Abstract:
This application relates to various methods and apparatus for rapidly obtaining accurate formation property data from a drilled earthen borehole. Quickly obtaining accurate formation property data, including formation fluid pressure, is vital to beneficially describing the various formations being intersected. For example, methods are disclosed for collecting numerous property values with a minimum of downhole tools, correcting and calibrating downhole measurements and sensors, and developing complete formation predictors and models by acquiring a diverse set of direct formation measurements, such as formation fluid pressure and temperature. Also disclosed are various methods of using of accurately and quickly obtained formation property data.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present application claims the benefit of U.S. Provisional Application Ser. No. 60/573,289, filed May 21, 2004, entitled Methods and Apparatus for Measuring Formation Properties, which is hereby incorporated herein by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     Not applicable.  
       BACKGROUND  
       [0003]     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.  
         [0004]     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 packers also, although the 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 across the axial span of the circumference of the borehole wall.  
         [0005]     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.  
         [0006]     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 isolation pad is critical 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.  
         [0007]     Examples of isolation pads and probes used in WFTs can be found in Halliburton&#39;s DT, SFTT, SFT4, and RDT tools. Isolation pads that are used with WFTs are typically 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 within the specific diameter hole in which they will be operating.  
         [0008]     With the use of WFTs and DSTs, the drill string with the drill bit must be retracted from the borehole. Then, a separate work string containing the testing equipment, or, with WFTs, the wireline tool string, must be lowered into the well to conduct secondary operations. Interrupting the drilling process to perform formation testing can add significant amounts of time to a drilling program.  
         [0009]     DSTs and WFTs may also cause tool sticking or formation damage. There may also be difficulties of running WFTs in highly deviated and extended reach wells. WFTs also do not have flowbores for the flow of drilling mud, nor are they designed to withstand drilling loads such as torque and weight on bit.  
         [0010]     Further, the formation pressure measurement accuracy of drill stem tests and, especially, of wireline formation tests may be affected by filtrate invasion and mudcake buildup because significant amounts of time may have passed before a DST or WFT engages the formation. Mud filtrate invasion occurs when the drilling mud fluids displace formation fluids. Because the mud filtrate ingress into the formation begins at the borehole surface, it is most prevalent there and generally decreases further into the formation. When filtrate invasion occurs, it may become impossible to obtain a representative sample of formation fluids or, at a minimum, the duration of the sampling period must be increased to first remove the drilling fluid and then obtain a representative sample of formation fluids. The mudcake is made up of the solid particles that are plastered to the side of the well by the circulating drilling mud during drilling. The prevalence of the mudcake at the borehole surface creates a “skin.” Thus there may be a “skin effect” because formation testers can only extend relatively short distances into the formation, thereby distorting the representative sample of formation fluids due to the filtrate. The mudcake also acts as a region of reduced permeability adjacent to the borehole. Thus, once the mudcake forms, the accuracy of reservoir pressure measurements decreases, affecting the calculations for permeability and producibility of the formation.  
         [0011]     Another testing apparatus is the formation tester while drilling (FTWD) tool. Typical FTWD formation testing equipment is suitable for integration with a drill string during drilling operations. Various devices or systems are used for isolating a formation from the remainder of the borehole, drawing fluid from the formation, and measuring physical properties of the fluid and the formation. For example, the FTWD may use a probe similar to a WFT that extends to the formation and a small sample chamber to draw in formation fluids through the probe to test the formation pressure. To perform a test, the drill string is stopped from rotating and the test procedure, similar to a WFT described above, is performed. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     For a more detailed description of the embodiments of the present invention, reference will now be made to the accompanying drawings, wherein:  
         [0013]      FIG. 1  is a schematic elevation view, partly in cross-section, of an embodiment of a formation tester apparatus disposed in a subterranean well;  
         [0014]      FIGS. 2A-2E  are schematic elevation views, partly in cross-section, of portions of the bottomhole assembly and formation tester assembly shown in  FIG. 1 ;  
         [0015]      FIG. 3  is an enlarged elevation view, partly in cross-section, of the formation tester tool portion of the formation tester assembly shown in  FIG. 2D ;  
         [0016]      FIG. 3A  is an enlarged cross-section view of the draw down piston and chamber shown in  FIG. 3 ;  
         [0017]      FIG. 3B  is an enlarged cross-section view along line  3 B- 3 B of  FIG. 3 ;  
         [0018]      FIG. 4  is an elevation view of the formation tester tool shown in  FIG. 3 ;  
         [0019]      FIG. 5  is a cross-sectional view of the formation probe assembly taken along line  5 - 5  shown in  FIG. 4 ;  
         [0020]      FIGS. 6A-6C  are cross-sectional views of a portion of the formation probe assembly taken along the same line as seen in  FIG. 5 , the probe assembly being shown in a different position in each of  FIGS. 6A-6C ;  
         [0021]      FIG. 7  is an elevation view of the probe pad mounted on the skirt employed in the formation probe assembly shown in  FIGS. 4 and 5 ;  
         [0022]      FIG. 8  is a top view of the probe pad shown in  FIG. 7 ;  
         [0023]      FIG. 9  is a schematic view of a hydraulic circuit employed in actuating the formation tester apparatus;  
         [0024]      FIG. 10  is a graph of the formation fluid pressure as compared to time measured during operation of the tester apparatus;  
         [0025]      FIG. 11  is another graph of the formation fluid pressure as compared to time measured during operation of the tester apparatus and showing pressures measured by different pressure transducers employed in the formation tester;  
         [0026]      FIG. 12  is another graph of the formation fluid pressure as compared to time measured during operation of the tester apparatus that can be used to calibrate the pressure transducers; and  
         [0027]      FIG. 13  is a graph of the annulus and formation fluid pressures in response to pressure pulses. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0028]     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.  
         [0029]     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 electrical connections are each intended to mean and refer to either an indirect or a direct 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.  
         [0030]     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 pore spaces, within certain types of rock. Therefore, it is critical to know certain properties of both the formation and the fluid contained therein. At various times during the following discussion, certain formation and formation fluid properties will be referred to in a general sense. Such formation properties include, but are not limited to: pressure, permeability, viscosity, mobility, spherical mobility, porosity, saturation, coupled compressibility porosity, skin damage, and anisotropy. Such formation fluid properties include, but are not limited to: viscosity, compressibility, flowline fluid compressibility, density, resistivity, composition and bubble point.  
         [0031]     Permeability is the ability of a rock formation to allow hydrocarbons to move between its pores, and consequently into a wellbore. Fluid viscosity is a measure of the ability of the hydrocarbons to flow, and the permeability divided by the viscosity is termed “mobility.” Porosity is the ratio of void space to the bulk volume of rock formation containing that void space. Saturation is the fraction or percentage of the pore volume occupied by a specific fluid (e.g., oil, gas, water, etc.). Skin damage is an indication of how the mud filtrate or mudcake has changed the permeability near the wellbore. Anisotropy is the ratio of the vertical and horizontal permeabilities of the formation.  
         [0032]     Resistivity of a fluid is the property of the fluid which resists the flow of electrical current. Bubble point occurs when a fluid&#39;s pressure is brought down at such a rapid rate, and to a low enough pressure, that the fluid, or portions thereof, changes phase to a gas. The dissolved gases in the fluid are brought out of the fluid so gas is present in the fluid in an undissolved state. Typically, this kind of phase change in the formation hydrocarbons being tested and measured is undesirable, unless the bubblepoint test is being administered to determine what the bubblepoint pressure is.  
         [0033]     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.  
         [0034]     Referring to  FIG. 1 , an MWD formation tester tool  10  is illustrated as a part of bottom hole assembly  6  (BHA) which includes an MWD sub  13  and a drill bit  7  at its lower most end. BHA  6  is lowered from a drilling platform  2 , such as a ship or other conventional platform, via drill string  5 . Drill string  5  is disposed through riser  3  and well head  4 . Conventional drilling equipment (not shown) is supported within derrick  1  and rotates drill string  5  and drill bit  7 , causing bit  7  to form a borehole  8  through the formation material  9 . The borehole  8  penetrates subterranean zones or reservoirs, such as reservoir  11 , that are believed to contain hydrocarbons in a commercially viable quantity. It should be understood that formation tester  10  may be employed in other bottom hole assemblies and with other drilling apparatus in land-based drilling, as well as offshore drilling as illustrated in  FIG. 1 . In all instances, in addition to formation tester  10 , the bottom hole assembly  6  contains various conventional apparatus and systems, such as a down hole drill motor, mud pulse telemetry system, measurement-while-drilling sensors and systems, and others well known in the art.  
