Patent Application
20080264629
1. Field of the Invention
The present invention relates primarily to centralizers and methods for centering a casing string downhole within the borehole of a well. More particularly, the present invention relates to a centralizer that may be shipped more efficiently and is easily assemblable in the field.
2. Description of the Related Art
Centralizers are commonly secured at intervals along a casing string to radially offset the casing string from the wall of a borehole in which the casing string is subsequently positioned. The centralizers generally include evenly-spaced ribs that project radially outwardly from the casing string to provide the desired offset. Centralizers ideally center the casing string within the borehole to provide a generally continuous annulus between the casing string and the interior wall of the borehole. This positioning of the casing string within a borehole promotes uniform and continuous distribution of cement slurry around the casing string during the subsequent step of cementing the casing string in a portion of the borehole. Uniform cement slurry distribution results in a cement liner that reinforces the casing string, isolates the casing from corrosive formation fluids, and prevents unwanted fluid flow between penetrated geologic formations.
A bow-spring centralizer is a common type of centralizer that employs flexible bow-springs as the ribs. Bow-spring centralizers typically include a pair of axially-spaced and generally aligned collars that are coupled by multiple bow-springs. The bow-springs bow outwardly from the axis of the centralizer to engage the borehole to center a pipe received axially through the generally aligned bores of the collars. Configured in this manner, the bow-springs provide stand-off from the borehole, and flex inwardly as they encounter borehole obstructions, such as tight spots or protrusions into the borehole, as the casing string is installed into the borehole. Elasticity allows the bow-springs to spring back to substantially their original shape after passing an obstruction to maintain the desired stand-off between the casing string and the borehole.
Centralizers are usually assembled at a manufacturing facility and then shipped to the well site for installation on a casing string. The centralizers, or subassemblies thereof, may be assembled by welding or by other means such as displacing a bendable and/or deformable tab or coupon into an aperture to restrain movement of the end of a bow-spring relative to a collar. Other centralizers may be assembled into their final configuration by riveting the ends of a bow-spring to a pair of spaced-apart and opposed collars. The partially or fully assembled centralizers may then be shipped in trucks or by other transportation to the well site.
Pre-assembly of the centralizers reduces the amount of labor and tooling required at the well site, but partially or fully assembling the centralizers at a manufacturing facility greatly increases shipping costs due to a dramatically decreased shipping density since, due to the structure and the intended function of centralizers, assembled centralizers take up a very large amount of cargo space for relatively little weight. For example, a single casing string may require many truckloads of centralizers to be transported over very long distances and to remote locations, and the inefficiency of shipping pre-assembled centralizers adds substantially to the cost of completing a well.
Another factor that increases the costs of centralizers is the cost of assembly. Welding the end of each bow-spring to two opposed and spaced-apart collars is time-consuming and tedious. A typical bow-spring centralizer requires 12 or more welds by highly-skilled welder using specialized welding equipment, rods, and a special power supply. Furthermore, welding a bow-spring to a centralizer collar creates an undesirable heat-affected zone (HAZ) in the bow-spring and in the collar around each weld, and this HAZ can possibly weaken the material and render the centralizer more subject to mechanical failure.
Conventional fasteners, such as rivets and bendable tabs received within slots or apertures, may also be used to join a bow-spring to collars in a non-welded centralizer, but conventional fasteners such as these may present protrusions that may hang up during installation of the pipe string into the borehole thereby making installation of the centralizers and pipe string more difficult and time-consuming. Also, riveting and bending tabs into slots requires special equipment, such as mechanical presses and special tools, that is difficult to power, use, and maintain in the field.
U.S. Pat. No. 6,871,706 discloses a centralizer that requires a step of bending a retaining portion of the collar material into a plurality of aligned openings, each to receive one end of each bow-spring. This step requires that the coupling operation needs to be performed in a manufacturing facility using a press. As shown in
Improved centralizers and methods continue to be sought, particularly in view of the limitations of the prior art and the need for better or stronger centralizers. Considerations for the development of new centralizers and of new methods of assembling the centralizers include manufacturing costs, shipping costs, the costs associated with installing the centralizers onto pipe strings and the ease of running the pipe string into the well.
A field-assemblable casing centralizer and method are disclosed. One embodiment provides a method of manufacturing a casing centralizer. A plurality of bow-springs are formed, each having two opposed ends, each end having an aperture adjacent to a foot for being received in an aligning slot in a collar to position the bow spring for coupling to the collar at the aperture. A first collar and a second collar are also formed, each collar having a plurality of circumferentially spaced aligning slots, each slot for receiving a foot at the end of a bow-spring. A plurality of collar through-holes are formed in the collar, in positions to align with apertures in the ends of a plurality of bow-springs. Each collar through-hole is formed adjacent to an aligning slot by positioning the collar on a supporting back-up member having an opening for receiving an aligned punch, and by then forcibly driving the punch through the wall of the collar and into the opening in the supporting back-up member to extrude a portion of the collar wall material into the opening. The preferred width of the punch is less than about 80% of the width of the opening in the supporting back-up member, or an amount sufficient to draw a portion of the collar wall material onto the annular space between the received punch and the interior wall of the opening to form an extruded through-hole. Each collar through-hole is then threaded using a tap.
