted when not in use, such as when the drill string is rotating while drilling if assembly  50  is used for an MWD application, or when the wireline testing tool is being lowered into the borehole if assembly  50  is used for a wireline testing application. 
   Upon an appropriate command to formation probe assembly  50 , a force is applied to the base portion of piston  96 , preferably by using hydraulic fluid. Piston  96  rises relative to the other portions of probe assembly  50 . The seal pad  178  is advanced until its outer surfaces contact the mud cake  24 . Mud cake  24  then enters the space  195  and helps form a seal, along with first inner metallic ring  182 , second outer metallic ring  184 , and o-ring  186 . The highly viscous mud cake  24  is trapped between the two metallic rings  182 ,  184  and forms a liquid o-ring to become an effective seal against the well bore. After the seal pad  178  is set, the formation draw down procedure, or other downhole procedure, is started. Continued force from hydraulic fluid causes snorkel assembly  98  to extend such that the outer end of the snorkel extends beyond the seal pad  178  surface through seal pad aperture  186 . 
   To retract probe assembly  50 , forces, or pressure differentials, may be applied to snorkel  98  and piston  96  in opposite directions relative to the extending forces. Simultaneously, the extending forces may be reduced or ceased to aid in probe retraction. 
   In one embodiment, the probe assembly  50  can be a telescoping probe including a second inner piston to further extend the probe assembly. In other embodiments, formation tester tool  10  can further include fins or hydraulic stabilizers or a heave compensator located proximate formation probe assembly  50  so as to anchor the tool and dampen motion of the tool in the bore hole. 
   Although the discussed embodiments describe several methods that improve the ability to seal a formation for the borehole for the purpose of formation testing in hostile environments, the embodiments may be suitable to both hostile and non hostile borehole conditions. 
   Moreover, although the above discussion relates generally to formation tester pads used to form a seal from the borehole to the formation for pressure testing, fluid sampling and fluid analysis, the seal pads may also be used for other applications of downhole measuring where isolations mechanical, electrically or pressure is required. 
   The disclosures above assume a borehole with drilling fluids and mud cake. However, the disclosures are not limited to fluid filled boreholes but air-filled holes will not be discussed in the disclosures. 
   The above discussion is meant to be illustrative of the principles and various embodiments of the present invention. While the preferred embodiment of the invention and its method of use have been shown and described, modifications thereof can be made by one skilled in the art without departing from the spirit and teachings of the invention. The embodiments described herein are exemplary only, and are not limiting. Many variations and modifications of the invention and apparatus and methods disclosed herein are possible and are within the scope of the invention. Accordingly, the scope of protection is not limited by the description set out above, but is only limited by the claims which follow, that scope including all equivalents of the subject matter of the claims.