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FIELD OF THE INVENTION  
         [0001]    This invention relates to the field of testing formations surrounding an earth borehole with a formation testing tool and, more particularly, to improvements in formation sampling, pressure measurement, and other measurements.  
         BACKGROUND OF THE INVENTION  
         [0002]    Existing well logging devices can provide useful information about hydraulic properties of formations, such as pressures and fluid flow rates, and can obtain formation fluid samples for uphole analysis. Reference can be made, for example, to U.S. Pat. Nos. 3,934,468 and 4,860,581. In a logging device of this general type, a setting arm or setting pistons can be used to controllably urge the body of the logging device against a side of the borehole at a selected depth. The side of the device that is urged against the borehole wall includes a packer which surrounds a probe. As the setting arm extends, the probe is inserted into the formation, and the packer then sets the probe in position and forms a seal around the probe, whereupon formation pressure can be measured and fluids can be withdrawn from the formation.  
           [0003]    In certain prior art application of formation testing, a gravel pack is provided at the place where the formation is perforated, to help maintain the integrity of the perforation and to prevent sand from clogging the opening. This gravel pack is generally not made too fine, as it would then tend to clog easily. From experience, it has been recognized that even when a gravel pack is employed at the probe, some sand tends to flow with fluid being sampled. This sand can cause problems with down hole pumps either plugging or by compromising the seals in the pump. As a result of the sand entering the pump, formation tester logging jobs may have to be terminated prematurely or multiple trips in the well may be necessary to acquire all the desired fluid samples. In addition, sampling during cased hole formation tester jobs can be compromised due to metal shavings from the casing entering the formation tester&#39;s down hole pump.  
           [0004]    It is among the objects of the present invention to address this problem of prior art formation testing tools.  
           [0005]    It is also among the objects of the present invention to improve formation testing methods and equipment to obtain enhanced information about the nature of formations being tested, including characteristics of solid components of the formations being tested.  
         SUMMARY OF THE INVENTION  
         [0006]    In accordance with a form of the invention, there is set forth an apparatus for testing formations surrounding an earth borehole. A tool, movable through the borehole, is provided, the tool having a flow line running therethrough. Means, in the tool, are provided for establishing fluid communication between the formations and the flow line in the tool. A sand trap is provided in the tool for trapping sand in the fluid from the formations travelling in the flow line. In an embodiment of this form of the invention, the sand trap comprises a receptacle containing a screen that is operative to cause precipitation of sand in the formation fluid and also to filter sand from the formation fluid. In this embodiment, the tool includes a pump in the flow line, and the sand trap is located so that sand-containing fluid from the formations reaches the sand trap before reaching the pump.  
           [0007]    In accordance with another form of the invention a method is set forth for testing formations surrounding an earth borehole, including the following steps: providing a tool movable through the borehole; providing a flow line in the tool; establishing fluid communication between the formations and the flow line of the tool; and providing a sand trap in communication with the flow line of the tool for trapping sand flowing with fluid from the formations. An embodiment of this form of the invention further comprises bringing the tool to the earth&#39;s surface, collecting the sand from the sand trap, and analyzing the sand to determine properties thereof.  
           [0008]    In accordance with a feature of the present invention, the obtained sand is a sample of recovered reservoir rock at the point or points of measurement and sampling in the borehole. Among the advantages of having such samples are the following: knowing of sand texture (grain size, shape and sorting) can provide key indicators of depositional environment; the nature of the sand sample can provide information on types and degree of cementation, as well as porosity and permeability indications, and lithologic and facies verification; and indication of clay minerals in the sampled zone can help in deposition environment models and completion and production design. The foregoing can be especially useful in thin beds where formation properties change drastically within very short vertical distances. Evidence of good quality sands within the interbeds would reinforce reservoir characterization. Also, sand grain size and distribution can provide important information in sand control completion design, and gravel pack mesh size and gravel pack screen design can be significantly improved if one has a sand sample.  
           [0009]    In accordance with a further feature of the invention, a useful pressure measurement can be taken when sand starts to flow, without fouling of the tool. The pressure inside the tool at which formation sand grains are mobilized represents a measurement of the formation failure pressure or differential pressure (difference between formation pressure and tool pressure). This measurement represents the condition of differential pressure at which the well will start producing sand along with formation fluids during the production phase. This value of differential pressure at which the formation will fail can be used to design well completion and production strategies.  
           [0010]    Further features and advantages of the invention will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a diagram, partially in block form, of a logging device in which embodiments of the invention can be employed.  
         [0012]    [0012]FIG. 2 is a block diagram of an example of a modular tool in which embodiments of the invention cam be employed.  
         [0013]    [0013]FIG. 3 is a block diagram of a portion of a system employing an embodiment of the invention.  
         [0014]    [0014]FIG. 4 is a diagram showing a portion of the FIG. 3 system employing a plurality of sand trap assemblies in accordance with an embodiment of the invention.  
         [0015]    [0015]FIG. 5 is a cross-sectional diagram of a sand trap assembly in accordance with an embodiment of the invention.  
