Apparatus for housing and sealing of instruments in a downhole tool

Abstract: An apparatus for sealing a first pressure environment from a second, lower pressure environment, includes a plug having a desired cross section for insertion into a barrier wall aperture shaped to receive the plug. The barrier wall has a first pressure environment on its outside surface and a second environment on its inside surface. A bearing surface on the plug bears against the wall of the aperture. A flexing member, subject to and reactive to the first pressure environment, has an outer surface providing at least a portion of the bearing surface. The flexing member, in reacting to the first pressure environment, flexes at least a portion of the bearing surface to bear more tightly against the wall of the aperture and tends to seal the first pressure environment from the second pressure environment and maintain the seal under a variety of mechanical loads. A chassis is secured within the barrier wall. The chassis has a pocket located such that when the plug is inserted into the barrier wall aperture a portion of the pocket aligns with the aperture. The plug is secured by attachment to the chassis.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates generally to apparatus and methods for performing measurements in a borehole. More particularly, the invention relates to sealing measurement instruments in a housing inserted in a borehole. Description of the Related Art In well logging, performed after a borehole has been drilled, and in measurement-while-drilling ("MWD"), performed during drilling of a borehole, measurement instruments are housed in a hermetically sealed housing, typically a metal cylinder, inserted down the borehole. The housing maintains a relatively lower internal pressure environment than that external to the housing and protects the instruments against fluid invasion and damage due to mechanical impact and compression forces within the borehole. The instruments are typically secured in a housing and an access port in the housing is sealed by bolting a cover plate over the port. It is well known to improve the sealing action of such cover plates with a resilient gasket between the cover plate and the housing. Also, a plug and resilient o-ring may be substituted for the cover plate and gasket. Certain problems exist with the above described sealing arrangement. Twisting and bending of the housing and cover plate or plug are particularly severe in MWD applications because of the forces associated with the vertical loading, friction, bending, and rotation of the drill string and the housing connected to the drill string. Moreover, this twisting and bending is repeated with each rotation of the drill string. Also, for MWD applications the housing is subjected to the flow of drilling mud at high pressures. A different approach to sealing a port is described in U.S. patent 5,337,598. This patent discloses a leak test port plug device which includes a resilient plug sized approximately to a port in a component to be tested. The device is designed so that when the center of the plug is compressed by application of mechanical force from a piston rod head, the plug expands radially to seal the port and allow pressure testing of the component. Because of the above factors, seal failure is a problem for these borehole related applications. For example, o-ring failure is commonly caused by a phenomenon which may be described as "dynamic extrusion failure." In this type of failure, the o-ring seals a gap between a plug and a port in the housing in which the plug is held. The gap repeatedly changes as the changing forces, such as bending and pressure, act on the housing, plug, o-ring and port. With the fluid pressure acting on the o-ring, the o-ring is repeatedly extruded into the gap as the gap increases with each cycle and is then pinched as the gap decreases. Over a period of many cycles an o-ring subjected to these conditions tends to fatigue and fail. U.S. patent 4,457,489 describes a different sealing mechanism in the form of a multiple subsea fluid conduit connection for remote controlled valves comprising a socket and a plug, and pistons being utilized in a locking mechanism to lock the plug in the socket. SUMMARY OF THE INVENTION An object of the invention is to provide an improved seal, such as for sealing instruments in a downhole tool. Another object of the invention is to provide an improved housing for instruments in a downhole tool. According to a first aspect of the invention, these and other objects are attained by a system for sealing an aperture comprising a plug adapted for insertion into the aperture, the plug having a flexing member equipped with a bearing side providing a bearing surface that bears against a wall of aperture and a fluid side, characterized in that said system further comprises a drill collar of a drill string, in that the aperture is formed in an outer wall of the drill collar that contacts pressurized fluid when the drill string is disposed in a borehole, and in that the fluid side of the flexing member is placed in communication with