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FIELD OF THE INVENTION 
     The invention relates generally to formation fluid testing and collection apparatus and more particularly to a formation tester that reduces the contamination caused by borehole fluids in recovered formation fluids. 
     BACKGROUND OF THE INVENTION 
     In the oil and gas industry, formation testing tools have been used for monitoring formation pressures along a wellbore, obtaining formation fluid samples from the wellbore and predicting performance of reservoirs around the wellbore. Such formation testing tools typically contain an elongated body having an elastomeric packer that is sealingly urged against the zone of interest in the wellbore to collect formation fluid samples in storage chambers placed in the tool. 
     During drilling of a wellbore, a drilling fluid (“mud”) is used to facilitate the drilling process and to maintain a pressure in the wellbore greater than the fluid pressure in the formations surrounding the wellbore. This is particularly important when drilling into formations where the pressure is abnormally high: if the fluid pressure in the borehole drops below the formation pressure, there is a risk of blowout of the well. As a result of this pressure difference, the drilling fluid penetrates into or invades the formations for varying radial depths (referred to generally as invaded zones) depending upon the types of formation and drilling fluid used. The formation testing tools retrieve formation fluids from the desired formations or zones of interest, test the retrieved fluids to ensure that the retrieved fluid is substantially free of mud filtrates, and collect such fluids in one or more chambers associated with the tool. The collected fluids are brought to the surface and analyzed to determine properties of such fluids and to determine the condition of the zones or formations from where such fluids have been collected. 
     One feature that all such testers have in common is a fluid sampling probe. This may consist of a durable rubber pad that is mechanically pressed against the rock formation adjacent the borehole, the pad being pressed hard enough to form a hydraulic seal. Through the pad is extended one end of a metal tube that also makes contact with the formation. This tube (“probe”) is connected to a sample chamber that, in turn, is connected to a pump that operates to lower the pressure at the attached probe. When the pressure in the probe is lowered below the pressure of the formation fluids, the formation fluids are drawn through the probe into the well bore to flush the invaded fluids prior to sampling. In some prior art devices, a fluid identification sensor determines when the fluid from the probe consists substantially of formation fluids; then a system of valves, tubes, sample chambers, and pumps makes it possible to recover one or more fluid samples that can be retrieved and analyzed when the sampling device is recovered from the borehole. 
     It is critical that only uncontaminated fluids are collected, in the same condition in which they exist in the formations. Commonly, the retrieved fluids are found to be contaminated by drilling fluids. This may happen as a result of a poor seal between the sampling pad and the borehole wall, allowing borehole fluid to seep into the probe. The mudcake formed by the drilling fluids may allow some mud filtrate to continue to invade and seep around the pad. Even when there is an effective seal, borehole fluid (or some components of the borehole fluid) may “invade” the formation, particularly if it is a porous formation, and be drawn into the sampling probe along with connate formation fluids. 
     In prior art operations, the pressure in the probe, and their connecting hydraulics flow line is lowered below the pressure of the fluid in the formation, drawing fluid from the formation into the probe, through the hydraulic flow line to the well bore. A fluid identification sensor may be installed in the hydraulic flow line, the fluid identification sensor producing a signal indicative of the composition of the fluid passing through it. When the fluid identification sensor determines that the fluid being pumped is primarily formation fluid, a sample chamber valve is opened and the sample chamber is filled. 
     Additional problems arise in Drilling Early Evaluation Systems (EES) where fluid sampling is carried out very shortly after drilling the formation with a bit. Inflatable packers or pads cannot be used in such a system because they are easily damaged in the drilling environment. In addition, when the packers are extended to isolate the zone of interest, they completely fill the annulus between the drilling equipment and the wellbore and prevent circulation during testing. Additionally, when an EES is used, there may be little or no mud cake formation prior to the test. A mud cake helps in sealing the formation from well bore fluids whereas in the absence of a mudcake, fluid leakage can be a serious problem. Pads are not adequate to provide a seal in the absence of a mudcake. 
     There is a need for an invention that reduces the leakage of borehole fluid into the sampling probe by isolating the probe from the borehole fluid. Such an invention should also reduce the amount of borehole fluid contaminating the connate fluid being withdrawn from the formation by the probe. Additionally, the invention should be able to sample formation fluids even when the mudcake is thin or non existent. There is a need for an invention that reduces the time spent on sampling and flushing of contaminated samples. The present invention satisfies this need. 
