CROSS-REFERENCE TO RELATED APPLICATIONS
- STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
- BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention is related generally to the field of centralizers, such as used on casing inserted in wellbores drilled through the earth. More specifically, the invention is related to centralizers which can pass through an opening that is smaller than the opening in which a device is to be centralized.
2. Background Art
Wellbores drilled through the earth to extract petroleum and the like are commonly “completed” by cementing a steel pipe or casing in the wellbore after it is drilled. The casing serves to maintain the mechanical integrity of the wellbore, provides a conduit for produced fluids to move to the earth's surface, and hydraulically isolates earth formations from each other so that high fluid pressure earth formations do not discharge fluid into lower fluid pressure earth formations.
The casing is typically inserted into the drilled wellbore by coupling segments of the casing together and lowering the coupled segments into the wellbore. To cement the casing in place in the wellbore, cement is typically pumped through the interior of the casing, and is discharged into an annular space between the casing and the wellbore from the bottom of the casing. An important aspect of properly cementing the casing in place to complete a wellbore is that the casing have a substantially uniform annular space around it at all places along the length of the wellbore. Uniformity of the annular space increases the likelihood that the cement will completely and uniformly fill the annular space, thereby ensuring that the wellbore properly hydraulically isolates earth formations from each other. Uniformity of the annular space is affected by the trajectory of the wellbore and the final shape of the wellbore, among other factors. Frequently wellbores are drilled along trajectories other than vertical, so earth's gravity and bends in the wellbore cause the casing to rest on the wall of the wellbore in some places along the wellbore. In other cases, the wall of the wellbore may include out of round sections, for example washouts or keyseats, which make cementing operations more difficult.
It is known in the art to use centralizers to keep the casing as close as possible to the center of the wellbore for proper cementing. Typical centralizers known in the art are shown, for example, in a sales brochure published by Antelope Oil Tool & Manufacturing Company, Mineral Wells, Tx. (not dated). Centralizers are typically coupled to the exterior surface of the casing at selected locations along the casing prior to inserting the casing into the wellbore. Spring blades on the centralizers provide a restoring force which tends to push the casing into the center of the wellbore. Specifications for the amount of restoring force, and proper use of centralizers are described in a document entitled, Specifications for Bow-Spring Centralizers, API Specification 10D, fifth edition, American Petroleum Institute, Washington, D.C. (1994). Generally speaking, casing centralizers are made to center a particular outside diameter (OD) casing within a particular nominal diameter wellbore. The casing OD is selected by the wellbore operator to closely match the wellbore diameter, which is primarily related to the diameter of the drill bit used to drill that segment of the wellbore.
More recently, it has become known in the art to drill wellbores to a depth greater than a depth to which casing has been set, in which the greater depth portion of the wellbore has a diameter larger than the diameter of the casing. This type of drilling can be performed using various types of reaming tools such as hydraulic underreamers or specialized drill bits known as bi-center bits. See, for example, U.S. Pat. No. 6,039,131 issued to Beaton. Drilling this type of wellbore makes it possible to insert a larger completion device in the deeper portion of the wellbore, such as gravel pack or sand screens, than would be possible using conventional drilling techniques. Completing wellbores having such deeper sections including oversize diameters using centralizers known in the art has proven difficult because it is impracticable to move a larger outside diameter centralizer through a smaller internal diameter casing or other opening.
It is desirable to have a centralizer which can position a casing inside a larger diameter wellbore than the opening through which the centralizer can freely pass. One such type of centralizer known in the art includes a plurality of bow shaped spring blades coupled at each end thereof to a collar. One of the collars is affixed to an exterior surface of the casing. The other collar is allowed to “float” or move axially along the outer surface of the casing. Typically, the exterior surface of the casing will be machined or otherwise reduced in diameter under the position of the spring blades. The centralizer is thus able to laterally compress when the centralizer is pushed through an opening smaller than the uncompressed diameter of the spring blades. When the spring blades are compressed, they extend a longer distance along the length of the casing. The floating collar is able to move in response thereto. When the compressible centralizer is moved into a larger diameter opening after passing through the restriction, the spring blades expand laterally, providing a restoring force to centralize the casing inside the larger diameter opening.