Another embodiment provides a bow-spring centralizer that may be field-assembled. First and second collars comprise a plurality of circumferentially-distributed, extruded through-holes that are threaded for receiving a fastener. The extruded and threaded through-holes have a protruding flange height resulting from the extrusion by the punch that is greater than the thickness of the collar wall adjacent to the extruded through-holes. A plurality of bow-springs each have a foot at protruding from each end adjacent to an aperture for receiving a fastener. The foot at one end of each of the plurality of bow-springs is disposed in an aligning slot of the first collar and secured in place by a threaded fastener installed through the aperture of the end of the bow-spring and threaded into an extruded through-hole. The foot at the other end of each bow-spring of each of the plurality of bow-springs is disposed in an aligning slot of the second collar and secured in place in the same manner. In this manner, only two fasteners, and no other structures, are required to secure each bow-spring to the pair of opposed and spaced-apart collars, and only one relatively portable tool may be required to assemble the centralizer in the field. Also, the foot and the aligning slot serve a dual purpose. The first, as discussed above, is that of aligning the aperture in the end of the bow-spring with an extruded through-hole in the collar for receiving a fastener. The second purpose is to reinforce the shear resistance of the mechanical coupling between the bow-spring and the collar. The foot is strategically placed in alignment with the anticipated direction of the shearing force applied to the coupling when the centralizer is secured to a tubular string and installed in a borehole. The aligning slots may conveniently be punched in the collar and adjacent to the extruded through-holes at the same time that the through-holes are punched and extruded.
Other embodiments, aspects, and advantages of the invention will be apparent from the following description and the appended claims.
The present invention provides an improved centralizer that can be quickly and inexpensively assembled in the field. The few components that make up the centralizer may be shipped unassembled and efficiently packed to a well site to maximize shipping density and minimize shipping costs. Once at the well site, centralizers of the present invention may be quickly and inexpensively assembled using basic tools and with minimal skilled labor. A centralizer according to the invention requires no welding and is not subject to complications from HAZ's.
In one embodiment of the centralizer of the present invention, a first collar and a second collar each have a plurality of circumferentially spaced aligning slots and a corresponding plurality of threaded, extruded through-holes. A plurality of bow-springs each have a foot at each end and an aperture generally adjacent to the foot. Each bow-spring is axially extendable between the first and second collars when the collars are generally aligned and spaced one from the other to receive a bow-spring. The foot at one end of each bow-spring is disposed in one of the aligning slots of the first collar and fastened to the first collar with a threaded fastener. The foot at the other end of that bow-spring is disposed in one of the retaining slots on the second collar and fastened to the second collar with a threaded fastener. A threaded fastener is inserted through each aperture at each end of the bow-spring and then threaded into an extruded through-hole in a collar to secure the ends of each bow-spring to the two opposed collars at opposed and corresponding through-holes to form a centralizer.
To maximize the ease and speed of assembly and/or disassembly of the centralizer, the bow-springs may be secured to a radially outward, exterior surface of each collar. This positioning of the bow-springs is, in contrast to that of conventional bow-springs on prior-art centralizers that typically retain the ends bow-springs inside a collar, allows each bow spring in the centralizer to be easily and independently installed, removed or replaced without necessarily removing any of the other bow-springs from the centralizer.
A method of manufacturing the casing centralizer is also provided, which includes the steps of extruding and threading holes in each collar, each for threadedly receiving a threaded fastener. The extruded through-holes have a radially-inwardly extruded flange height greater than the collar thickness immediately adjacent to the extruded through-holes. Preferably, the extruded flange height is at least 1.5 times a material thickness of the wall of the collar, and more preferably, the extruded height is between 2.0 and 3.0 times the material thickness of the wall of the collar. The increased extruded height allows more threads to be formed within the extruded holes, resulting in a much stronger threaded connection with the threaded fasteners and a correspondingly stronger and more durable centralizer.
The bow-springs 18 are angularly spaced one from the others along the collars 12, 14 and about the centralizer axis 15, and are typically evenly spaced, i.e. with substantially the same angular spacing between each pair of adjacent bow-springs 18.
