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CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority to U.S. Provisional Application No. 60/682,498, entitled “APPARATUS AND METHOD FOR OBTAINING DOWNHOLE SAMPLES” filed on May 19, 2005, which is hereby incorporated in its entirety. 
     
    
     BACKGROUND  
       [0002]     The present invention relates to sampling downhole fluids in a wellbore penetrating a subterrean formation. In particular, this invention relates to techniques for collecting downhole fluid samples and retrieving the samples to a surface location.  
         [0003]     Wellbores, which are also known as boreholes, are drilled for hydrocarbon prospecting and production. It is often desirable to perform various evaluations of the formations penetrated by a wellbore during drilling operations, such as during periods when actual drilling has temporarily stopped. In some cases, the drill string may be provided with one or more drilling tools to test and/or sample the surrounding formation. In other cases, the drill string may be removed from the wellbore, in a sequence called a “trip,” and a wireline tool may be deployed into the wellbore to test and/or sample the formation. The samples or tests performed by such downhole tools may be used, for example, to locate valuable hydrocarbon-producing formations and manage the production of hydrocarbons therefrom.  
         [0004]     Such drilling tools and wireline tools, as well as other wellbore tools conveyed on coiled tubing, drill pipe, casing or other conveyors, are also referred to herein simply as “downhole tools.” Such downhole tools may themselves include a plurality of integrated modules, each for performing a separate function, and a downhole tool may be employed alone or in combination with other downhole tools in a downhole tool string.  
         [0005]     More particularly, formation evaluation often requires that fluid from the formation be drawn into a downhole tool, or module thereof, for testing in situ and/or sampling. Various devices, such as probes and/or packers, are extended from the downhole tool to isolate a region of the wellbore wall, and thereby establish fluid communication with the formation surrounding the wellbore. Fluid may then be drawn into the downhole tool using the probe and/or packers.  
         [0006]     A typical probe employs a body that is extendable from the downhole tool and carries a packer at an outer end thereof for positioning against a sidewall of the wellbore. Such packers are typically configured with one relatively large element that can be deformed easily to contact the uneven wellbore wall (in the case of open hole evaluation), yet retain strength and sufficient integrity to withstand the anticipated differential pressures. These packers may be set in open holes or cased holes. They may be run into the wellbore on various downhole tools.  
         [0007]     Another device used to form a seal with the wellbore sidewall is referred to as a dual packer. With a dual packer, two elastomeric rings are radially expanded about a downhole tool to isolate a portion of the wellbore wall therebetween. The rings from a seal with the wellbore wall and permit fluid to be drawn into the downhole tool via the isolated portion of the wellbore.  
         [0008]     The mudcake lining the wellbore is often useful in assisting the probe and/or dual packers in making the appropriate seal with the wellbore wall. Once the seal is made, fluid from the formation is drawn into the downhole tool through an inlet therein by lowering the pressure in the downhole tool. Examples of probes and/or packers used in various downhole tools are described in U.S. Pat. Nos. 6,301,959, 4,860,581, 4,936,139, 6,585,045, 6,609,568, and 6,719,049, and U.S. Patent Application Publication No. 2004/0000433, which are incorporated herein by reference.  
         [0009]     Fluid is drawn into the down tool through an inlet in the probes or packers. Fluid flows into a flowline and is selectively delivered to a sample chamber or bottle for collection therein. Examples of sample chambers and related techniques used in downhole tools are depicted in U.S. Pat. Nos. 6,745,835, 6,688,390, 6,659,177, 5,803,186, 5,233,866, 5,303,775, and 5,377,755, among others. Sample chambers are containers typically provided with an internal piston that retains the collected fluid under pressure. Once fluid is collected in the sample chamber, the tool is retrieved to the surface, and the sample chambers are removed for further analysis. In some cases, the sample chambers are removed at the surface for evaluation. In other cases, the sample chambers are taken to an offsite facility for further testing.  
         [0010]     Despite the advances in sampling technology, there remains a need to obtain samples without interrupting the downhole operations being performed by the downhole tool. In some instances, sample chambers may become defective, full or other wise inoperable during operations. These remains a need for techniques for obtaining samples more quickly and/or without having to remove the tool. In such cases, it is desirable to retrieve one or more sample chambers from the downhole tool without withdrawing the tool.  