- SUMMARY OF THE INVENTION
A limitation to the laterally compressible centralizer known in the art is that it requires a relatively high starting force and running force to move axially through the restricted diameter opening. Another limitation to these centralizers is that they may require machining or other form of reducing the diameter of the casing to enable sufficient compression of the spring blades.
One aspect of the invention is a centralizer for laterally positioning a pipe in an opening larger in diameter than an opening through which the centralizer may freely pass. The centralizer includes a plurality of helically shaped spring blades affixed at each end thereof to a slip collar. The slip collars are adapted to slide and rotate about the exterior surface of the pipe. The retaining sleeve is disposed axially between the slip collars, and is adapted to be affixed to the pipe so as to limit axial motion of the slip collars.
In one embodiment, the slip collars include grooves in the exterior surface adapted to receive the ends of the spring blades. In one embodiment, the sides of the grooves are radiused to avoid distorting the slip collars. In one embodiment, a thickness of the slip collar material below the groove is substantially the same as that of the retaining sleeve. In one embodiment, the centralizer includes an anti-friction bearing between at least one of the slip collars and the retaining sleeve. The anti-friction bearing is adapted to reduce rotational friction of the slip collar about the casing when the spring blades are laterally compressed. In one embodiment, the retaining sleeve includes a plurality of set screws arranged in a helical pattern corresponding to the shape of the spring blades to enable access to the set screws. In one embodiment, the slip collars include mud channels on an exterior surface to avoid surge and swab effects when running the centralizer.
BRIEF DESCRIPTION OF THE DRAWINGS
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.
FIG. 1 shows a laterally compressible centralizer assembly according to one aspect of the invention.
FIG. 2 shows a cross section of one end of an embodiment of the centralizer.
FIG. 3 shows a partial side view of a particular embodiment of one of the slip collars.
FIG. 4 shows an end view of the slip collar embodiment shown in FIG. 3.
FIG. 5 shows two centralizers according to the invention affixed to a pipe being passed through a restricted diameter opening.
FIG. 1 shows a laterally compressible centralizer according to one aspect of the invention. The centralizer is placed on the exterior of a joint of casing 10 which is to be moved through an opening having a smaller diameter than the opening in which the centralizer ultimately will be positioned. Such smaller diameter openings may include, for example, a casing string in a wellbore drilled through the earth, below which open wellbore is “underreamed” to a larger diameter than the internal diameter of the casing string. The centralizer includes slip collars 14 disposed on the exterior of the casing 10 at axially spaced apart locations thereon. The slip collars 14 are interconnected by “bow” shaped spring blades 16, of a type conventional for bow spring type centralizers. Each of the spring blades 16 is affixed at each of its ends to one of the slip collars 14. In their uncompressed state, the spring blades 16 extend laterally a predetermined radial distance beyond the radius of the casing 10. This distance, and the number of spring blades 16 are selected based on factors such as the diameter of the opening into which the casing 10 is to be centralized and the weight of the casing 10, among other factors. In this embodiment, the spring blades 16 are preferably helically shaped. More preferably, the helical angle of each spring blade 16 is within a range of about 15 to 20 degrees away from being parallel to the longitudinal axis (not shown) of the casing 10. The spring blades 16 may be welded, adhesively bonded, or otherwise affixed to the slip collars 14. The spring blades 16 and slip collars 14 may also be formed from a single sheet, tube, or other piece of material. The material is typically steel, but may be any other suitable material capable of providing the required restoring force.