the pressurized fluid when plug is inserted into aperture, and in that the flexing member is subject and reactive to the pressure of the pressurized fluid such that said pressure on said fluid side forces the bearing surface of the bearing wall to bear more tightly against the wall of the aperture. According to a second aspect of the invention, these and other objects are attained by a method for sealing an aperture of a drill collar of a drill string, the aperture being formed in an outer wall of the drill collar that contacts pressurized fluid when the drill string is disposed in a borehole, the method being characterized by: equipping a plug with a flexing member having a bearing side and a fluid side; and positioning the plug within the aperture wherein the pressurized fluid contacts the fluid side of the flexing member when the drill string is disposed in the borehole and forces the bearing side of the flexing member to bear against a wall of the aperture. The barrier wall includes a drill collar. The drill collar has a port which provides the barrier wall aperture. A chassis having a desired cross section is inserted into the drill collar. The chassis also has a pathway for transmitting drilling mud through the drill string, and a pocket for housing an instrument. The pocket is located in the chassis such that when inserted into the drill collar at least a portion of the pocket aligns with the drill collar port. The plug has an interior end which inserts at least partly into the chassis pocket. The plug is secured in the drill collar at least partly by attachment of the plug to the chassis. In a still further aspect of the invention the chassis has a locking member, and the plug has a second locking member on the plug interior end for engaging the first locking member and securing the plug within the drill collar. With the chassis in a first position within the drill collar the plug is received partly into the chassis. With the chassis secured in a second position within the drill collar the locking members engage. Still other objects and advantages of the present invention will become apparent to those skilled in the art from the following detailed description. DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic view of the apparatus of the present invention attached to a drill string in a borehole. FIG. 2 is cross section view of a housing and chassis of the present invention. FIG. 3 is an enlarged portion of one wall of the housing and a portion of the chassis of FIG. 2, also showing a plug inserted into the housing. FIG. 4 is a perspective view of the plug of FIG. 3. FIG. 5 is a side view of another embodiment of the plug of the present invention. FIG. 6 is another side view of the embodiment of FIG. 5, with the plug rotated 90 degrees in relation to the view of FIG. 5. FIG. 7 is a perspective view of an embodiment of the chassis of the present invention. FIG. 8 is a section view of the chassis of FIG. 7 inserted in an initial position in a drill collar and having the plug of FIGS. 5 and 6 inserted into the drill collar and chassis. FIG. 9 is the same view of FIG. 8 with the chassis in a final position in the drill collar to lock the plug in the housing. FIG. 10 is a side view of another plug illustrating an alternative locking arrangement. FIG. 11 is a section view of a chassis of the present invention inserted in an initial position in a drill collar and having the plug of FIG. 10 inserted into the drill collar and chassis. FIG. 12 is the same view of FIG. 11 with the chassis in a final position in the drill collar to lock the plug in the housing. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, an MWD application of the present invention is shown. An MWD tool 10 comprises a housing 12 and at least one instrument accessible through an opening or port 14 in the housing 12. In this preferred embodiment the housing 12 is a cylindrical steel drill collar which comprises a subsection of the drill string 18. The drill string 18 and an attached drill bit 20 at its terminus are rotated by a drilling rig (not shown), creating a borehole 24 in the earth 16. Drilling mud 17 is injected at the surface into a bore 22 within the drill string 18, forcing the mud 17 out of the drill bit 20 and into the borehole 24. Consequently, some of the mud 17 is forced into the formation surrounding the borehole 24, while the remainder travels up the borehole 24 and is forced back to the surface. Due to friction and the weight of the mud, at the bottom of a deep borehole the drilling mud pressure within the borehole and external to the drill string may be very high. Pressures of 138,000 KPa (20,000 psi) and higher are not uncommon. Since the MWD tool 10 is normally located in the drill string 18 relatively close to the drill bit 20, the housing 12 is subjected to nearly maximum mud pressure. Referring now to FIG. 2, a cross section of part of the MWD tool 10 is shown. The housing 12 comprises a drill collar into which a chassis 30 is removably fitted. The chassis 30 has a pocket 32 for an electronic instrument (not shown) and a bore 22 for the passage of drilling mud 