         [0011]     Techniques have been developed for retrieving, measurement and logging while drilling tools (MWD, LWD) from downhole drilling tools. These MWD and LWD tools are typically deployed into and retrieved from downhole drilling tools via wireline or slickline devices. In such cases, the component is sent downhole through a mud channel extending through the downhole drilling tool and operatively inserted into the bottom hole assembly of the downhole drilling tool. Examples of such devices and related techniques are described in U.S. Pat. No. 6,577,244. However, no known techniques exist for retrieving sample chambers from downhole devices or tools. Difficulty exists in maintaining samples under the desired pressure, and preventing contamination of the sample during extraction and/or transport.  
         [0012]     A need therefore exists for a system and method capable of collecting a sample and transporting it to the surface without requiring the removal of the downhole tool. It is desirable that such a system be operable even under harsh drilling environments, such as offset drilling conditions. It is further desirable that such a system be capable of isolating the sample from contamination and/or damage during transportation to the surface. These and other features of the invention are set forth herein.  
       SUMMARY OF THE INVENTION  
       [0013]     In an aspect, the invention relates to a downhole drilling tool positionable in a wellbore penetrating a subterranean formation. The tool includes a formation evaluation tool having fixed and retrievable portions. The fixed portion is operatively connected to a drill collar of the downhole tool. The fixed portion is for establishing fluid communication with a subterranean formation. The retrievable portion is fluidly connected to the fixed portion and retrievable therefrom to a surface location. The retrievable portion is for receiving a formation fluid from the subterranean formation.  
         [0014]     In another aspect, the invention relates to a formation evaluation while drilling tool positionable in a wellbore penetrating a subterrean formation. The tool includes a fluid communication device extendable from the drilling tool for establishing fluid communication with the subterranean formation. The fluid communication device has an inlet for receiving formation fluid from the subterranean formation and at least one sample chamber for receiving the formation fluid. The sample chambers are operatively connected to the fluid communication device via at least one flowline. The sample chambers are also positioned in the drill collar and retrievable therefrom to the surface.  
         [0015]     In yet another aspect, the invention relates to A method of performing formation evaluation via a downhole drilling tool positionable in a wellbore penetrating a subterranean formation. The method involves establishing fluid communication between a fixed portion of the downhole drilling tool and the formation, drawing a formation fluid from the formation and into the fixed portion, passing the formation fluid from the fixed portion to a retrievable portion of the downhole drilling tool and retrieving the retrievable portion of the downhole drilling tool to a surface location. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     So that the above recited features and advantages of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof that are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.  
         [0017]      FIG. 1  is a schematic view, partially in cross-section of drilling rig with a downhole drilling tool advanced into a wellbore via a drill string, the downhole drilling tool includes a formation evaluation assembly therein.  
         [0018]      FIG. 2A  is a schematic view of the formation evaluation assembly of  FIG. 1  including a retrievable sampling tool.  
         [0019]      FIG. 2B  is a schematic view of an alternate formation evaluation assembly including an alternate retrievable sampling tool.  
         [0020]      FIG. 2C  is a schematic view of an alternate formation evaluation assembly including a retrievable sample chamber.  
         [0021]      FIG. 3A  is a schematic view of the retrievable sample chamber of  FIG. 2C .  
         [0022]      FIG. 3B  is a schematic view of an alternate retrievable sample chamber. 
     
    
     DETAILED DESCRIPTION  
       [0023]     Referring now to  FIG. 1 , a conventional drilling rig and drill string are shown wherein a land-based platform and derrick assembly  10  is positioned over a wellbore  11  penetrating subsurface formation F. In the illustrated embodiment, the wellbore  11  is formed by rotary drilling in a manner that is well known. Those of ordinary skill in the art given the benefit of this disclosure will appreciate, however, that the present invention also finds application in directional drilling applications as well as rotary drilling, and is not limited to land-based rigs.  
         [0024]     A drill string  12  is suspended within the wellbore  11  and includes a drill bit  15  at its lower end. The drill string  12  is rotated by a rotary table  16 , energized by means not shown, which engages a kelly  17  at the upper end of the drill string  12 . The drill string  12  is suspended from a hook  18 , attached to a traveling block (also not shown), through the kelly  17  and a rotary swivel  19 , which permits rotation of the drill string  12  relative to the hook  18 .  