     SUMMARY OF THE INVENTION 
     One embodiment of the invention, suitable for use on a wireline, employs a hydraulic guard ring surrounding the probe tube to isolate the probe from the borehole fluid. The guard ring is provided with its own flow line and sample chamber, separate from the flow line and the sample chamber of the probe. By maintaining the pressure in the guard ring at or slightly below the pressure in the probe tube, most of the fluid drawn into the probe will be connate formation fluid. The same result is also obtained by using inflatable packer elements to create a guard ring above and below the sampling section. An alternate embodiment of the invention useful in Drilling Early Evaluation Systems uses two sets of seal elements are used to obtain an uncontaminated fluid sample. Two thin seals, such as the wall of a small pipe are employed to isolate two areas of the formation at the borehole wall: one between the inner and outer seals and the second in the center of the inner seal. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 is a simplified schematic illustration of an embodiment of the present invention; 
     FIG. 2 shows a detail of the arrangement of the guard ring in the embodiment illustrated in FIG. 1; 
     FIG. 3 is a simplified schematic illustration of an alternate embodiment of the present invention using inflatable packers on a wireline; 
     FIG. 4 is a simplified schematic illustration of an embodiment of the invention for use in a drilling Early Evaluation System using snorkel tubes; 
     FIG. 5 illustrates some possible arrangements of the tubes in the invention of FIG. 4; 
     FIG. 6 is a simplified schematic illustration of the invention for use in a drilling Early Evaluation System using inflatable packers on a drill pipe; 
     FIG. 7 shows the simulation of fluid flow in a prior art device; 
     FIG. 8 shows a simulation of the direction of fluid flow in the vicinity of a fluid sampling pad. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is best understood by reference to FIGS. 1-3. FIG. 1 is a schematic illustration of the preferred embodiment of the present invention. A portion of a borehole  1  is shown in a subterranean formation  7 . The borehole wall is covered by a mudcake  5 . The formation tester body  9  is connected to a wireline  3  leading from a rig at the surface (not shown). Alternatively, the formation tester body may be carried on a drillstring. The details of the method of connection of the tester body to a wireline or drillstring would be familiar to those versed in the art. 
     The formation tester body is provided with a mechanism, denoted by  10 , to clamp the tester body at a fixed position in the borehole. This clamping mechanism is at the same depth as a probe and guard ring arrangement, details of which are seen in FIG.  2 . 
     By means of the clamping mechanism,  10 , a fluid sampling pad,  13 , is mechanically pressed against the borehole wall. A probe tube,  17 , is extended from the center of the pad, through the mud cake,  5 , and pressed into contact with the formation. The probe is connected by a hydraulic flow line,  23   a,  to a probe sample chamber,  27   a.    
     The probe is surrounded by a guard ring,  15 . The guard ring is a hydraulic tube, formed into a loop, that encircles the probe. The guard ring has suitable openings along its length, the openings being in contact with the formation. The guard ring is connected by its own hydraulic flow line,  23   b , to a guard sample chamber,  27   b . Because the flow line  23   a  of the probe,  17 , and flow line  23   b  of the guard ring,  15 , are separate, the fluid flowing into the guard ring does not mix with the fluid flowing into the probe. The guard ring isolates the flow into the probe from the borehole beyond the pad  13 . Thus three zones are defined in the borehole: a first zone consisting of the borehole outside the pad  13 , a second zone (the guard zone) consisting of the guard ring  15  and a third zone (probe zone) consisting of the probe  17 . The probe zone is isolated from the first zone by the guard zone. 
     The hydraulic flow lines  23   a  and  23   b  are each provided with pressure transducers  11   a  and  11   b.  The pressure maintained in the guard flowline is the same as, or slightly less than, the pressure in the probe flowline. With the configuration of the pad and the guard ring, borehole fluid that flows around the edges of the pad is preferentially drawn into the guard ring,  15 , and diverted from entry into the probe,  17 . 
     The flow lines  23   a  and  23   b  are provided with pumps  21   a  and  21   b.  These pumps are operated long enough to substantially deplete the invaded zone in the vicinity of the pad and to establish an equilibrium condition in which the fluid flowing into the probe is substantially free of contaminating borehole filtrate. 