         [0035]     It should also be understood that, even though the MWD formation tester  10  is illustrated as part of a drill string  5 , the embodiments of the invention described below may be conveyed down the borehole  8  via wireline technology, as is partially described above. It should also be understood that the exact physical configuration of the formation tester and the probe assembly is not a requirement of the present invention. The embodiment described below serves to provide an example only. Additional examples of a probe assembly and methods of use are described in U.S. patent application Ser. No. 10/440,593, filed May 19, 2003 and entitled “Method and Apparatus for MWD Formation Testing”; Ser. No. 10/440,835, filed May 19, 2003 and entitled “MWD Formation Tester”; and Ser. No. 10/440,637, filed May 19, 2003 and entitled “Equalizer Valve”; each hereby incorporated herein by reference for all purposes. Further examples of formation testing tools, probe assemblies and methods of use, whether conveyed via a drill string or wireline, or any other method, include U.S. patent application Ser. No. ______ entitled “Downhole Probe Assembly,” having U.S. Express Mail Label Number EV 303483549 US and Attorney Docket Number 1391-52601; U.S. patent application Ser. No. ______ entitled “Formation Tester Tool Assembly and Methods of Use,” having U.S. Express Mail Label Number EV 303483552 US and Attorney Docket Number 1391-53801; U.S. patent application Ser. No. ______ entitled “Methods and Apparatus for Using Formation Property Data,” having U.S. Express Mail Label Number EV 303483570 US and Attorney Docket Number 1391-54001; U.S. patent application Ser. No. ______ entitled “Methods and Apparatus for Controlling a Formation Tester Tool Assembly,” having U.S. Express Mail Label Number EV 303483362 US and Attorney Docket Number 1391-54101; and U.S. patent application Ser. No. ______ entitled “Methods for Measuring a Formation Supercharge Pressure,” having U.S. patent application Ser. No. 11/069,649; each hereby incorporated herein by reference for all purposes.  
         [0036]     The formation tester tool  10  is best understood with reference to  FIGS. 2A-2E . Formation tester  10  generally comprises a heavy walled housing  12  made of multiple sections of drill collar  12   a ,  12   b ,  12   c , and  12   d  which threadedly engage one another so as to form the complete housing  12 . Bottom hole assembly  6  includes flow bore  14  formed through its entire length to allow passage of drilling fluids from the surface through the drill string  5  and through the bit  7 . The drilling fluid passes through nozzles in the drill bit face and flows upwards through borehole  8  along the annulus  150  formed between housing  12  and borehole wall  151 .  
         [0037]     Referring to  FIGS. 2A and 2B , upper section  12   a  of housing  12  includes upper end  16  and lower end  17 . Upper end  16  includes a threaded box for connecting formation tester  10  to drill string  5 . Lower end  17  includes a threaded box for receiving a correspondingly threaded pin end of housing section  12   b . Disposed between ends  16  and  17  in housing section  12   a  are three aligned and connected sleeves or tubular inserts  24   a,b,c  which creates an annulus  25  between sleeves  24   a,b,c  and the inner surface of housing section  12   a . Annulus  25  is sealed from flowbore  14  and provided for housing a plurality of electrical components, including battery packs  20 ,  22 . Battery packs  20 ,  22  are mechanically interconnected at connector  26 . Electrical connectors  28  are provided to interconnect battery packs  20 ,  22  to a common power bus (not shown). Beneath battery packs  20 ,  22  and also disposed about sleeve insert  24   c  in annulus  25  is electronics module  30 . Electronics module  30  includes the various circuit boards, capacitors banks and other electrical components, including the capacitors shown at  32 . A connector  33  is provided adjacent upper end  16  in housing section  12   a  to electrically couple the electrical components in formation tester tool  10  with other components of bottom hole assembly  6  that are above housing  12 .  
         [0038]     Beneath electronics module  30  in housing section  12   a  is an adapter insert  34 . Adapter  34  connects to sleeve insert  24   c  at connection  35  and retains a plurality of spacer rings  36  in a central bore  37  that forms a portion of flowbore  14 . Lower end  17  of housing section  12   a  connects to housing section  12   b  at threaded connection  40 . Spacers  38  are disposed between the lower end of adapter  34  and the pin end of housing section  12   b . Because threaded connections such as connection  40 , at various times, need to be cut and repaired, the length of sections  12   a ,  12   b  may vary in length. Employing spacers  36 ,  38  allow for adjustments to be made in the length of threaded connection  40 .  
         [0039]     Housing section  12   b  includes an inner sleeve  44  disposed therethrough. Sleeve  44  extends into housing section  12   a  above, and into housing section  12   c  below. The upper end of sleeve  44  abuts spacers  36  disposed in adapter  34  in housing section  12   a . An annular area  42  is formed between sleeve  44  and the wall of housing  12   b  and forms a wire way for electrical conductors that extend above and below housing section  12   b , including conductors controlling the operation of formation tester  10  as described below.  
         [0040]     Referring now to  FIGS. 2B and 2C , housing section  12   c  includes upper box end  47  and lower box end  48  which threadingly engage housing section  12   b  and housing section  12   c , respectively. For the reasons previously explained, adjusting spacers  46  are provided in housing section  12   c  adjacent to end  47 . As previously described, insert sleeve  44  extends into housing section  12   c  where it stabs into inner mandrel  52 . The lower end of inner mandrel  52  stabs into the upper end of formation tester mandrel  54 , which is comprised of three axially aligned and connected sections  54   a, b , and  c . Extending through mandrel  54  is a deviated flowbore portion  14   a . Deviating flowbore  14  into flowbore path  14   a  provides sufficient space within housing section  12   c  for the formation tool components described in more detail below. As best shown in  FIG. 2E , deviated flowbore  14   a  eventually centralizes near the lower end  48  of housing section  12   c , shown generally at location  56 . Referring momentarily to  FIG. 5 , the cross-sectional profile of deviated flowbore  14   a  may be a non-circular in segment  14   b , so as to provide as much room as possible for the formation probe assembly  50 .  
         [0041]     As best shown in  FIGS. 2D and 2E , disposed about formation tester mandrel  54  and within housing section  12   c  are electric motor  64 , hydraulic pump  66 , hydraulic manifold  62 , equalizer valve  60 , formation probe assembly  50 , pressure transducers  160 , and draw down piston  170 . Hydraulic accumulators provided as part of the hydraulic system for operating formation probe assembly  50  are also disposed about mandrel  54  in various locations, one such accumulator  68  being shown in  FIG. 2D .  
         [0042]     Electric motor  64  may be a permanent magnet motor powered by battery packs  20 ,  22  and capacitor banks  32 . Motor  64  is interconnected to and drives hydraulic pump  66 . Pump  66  provides fluid pressure for actuating formation probe assembly  50 . Hydraulic manifold  62  includes various solenoid valves, check valves, filters, pressure relief valves, thermal relief valves, pressure transducer  160   b  and hydraulic circuitry employed in actuating and controlling formation probe assembly  50  as explained in more detail below.  
         [0043]     Referring again to  FIG. 2C , mandrel  52  includes a central segment  71 . Disposed about segment  71  of mandrel  52  are pressure balance piston  70  and spring  76 . Mandrel  52  includes a spring stop extension  77  at the upper end of segment  71 . Stop ring  88  is threaded to mandrel  52  and includes a piston stop shoulder  80  for engaging corresponding annular shoulder  73  formed on pressure balance piston  70 . Pressure balance piston  70  further includes a sliding annular seal or barrier  69 . Barrier  69  consists of a plurality of inner and outer o-ring and lip seals axially disposed along the length of piston  70 .  
         [0044]     Beneath piston  70  and extending below inner mandrel  52  is a lower oil chamber or reservoir  78 , described more fully below. An upper chamber  72  is formed in the annulus between central portion  71  of mandrel  52  and the wall of housing section  12   c , and between spring stop portion  77  and pressure balance piston  70 . Spring  76  is retained within chamber  72 . Chamber  72  is open through port  74  to annulus  150 . As such, drilling fluids will fill chamber  72  in operation. An annular seal  67  is disposed about spring stop portion  77  to prevent drilling fluid from migrating above chamber  72 .  
         [0045]     Barrier  69  maintains a seal between the drilling fluid in chamber  72  and the hydraulic oil that fills and is contained in oil reservoir  78  beneath piston  70 . Lower chamber  78  extends from barrier  69  to seal  65  located at a point generally noted as  83  and just above transducers  160  in  FIG. 2E . The oil in reservoir  78  completely fills all space between housing section  12   c  and formation tester mandrel  54 . The hydraulic oil in chamber  78  may be maintained at slightly greater pressure than the hydrostatic pressure of the drilling fluid in annulus  150 . The annulus pressure is applied to piston  70  via drilling fluid entering chamber  72  through port  74 . Because lower oil chamber  78  is a closed system, the annulus pressure that is applied via piston  70  is applied to the entire chamber  78 . Additionally, spring  76  provides a slightly greater pressure to the closed oil system  78  such that the pressure in oil chamber  78  is substantially equal to the annulus fluid pressure plus the pressure added by the spring force. This slightly greater oil pressure is desirable so as to maintain positive pressure on all the seals in oil chamber  78 . Having these two pressures generally balanced (even though the oil pressure is slightly higher) is easier to maintain than if there was a large pressure differential between the hydraulic oil and the drilling fluid. Between barrier  69  in piston  70  and point  83 , the hydraulic oil fills all the space between the outside diameter of mandrels  52 ,  54  and the inside diameter of housing section  12   c , this region being marked as distance  82  between points  81  and  83 . The oil in reservoir  78  is employed in the hydraulic circuit  200  ( FIG. 9 ) used to operate and control formation probe assembly  50  as described in more detailed below.  
         [0046]     Equalizer valve  60 , best shown in  FIG. 3 , is disposed in formation tester mandrel  54   b  between hydraulic manifold  62  and formation probe assembly  50 . Equalizer valve  60  is in fluid communication with hydraulic passageway  85  and with longitudinal fluid passageway  93  formed in mandrel  54   b . Prior to actuating formation probe assembly  50  so as to test the formation, drilling fluid fills passageways  85  and  93  as valve  60  is normally open and communicates with annulus  150  through port  84  in the wall of housing section  12   c . When the formation fluids are being sampled by formation probe assembly  50 , valve  60  closes the passageway  85  to prevent drilling fluids from annulus  150  entering passageway  85  or passageway  93 .  