As shown in
A first end 20 of each bow-spring 18 comprises a foot 28 for being disposed in an aligning slot 21 of a collar, which as shown in the embodiment illustrated in
Each bow-spring 18 is further secured at its first end 20 to the first collar 12 using a threaded fastener 24 inserted through an aperture (not shown in
To maximize the ease and speed of assembly and/or disassembly of the centralizer 10, the bow-springs 18 have been configured so that they may be secured as shown to the radially outward, exterior wall 25 of each collar 12, 14. This positioning of the ends 20, 22 of the bow-springs 18 on the radially outward, exterior wall 25 is in contrast to that of conventional bow-springs of prior-art centralizers that typically secure each end of each bow-spring inside a collar, i.e. between the interior wall of the collar and the exterior wall of the casing string to be installed within the bore of the centralizer along its axis 15. Thus, unlike with prior art centralizers, any of the bow-springs 18 in the centralizer 10 of the present invention may be independently installed or removed without removing the centralizer 10 from the casing and without removing any of the other bow-springs 18 from the centralizer. This feature increases the speed and reduces the costs of assembling the centralizer 20. Also, being field-assemblable allows for two or more types of bow-springs to be usable with the same type of centralizer collars. A user can maintain a smaller inventory, but still select the specific type of bow-spring centralizer according to the length, shape, material and strength needed to serve a particular application and have bow-springs of that type delivered to the well site for assembly with a single, or “universal,” type of collars for that given diameter of tubular string.
The punches may have selected dimensions favorable for perforating the specific thickness and grade of sheet metal or other relatively thin-walled workpieces such as the collar 12. The heads 42, 44 may be powered to deliver sufficient punching forces to the punches 51, 53. For example, the heads 42, 44 may operated together or independently, and they may be hydraulically and/or pneumatically powered, or the may be driven with a pair of rotatable threaded guide axles (not shown). Alternatively, the heads 42, 44 may be either automatically or manually driven by a weighted assembly, or driven manually by an operator using a leveraged actuator (not shown). Other ways of driving the heads 42, 44 or otherwise imposing sufficient penetrating forces to the punches 51, 53 to perforate the collar 12 may be devised according to the present invention.
Still referring to
It should be noted that the dimensions d1, d2, d3, and d4 are shown in the plane of the page, and do not necessarily indicate the dimensions 51, 53 of the punches relative to the dimensions of opening 52, 54. For example, the slot retainer 21 formed in
Only one aligning slot 21 and one extruded through-hole 36 are shown in
The movement of the heads 42, 44 and the punches 51, 53 may be relatively rapid and “explosive,” such as performed by a stamping operation, or it may be slow and controlled, such as may be performed by a controlled movement of the heads 42, 44. A number of factors may be considered in selecting the speed of movement of the heads 42, 44 and the punches 51, 53, such as the strength of the collar material, the thickness, t, of the collar material, and the mechanical properties of the punches collar material and of the 51, 53. For forming the slot 21 with the punch 51, the movement may be rapid, such as to maximize manufacturing productivity when forming multiple slots 21 in many collars 12. A quick movement of the punch 51 may maximize the shearing effect, as well. By contrast, the speed of movement of the punch 53 in forming the extruded hole 36 may affect the flange height, h, of the extruded hole 36. For example, slower, more controlled punch movement may result in more plastic deformation (elongation) of the extruded hole 36 prior to shear. The temperature of the collar 12 is also likely to affect the extent of the elongation. If the collar 12 is sufficiently heated, the collar material may allow a generally longer extrusion. If the collar 12 is instead worked at or near ambient temperature, the movement of the punch 53 may be relatively slow to minimize the possibility of premature shear in the extruded through-hole 36.
The invention, therefore, includes both the provision of a field-assemblable casing centralizer and a method of manufacturing the field-assemblable centralizer. The centralizer may be shipped unassembled to increase shipping density and decrease associated shipping costs. Once at the well site, the components of the centralizer, such as the collars and the bow-springs, may be assembled easily using minimal tools, skills, and labor. The bow-springs are desirably secured to outer portions of the collars and may therefore be quickly and easily removed or replaced without having to remove the casing centralizer from the casing. The extruded through-holes in the collar are extruded to increase the number of threads that may be disposed within the through-holes, which increases the strength of the threaded connection made up using the threaded fasteners and improves the overall strength and durability of the centralizer. 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 scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.
There is a generally large number of combinations of dimensions and materials that may be used to implement the present invention, and there are no specific ratios or parameters that are required to implement the present invention. Those skilled in the art will recognize that the extruded hole can be obtained in the manner disclosed herein for those materials having sufficient ductility, but may not be obtained using materials having excessive hardness, and that the material will often dictate the ratio of the width of the punch to the width of the opening in the backing member that will produce a satisfactory extruded hole that can be successfully tapped and used to implement the present invention.
The terms “comprising,” “including,” and “having,” as used in the claims and specification herein, shall be considered as indicating an open group that may include other elements not specified. The terms “a,” “an,” and the singular forms of words shall be taken to include the plural form of the same words, such that the terms mean that one or more of something is provided. The term “one” or “single” may be used to indicate that one and only one of something is intended. Similarly, other specific integer values, such as “two,” may be used when a specific number of things is intended. The terms “preferably,” “preferred,” “prefer,” “optionally,” “may,” and similar terms are used to indicate that an item, condition or step being referred to is an optional (not required) feature of the invention.
The term “pair” may be used to indicate two items that are not identical in structure, size, shape and material, but are substantially identical with respect to the properties or structure related to the characteristic or quality being referred to in the context of the disclosure using that term. An insubstantial change that does not materially affect the use of the present invention does not make one item not form a “pair” with the substantially similar item.