         [0025]     Drilling fluid or mud  26  is stored in a pit  27  formed at the well site. A pump  29  delivers drilling fluid  26  to the interior of the drill string  12  via a port in the swivel  19 , inducing the drilling fluid  26  to flow downwardly through the drill string  12  as indicated by directional arrow  9 . The drilling fluid  26  exits the drill string  12  via ports in a drill bit  15 , and then circulates upwardly through the region between the outside of the drill string  12  and the wall of the wellbore  11 , called the annulus, as indicated by direction arrows  32 . In this manner, the drilling fluid lubricates the drill bit  15  and carries formation cuttings up to the surface as it is returned to the pit  27  for recirculation.  
         [0026]     The drill string  12  further includes a downhole tool or bottom hole assembly (BHA), generally referred to as  100 , near the drill bit  15 . The BHA  100  includes drill collars  150  housing various components capable of measuring, processing, and storing information, as well as communicating with the surface. One such component is a measuring and local communications apparatus  200  for determining and communicating the resistivity of formation F surrounding the wellbore  11 . Another component is a formation evaluation assembly  300 . The formation evaluation assembly  300  includes stabilizers or ribs  314 , and a probe  316  positioned in a stabilizer.  
         [0027]     Referring now to  FIG. 2A , the formation evaluation assembly  300  is positioned in a drill collar  150 . The formation evaluation assembly  300  includes a fixed section or portion  403  and a retrievable section or portion  400 . The drill collar  150  has an annulus  401  extending therethrough for the passage of mud or drilling fluid. As shown, the fixed portion  403  is positioned in the drill collar  150  with a passage defined and extending therethrough. The retrievable portion  400  is positioned centrally within the annulus  401 . However, it will be appreciated that the tools may be positioned and/or supported within the drill collar in a manner that facilitates formation evaluation and/or mud flow operations. The portions may be in one or more drill collars. The portions may be adjacent, or extended a distance across the downhole tool.  
         [0028]     The probe  316  is positioned in the fixed portion  403  and extends therefrom to contact the wall of the wellbore  11  and establish fluid communication with an adjacent formation. The fixed portion  403  includes a pretest piston  404  and pressure gauge  406 . Other devices, such as sensors, fluid analysis, hydraulics, electronics, etc., may also be provided.  
         [0029]     The retrievable portion  400  has a latching mechanism  408  as a downhole end thereof, and a fishing/wireline head  410  at an uphole end thereof. The latching mechanism  408  removably connects the retrievable sampling tool (or the retrievable portion  400 ) to the drill collar  150 . The fishing head  410  is preferably adapted for connection to a wireline  411 . Alternatively, a slickline or other retrieval mechanism may be used to facilitate retrieval to the surface. The retrievable portion  400  may also be deployed into the downhole tool or formation evaluation assembly  300  using a tractor, mud flow, gravity or other conveyance. The retrievable portion  400  is then secured in place using the latching mechanism  408 .  
         [0030]     The wireline  411  may be used to provide power to the retrievable and/or fixed portions, as well as other portions of the downhole tool. In such cases, the downhole tool may be operated using power from the wireline  411  to supplement or replace power from mud flow. The downhole tool is thereby enable to operate in an LWD mode, or in wireline mode. In LWD mode, the downhole tool receives power from the flow of mud through a downhole generator (not shown). In wireline mode, the wireline  411  electrically conveys power to the downhole tool. The wireline mode permits operation when mud cannot be passed through the downhole tool, for example when the tool is ‘tripping.’ 
         [0031]     The latching mechanism  408  is adapted to make fluid connection of a flowline  402  between the retrievable portion  400  and the fixed portion  403 . The latching mechanism  408  includes a self-sealing mechanism (not shown) to seal the fixed portion  403  and prevent fluid flow therein when the retrievable portion  400  is detached. This self-sealing mechanism is preferably robust enough to withstand the high mud flow-rate in the mud channel following removal of the retrievable portion  400 .  