         [0047]     As shown in  FIGS. 3 and 4 , housing section  12   c  includes a recessed portion  135  adjacent to formation probe assembly  50  and equalizer valve  60 . The recessed portion  135  includes a planar surface or “flat”  136 . The ports through which fluids may pass into equalizing valve  60  and probe assembly  50  extend through flat  136 . In this manner, as drill string  5  and formation tester  10  are rotated in the borehole, formation probe assembly  50  and equalizer valve  60  are better protected from impact, abrasion and other forces. Flat  136  is recessed at least ¼ inch and may be at least 12 inch from the outer diameter of housing section  12   c . Similar flats  137 ,  138  are also formed about housing section  12   c  at generally the same axial position as flat  136  to increase flow area for drilling fluid in the annulus  150  of borehole  8 .  
         [0048]     Disposed about housing section  12   c  adjacent to formation probe assembly  50  is stabilizer  154 . Stabilizer  154  may have an outer diameter close to that of nominal borehole size. As explained below, formation probe assembly  50  includes a seal pad  140  that is extendable to a position outside of housing  12   c  to engage the borehole wall  151 . As explained, probe assembly  50  and seal pad  140  of formation probe assembly  50  are recessed from the outer diameter of housing section  12   c , but they are otherwise exposed to the environment of annulus  150  where they could be impacted by the borehole wall  151  during drilling or during insertion or retrieval of bottom hole assembly  6 . Accordingly, being positioned adjacent to formation probe assembly  50 , stabilizer  154  provides additional protection to the seal pad  140  during insertion, retrieval and operation of bottom hole assembly  6 . It also provides protection to pad  140  during operation of formation tester  10 . In operation, a piston extends seal pad  140  to a position where it engages the borehole wall  151 . The force of the pad  140  against the borehole wall  151  would tend to move the formation tester  10  in the borehole, and such movement could cause pad  140  to become damaged. However, as formation tester  10  moves sideways within the borehole as the piston is extended into engagement with the borehole wall  151 , stabilizer  154  engages the borehole wall and provides a reactive force to counter the force applied to the piston by the formation. In this manner, further movement of the formation test tool  10  is resisted.  
         [0049]     Referring to  FIG. 2E , mandrel  54   c  contains chamber  63  for housing pressure transducers  160  a, c, and d as well as electronics for driving and reading these pressure transducers. In addition, the electronics in chamber  63  contain memory, a microprocessor, and power conversion circuitry for properly utilizing power from a power bus (not shown).  
         [0050]     Referring still to  FIG. 2E , housing section  12   d  includes pins ends  86 ,  87 . Lower end  48  of housing section  12   c  threadedly engages upper end  86  of housing section  12   d . Beneath housing section  12   d , and between formation tester tool  10  and drill bit  7  are other sections of the bottom hole assembly  6  that constitute conventional MWD tools, generally shown in  FIG. 1  as MWD sub  13 . In a general sense, housing section  12   d  is an adapter used to transition from the lower end of formation tester tool  10  to the remainder of the bottom hole assembly  6 . The lower end  87  of housing section  12   d  threadedly engages other sub assemblies included in bottom hole assembly  6  beneath formation tester tool  10 . As shown, flowbore  14  extends through housing section  12   d  to such lower subassemblies and ultimately to drill bit  7 .  
         [0051]     Referring again to  FIG. 3  and to  FIG. 3A , drawdown piston  170  is retained in drawdown manifold  89  that is mounted on formation tester mandrel  54   b  within housing  12   c . Piston  170  includes annular seal  171  and is slidingly received in cylinder  172 . Spring  173  biases piston  170  to its uppermost or shouldered position as shown in  FIG. 3A . Separate hydraulic lines (not shown) interconnect with cylinder  172  above and below piston  170  in portions  172   a ,  172   b  to move piston  170  either up or down within cylinder  172  as described more fully below. A plunger  174  is integral with and extends from piston  170 . Plunger  174  is slidingly disposed in cylinder  177  coaxial with 172. Cylinder  175  is the upper portion of cylinder  177  that is in fluid communication with the longitudinal passageway  93  as shown in  FIG. 3A . Cylinder  175  is flooded with drilling fluid via its interconnection with passageway  93 . Cylinder  177  is filled with hydraulic fluid beneath seal  166  via its interconnection with hydraulic circuit  200 . Plunger  174  also contains scraper  167  that protects seal  166  from debris in the drilling fluid. Scraper  167  may be an o-ring energized lip seal.  
         [0052]     As best shown in  FIG. 5 , formation probe assembly  50  generally includes stem  92 , a generally cylindrical adapter sleeve  94 , piston  96  adapted to reciprocate within adapter sleeve  94 , and a snorkel assembly  98  adapted for reciprocal movement within piston  96 . Housing section  12   c  and formation tester mandrel  54   b  include aligned apertures  90   a ,  90   b , respectively, that together form aperture  90  for receiving formation probe assembly  50 .  
         [0053]     Stem  92  includes a circular base portion  105  with an outer flange  106 . Extending from base  105  is a tubular extension  107  having central passageway  108 . The end of extension  107  includes internal threads at  109 . Central passageway  108  is in fluid connection with fluid passageway  91  that, in turn, is in fluid communication with longitudinal fluid chamber or passageway  93 , best shown in  FIG. 3 .  
         [0054]     Adapter sleeve  94  includes inner end  111  that engages flange  106  of stem number  92 . Adapter sleeve  94  is secured within aperture  90  by threaded engagement with mandrel  54   b  at segment  110 . The outer end  112  of adapter sleeve  94  extends to be substantially flushed with flat  136  formed in housing member  12   c . Circumferentially spaced about the outermost surface of adapter sleeve  94  is a plurality of tool engaging recesses  158 . These recesses are employed to thread adapter  94  into and out of engagement with mandrel  54   b . Adapter sleeve  94  includes cylindrical inner surface  113  having reduced diameter portions  114 ,  115 . A seal  116  is disposed in surface  114 . Piston  96  is slidingly retained within adapter sleeve  94  and generally includes base section  118  and an extending portion  119  that includes inner cylindrical surface  120 . Piston  96  further includes central bore  121 .  
         [0055]     Snorkel  98  includes a base portion  125 , a snorkel extension  126 , and a central passageway  127  extending through base  125  and extension  126 .  
         [0056]     Formation tester apparatus  50  is assembled such that piston base  118  is permitted to reciprocate along surface  113  of adapter sleeve  94 . Similarly, snorkel base  125  is disposed within piston  96  and snorkel extension  126  is adapted for reciprocal movement along piston surface  120 . Central passageway  127  of snorkel  98  is axially aligned with tubular extension  107  of stem  92  and with screen  100 .  
         [0057]     Referring to  FIGS. 5 and 6 C, screen  100  is a generally tubular member having a central bore  132  extending between a fluid inlet end  131  and outlet end  122 . Outlet end  122  includes a central aperture  123  that is disposed about stem extension  107 . Screen  100  further includes a flange  130  adjacent to fluid inlet end  131  and an internally slotted segment  133  having slots  134 . Apertures  129  are formed in screen  100  adjacent end  122 . Between slotted segment  133  and apertures  129 , screen  100  includes threaded segment  124  for threadedly engaging snorkel extension  126 .  
         [0058]     Scraper  102  includes a central bore  103 , threaded extension  104  and apertures  101  that are in fluid communication with central bore  103 . Section  104  threadedly engages internally threaded section  109  of stem extension  107 , and is disposed within central bore  132  of screen  100 .  
         [0059]     Referring now to  FIGS. 5, 7  and  8 , seal pad  140  may be generally donut-shaped having base surface  141 , an opposite sealing surface  142  for sealing against the borehole wall, a circumferential edge surface  143  and a central aperture  144 . In the embodiment shown, base surface  141  is generally flat and is bonded to a metal skirt  145  having circumferential edge  153  with recesses  152  and corners  2008 . Seal pad  140  seals and prevents drilling fluid from entering the probe assembly  50  during formation testing so as to enable pressure transducers  160  to measure the pressure of the formation fluid. The rate at which the pressure measured by the formation test tool increases is an indication of the permeability of the formation  9 . More specifically, seal pad  40  seals against the mudcake  49  that forms on the borehole wall  151 . Typically, the pressure of the formation fluid is less than the pressure of the drilling fluids that are circulated in the borehole. A layer of residue from the drilling fluid forms a mudcake  49  on the borehole wall and separates the two pressure areas. Pad  140 , when extended, conforms its shape to the borehole wall and, together with the mudcake  49 , forms a seal through which formation fluids may be collected.  
         [0060]     As best shown in  FIGS. 3, 5 , and  6 , pad  140  is sized so that it may be retracted completely within aperture  90 . In this position, pad  140  is protected both by flat  136  that surrounds aperture  90  and by recess  135  that positions face  136  in a setback position with respect to the outside surface of housing  12 . Pad  140  is preferably made of an elastomeric material, but is not limited to such a material.  
         [0061]     To help with a good pad seal, tool  10  may include, among other things, centralizers for centralizing the formation probe assembly  50  and thereby normalizing pad  140  relative to the borehole wall. For example, the formation tester may include centralizing pistons coupled to a hydraulic fluid circuit configured to extend the pistons in such a way as to protect the probe assembly and pad, and also to provide a good pad seal.  