17 (FIG. 1). The instrument in the chassis 30 may be, for example, a radiation sensor or source such as for a nuclear measuring application, an electromagnetic sensor or source such as for a resistivity measurement, a sensor or source for an acoustic measurement, or some other device. The drill collar 12 has a port 14 which is located such that when the chassis 30 is fitted into the collar 12, the port 14 is in alignment with at least a portion of the pocket 32 in the chassis 30. As shown in FIG. 3, a plug 40 fits removably into the drill collar port 14 and partly into the chassis pocket 32. The plug 40 has an interior end section 41 and an exterior end section 42. The exterior end section 42 contains a cavity 44 in contact with the high pressure mud 17 in the borehole 24. Surrounding the cavity 44 is a peripheral segment 50 of the exterior end section 42 having a bearing surface 50a facing and bearing against the port wall 52 of the port 14 tending to seal the mud 17 from the chassis 30. The peripheral segment 50 acts as a flexing member that is subject to and reactive to the pressure of the mud 17, the mud 17 being communicated to the peripheral segment 50 through the cavity 44. That is, the peripheral segment 50 provides a bearing wall having a bearing surface 50a that bears against the port wall 52, and a fluid side 50b in communication with the mud 17. In reacting to the fluid pressure, the peripheral segment 50 forces the bearing surface 50a to bear more tightly against the port wall 52, tending to further seal the mud 17 out of the chassis 30. In the embodiment shown, the bearing surface 50a includes grooves 53 which are filled with a resilient sealing material 54, such as an o-ring, to further improve the ability of the bearing surface 50a to conform to the shape of the port wall 52. The peripheral segment 50 has a thickness 56, which is small enough so that when forces applied to the drill collar 12 deform the port 14 and plug 40, causing at least a portion of the port wall 52 to tend to move away from the bearing surface 50a, the mud 17 exerts a force on the peripheral segment 50, causing the bearing surface 50a to tend to conform to the deformed shape of the port 14 and move toward the port wall 52. In a preferred embodiment, the plug 40 comprises a material having relatively more elasticity than the material of the drill collar 12 so that under the effects of the fluid pressure on the collar 12 and the plug 40, the elasticity of the peripheral segment 50 further contributes to the bearing surface 50a tending to conform to the shape of the port wall 52. Also, the elasticity allows the required flexing of the peripheral segment 50 with a thickness 56 which is relatively greater than that required for the peripheral segment 50 if the material were not relatively more elastic than the material of the collar 12. In the embodiment shown in FIGS. 3 and 4, the plug 40 is cylindrical in shape, with a circular top. Referring now to FIG. 4, the cylinder side is oriented essentially vertically and provides the bearing surface 50a. The exterior end section 42 includes a cavity 44 which may be described as a circular slot or channel. The channel has a bottom and a first and second side, the channel sides being essentially parallel to the bearing surface 50a. One of the channel sides forms the fluid side 50b of the peripheral segment 50. In the alternative embodiment for the plug 40 shown in FIGS. 5 and 6, the cavity 44 in the exterior end section 42 includes a hollowed-out portion occupying the entire region surrounded by the peripheral segment 50. This cavity 44 acts to communicate fluid pressure to the peripheral segment 50 whether the cavity 44 is empty or filled with a flexible material, such as a high performance plastic or elastomer, reactive to fluid pressure. For various measurement applications, including nuclear, electromagnetic, acoustic, or others, with sources and/or detectors beneath the plug 40 it may be advantageous to fill the cavity 44 with plastic, elastomeric or other low density material having known transmission characteristics so that fluid or other material in the borehole 24 having unknown transmission characteristics is displaced from the cavity 44. FIGS. 3 and 4 also show an arrangement for securing the plug 40, in which the plug 40 has a threaded stud 43 extending from its interior end section 41. The chassis 30 has a matching threaded bore 33 for receiving the stud 43. A hexagonal or square socket 58 is included in the center of the exterior end section 42 of the plug 40 for receiving a wrench used to rotate the plug 40 and screw the stud 43 into the bore 33. Alternative arrangements for securing the plug 40 will be described; however, next an alternative configuration of the plug 40 is described. The plug 40 shown in FIGS. 5 and 6 is generally cylindrical and has a bifurcated interior end section 41 forming legs 68a and 68b extending downward. A circular groove 70 extends around the interior end section 41. The interior end section 41 in the plug as shown here is bifurcated so that an instrument (not shown) located in the pocket 32 may