The centralizer in this embodiment includes a retaining sleeve 12 which is affixed to the exterior surface of the casing 10. The retaining sleeve 12 in this embodiment includes a plurality of set screws 18 which are disposed inside threaded holes to enable tightening against the exterior of the casing 10. In this embodiment, the set screws 18 are disposed in a helical pattern about the exterior of the sleeve 12. The helical pattern of the set screws 18 is preferably selected so that they are easily accessible between the individual spring blades 16. Although set screws are shown in this embodiment to retain the sleeve 12 on the exterior of the casing 10, other means for retaining the retaining sleeve may be used, including welding and/or adhesive.
An outer edge of at least the lowermost slip collar 14 preferably includes thereon a beveled or tapered edge 14A to reduce the possibility of the centralizer becoming caught on any protrusions or shoulders in a wellbore (not shown) during movement of the casing therethrough.
The edges of the sleeve 12 preferably include thereon an anti-friction bearing 20 which may be a roller or ball bearing, a low friction coating or any other device which is adapted to reduce friction of rotation between the slip collar 14 and the sleeve 12 when the edges of the slip collar 14 and sleeve 12 come into contact. The reason for this contact will be further explained. Alternatively, the anti-friction bearing 20 may be formed or attached to the corresponding edge of the slip collars 14. Additionally, an anti-friction bearing surface 22 may be applied to or otherwise formed into the exterior surface of the casing 10, or on a corresponding interior surface of the slip collars 14, to enable the slip collars 14 to slide axially along the casing 10 and to rotate about the exterior of the casing 10.
A cross-sectional view of one end of the centralizer as affixed to the exterior of the casing is shown in FIG. 2. In the embodiment shown in FIG. 2, the slip collar 14 preferably includes therein a slot or groove 15 adapted to receive the end of each one of the spring blades 16. As shown in FIG. 2, preferably, the groove 15 is formed in the outer surface of the slip collar 14, such as by milling, grinding, or preforming into the exterior surface of the slip collar 14 material, so that the spring blade 16 is substantially flush with the exterior surface of the remainder of the slip collar 14 when inserted into the groove 15. The groove 15 preferably also has a depth selected so that the thickness of a tongue portion 17 therein substantially matches the thickness of the retaining sleeve 12. This configuration enables the spring blades 16 to compress laterally to nearly as small a diameter as the external diameter of the slip collars 14, without binding or sticking on the outer surface of the sleeve 12. Because in this embodiment, the spring blades 16 are flush with the exterior of the slip collars 14, the diameter of the restricted opening through which the centralizer may pass in its compressed state is minimized. This feature increases the useful diameter range of the centralizer.
FIG. 3 shows a side view of a segment of one of the slip collars 14 to explain another aspect of a centralizer according to the invention. The grooves 15 in this embodiment of the slip collar 14 have therein radiused sides 19. The radiused sides 19 preferably have substantially the same width as the spring blades 16 at the ends of the blades, but increase in width towards the tongue end (17 in FIG. 2). The radiused sides 19 better enable twisting of the spring blades 16, as a result of relative rotation of the slip collars 14, without distorting the shape of the slip collars 14. The term “radiused” as used herein to describe the shape of the sides of the grooves 15 is meant to include within its scope any form of curved or tapered change in width along the length of the groove such that the spring blades 16 may twist around the axis (not shown) of the casing (10 in FIG. 1) substantially without distorting the shape of the slip collars 14.
The embodiment shown in FIG. 3 also includes “mud channels” shown at 21, which may be slots or grooves, cut or otherwise formed into the exterior surface of the slip collar 14. The mud channels 21 are preferably located on the circumference of the slip collars 14 between the spring blade grooves 15, and are cut to a depth such that the thickness of the remaining portion of the slip collars 14 underneath the mud channels 21 is substantially the same as that of the retaining sleeve (12 in FIG. 1). The mud channels 21 enable passage of fluid by the slip collars 14 when the centralizer is inserted into a restricted opening having an internal diameter only slightly larger than the external diameter of the slip collars 14. The mud channels therefore reduce “surge” and “swab” effects as the centralizer is moved along the inside of the restricted opening (not shown). Surge and swab are terms used in the art to describe a piston-like hydraulic action when a member is moved inside a wellbore. An end view of the embodiment of the slip collar 14 is shown in FIG. 4.