         [0062]     The hydraulic circuit  200  used to operate probe assembly  50 , equalizer valve  60 , and draw down piston  170  is illustrated in  FIG. 9 . A microprocessor-based controller  190  is electrically coupled to all of the controlled elements in the hydraulic circuit  200  illustrated in  FIG. 10 , although the electrical connections to such elements are conventional and are not illustrated other than schematically. Controller  190  is located in electronics module  30  in housing section  12   a , although it could be housed elsewhere in bottom hole assembly  6 . Controller  190  detects the control signals transmitted from a master controller (not shown) housed in the MWD sub  13  of the bottom hole assembly  6  which, in turn, receives instructions transmitted from the surface via mud pulse telemetry, or any of various other conventional means for transmitting signals to downhole tools.  
         [0063]     When controller  190  receives a command to initiate formation testing, the drill string has stopped rotating. As shown in  FIG. 9 , motor  64  is coupled to pump  66  that draws hydraulic fluid out of hydraulic reservoir  78  through a serviceable filter  79 . As will be understood, the pump  66  directs hydraulic fluid into hydraulic circuit  200  that includes formation probe assembly  50 , equalizer valve  60 , draw down piston  170  and solenoid valves  176 ,  178 ,  180 .  
         [0064]     The operation of formation tester  10  is best understood in reference to  FIG. 9  in conjunction with  FIGS. 3A, 5  and  6 A-C. In response to an electrical control signal, controller  190  energizes solenoid valve  180  and starts motor  64 . Pump  66  then begins to pressurize hydraulic circuit  200  and, more particularly, charges probe retract accumulator  182 . The act of charging accumulator  182  also ensures that the probe assembly  50  is retracted and that drawdown piston  170  is in its initial shouldered position as shown in  FIG. 3A . When the pressure in system  200  reaches a predetermined value, such as 1800 p.s.i. as sensed by pressure transducer  160   b , controller  190  (which continuously monitors pressure in the system) energizes solenoid valve  176  and de-energizes solenoid valve  180 , which causes probe piston  96  and snorkel  98  to begin to extend toward the borehole wall  151 . Concurrently, check valve  194  and relief valve  193  seal the probe retract accumulator  182  at a pressure charge of between approximately 500 to 1250 p.s.i.  
         [0065]     Piston  96  and snorkel  98  extend from the position shown in  FIG. 6A  to that shown in  FIG. 6B  where pad  140  engages the mudcake  49  on borehole wall  151 . With hydraulic pressure continued to be supplied to the extend side of the piston  96  and snorkel  98 , the snorkel then penetrates the mudcake as shown in  FIG. 6C . There are two expanded positions of snorkel  98 , generally shown in  FIGS. 6B and 6C . The piston  96  and snorkel  98  move outwardly together until the pad  140  engages the borehole wall  151 . This combined motion continues until the force of the borehole wall against pad  140  reaches a pre-determined magnitude, for example 5,500 lbs., causing pad  140  to be squeezed. At this point, a second stage of expansion takes place with snorkel  98  then moving within the cylinder  120  in piston  96  to penetrate the mudcake  49  on the borehole wall  151  and to receive formation fluids.  
         [0066]     As seal pad  140  is pressed against the borehole wall, the pressure in circuit  200  rises and when it reaches a predetermined pressure, valve  192  opens so as to close equalizer valve  60 , thereby isolating fluid passageway  93  from the annulus. In this manner, valve  192  ensures that valve  60  closes only after the seal pad  140  has entered contact with mudcake  49  that lines borehole wall  151 . Passageway  93 , now closed to the annulus  150 , is in fluid communication with cylinder  175  at the upper end of cylinder  177  in draw down manifold  89 , best shown in  FIG. 3A .  
         [0067]     With solenoid valve  176  still energized, probe seal accumulator  184  is charged until the system reaches a predetermined pressure, for example 1800 p.s.i., as sensed by pressure transducer  160   b . When that pressure is reached, controller  190  energizes solenoid valve  178  to begin drawdown. Energizing solenoid valve  178  permits pressurized fluid to enter portion  172   a  of cylinder  172  causing draw down piston  170  to retract. When that occurs, plunger  174  moves within cylinder  177  such that the volume of fluid passageway  93  increases by the volume of the area of the plunger  174  times the length of its stroke along cylinder  177 . This movement increases the volume of cylinder  175 , thereby increasing the volume of fluid passageway  93 . For example, the volume of fluid passageway  93  may be increased by 10 cc as a result of piston  170  being retracted.  
         [0068]     As draw down piston  170  is actuated, formation fluid may thus be drawn through central passageway  127  of snorkel  98  and through screen  100 . The movement of draw down piston  170  within its cylinder  172  lowers the pressure in closed passageway  93  to a pressure below the formation pressure, such that formation fluid is drawn through screen  100  and snorkel  98  into aperture  101 , then through stem passageway  108  to passageway  91  that is in fluid communication with passageway  93  and part of the same closed fluid system. In total, fluid chambers  93  (which include the volume of various interconnected fluid passageways, including passageways in probe assembly  50 , passageways  85 ,  93  [ FIG. 3 ], the passageways interconnecting  93  with draw down piston  170  and pressure transducers  160   a,c ) may have a volume of approximately 40 cc. Drilling mud in annulus  150  is not drawn into snorkel  98  because pad  140  seals against the mudcake. Snorkel  98  serves as a conduit through which the formation fluid may pass and the pressure of the formation fluid may be measured in passageway  93  while pad  140  serves as a seal to prevent annular fluids from entering the snorkel  98  and invalidating the formation pressure measurement.  
         [0069]     Referring momentarily to  FIGS. 5 and 6 C, formation fluid is drawn first into the central bore  132  of screen  100 . It then passes through slots  134  in screen slotted segment  133  such that particles in the fluid are filtered from the flow and are not drawn into passageway  93 . The formation fluid then passes between the outer surface of screen  100  and the inner surface of snorkel extension  126  where it next passes through apertures  123  in screen  100  and into the central passageway  108  of stem  92  by passing through apertures  101  and central passage bore  103  of scraper  102 .  
         [0070]     Referring again to  FIG. 9 , with seal pad  140  sealed against the borehole wall, check valve  195  maintains the desired pressure acting against piston  96  and snorkel  98  to maintain the proper seal of pad  140 . Additionally, because probe seal accumulator  184  is fully charged, should tool  10  move during drawdown, additional hydraulic fluid volume may be supplied to piston  96  and snorkel  98  to ensure that pad  140  remains tightly sealed against the borehole wall. In addition, should the borehole wall  151  move in the vicinity of pad  140 , the probe seal accumulator  184  will supply additional hydraulic fluid volume to piston  96  and snorkel  98  to ensure that pad  140  remains tightly sealed against the borehole wall  151 . Without accumulator  184  in circuit  200 , movement of the tool  10  or borehole wall  151 , and thus of formation probe assembly  50 , could result in a loss of seal at pad  140  and a failure of the formation test.  
         [0071]     With the drawdown piston  170  in its fully retracted position and formation fluid drawn into closed system  93 , the pressure will stabilize and enable pressure transducers  160   a,c  to sense and measure formation fluid pressure. The measured pressure is transmitted to the controller  190  in the electronic section where the information is stored in memory and, alternatively or additionally, is communicated to the master controller in the MWD tool  13  below formation tester  10  where it may be transmitted to the surface via mud pulse telemetry or by any other conventional telemetry means.  
         [0072]     When drawdown is completed, piston  170  actuates a contact switch  320  mounted in endcap  400  and piston  170 , as shown in  FIG. 3A . The drawdown switch assembly consists of contact  300 , wire  308  coupled to contact  300 , plunger  302 , spring  304 , ground spring  306 , and retainer ring  310 . Piston  170  actuates switch  320  by causing plunger  302  to engage contact  300  that causes wire  308  to couple to system ground via contact  300  to plunger  302  to ground spring  306  to piston  170  to endcap  400  that is in communication with system ground (not shown).  
         [0073]     When the contact switch  320  is actuated controller  190  responds by shutting down motor  64  and pump  66  for energy conservation. Check valve  196  traps the hydraulic pressure and maintains piston  170  in its retracted position. In the event of any leakage of hydraulic fluid that might allow piston  170  to begin to move toward its original shouldered position, drawdown accumulator  186  will provide the necessary fluid volume to compensate for any such leakage and thereby maintain sufficient force to retain piston  170  in its retracted position.  
         [0074]     During this interval, controller  190  continuously monitors the pressure in fluid passageway  93  via pressure transducers  160   a,c  until the pressure stabilizes, or after a predetermined time interval.  
         [0075]     When the measured pressure stabilizes, or after a predetermined time interval, controller  190  de-energizes solenoid valve  176 . De-energizing solenoid valve  176  removes pressure from the close side of equalizer valve  60  and from the extend side of probe piston  96 . Spring  58  then returns the equalizer valve  60  to its normally open state and probe retract accumulator  182  will cause piston  96  and snorkel  98  to retract, such that seal pad  140  becomes disengaged with the borehole wall. Thereafter, controller  190  again powers motor  64  to drive pump  66  and again energizes solenoid valve  180 . This step ensures that piston  96  and snorkel  98  have fully retracted and that the equalizer valve  60  is opened. Given this arrangement, the formation tool  10  has a redundant probe retract mechanism. Active retract force is provided by the pump  66 . A passive retract force is supplied by probe retract accumulator  182  that is capable of retracting the probe even in the event that power is lost. Accumulator  182  may be charged at the surface before being employed downhole to provide pressure to retain the piston and snorkel in housing  12   c.    