engage the plug 40 with the legs 68a and 68b straddling the instrument. For other applications the plug 40 interior end section 41 may not be bifurcated. The groove 70 is for accepting a locking member and is in a plane essentially parallel to the axis of the drill collar 12. FIGS. 7, 8 and 9 show one arrangement for securing the plug 40 of FIGS. 5 and 6 using the groove 70. In FIG. 7 chassis 30 is shown removed from the housing 12 (not shown). For this arrangement, a plate 76, attached to a flattened portion 77 (FIGS. 8 and 9) of the chassis 30 by recessed sockethead cap screws 78, provides a locking member. The plate 76 has an oval opening 80 large enough to receive the interior end section 41 of the plug 40 (not shown). An edge 82 of the opening 80 is thin enough to fit into the groove 70 of the plug 40 (not shown). Referring now to FIGS. 8 and 9, for installation the chassis 30 is initially placed within the drill collar 12 in a first position (FIG. 8) in which the opening 80 (not shown) in the plate 76 accepts the plug 40. The chassis 30 is then moved in the direction indicated by reference arrow 84 into a second position (FIG. 9) within the collar 12 by sliding the chassis 30 along the axis of the drill collar 12. The chassis 30 is locked into the second position by tightening a threaded sleeve(not shown) securely connecting and sealing an inside end of the drill collar 12 with the end of the chassis 30. In operation the chassis 30 is further held in position by fluid pressure acting on the chassis seal (not shown). In this second position, the edge 82 of the opening 80 in the plate 76 fits into at least a portion of the groove 70 of the plug 40 thereby locking the plug 40 into position in the drill collar 12. The plug 40 has a lip 86 around its circumference and the collar 12 has a corresponding lip 88 around the circumference of the port 14 which engage when the plug 40 is fully inserted. The lip 88 thus provides a stop which prevents the plug 40 from being pushed into the chassis 30 beyond the stop due to the fluid pressure of the drilling mud 17. FIG. 10 shows another configuration for the plug 40 in which the plug 40 is cylindrical and has one or more grooves 92 with a first portion 92a parallel to the axis of the plug 40 and extending toward the middle of the plug 40 through the interior end section 41. A second portion 92b of the groove 92 extends at an angle from the first portion 92a. The groove 92 thus forms an "L" shape. In the preferred embodiment of this locking arrangement, there are two such grooves 92 in the plug 40 located 180 degrees apart. Referring now to FIGS. 11 and 12, a locking arrangement is shown for the plug 40 of FIG. 10. Pins 94 in the chassis 30 match the grooves 92 in the plug 40 and are located such that a pin 94 fits in the first portion 92a of a groove 92 when the plug 40 is inserted in the collar 12 with the chassis 30 in a first position (FIG. 11). As shown in FIG. 12, when the chassis 30 is slid into a second position the pin 94 fits in the second portion 92b of the groove 92. With the chassis 30 secured in the second position, a pin 94 is held in the second portion 92b of the groove 92 thereby preventing the plug 40 from being removed from the port 14 in the drill collar 12. The above description illustrates the preferred embodiments of the invention, but other embodiment are possible without departing from the scope of the claims.

Claims: A system for sealing an aperture (14) comprising a plug (40) adapted for insertion into the aperture (14), the plug (40) having a flexing member (50) equipped with a bearing side providing a bearing surface (50a) that bears against a wall (52) of aperture (14) and a fluid side (50b), characterized in that said system further comprises a drill collar (12) of a drill string (18), in that the aperture (14) is formed in an outer wall of the drill collar that contacts pressurized fluid (17) when the drill string (18) is disposed in a borehole (24), and in that the fluid side (50b) of the flexing member (50) is placed in communication with the pressurized fluid (17) when plug (40) is inserted into aperture (14), and in that the flexing member (50) is subject and reactive to the pressure of the pressurized fluid (17) such that said pressure on said fluid side (50b) forces the bearing surface (50a) of the bearing wall (50) to bear more tightly against the wall (52) of the aperture (14). The system of claim 1 wherein the flexing member (50) comprises material selected to be more elastic than the material of the outer wall of the drill collar (12). The system of claim 2 wherein the plug (40) further comprises an exterior end (42) and the flexing member comprises a peripheral segment (50) surrounding a cavity (44) therein. The system of claim 3 wherein the cavity (44) comprises a slot approximating the cross section shape of the exterior end. The system of claim 3 wherein the cavity (44) comprises a hollowed-out portion of the exterior end. The system of claim 5 wherein the cavity (44) contains a flexible material reactive to the pressurized fluid. The system of claim 3 wherein the bearing