Apparatus for sealing instruments in a downhole tool

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.

BACKGROUND OF THE INVENTION 
1. 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. 
2. 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. 
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. 
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. 
These and other objects are attained by 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 
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. 
In accordance with another aspect of the invention, 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 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 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 embodiments are possible without departing from the 
scope of the invention. Accordingly, the drawings and description are to 
be regarded as illustrative, and not restrictive in nature.