         [0076]     Referring again briefly to  FIGS. 5 and 6 , as piston  96  and snorkel  98  are retracted from their position shown in  FIG. 6C  to that of  FIG. 6B  and then  FIG. 6A , screen  100  is drawn back into snorkel  98 . As this occurs, the flange on the outer edge of scraper  102  drags and thereby scrapes the inner surface of screen member  100 . In this manner, material screened from the formation fluid upon its entering of screen  100  and snorkel  98  is removed from screen  100  and deposited into the annulus  150 . Similarly, scraper  102  scrapes the inner surface of screen member  100  when snorkel  98  and screen  100  are extended toward the borehole wall.  
         [0077]     After a predetermined pressure, for example 1800 p.s.i., is sensed by pressure transducer  160   b  and communicated to controller  190  (indicating that the equalizer valve is open and that the piston and snorkel are fully retracted), controller  190  de-energizes solenoid valve  178  to remove pressure from side  172   a  of drawdown piston  170 . With solenoid valve  180  remaining energized, positive pressure is applied to side  172   b  of drawdown piston  170  to ensure that piston  170  is returned to its original position (as shown in  FIG. 3 ). Controller  190  monitors the pressure via pressure transducer  160   b  and when a predetermined pressure is reached, controller  190  determines that piston  170  is fully returned and it shuts off motor  64  and pump  66  and de-energizes solenoid valve  180 . With all solenoid valves  176 ,  178 ,  180  returned to their original position and with motor  64  off, tool  10  is back in its original condition and drilling may again be commenced.  
         [0078]     Relief valve  197  protects the hydraulic system  200  from overpressure and pressure transients. Various additional relief valves may be provided. Thermal relief valve  198  protects trapped pressure sections from overpressure. Check valve  199  prevents back flow through the pump  66 .  
         [0079]     The formation test tool  10  may operate in two general modes: pumps-on operation and pumps-off operation. During a pumps-on operation, mud pumps on the surface pump drilling fluid through the drill string  6  and back up the annulus  150  while testing. Using that column of drilling fluid, the tool  10  may transmit data to the surface using mud pulse telemetry during the formation test. The tool  10  may also receive mud pulse telemetry downlink commands from the surface. During a formation test, the drill pipe and formation test tool are not rotated. However, it may be the case that an immediate movement or rotation of the drill string will be necessary. As a failsafe feature, at any time during the formation test, an abort command may be transmitted from surface to the formation test tool  10 . In response to this abort command, the formation test tool will immediately discontinue the formation test and retract the probe piston to its normal, retracted position for drilling. The drill pipe may then be moved or rotated without causing damage to the formation test tool.  
         [0080]     During a pumps-off operation, a similar failsafe feature may also be active. The formation test tool  10  and/or MWD tool  13  may be adapted to sense when the mud flow pumps are turned on. Consequently, the act of turning on the pumps and reestablishing flow through the tool may be sensed by pressure transducer  160   d  or by other pressure sensors in bottom hole assembly  6 . This signal will be interpreted by a controller in the MWD tool  13  or other control and communicated to controller  190  that is programmed to automatically trigger an abort command in the formation test tool  10 . At this point, the formation test tool  10  will immediately discontinue the formation test and retract the probe piston to its normal position for drilling. The drill pipe may then be moved or rotated without causing damage to the formation test tool.  
         [0081]     The uplink and downlink commands are not limited to mud pulse telemetry. By way of example and not by way of limitation, other telemetry systems may include manual methods, including pump cycles, flow/pressure bands, pipe rotation, or combinations thereof. Other possibilities include electromagnetic (EM), acoustic, and wireline telemetry methods. An advantage to using alternative telemetry methods lies in the fact that mud pulse telemetry (both uplink and downlink) requires active pumping, but other telemetry systems do not. The failsafe abort command may therefore be sent from the surface to the formation test tool using an alternative telemetry system regardless of whether the mud flow pumps are on or off.  
         [0082]     The down hole receiver for downlink commands or data from the surface may reside within the formation test tool or within an MWD tool  13  with which it communicates. Likewise, the down hole transmitter for uplink commands or data from down hole may reside within the formation test tool  10  or within an MWD tool  13  with which it communicates. The receivers and transmitters may each be positioned in MWD tool  13  and the receiver signals may be processed, analyzed, and sent to a master controller in the MWD tool  13  before being relayed to local controller  190  in formation testing tool  10 .  
         [0083]     Commands or data sent from surface to the formation test tool may be used for more than transmitting a failsafe abort command. The formation test tool may have many preprogrammed operating modes. A command from the surface may be used to select the desired operating mode. For example, one of a plurality of operating modes may be selected by transmitting a header sequence indicating a change in operating mode followed by a number of pulses that correspond to that operating mode. Other means of selecting an operating mode will certainly be known to those skilled in the art.  
         [0084]     In addition to the operating modes discussed, other information may be transmitted from the surface to the formation test tool  10 . This information may include critical operational data such as depth or surface drilling mud density. The formation test tool may use this information to help refine measurements or calculations made downhole or to select an operating mode. Commands from the surface might also be used to program the formation test tool to perform in a mode that is not preprogrammed.  
         [0000]     Measuring Formation Properties  
         [0085]     Referring again to  FIG. 9 , the formation test tool  10  may include four pressure transducers  160 : two quartz crystal gauges  160   a ,  160   d , a strain gauge  160   c , and a differential strain gage  160   b . One of the quartz crystal gauges  160   a  is in communication with the annulus mud and also senses formation pressures during the formation test. The other quartz crystal gauge  160   d  is in communication with the flowbore  14  at all times. In addition, both quartz crystal gauges  160   a  and  160   d  may have temperature sensors associated with the crystals. The temperature sensors may be used to compensate the pressure measurement for thermal effects. The temperature sensors may also be used to measure the temperature of the fluids near the pressure transducers. For example, the temperature sensor associated with quartz crystal gauge  160   a  is used to measure the temperature of the fluid near the gage in chamber  93 . The third transducer is a strain gauge  160   c  and is in communication with the annulus mud and also senses formation pressures during the formation test. The quartz transducers  160   a ,  160   d  provide accurate, steady-state pressure information, whereas the strain gauge  160   c  provides faster transient response. In performing the sequencing during the formation test, chamber  93  is closed off and both the annulus quartz gauge  160   a  and the strain gauge  160   c  measure pressure within the closed chamber  93 . The strain gauge transducer  160   c  essentially is used to supplement the quartz gauge  160   a  measurements. When the formation tester  10  is not in use, the quartz transducers  160   a ,  160   d  may operatively measure pressure while drilling to serve as a pressure while drilling tool.  
         [0086]     Referring now to  FIG. 10 , a pressure versus time graph illustrates in a general way the pressure sensed by pressure transducers  160   a ,  160   c  during the operation of formation tester  10 . As the formation fluid is drawn within the tester, pressure readings are taken continuously by transducer  160   a ,  160   c . The sensed pressure will initially be equal to the annulus pressure shown at point  201 . As pad  140  is extended and equalizer valve  60  is closed, there will be a slight increase in pressure as shown at  202 . This occurs when the pad  140  seals against the borehole wall  151  and squeezes the drilling fluid trapped in the now-isolated passageway  93 . As drawn down piston  170  is actuated, the volume of the closed chamber  93  increases, causing the pressure to decrease as shown in region  203 . This is known as the pretest drawdown. The combination of the flow rate and snorkel inner diameter determines an effective range of operation for tester  10 . When the drawn down piston bottoms out within cylinder  172 , a differential pressure with the formation fluid exists causing the fluid in the formation to move towards the low pressure area and, therefore, causing the pressure to build over time as shown in region  204 . The pressure begins to stabilize, and at point  205 , achieves the pressure of the formation fluid in the zone being tested. After a fixed time, such as three minutes after the end of region  203 , the equalizer valve  60  is again opened, and the pressure within chamber  93  equalizes back to the annulus pressure as shown at  206 .  
         [0087]     In an alternative embodiment to the typical formation test sequence, the test sequence is stopped after pad  140  is extended and equalizer valve  60  is closed, and the slight increase in pressure is recorded as shown at  202  in  FIG. 10 . The normal test sequence is stopped so that a response to the increase in pressure  202  may be observed. Since the test sequence has been stopped before draw down piston  170  is actuated, no fluid flow has been induced by the formation probe assembly; the formation probe assembly is maintaining a substantially non-flow condition. The non-flow pressure response to increase  202  can be recorded and interpreted to determine properties of the mudcake, such as mobility. If the response to increase  202  is a quick equalization of the pressure back to hydrostatic  201 , then the mudcake has high permeability, and is most likely not very thick or durable. If the response is a slow decrease in pressure, then the mudcake is likely thicker and more impermeable.  
         [0088]     To assist in determining mudcake thickness, in addition to the method described above, the position indicator on the probe assembly, described in the U.S. patent application Ser. No. ______ entitled “Downhole Probe Assembly,” having U.S. Express Mail Label Number EV 303483549 US and Attorney Docket Number 1391-52601, may be used to measure how far the probe assembly extends after engagement with the mud filtrate. This measurement gives an indication of how thick the mud filtrate is, and may be used to bolster the data gathered using pressure response, described above. Again, this measurement may be taken under a non-flow condition of the formation probe assembly, as previously described.  