     The flow lines  23   a  and  23   b  are also provided with fluid identification sensors,  19   a  and  19   b . This makes it possible to compare the composition of the fluid in the probe flowline  23   a  with the fluid in the guard flowline  23   b . During initial phases of operation of the invention, the composition of the two fluid samples will be the same; typically, both will be contaminated by the borehole fluid. These initial samples are discarded. As sampling proceeds, if the borehole fluid continues to flow from the borehole towards the probe, the contaminated fluid is preferentially drawn into the guard ring. Pumps  21   a  and  21   b  discharge the sampled fluid into the borehole. At some time, an equilibrium condition is reached in which contaminated fluid is drawn into the guard ring and uncontaminated fluid is drawn into the probe. The fluid identification sensors  19   a  and  19   b  are used to determine when this equilibrium condition has been reached. At this point, the fluid in the probe flowline is free or nearly free of contamination by borehole fluids. Valve  25   a  is opened, allowing the fluid in the probe flowline  23   a  to be collected in the probe sample chamber  27   a . Similarly, by opening valve  25   b , the fluid in the guard flowline is collected in the guard sample chamber  27   b . The ability to pump from the guard ring into the guard sample chamber is one of the novel features of the invention: this results in an increased rate of flow from the formation into the probe and thereby improves the shielding effect of the guard ring. Alternatively, the fluid gathered in the guard ring can be pumped to the borehole while the fluid in the probe line is directed to the probe sample chamber  27   a . Sensors that identify the composition of fluid in a flowline would be familiar to those knowledgeable in the art. 
     FIG. 3 shows an alternate embodiment of the invention. A portion of a borehole  101  is shown in a subterranean formation  107 . The borehole wall is covered by a mudcake  105 . The formation tester body  109  is connected to a wireline  103  leading from a rig at the surface (not shown). The details of the method of connection of the tester body to the wireline would be familiar to those versed in the art. 
     The formation tester body is provided with inflatable flow packers  112  and  112 ′ and inflatable guard packers  110  and  110 ′. When the formation tester is at the depth at which formation fluids are to be sampled, the inflatable packers  110 ,  110 ′,  112  and  112 ′ are inflated to form a tight seal with the borehole wall and mudcake  105 . The mechanism for activating the packers would be familiar to those versed in the art. 
     A hydraulic flow line (probe flowline)  123   a  is connected to an opening  114  in the tester located between the flow packers  112  and  112 ′ and to a probe sample chamber  127   a . This serves to sample formation fluid that flows into the borehole between the two flow packers. A second hydraulic flow line (guard flowline)  123   b  is connected to openings  116  and  116 ′ in the tester located between the guard packer  110  and the flow packer  112  and between the guard packer  110 ′ and flow packer  112 ′ respectively. The guard flowline is connected to a guard sample chamber  127   b . Thus three zones are defined in the borehole: a first zone consisting of the borehole above the packer  110  and below the packer  110 ′, a second zone (the guard zone) consisting of the region between the packers  110  and  112  and between the packer  110 ′ and  112 ′; and a third zone (probe zone) consisting of the zone between the packers  112  and  112 ′. The probe zone is isolated from the first zone by the guard zone. 
     The hydraulic flow lines  123   a  and  123   b  are each provided with pressure transducers  111   a  and  111   b.  The pressure maintained between each of the flow packers and the adjacent guard packer is the same as, or slightly less than, the pressure between the two flow packers. With the configuration of the guard and flow packers, borehole fluid that flows around the edges of the guard packers is preferentially drawn into the guard flowline  123   b , and diverted from entry into the probe flowline  123   a.    
     The flow lines  123   a  and  123   b  are provided with pumps  121   a  and  121   b . These pumps are operated long enough to substantially deplete the invaded zone in the vicinity of the tool and to establish an equilibrium condition in which the fluid flowing into the probe flowline is substantially free of contaminating borehole filtrate. 