         [0016]    [0016]FIG. 6 is a diagram setting forth a sequence of steps in accordance with an embodiment of the invention.  
         [0017]    [0017]FIG. 7 is a diagram setting forth a sequence of steps in accordance with another embodiment of the imvention.  
     
    
     DETAILED DESCRIPTION  
       [0018]    Referring to FIG. 1 there is shown a representative embodiment of a formation tester apparatus for investigating subsurface formations  31  traversed by a borehole  32 , which can be used in practicing embodiments of the invention. The borehole  32  is typically filled with drilling fluid or mud which contains finely divided solids in suspension. A mudcake on the borehole wall is represented at  35 . However, the invention can also have application to other situations, for example, operation in a cased borehole. The investigating apparatus or logging device  100  is suspended in the borehole  32  on an armored multiconductor cable  33 , the length of which substantially determines the depth of the device  100 . Known depth gauge apparatus (not shown) is provided to measure cable displacement over a sheave wheel (not shown) and thus the depth of logging device  100  in the borehole  32 . The cable length is controlled by suitable means at the surface such as a drum and winch mechanism (not shown). Circuitry  51 , shown at the surface although portions thereof may typically be downhole, represents control and communication circuitry for the logging apparatus. Also shown at the surface are processor  50  and recorder  90 . These may all generally be of known type.  
         [0019]    The logging device or tool  100  has an elongated body  105  which encloses the downhole portion of the device, controls, chambers, measurement means, etc. One or more arms  123  can be mounted on pistons  125  which extend, e.g. under control from the surface, to set the tool. The logging device includes one or more probe modules each of which includes a probe assembly  210  which is movable with a probe actuator (not separately shown) and includes a probe (not separately shown) that is outwardly displaced into contact with the borehole wall, piercing the mudcake and communicating with the formations. The equipment and methods for taking pressure measurements and doing sampling are well known in the art, and the logging device  100  is provided with these known capabilities. Reference can be made, for example, to U.S. Pat. Nos. 3,934,468 and 4,860,581, which describe early versions of devices of this general type.  
         [0020]    Modern commercially available services utilizing, for example, a modular formation dynamics tester (“MDT”—trademark of Schlumberger), can provide a variety of measurements and samples, as the tool is modularized and can be configured in a number of ways. Examples of some of the modules employed in this type of tool, are as follows: An electric power module is generally provided. It does not have a flowline or hydraulic bus, and will typically be the first (top) module in the string. A hydraulic power module provides hydraulic power to all modules that may require same, and such power can be propagated via a hydraulic bus. Probe modules, which can be single or plural probes, includes pistons for causing engagement of probe(s) for fluid communication with the formations. Sample modules contain sample chambers for collecting samples of formation fluids, and can be directly connected with sampling points or connected via a flowline. A pumpout module can be used for purging unwanted fluids. An analyzer module uses optical analysis to identify characteristics of fluids. A packer module includes inflatable packer elements which can seal the borehole circumference over the length of the packer elements.  
         [0021]    Using the foregoing and other types of modules, the tool can be configured to perform various types of functions. Examples are permeability measurements, pressure gradient testing, PVT sampling, and interval testing. The present invention has application to all of these.  
         [0022]    Referring to FIG. 2, there is shown an example of a formation tester tool string in which embodiments of the invention can be employed. It is emphasized, that this particular configuration is an example, and the invention has application to many other tool configurations, modular or otherwise. In FIG. 2, 212 represents an electronics module that provides electrical power and control. The module  216  is of the type that contains an exit port (for returning formation fluids to the borehole) and a plurality of bottles for collecting samples. The module  220  is of the type that contains a single large volume bottle or receptacle for sampling. The module  224  is a pump-out module, and the module  230  is a fluid analyzer module, for example of the optical type noted briefly above. The module  250 , in the present example, is the type of module that ordinarily would contain several (e.g. six) sample chambers or bottles, each capable of holding a sample of, for example, 450 cc. In accordance with an embodiment of the invention, one or a plurality of the sample chambers are replaced with respective sand traps which can be, for example, of the type described hereinbelow. The blocks  262  and  274  are hydraulic power and control modules, and the modules  268  and  280  are pad/probe modules.  
         [0023]    As an example of a job that includes sampling, the tool is set, a pretest is taken, the pump is turned on and the formation fluid goes through the flow line of all the modules until reaching the exit port at which, after the contamination level reaches an acceptable level (as monitored by the fluid analyzer module), the exit port is shut off and the sample is routed into a chamber (for example, one of the bottles in module  250  and/or the large volume sample chamber of module  220 ). In order to capture sand in the formation fluid before it reaches the pump-out module, it is desirable to put the sand trap below the pump-out module. In an embodiment hereof, sand-containing formation fluid is routed through the sand trap in the module  250 . The formation fluid then continues through the water line of the module, back into the flow line, through the pump-out module, and out the exit port to the well bore. The chambers above the pump-out, in module  216 , can be filled in the same fashion as they would be conventionally.  