         [0089]     When taking pressure measurements, it is also possible to use the different pressure transducers to verify each gauge&#39;s reading compared to the others. Additionally, with multiple transducers, hydrostatic pressure in the borehole may be used to reverify gauges in the same location, by confirming that they are taking similar hydrostatic measurements. Because quartz gauges are more accurate, the quartz gauge response may be used to calibrate the strain gauge if the response is not highly transient.  
         [0090]      FIG. 11  illustrates representative formation test pressure curves. The solid curve  220  represents pressure readings P sg  detected and transmitted by the strain gauge  160   c . Similarly, the pressure P q , indicated by the quartz gauge  160   a , is shown as a dashed line  222 . As noted above, strain gauge transducers generally do not offer the accuracy exhibited by quartz transducers and quartz transducers do not provide the transient response offered by strain gauge transducers. Hence, the instantaneous formation test pressures indicated by the strain gauge  160   c  and quartz  160   a  transducers are likely to be different. For example, at the beginning of a formation test, the pressure readings P hyd1  indicated by the quartz transducer P q  and the strain gauge P sg  transducer are different and the difference between these values is indicated as E offs1  in  FIG. 11 .  
         [0091]     With the assumption that the quartz gauge reading P q  is the more accurate of the two readings, the actual formation test pressures may be calculated by adding or subtracting the appropriate offset error E off1  to the pressures indicated by the strain gauge P sg  for the duration of the formation test. In this manner, the accuracy of the quartz transducer and the transient response of the strain gauge may both be used to generate a corrected formation test pressure that, where desired, is used for real-time calculation of formation characteristics or calibration of one or more of the gauges.  
         [0092]     As the formation test proceeds, it is possible that the strain gauge readings may become more accurate or for the quartz gauge reading to approach actual pressures in the pressure chamber even though that pressure is changing. In either case, it is probable that the difference between the pressures indicated by the strain gauge transducer and the quartz transducer at a given point in time may change over the duration of the formation test. Hence, it may be desirable to consider a second offset error that is determined at the end of the test where steady state conditions have been resumed. Thus, as pressures P hyd2  level off at the end of the formation test, it may be desirable to calculate a second offset error E offS2 . This second offset error E offs2  might then be used to provide an after-the-fact adjustment to the formation test pressures, or calibration of the strain gauge.  
         [0093]     The offset values E offs1  and E offs2  may be used to adjust specific data points in the test. For example, all critical points up to P fu  might be adjusted using errors E offs1 , whereas all remaining points might be adjusted offset using error E offs2 . Another solution may be to calculate a weighted average between the two offset values and apply this single weighted average offset to all strain gauge pressure readings taken during the formation test. Other methods of applying the offset error values to accurately determine actual formation test pressures may be used accordingly and will be understood by those skilled in the art.  
         [0094]     As previously generally described, quartz gauges are used for accuracy because they are steady and stable over time and retain their calibration over a wide variety of conditions. However, they are slow to respond to their environment. There are changes in pressure taking place during the measurement that the quartz gauge cannot detect. On the other hand, strain gauges are susceptible to change and to calibration effects. However, they are quick to respond to changes in their environment. Thus, both gauges may be used, with the quartz gauge used to get an accurate pressure reading while the strain gauge is used to look at the differences in pressure.  
         [0095]     In another embodiment for calibrating the strain gauge using the quartzdyne gauge, a simple linear fit may be used. Referring to  FIG. 12 , pressure curve  500  is illustrated representing a typical drawdown and buildup curve measured during a pressure formation test. Portion  502  of curve  500  shows a stable pressure, which is typically a measure of the annulus pressure because the formation test has not begun yet. The annulus pressure will usually be higher than the formation pressure because most wells are drilled in overbalanced situations, where the drilling fluid in the annulus is kept at a higher pressure than the formation so as to stabilize the borehole and prevent borehole deterioration and blowout.  
         [0096]     The pressures measured by the quartz gauge, P Q1 , and the corrected strain gauge, P SG1 , will be the same in curve portion  502 , where the pressure is stable and near hydrostatic, and before any dynamic responses are detected by either gauge. Once the formation pressure test has begun, a slight increase in pressure is illustrated at  501  before the drawdown is commenced, illustrated by curve portion  504 . After drawdown is completed, the formation pressure is allowed to build back up until it stabilizes, illustrated at curve portion  506 . Now, a second set of stabilized pressures may be taken, P Q2  and P SG2 , and they will most likely be different because the dynamic response of the strain gauge is much less accurate than the dynamic response of the quartz gauge.  
         [0097]     To recalibrate the strain gauge, two unknown values are identified and a simple linear fit is applied to the known and unknown values. The unknown values may be identified as P off , representing the pressure offset between the two sets of stable pressure measurements, and P slope , representing the slope of the curve between the two sets of stable pressure measurements. The known values are P Q1 , P SG1 , P Q2  and P SG2 . The linear fit equations may be represented as: 
 
 P   Q1   =P   off +( P   slope   *P   SG1 ), and 
 
 P   Q2   =P   off +( P   slope   *P   SG2 ); which may be expressed as: 
 
 P   slope =( P   Q1   −P   Q2 )/( P   SG1   −P   SG2 ), and 
 
 P   off   =P   Q1 −( P   Q1   −P   Q2 )/( P   SG1   −P   SG2 )* P   SG1 ; which may be expressed as: 
 
 P   SG corrected   =P   off +( P   slope   *P   SG ). 
 
         [0098]     With two equations and two unknowns, the equations may be solved as above to arrive at P SGcorrected , a corrected value obtained from the strain gauge. Alternatively, the strain gauge may be corrected based on the known values alone, substituting for P off  and P slope  to acquire the equation: 
 
 P   SG corrected   =P   Q1 −( P   Q1   −P   Q2 )/( P   SG1   −P   SG2 )*( P   SG1   −P   SG2 ). 
 
         [0099]     Further, these gauge corrections may be done “on the fly,” or after each test as each sequential test is completed in the wellbore. The corrections may be done on the fly using real time streaming of the data to the surface using telemetry means, or, alternatively, using downhole processors and software placed in the tool.  
         [0100]     Using the MWD tool&#39;s embedded software (and neural network techniques) and a downhole reference standard, such as the quartz gauge, every depth point in the borehole may be corrected to the reference. In a formation tester, there will typically be various types of pressure gauges for measuring pressure in the flow lines that carry formation fluids. For example, the formation fluid flow lines, such as lines  91 ,  93  may be in fluid communication with quartz gauges and strain gauges, such as transducers  160   a ,  160   c  of  FIG. 9 . After a drawdown, where formation fluids are drawn into the formation tester, drawing in of fluids is stopped and the fluids are allowed to build back up to the pressure of the surrounding formation. After several of these drawdowns and buildups, the strain gauges may exhibit large errors in their readings. Thus, as mentioned before, these strain gauge pressure transducers need to be calibrated. In one embodiment, the pressure readings at every point in the well where pressure was measured may be used as a reference point for continual calibration of the strain gauges, thereby eliminating the need to calibrate and recalibrate the strain gauges.  
         [0101]     Every location in the well has a discrete pressure and associated temperature as well stabilization occurs. Each time a pressure test is run, the pressure taken by the quartz gauge may be used as a continual calibration point for the strain gauges. If the data is continuously collected, a three-dimensional, contour-type plot of pressure vs. temperature may be created. The three dimensions that may be used are measured pressure, reference pressure, as described above, and temperature. Then, neural network techniques found in the tool&#39;s embedded software may be applied to the collected data such that the strain gauge transducers do not require recalibration.  
         [0102]     Pressure transducers typically have a pressure data input range to which their accuracy is defined, such as zero to 10,000 p.s.i. or zero to 20,000 p.s.i. Accuracy is commonly measured as a percentage of full scale, thus the accuracy of a 10,000 p.s.i. gauge will be greater because the percentage number of that gauge will be less than the same percentage number of 20,000. To improve accuracy of the formation testing tool, several gauges may be used to cover the possible ranges of pressures to be tested, instead of using one gauge that covers the whole range. Therefore, to make the tool more accurate, multiple pressure gauges are used.  
         [0103]     Alternatively, the range of a gauge may be calibrated for a smaller range to make the gauge more accurate. The manufacturer of the pressure gauge may set the electronics to detect a broad range of pressures. The electronics, which are very similar between gauges, may be adjusted to scale the transducer over a smaller range, thereby improving accuracy. Similarly, the same transducer may be used for different pressure ranges by using two or more calibration tables. The pressure data output effect of the transducer for the full pressure input range may be determined for one pressure transducer, and then two or more calibration tables may be established to interpret the output information given by the transducers for different pressure input ranges. Therefore, accuracy may be improved without the use of multiple transducers.  
         [0104]     Accurate determination of formation pressure is vital to proper use of the measured formation pressures. However, changing densities of fluids in the formation testing tool&#39;s flow lines can be problematic. The measured pressure can be corrected for the density of the fluid in the vertical column of the flow line. The pressure transducers may be measuring accurate pressures of the formation fluids the transducers communicate with, but these transducers are removed from the location of the probe that gathers the formation fluids. For example, transducers  160   a ,  160   c ,  160   d  are located below the probe assembly, as illustrated in  FIG. 2D -E. Thus, the pressure at the probe may be different from the pressure measured at the transducers due to this location offset.  
         [0105]     Preferably, the vertical offset between the reference point of the transducer and the fluid inlet point at the probe is a known distance. Additionally, if the formation testing tool is located in a deviated or inclined well, the orientation of the tool may be known from a navigational package. Thus, vertical known distance between the transducer and the probe inlet may be calculated for any inclination of the tool in the well. Lastly, if the fluid present in the flow line connecting the transducer and the probe inlet is known, then the pressure gradient of that fluid may be used to calculate the pressure at the probe inlet with respect to the pressure at the transducer.  