surface (50a) comprises a resilient sealing material (54). The system of claim 1 in which the drill collar includes a housing (12) for an instrument and the aperture (14) comprises a walled port shaped to receive the plug (40). The system of claim 8 further comprising a chassis (30) for insertion into the housing (12), the chassis having a pocket (32) located therein such that when the chassis is inserted into the housing at least a portion of the pocket (32) aligns with the port (14), and wherein the plug (40) is secured at least partly by attachment of the plug to the chassis. The system of claim 9 wherein the chassis (30) further comprises a first locking member (76), and the plug further comprises a second locking member (70) for engaging the first locking member. The system of claim 10 wherein, with the chassis is situated in a first position inside the drill collar (12), the plug (40) is received into the chassis, and, with the chassis situated in a second position inside the drill collar, the first locking member (76) engages the second locking member (70). The system of claim 9 wherein the chassis (30) further comprises a first locking member (76), and wherein the plug further comprises an interior end (41) and a second locking member (70) on the plug interior end for engaging the first locking member. The system of claim 10 wherein, with the chassis situated in a first position within the housing, the plug (40) is received partly into the chassis, and, with the chassis situated in a second position within the housing, the first locking member (76) engages the second locking member (70). The system of claim 9 wherein the bearing surface (50a) further comprises a resilient seal (54). The system of claim 9 wherein the plug (40) comprises material selected to be more elastic than the material of the housing wall. The system of claim 13 wherein the second locking member (70) comprises a groove extending around the plug interior end and the first locking member comprises and edge (82) around the pocket for engaging the groove when the chassis is in the second position. The system of claim 13 wherein the second locking member comprises a groove (92) on the plug interior end (41), the groove having a first portion (92a) extending substantially along the axis of the plug and a second portion (92b) at an angle to the first portion, and wherein the first locking member comprises a pin (94) for engaging the groove (92), wherein in the chassis first position the pin (94) engages the first portion (92a) of the groove, and in the chassis second position the pin (94) engages the second portion (92b) of the groove and locks the plug into the housing. The system of claim 12 wherein the second locking member comprises a threaded shaft (43), and the first locking member comprises a threaded bore (33) for receiving the shaft so that threading the shaft into the bore locks the plug into the housing. The system of claim 9 wherein the flexing member (50) comprises: an exterior end (42) of the plug having a cavity (44) therein in contact with the pressurized fluid; and a peripheral segment (50) of the exterior end; wherein the pressurized fluid communicates with the peripheral segment (50) through the cavity (44). The system of claim 19 wherein the cavity (44) comprises a slot in the exterior end approximating the shape of the cross section of the plug. The system of claim 19 wherein the cavity (44) comprises a hollowed-out portion of the exterior end. The system of claim 21 wherein the cavity (44) contains a flexible material reactive to the pressurized fluid. The system of claim 1 wherein the plug (40) is cylindrical, the cylinder side providing the bearing surface (50a) of the flexing member (50), and wherein the circular top of the plug comprises a channel (44) substantially concentric with the plug bearing surface with one of the channel sides providing the fluid side (50b) of the flexing member. A method for sealing an aperture (14) of a drill collar (12) of a drill string, the aperture (14) being formed in an outer wall of the drill collar (12) that contacts pressurized fluid when the drill string is disposed in a borehole, the method being characterized by : equipping a plug (40) with a flexing member (50) having a bearing side (50a) and a fluid side (50b); and positioning the plug (40) within the aperture (14) wherein the pressurized fluid contacts the fluid side (50b) of the flexing member (50) when the drill string is disposed in the borehole and forces the bearing side (50a) of the flexing member (50) to bear against a wall (52) of the aperture (14). The method of claim 24, further characterized by : inserting a chassis into the drill collar (12), the chassis having a pocket (32) located therein such that when the chassis is inserted inside the drill collar (12) at least a portion of the pocket aligns with the aperture (14).

Inventors: MORIARTY KEITH A, MORIARTY, KEITH A.
Applicants: ANADRILL INTERNAT SA, SCHLUMBERGER SERVICES PETROL, ANADRILL INTERNATIONAL SA, SERVICES PETROLIERS SCHLUMBERGER
Application year: 1996
Main classification: E21B47
IPC codes: E21B17, E21B47