         [0024]    [0024]FIG. 3 shows an embodiment of the module  250  of FIG. 2, wherein a sand trap is provided in accordance with an embodiment of the invention. In FIG. 3, the reference numerals  341 ,  342 ,  343 ,  344 , and  345  refer to sample bottles. A sand trap assembly  350 , in accordance with an embodiment of the invention, is substituted for one of the original six bottles. It will be understood that more than one sand trap assembly can be employed, consistent with the principles hereof. For example, another sandtrap assembly ( 355 ) can be substituted for sample bottle  345 , as shown in FIG. 4.  
         [0025]    Referring again to FIG. 3, reference numerals  303  and  304  represent bleed plugs, and reference numerals  307  and  308  represent drain valves. Reference numeral  311  represents a charge valve, and reference numerals  315  and  316  represent pressure relief valves. The reference numerals  322  and  323  represent flow restrictors, and the reference, numerals  327  and  328  represent bypasses. Also, the reference numerals  331 ,  332 ,  333 ,  334 ,  335  and  336  represent check valves with respective bypasses. The reference numeral  338  represents an upper throttle/seal valve, and the reference numeral  339  represents a lower throttle/seal valve. The reference numerals  371 ,  372 ,  373 ,  374 ,  375  and  380 , represent control valves for the sample bottles  341  through  345 , and sand trap assembly  350 , respectively. In the present embodiment, these are single-shot control valves, two per bottle or sand trap assembly, as the case may be. In the FIG. 3 diagram, the double arrows show the flow path.  
         [0026]    Referring to FIG. 5, there is shown a sand trap assembly  350  in accordance with an embodiment of the invention. A cylindrical tube or receptacle housing  512  has a top endcap  515  with apertures  516 . Inside the housing  512  is a tubular screen  525 , and inside the screen is a stand pipe  530 , one end of which ( 531 ), near the top endcap, is opened, and the other end of which fits into a central aperture in a bottom endcap  545 .  
         [0027]    In operation, the formation fluid (typically a slurry of the fluid with some sand) from the flow line enters the sand trap assembly by flowing through apertures  516  of the top endcap and around the screen  525 . At this time, the change in volume reduces the flow rate causing sand to precipitate to the bottom of the receptacle (e.g. at  581 ). The formation fluid passes through the screen providing further filtering. Then, the formation fluid, absent the sand, enters the top ( 531 ) of the stand pipe  530  and flows out through the bottom of the bottle into the water line, which returns the formation fluid to the main flow line.  
         [0028]    The upper seal valve  338  may be opened and sample chambers above the pump-out may be filled. By opening the upper seal valve, any pressure drop occurring across the sand trap may be avoided (unlike a gravel packed probe where sample quality is compromised due to a large pressure drop across the gravel pack). It is not necessary to close the inlet to the sand trap before filling the sample chambers above the pump-out. Sand from the sand trap will not flow back out the top of the sand trap because the upper pressure relief valve ( 315 ) will not allow fluid to displace what is already inside the receptacle. The remaining bottles of the multisampler below the pump out may be filled with formation fluid by closing the upper seal valve, closing the inlet to the sand trap, and opening the valve to the desired sample bottle. The sample bottle may then be filled by pulling down the sample piston from the back side with the pump-out module.  
         [0029]    Referring to FIG. 6, there is shown a diagram of steps of a method, in accordance with an embodiment of the invention, for obtaining samples of formation matrix, in the form of sand, bringing it to the surface, and then analyzing the sand to obtain, for example, particle size distribution and other information. The block  610  represents positioning of a formation tester tool having a sand trap in accordance with the invention, at a desired depth level in the borehole. The block  620  represents activation of probe(s), in the tool, the causing of formation fluid and sand to flow in the tool, and the collecting of sand in the sand trap of the tool, as described herein. The block  630  represents bringing the sand sample to the surface. The block  640  represents the analyzing of the sand sample. This can include any known type of analysis to determine properties of the sand, for example, particle size distribution.  
         [0030]    In accordance with a further feature of the invention, a useful pressure measurement can be taken when sand starts to flow, without fouling of the tool. The pressure inside the tool at which formation sand grains are mobilized represents a measurement of the formation failure pressure or differential pressure (difference between formation pressure and tool pressure). This measurement represents the condition of differential pressure at which the well will start producing sand along with formation fluids during the production phase. This value of differential pressure at which the formation will fail can be used to design well completion and production strategies.  
         [0031]    Referring to FIG. 7, there is shown a diagram of steps of a method, in accordance with an embodiment of the invention, for determining the differential pressure at which the well can be expected to start producing sand along with formation fluids. The block  710  represents positioning of a formation tester tool having a sand trap in accordance with the invention, at a desired depth level in the borehole. The block  720  represents activation of probe(s) in the tool, the causing of formation fluid to flow in the tool. The block  730  represents determining of the pressure in the tool at the time that formation sand grains are detected as flowing in the tool.

Summary:
A method and apparatus for testing formations surrounding an earth borehole. The method includes the following steps: providing a tool movable through the borehole; providing a flow line in the tool; establishing fluid communication between the formations and the flow line of the tool; and providing a sand trap in communication with the flow line of the tool for trapping sand flowing with fluid from the formations.