         [0106]     For example, water has a pressure gradient of 0.433 p.s.i. per foot. If it was known that water was present in the flow line and that there was a foot difference between the pressure transducer and the probe inlet, a 0.433 p.s.i. correction may be made in the reading of the pressure transducer.  
         [0107]     Thus, it is preferred that the pressure transducers be disposed as close to the probe assembly as possible.  
         [0108]     In another embodiment of formation testing, while the formation probe assembly is engaged with the borehole, instead of pulling fluids into the probe assembly, or after pulling fluids into the probe assembly, fluids can be pushed out of the assembly into the formation. Thus, fluid communication may be established with the formation in the direction that is opposite to that of draw down, with such communication tending to pressure up the formation. This may be accomplished by adjustments to the sequence of events described previously. Now, the response to this pressure up can be recorded, and the pressure over time can observed for a portion of the formation. How the formation responds can be interpreted to obtain many of the formation properties previously described. Specifically, the pressure transient response to the change in formation pressure may be used to determine permeability of the mud cake, estimating the damage to the near wellbore formation and calculating mobility of the formation. For further detail on the process just described, reference may be made to the Society of Petroleum Engineers paper number 36524 entitled “Supercharge Pressure Compensation Using a New Wireline Method and Newly Developed Early Time Spherical Flow Model” and U.S. Pat. No. 5,644,075 entitled “Wireline Formation Tester Supercharge Correction Method,” each hereby incorporated herein by reference for all purposes.  
         [0109]     Furthermore, the formation may be pressured up as just described, except to the point where the formation material breaks or fractures. This is called an injectivity test, and may be done with fluid from the same area (at the present measurement location), or fluid, such as water, which may be obtained from another area of the formation. The fluids obtained from another area may be stored in either a pressure vessel or in the drawdown piston assembly, and then injected into another area that contains a different fluid. Fluids may also be carried from the surface and selectively injected into the formation.  
         [0110]     If injection rates are high enough to materially break or induce fracture in the formation, a change in pressure can be observed and interpreted, as has been previously described, to obtain formation properties, such as fracture pressure, which may be used to efficiently design future completion and stimulation programs. It should be noted that the injectivity may be performed to test the mud cake&#39;s ability to prevent fluid ingress to the formation. Alternatively, the test may be performed after a draw down and the mud cake is no longer present.  
         [0111]     Formation testers may also be used to gather additional information aside from properties of the producible hydrocarbon fluids. For example, the formation tester tool instruments may be used to determine the resistivity of the water, which can be used in the calculation of the formation&#39;s water saturation. Knowing the water saturation helps in predicting the producibility of the formation. Sensor packages, such as induction packages or button electrode packages, may be added adjacent the probe assembly that are tailored to measuring the resistivity of the bound water in the formation. These sensors, preferably, would be disposed on the extending portions of the probe assembly, such as the snorkel  98  that may penetrate the mudcake and formation, as illustrated in  FIG. 6C . In addition, sensors may be disposed in the flow lines, such as flow lines  91 ,  93 , to measure water properties in the fluids that are drawn into the formation tester assembly.  
         [0112]     The advantage of the probe style formation test tool described herein is the flexibility to place the probe in a specific position upon the borehole to best obtain a formation pressure, or, alternatively, to not place the probe in an undesirable location. A tool such as an acoustic imaging device can provide a real time image of the borehole so the operator can determine where to take a pressure test. Additionally, the image from a porosity-type tool may provide information on porosity quality at an orientation within a portion of the well at constant depth, or at a direction along the wellbore (constant azimuth). It may also provide a real-time image of fractures intersecting the wellbore, providing the opportunity to avoid these fractures to obtain a good test for matrix pressures, or to test at these fractures to determine fracture properties. The image from these tools may be sensitive enough to determine that the probe from the pressure device actually tested at the pre-determined position and verify that the test was taken at the chosen position. These tools may also be used to examine the condition of the wellbore. This may be significant in high angle or horizontal wellbores where debris such as unremoved cuttings may still be in place and could interfere with obtaining an accurate formation pressure measurement.  
         [0113]     It is common for the borehole to exhibit abnormalities due to erosion from the drill string or circulated drilling fluids. Abnormalities also exist due to fault lines and different types of formations abutting each other. Thus, often it is necessary to have a pre-existing image of the formation so that pressure measurements may be taken at pinpoint locations rather than at random locations in the formation. Acoustic, sonic, density, resistivity, gamma ray and other imaging techniques may be used to image the formation in real time. Then, the formation testing tool may be azimuthally oriented to locations of greatest or least porosity, permeability, density or other formation property, depending on what is to be gained from the pressure or other formation testing tool measurement. In cases where imaging tools indicate a sealing or “tight” zone, pressure measurements may be used to verify whether there is fluid communication or not. Alternatively, the imaging tools may be used to find zones that should not be pressure tested, such as highly dense or impermeable zones.  
         [0114]     Afterwards, the previously mentioned imaging techniques may be used to verify where the pressure or other measurement was taken. The seal pad may leave an imprint on the borehole wall, thus an electrical imaging tool or acoustic scanning tool may be used to image after the test to verify the pad location on the borehole wall.  
         [0115]     Pressure and other formation testing tool measurements may be taken with the mud pumps on or off. Pressure in the annulus is higher with pumps on than with pumps off, and the pressure drops in the direction of flow. With higher pressures from circulating, there is a higher rate of influx of drilling fluids and filtrate going into the formation, thus forming the mudcake more rapidly. The equivalent circulating density (ECD) is a measure of the drilling fluid density taking into account suspended drilling cuttings, fluid compressibility and the frictional pressure losses related to fluid flow. ECD will decrease with time if circulation continues but drilling stops because, as the drilling mud circulates, more of the drilling cuttings are filtered out while new cuttings are not being added. If pressure measurements are being taken by the formation tester, a difference may be noticed in the formation pressure because of the change in ECD from pumps-on to pumps-off.  
         [0116]     For example, the formation probe assembly may be extended and a drawdown test performed wherein the pressure decreases as the fluids are drawn into the formation tester. Then, after the drawdown chamber is full, the pressure may build back up to equilibrate with the pressure in the undisturbed formation. Now, if the pumps are turned on, the ECD in the annulus increases, increasing the pressure sensed by the formation tester. If the pumps are turned off, the pressure will return to the original pressure before pumps were turned on. This pressure difference is due to the difference in the ECD and the hydrostatic pressure, and may be used to indicate how much drilling fluid is penetrating the formation, or how much communication there is between the drilling fluids and the formation. This difference may be equated to mobility or pressure transients, thereby obtaining more accurate measurements. These effects are associated with supercharge pressures and effects, which are more thoroughly described in various of the previously incorporated references.  
         [0117]     With the pumps on, pressure pulses are sent downhole by the mud pumps, communication pulsers or other devices, and the pulses may be seen to exhibit sinusoidal behavior. During a pressure test, with the probe assembly extended, the probe may detect these pressure pulses through the formation because the inside of the probe assembly is relatively isolated from the wellbore fluids. The pressure pulses as detected in the wellbore may be compared with the pressure pulses as detected by the formation tester.  
         [0118]     Referring now to  FIG. 13 , a pressure pulse curve  600  represents pressures created by the mud pumps or pulsers and detected by a pressure sensor in communication with the annulus such as a PWD sensor in the MWD tool  13 , or other LWD tool. Pressure curve  602  represents pressures detected by the formation probe assembly, which are the pressure pulses that have traveled from the annulus, through the formation, and into the isolated probe assembly. Pressure curves  600  and  602  have peaks  604 ,  606  and  608 ,  610 , respectively. These peaks may be used to determine peak shifts or phase delay  612  and amplitude difference  614 . With the phase delay  612  and amplitude difference  614 , mudcake properties, such as permeability, porosity and thickness may be determined. Further, similar formation properties may be determined.  
         [0119]     In an alternative embodiment to the embodiment just described, the formation testing tool includes more than one formation probe assembly. Instead of creating pressure pulses at the surface of the wellbore, the pulses may be created by one probe assembly while the other probe assembly takes measurements. While at least two formation probe assemblies are extended and engaged with the borehole wall, one probe assembly may pulse fluid into the assembly and back out into the formation by reciprocating the draw down pistons. Meanwhile, the other probe assembly takes measurements as described above.  
         [0120]     Formation tests may be taken with the formation tester tool very soon after the drill bit has penetrated the formation. For example, the formation tests may be taken immediately after the formation has been drilled through, such as within ten minutes of penetration. Taking tests at this time means there is less mud invasion and less mudcake to contend with, resulting in better pressure and/or permeability tests, better formation fluid samples (less contamination) and less rig time required to obtain these data. Taking tests immediately after drilling will also allow the drilling operator look for casing points immediately. These tests may also indicate whether the zone is depleted, or whether hole collapse is imminent. Corrective actions may then be taken, such as casing the hole, changing mud properties, continuing drilling, or others.  
         [0121]     Additionally, the formation may be tested on the way into a drilled hole and on the way out to observe changes in the mudcake and formation over time. The two sets of measurements may be compared to identify changes that are occurring to the borehole and surrounding formation. The differences over time may indicate supercharging effects, more fully developed in the various references previously mentioned, and may be used to correct a model of the formation to account for the supercharge pressure.  