         [0032]     The retrievable portion  400  includes a pump  412  and sample chambers or bottles  414 . One or more sample bottles of a desired size may be used. Preferably the sample chambers are slim to allow for passage of mud. Sample bottles longer than a drill collar may be used and extend through the retrievable portion  400 . The flowline  402  extends through the fixed portion  403  and the retrievable portion  400 . The flowline  402  fluidly connects the probe  316  to the sample chambers  414  in the retrievable portion  400 . Additional valving, sample chambers, pumps, exit ports, charging chambers and other devices may be provided in the sampling assembly to facilitate the formation evaluation process. While the pump  412  is depicted in the sampling tool or retrievable portion  400 , and the pretest and gauge are depicted as being in the drill collar portion or fixed portion  403  of the formation evaluation tool, these devices may be positioned in various locations about the formation evaluation tool.  
         [0033]     Referring now to  FIG. 2B , an alternate formation evaluation assembly  300   a  is depicted. The formation evaluation assembly  300   a  is similar to the formation evaluation assembly  300  of  FIG. 2A , except that the fixed portion  403   a  contains the probe  316 , and the retrievable portion  400   a  contains the pretest piston  404 , pressure gauge  406 , electronics  502  and hydraulics  504 . With this configuration, additional components are positioned in the retrievable portion  400   a  and may be retrieved to the surface for replacement or adjustment as necessary.  
         [0034]     As depicted in  FIG. 2B , the formation evaluation tool  300   a  has no sample chambers or pumps. The configuration of  FIG. 2B  may be used for performing formation testing without sampling. However, these and other components may optionally be provided to enable sampling operations.  
         [0035]     Referring now to  FIG. 2C , another alternate formation evaluation assembly  300   b  is shown having a retrievable portion  400   b  and a fixed portion  403   b . This configuration is similar to the formation evaluation assembly  300  of  FIG. 2A , except that the pump  412  has been removed from retrievable portion  400   b  and positioned in the fixed portion  403   b.    
         [0036]      FIGS. 3A and 3B  depict flowline configurations for the downhole formation evaluation assembly. As shown in  FIG. 3A , the flowline  402  branches into flowlines  602  and  604 . A valve  606  selectively permits fluid flow from the flowline  402  into a sample chamber  614 . When the valve  606  is closed, the flowline  402  may bypass the flowline  604  and the sample chamber  614  and proceed to other sample chambers or portions of the downhole tool. This enables a single flow line entering and exiting the bottle that will allow multiple bottles to be placed in series.  
         [0037]     As shown in  FIG. 3B , the flowline  402  branches to flowline  620  and  622 . Valves  624  and  626  permit fluid to selectively pass into flowlines  620 ,  622 , respectively. In this case, the valves are located remotely from the bottles, for example within the fixed portion or latch section. In this configuration, the valves  624  and  626  permit operation without the use of electrically operated valves in the bottles. Such a configuration obviates the need for wires. A separate flow  622  is provided for each sample chamber in series.  
         [0038]     Referring now to  FIGS. 3A and 3B , the sample chamber  614  includes a piston  628  slidably positioned therein. The piston defines a sample cavity  630  and a buffer cavity  632 . The buffer cavity  632  has an exit port  634  in fluid communication with the wellbore. Other flowline configurations, valving and additional devices, such as nitrogen chambers, may also be used.  
         [0039]     Preferably the pump  412 , which is shown in  FIG. 2C , is positioned adjacent the sample chambers to circulate formation fluid near the valves  624  and  626 . The pump  412  may be positioned to minimize the amount of stagnant, contaminated fluid that will enter the sample chamber upon opening the valves.  
         [0040]     It will be understood from the foregoing description that various modifications and changes may be made in the preferred and alternative embodiments of the present invention without departing from its true spirit. Furthermore, this description is intended for purposes of illustration only and should not be construed in a limiting sense. The scope of this invention should be determined only by the language of the claims that follow. The term “comprising” within the claims is intended to mean “including at least” such that the recited listing of elements in a claim are an open set or group. Similarly, the terms “containing,” having,” and “including” are all intended to mean an open set or group of elements. “A,” “an” and other singular terms are intended to include the plural forms thereof unless specifically excluded.

Summary:
A downhole drilling tool positionable in a wellbore penetrating a subterranean formation is provided. The tool includes a formation evaluation tool having fixed and retrievable portions. The fixed portion is operatively connected to a drill collar of the downhole tool. The fixed portion is for establishing fluid communication with a subterranean formation. The retrievable portion is fluidly connected to the fixed portion and retrievable therefrom to a surface location. The retrievable portion is for receiving a formation fluid from the subterranean formation.