     The flow lines  123   a  and  123   b  are also provided with fluid identification sensors,  119   a  and  119   b . This makes it possible to compare the composition of the fluid in the probe flowline  123   a  with the fluid in the guard flowline  123   b . During initial phases of operation of the invention, the composition of the two fluid samples will be the same; typically, both will be contaminated by the borehole fluid. These initial samples are discarded. As sampling proceeds, if the borehole fluid continues to flow from the borehole towards the opening  114 , the contaminated fluid is preferentially drawn into the openings  116  and  116 ′. Pumps  121   a  and  121   b  discharge the sampled fluid into the borehole. At some time, an equilibrium condition is reached in which contaminated fluid is drawn into the guard flowline and uncontaminated fluid is drawn into the probe flowline. The fluid identification sensors  119   a  and  119   b  are used to determine when this equilibrium condition has been reached. At this point, the fluid in the probe flowline is free or nearly free of contamination by borehole fluids. Valve  125   a  is opened, allowing the fluid in the probe flowline  123   a  to be collected in the probe sample chamber  127   a . Similarly, by opening valve  125   b , the fluid in the guard flowline is collected in the guard sample chamber  127   b . The ability to pump from the guard ring into the guard sample chamber is one of the novel features of the invention: this results in an increased rate of flow from the formation into the probe and thereby improves the shielding effect of the guard ring. 
     FIG. 4 shows an alternate embodiment of the invention suitable for use in a drilling early evaluation system (EES). The borehole wall  205  in a formation  207  is indicated. The EES tool  209  is inside the borehole and attached to the drilling means (not shown). For simplicity of illustration, only one side of the EES tool is shown. Contact with the formation is accomplished by means of an outer snorkel tube  215  and an inner snorkel tube  217 . The two tubes are independently movable, the inner snorkel tube  217  having the capability of penetrating deeper into the formation. Means for operating snorkel tubes of this kind would be familiar to those knowledgeable in the art. 
     The inner snorkel tube  217  is connected to probe flowline  223   a  while the region between the inner snorkel tube  217  and the outer snorkel tube  215  defines a guard zone that is connected to the guard flowline  223   b . Flowlines  223   a  and  223   b  are provided with pumps and sample chambers (not shown). The inner snorkel tube  217  defines a probe zone that is isolated by the outer snorkel tube  215  from the portion of the borehole outside the outer snorkel tube. These pumps are operated long enough to substantially deplete the invaded zone in the vicinity of the outer snorkel tube  215  and to establish an equilibrium condition in which the fluid flowing into the inner snorkel tube is substantially free of contaminating borehole filtrate. When the equilibrium condition is reached, contaminated fluid is drawn into the guard zone and uncontaminated fluid is drawn into the inner snorkel tube. At this time, sampling is started with the pumps continuing to operate for the duration of the sampling. As sampling proceeds, the borehole fluid continues to flow from the borehole towards the probe, while the contaminated fluid is preferentially drawn into the outer snorkel tube. Pumps (not shown) discharge the contaminated fluid into the borehole. The fluid from the inner snorkel tube is retrieved to provide a sample of the formation fluid. 
     FIGS. 5 a - 5   c  show alternative arrangements of the snorkel tube. In FIG. 5 a , the inner snorkel tube  241  and the outer snorkel tube  243  are shown as concentric cylinders. In FIG. 5 b , the annular region between the inner snorkel tube  245  and the outer snorkel tube  247  is segmented by means of a plurality of dividers  249 . FIG. 5 c  shows an arrangement in which the guard zone is defined by a plurality of tubes  259  interposed between the inner snorkel tube  255  and the outer snorkel tube  257 . In any of these configurations, a wire mesh or a gravel pack may also be used to avoid damage to the formation. 
     FIG. 6 shows an alternative EES tool that uses short packers instead of the snorkel tubes. The packers may be inflatable or may be expandable metal packers. A portion of a borehole  301 , is shown in a subterranean formation,  307 . The borehole wall is shown at  305 . The formation tester body  309 , is connected to a drilling apparatus. The EES tool is provided with short flow packers  312  and  312 ′ and guard packers  310  and  310 ′. The zone between the flow packers  312  and  312 ′ defines the probe zone while the zone between the flow packers and the guard packers  310  and  310 ′ defines the guard zone. When the formation tester is at the depth at which formation fluids are to be sampled, the inflatable packers  310 ,  310 ′,  312  and  312 ′ are inflated to form a tight seal with the borehole wall  305 . The mechanism for activating the packers would be familiar to those versed in the art. Thus three zones are defined in the borehole: a first zone consisting of the borehole above the packer  310  and below the packer  310 ′, a second zone (the guard zone) consisting of the region between the packers  310  and  312  and between the packer  310 ′ and  312 ′; and a third zone (probe zone) consisting of the zone between the packers  312  and  312 ′. The probe zone is isolated from the first zone by the guard zone. 