         [0122]     Predicting pore pressure is typically accomplished by measuring the magnitude of formation compaction. Formation compaction typically occurs in shales, thus shale formations must be drilled and logged to obtain the necessary data to create pore prediction models. The formation testing tool described herein may measure pore pressure directly. This measurement is more accurate and may be used to calibrate pore pressure predictor models.  
         [0000]     Using Formation Property Data  
         [0123]     After measuring formation pressure, permeability and other formation properties, this information may be sent to the surface using mud pulse telemetry, or any of various other conventional means for transmitting signals from downhole tools. At the surface, the drilling operator may use this information to optimize bit cutting properties or drilling parameters.  
         [0124]     Knowing mudcake properties allows adjustments to certain drilling parameters if the mudcake differs from a known, predetermined, or desirable value; adjustments to the mud system itself may also be made, to enhance the mud properties and reduce mud cake thickness or filtrate invasion rate. For example, if the mudcake is found to be contaminated or impermeable, the drilling mud properties can be adjusted to reduce the pressure on the mudcake or reduce the amount of contaminants ingressing into the mudcake, or chemicals may be added to the mud system to correct mud cake thickness.  
         [0125]     Furthermore, pressure measurements taken downhole may indicate the need to make downhole pressure adjustments if, again, the downhole measurements differ from a desirable known or predetermined value. However, instead of adjusting mud properties, other mechanical means may be use to control the downhole pressure. For example, with a choke control or a rotating blowout preventer (BOP), the choke or rotating BOP restriction may be manipulated to mechanically increase or decrease the resistance to flow at the surface, thereby adjusting the downhole pressure.  
         [0126]     An exemplary drilling parameter that may be adjusted is the rate of drill bit penetration. Using the formation tester in the ways described above, certain rock properties, also described above, can be measured. These properties may be directed to the surface in real time so as to optimize the rate of penetration while drilling. With a certain shape of the probe and knowing the shape of the frontal contact area of the borehole wall, certain formation properties may be measured. If a formation probe assembly such as that illustrated in  FIGS. 5 and 6 A-C, or in the U.S. patent application Ser. No. ______ entitled “Downhole Probe Assembly,” previously mentioned and incorporated by reference, is used to engage the formation, force vs. displacement of the probe assembly may then be determined using an extensiometer or potentiometer. The force vs. displacement information may be used to calculate compressive strength, compressive modulus and other properties of the formation materials themselves. These formation material properties are useful in determining and optimizing the rate of drill bit penetration.  
         [0127]     Measurements taken by the formation testing tool may be used for optimizing additional drilling applications. For example, formation pressure may be used to determine casing requirements. The formation pressures taken downhole may be used to determine the optimal size and strength of the casing required. If the formation is found to have a high formation pressure, then the hole may be cased with a relatively strong casing material to ensure that the integrity of the borehole is maintained in the high pressure formation. If the formation is found to have a low pressure, the casing size may be reduced and different materials may be used to save costs. Rock strength measurements taken with the tool may also assist with casing requirements. Solid rock formations require less casing material because they are stable, while formations composed of sediments require thicker casing.  
         [0128]     In inclined or horizontal wells, and particularly when the drilling fluid has stopped circulating, heavier density particles in the drilling fluid settle toward the lower side of the borehole. This condition is undesirable because the effective density of the fluid is lowered. When the surrounding formation is at a higher pressure than the drilling fluid, hole blowout becomes more likely. To detect this condition, the formation testing tool may be oriented to the low side of the borehole, where measurements may now be taken. In one embodiment, the probe assembly may be extended and pressures taken. Preferably, the pressure transducers that are in communication with the annulus, such as transducer  160   c  or the PWD sensor in the MWD tool, can be used to take the pressure of the annulus fluid without extending the probe. If the fluid on the low side of the borehole is found to have a higher density or weight than the equivalent drilling fluid density or weight, then the drilling fluid properties may be adjusted to correct this condition. Alternatively, or in addition, the measurements may be taken at other locations in the borehole, such as at the upper side.  
         [0129]     Anisotropic formations exhibit properties, any property, with different values when measured in different directions. For example, resistivity may be different in the horizontal direction than in the vertical direction, which may be due to the presence of multiple formation beds or layering within certain types of rocks.  
         [0130]     For example, formation anisotropy may be determined by taking formation measurements, such as pressure and temperature, re-orienting the tool rotationally and taking additional measurements at additional angles around the borehole. Alternatively, if multiple probe assemblies or other measuring devices are disposed about the tool, these measurements taken about the tool may be taken simultaneously. In addition to taking direct formation measurements, the tool may take other measurements, such as sonic and electromagnetic measurements. After all such measurements have been taken, the formation anisotropy for each type of measurement may be calculated. A formation anisotropy value may be tied to or compared with acoustic, resistivity and other measurements taken by other tools. This would allow, for example, resistivity to be correlated with permeability changes using known formation models (more fully described below).  
         [0131]     Typically, formation pressure measurements are estimated and/or predicted by interpreting certain formation measurements other than the direct measurement of formation pressure. For example, pressure while drilling (PWD) and logging while drilling (LWD) measurements are gathered and analyzed to predict what the actual formation pressure is. Analysis of data such as rock properties and stress orientation, and of models such as fracture-gradient models and trend-based models, can be used to predict actual formation pressure. Furthermore, direct formation measurements may be used too supplement, correct or adjust these data and models to more accurately predict formation pressures. The advantage with the formation testing tools described and referenced herein is that the pressure and other formation data may be sent uphole real time, thereby allowing the models to be updated real time.  
         [0132]     Additionally, each measured formation property, including those previously listed and defined, may themselves be used to map or image the formation. Ultimately, a formation model is developed so it is known what the formation looks like on a computer screen at the surface of the borehole. An example of such a formation model is the Landmark earth model. Each additional measured property of the formation may be used to make complementary images, with each new property and image adding to the accuracy of the formation model or image. Thus, the properties gathered by the formation tester tools referenced herein, particularly pressure data, may be used to create better models or enhance existing ones, to better understand the formations that are being penetrated. As described before, these models and data may be updated “on the fly” to calibrate various models for better formation pressure predictions.  
         [0133]     Similarly, formation test data, such as pressure, temperature and other previously described data, gathered using a formation testing tool  10  may be used to improve or correct other measurements, and vice-versa. Other measurements that may benefit from real time pressure data and pressure gradient information include: pressure while drilling (PWD), sonic or acoustic tool measurements, nuclear magnetic resonance imaging, resistivity, density, porosity, etc. These measurements or interpretive tools, such as pore-pressure prediction tools or models, may be updated based on physical measurements, and are at least somewhat dependent on pressure or other formation properties. Drilling mud properties may also be adjusted in a similar fashion, based on the formation measurements taken real time. Further, the formation data may be used to assist other services, including drilling fluid services and completion services, and operation of other tools.  
         [0134]     While drilling, LWD tools may be measuring the resistivity of the formation fluids and creating resistivity logs. From the resistivity log and other data, water saturation of the formation may be calculated. Changes in water saturation with depth may be observed and may be consolidated into a gradient. The water saturation level is related to how far above the 100% free water level the test depth is. The water saturation levels and gradient may be used to create a capillary pressure curve. The pressure data from the formation testing tool may be matched up with the capillary pressure curve, which may then be projected downhole to estimate the free water level. The free water level may be used to determine the amount of hydrocarbons, especially gas, that are available for production. At the 100% free water level, production is not viable. Thus, the free water level may be determined without having to test down to the actual free water level.  
         [0135]     Pressure measurements may also be used to steer the bottom hole assembly (BHA). If formation pressure measurements indicate that the current zone is not producible or otherwise unattractive for drilling, then the BHA, including the drill bit, may be steered in another direction. An example of a steerable BHA assembly is Halliburton&#39;s GeoPilot system. Such directional drilling is intended to steer the BHA into the highest pressure portions of the reservoir, maintain the BHA in the same pressure zone, or avoid a decreased pressure zone. Again, petrophysical data, such as those formation properties previously mentioned, may also be used to more accurately steer the BHA.  
         [0136]     The bubble point, as previously defined, can be a beneficial real time measurement. Measuring changes in the bubble point of formation fluids with depth of the formation tester tool in the wellbore allows a bubble point gradient to be determined. Plotting the bubble point gradient generally allows transitions back and forth between gas, water and oil and to be observed, or identification of a zone that is not connected to another zone based on downhole pressure measurements. The bubble point gradient may be used to steer the BHA. Steering downward toward denser fluids is desirable, as the lighter fluids, i.e., the ones having higher bubble points due to retaining more dissolved gases, tend to move upward. Therefore, as fluids with lower bubble points are encountered, the BHA is steered toward these fluids.  
         [0137]     The bubble point gradient, as well as other gradients, may be computed on the fly as bubble points and pressure measurements are taken at different depths during the same trip into the borehole. The data is sent to the surface real time for the gradients to be calculated and used.  
         [0138]     As described above, pressure while drilling, taken in the annulus, and actual formation pressure are two distinct measurements. With the ability to obtain actual formation pressure, these two measurements may be combined and interpreted for flags, or warnings, and the flags may then be sent to the surface. Prior to the advent of FTWD, these measurements had to combined and interpreted at the surface because actual formation pressure could only be obtained after drilling had stopped. Therefore, the warning could only be determined after the fact. The types of flags that may be sent to the surface include the annulus pressure being below the formation pressure and the annulus pressure being above the fracture gradient.  
         [0139]     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.