Having explained the general structure of a centralizer according to the various aspects of this invention, the manner in which the invention is used in a wellbore will now be explained. Referring to FIG. 5, when the sleeve 12 and centralizer (including slip collars 14 and spring blades 16) are attached to a joint of casing 10, the spring blades extend to an uncompressed diameter D1. When the joint of casing 10 is to be passed through a restricted diameter opening (such as a casing string 10A in a wellbore), typically the joint of casing 10 will be coupled to similar joints of casing, some of which may include centralizers such as shown in FIG. 1 and previously explained. As the joint of casing 10 is moved into the restricted diameter opening 10A, the spring blades 16 are laterally compressed, to D2 in FIG. 5, which is the internal diameter of the restricted diameter opening 10A. Lateral compression extends the blades 16 axially along the joint of casing 10. Because the slip collars 14 are free to slide along the casing 10, axial extension of the spring blades 16 is unhindered. Further, the bearing surface (22 in FIG. 2) used in some embodiments reduces axial friction between the slip collars 14 and the joint of casing 10. Note that the retaining sleeve 12 limits the axial movement of one of the slip collars 14, but does not limit the axial separation between the slip collars 14. Therefore, the axial position of the centralizer is ultimately limited. Because the spring blades 16 of the embodiment shown in FIG. 5 are also helically shaped, lateral compression of the spring blades 16 also results in some rotation of the slip collars 14 about the exterior of the casing 10, as the spring blades 16 twist under the lateral compression. The slip collars 14, as previously explained, are also free to rotate about the casing joint 10. Further, as one of the slip collars 14 comes into contact with the edge of the sleeve 12, the anti-friction bearing used in some embodiments (20 in FIG. 2) reduces rotational friction between the sleeve 12 and the slip collar 14. The combination of axial extension and twisting of the spring blades 16 enables the spring blades 16 to more easily compress to fit within the restricted diameter opening (not shown), and then to laterally expand as the centralizer is moved into a larger diameter opening (not shown).
In the invention, it has been determined that using helical spring blades 16 on a centralizer such as shown in FIG. 1 reduces the amount of axial force needed to move the centralizer and casing (10 in FIG. 1) through any particular restricted diameter opening, while retaining enough lateral expansion force to provide enough restoring force to lift the casing 10 away from the wall of the larger diameter opening. Such forces are referred to in the art as “starting force” for initiating movement into the restricted diameter opening, and “running force” to continue movement along the restricted diameter opening.
As a practical matter, a centralizer as shown in FIG. 1 including the sleeve 12 therein may be made, for example, by welding or otherwise affixing the spring blades 16 to the slip collars 14 while the sleeve 12 is disposed axially between the slip collars 14. This may be performed, for example, on a suitable diameter mandrel (not shown) for convenience of positioning the sleeve 12 and slip collars 14 prior to welding the spring blades 16 thereon. The mandrel (not shown) may also be used to store and transport the centralizer including the sleeve 12 therein until it is to be affixed to the casing 10.
It is known in the art to affix a helical bow spring blade centralizer so as to straddle a casing collar (not shown in the Figures) wherein the slip collars are free both to rotate and slide along the casing. Typically, the centralizer used to straddle a casing collar is in the form of “half shells” or a “clam shell” which is opened to clamp around the casing. The centralizer and method for attaching to the casing in the invention are different from the prior art “straddle” type centralizers in that the straddle type centralizer is limited as to its amount of lateral compressibility by the diameter of the casing collar. Typically, straddle type centralizers are not intended to be moved through a restricted diameter opening and therefore do not have the useful diameter range of the centralizer of the present invention.
While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the spirit of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.