     A hydraulic flow line (probe flowline),  323 , is connected to an opening,  314 , in the tester located in the probe zone and to a pump (not shown). This serves to sample formation fluid that flows into the borehole between the two flow packers. A second hydraulic flow line (guard flowline),  323   b , is connected to openings  316  and  316 ′ in the tester located between the guard zone. The pumps are operated long enough to substantially deplete the invaded zone in the vicinity of the pad and to establish an equilibrium condition in which the fluid flowing into the inner snorkel tube is substantially free of contaminating borehole filtrate. As sampling proceeds, if the borehole fluid continues to flow from the borehole towards the probe, the contaminated fluid is preferentially drawn into the guard ring. Pumps (not shown) discharge the sampled fluid into the borehole. At some time, an equilibrium condition is reached in which contaminated fluid is drawn into the guard zone and uncontaminated fluid is drawn into the inner snorkel tube. This fluid is retrieved to provide a sample of the formation fluid. The pumps continue to operate during the process of retrieval of the formation fluid from the inner snorkel tube. 
     The walls of the packers need only be thick enough to provide the necessary structural arrangement wherein the flow into the inner tube is isolated from the flow outside; this means that problems encountered in prior art where, in the absence of a mudcake, leakage occurs around the packers is circumvented. 
     EXAMPLES 
     The effectiveness of the focused type probe is demonstrated by the results of a finite element simulation shown in FIGS. 7 and 8. In both figures, one fourth of the pad area is shown with the remaining portion cut away to see into the formation. FIG. 7 is for the simulation of an unfocussed flow, i.e., a conventional probe according to prior art. In FIG. 7, the direction labeled  421  is radial and into the formation,  425  follows the borehole wall vertically and  423  follows the borehole wall circumferentially. The center of the probe is at the intersection of  421 ,  423  and  425 . The arrows in FIG. 7 show the direction of fluid flow in the simulation. The zones labeled  427  and  427 ′ show that borehole fluid is flowing into the probe and contaminating the fluid drawn into the probe. In addition, the zone labeled as  429  generally corresponds to borehole fluids that have invaded the formation and are flowing back into the probe. 
     FIG. 8 is for the simulation of a focused flow, i.e., a probe according to the present invention. The direction labeled  431  is radial and into the formation,  435  follows the borehole wall vertically and  433  follows the borehole wall circumferentially. The center of the probe is at the intersection of  431 ,  433  and  435 . The arrows in show the direction of fluid flow in the simulation. It can be seen in FIG. 8 that in the zones corresponding to  427  and  427 ′ in FIG. 7, the flow direction is radial, i.e., the borehole fluid is not being drawn into the probe. Instead, the borehole fluid flows into the zone labeled as  437 . This corresponds to the position of the guard ring, packer or snorkel tube. Furthermore, in the zone corresponding to  429  in FIG. 7, the flow direction is radial, indicating that the probe is effectively draining fluid from deeper into the formation with less contamination by invaded borehole fluids. 
     The foregoing description has been limited to specific embodiments of this invention. It will be apparent, however, that variations and modifications may be made to the disclosed embodiments, with the attainment of some or all of the advantages of the invention. Therefore, it is the object of the appended claims to cover all such variations and modifications as come within the true spirit and scope of the invention.

Summary:
A formation fluid sampling probe uses two hydraulic lines to recover formation fluids from two zones in a borehole. One of the zones is a guard zone and the other is a probe zone. The guard zone and the probe zone are isolated from each other by mechanical means, with the guard zone surrounding the probe zone and shielding it from the direct access to the borehole fluids. Operation of the tool involves withdrawal of fluid from both zones. Borehole fluids are preferentially drawn into the guard zone so that the probe zone recovers the formation fluid substantially free of borehole fluids. Separation of the guard zone from the probe zone may be accomplished by means of an elastomeric guard ring, by inflatable packers or by tubing. The device can be adapted for use either on a wireline or in an early evaluation system on a drillstring.