US 7845061 B2
A bow-spring centralizer includes a center assembly having a plurality of bow springs and a pair of generally tubular moving collars secured one to each end of each bow spring, the center assembly formed intermediate a pair of generally tubular stop collars. Each moving collar/stop collar combination forms an interlocking and axially extendable collar. The centralizer is formed from a single tube. The tube is cut using a laser according to a cut pattern that creates interlocked stop collar/moving collar combinations that are rotatably locked but axially movable one relative to the other. Each of the interlocked stop collars and moving collars includes a plurality of circumferentially spaced heads, each head integrally formed on one of a plurality of circumferentially spaced extensions protruding from the stop collar or moving collar. The heads may have a variety of projected shapes, such as a rectangular, arrow or a teardrop shaped. Each head is axially slidably captured within one of a plurality of chambers on the other tubular member (stop collar or moving collar) to which the first member is coupled. The extensions of each interlocked tubular member define the outer walls of the chamber in which a head of the opposing tubular member is slidably captured.
1. A method of making a bow spring centralizer having extendable collars, comprising:
cutting elongate portions from the center portion of a tube intermediate the first and second tubular ends to form a generally elongate cage with a plurality of generally angularly distributed ribs intermediate the first and second tubular ends;
forming a first extendable collar from the first tubular end by cutting the first tubular end into first and second interlocking tubular members and by removing portions of a wall of the first tubular end along the interlocking interface to form a first stop collar that is rotatably locked to, but slidably interlocked with, a first moving collar;
forming a second extendable collar from the second tubular end by cutting the second tubular end into first and second interlocking tubular members and by removing portions of a wall of the second tubular end to form a second stop collar that is rotatably locked to, but slidably interlocked with, a second moving collar that is opposite the elongate cage from the first moving collar, with both moving collars intermediate the first and second stop collars; and
displacing the angularly distributed ribs radially outwardly from the axis of the tube to form a plurality of bow springs;
wherein the moving collars are integral with the plurality of bow springs, and
wherein a bore of the first extendable collar and a bore of the second extendable collar are generally aligned for receiving a casing.
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1. Field of the Invention
This invention is directed to casing centralizers having flexible bow springs for use in borehole completion operations, and particularly to centralizers that may be radially collapsed to pass through a small annular space, and that can deploy to generally center a casing within a borehole. More specifically, the present invention is directed to an integrally-formed centralizer in which the collars and the bow springs may be formed from a single tube.
2. Description of the Related Art
Centralizers are commonly secured at spaced intervals along a casing or tubing string to provide radial stand-off of the casing or tubing from the interior wall of a borehole in which the string is subsequently installed. The centralizers generally comprise generally aligned collars defining a bore there through for receiving the casing, and a plurality of angularly-spaced ribs that project radially outwardly from the casing string to provide the desired stand-off from the interior wall of the borehole. Centralizers ideally center the casing within the borehole to provide a generally uniform annulus between the casing string exterior and the interior wall of the borehole. This centering of the casing string within the borehole promotes uniform and continuous distribution of cement slurry around the casing string during the subsequent step of cementing the casing string within an interval 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 one to the other by a plurality of bow-springs. The flexible bow-springs are predisposed to deploy and bow radially outwardly away from the axis of the centralizer to engage the interior wall of the borehole and to center a casing received axially through the generally aligned bores of the collars. Configured in this manner, the bow-springs provide stand-off from the interior wall of the borehole, and may flex or collapse radially inwardly as the centralizer encounters borehole obstructions or interior wall of the borehole 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 collapsing to pass a borehole obstruction, and to thereby maintain the desired stand-off between the casing string and the interior wall of the borehole.
Some centralizers include collars that move along the length of the casing in response to flexure of the bow springs. For example, U.S. Pat. No. 6,679,325 discloses, in part, a low-clearance centralizer having an extendable collar at each end, each extendable collar comprising a moving collar and a stop collar that cooperate to form an extendable collar. The extendable collar at each end of the centralizer of the '325 Patent includes a longitudinal bore within the aligned extendable collars for receiving the casing to which the stop collars are secured to position the centralizer on the casing. Each moving collar has a collet with a radially outwardly flanged portion for being movably received within an interior circumferential groove or bore within the mating stop collar. A plurality of flexible bow springs are secured at each end to a moving collar, and the two moving collars are maintained in a variable spaced-apart relationship by the bow springs and the stop collars.
A shortcoming of the centralizer of the '325 Patent is that the stop collar and the moving collar require axially overlapping structures in order to slidably interface one with the other. This overlapping structure adds to the radial thickness of a centralizer of comparable strength, thereby increasing the minimum collapsed diameter of the casing centralizer and limiting the borehole restrictions through which the centralizer and a casing can pass.
The radial thickness added to the exterior of a casing string by an installed centralizer is but one factor to be considered in selecting a centralizer for a given application. The cost of manufacturing the centralizer is also an important consideration. Many movable collars require the manufacture of complicated mechanisms as compared with simple stationary collars. Even less complicated designs include moving collars that are assembled using multiple components, each of which must be separately manufactured and subsequently assembled into a moving collar. While the end result is useful, the costs of manufacturing multiple components, and the costs associated with assembling the components into a centralizer, make these devices relatively expensive. Thus, there is an ongoing need for centralizers having extendable collars that are radially thinner, but less expensive to manufacture and assemble.
The present invention provides a low-clearance and efficiently manufactured centralizer for use in centering a casing within an earthen borehole. The low-clearance centralizer comprises a stop collar having a bore, the stop collar securable to the exterior of a casing in a spaced-apart relationship to an opposing stop collar having a generally aligned bore, the opposing stop collar also securable to the exterior of the casing. Each stop collar is movably interlocked with and cooperates with a moving collar that is formed along with the stop collar from a single tube. Each moving collar is secured to its stop collar using a circumferentially interlocking structure to form an extendable collar. The moving end of the extendable collar receives and secures to the ends of a plurality of bow-springs that may also be formed from the same single tube from which the extendable collar is formed.
The bow springs of the centralizer of the present invention are modified—after being cut from the tube—to bow radially outwardly and thereby deploy against a interior wall of the borehole to provide stand-off between the casing and the interior wall of the borehole. The bow springs are sufficiently flexible to elastically collapse from the deployed condition to a collapsed condition to lie generally along the length of the exterior wall of the casing received within the centralizer. A portion of the arc length of the bow springs in their deployed (or bowed) condition is receivable within the retracted length of one of the extendable collars. The centralizer of the present invention is adapted for being pulled through a tight restriction in the borehole by the leading extendable collar. The extendable collars may be designated as a leading collar and a trailing collar, depending on the direction of movement of the casing string and the centralizer affixed thereon. As the deployed bow springs encounter the borehole restriction, the leading extendable collar is extended to its greatest length upon being introduced into the borehole restriction; that is, the leading moving collar, and the bow springs secured at a leading end to the leading moving collar, slide—according to the collapsing force imparted to the bow springs by the borehole restriction—to an extreme configuration for separation of the leading stop collar from the leading moving collar to fully extend the leading extendable collar. As the bow springs continue to collapse to lie generally flat along the exterior surface of the portion of the casing between the leading and trailing extendable collars, a portion of the arc length from previously bowed and deployed bow springs is generally straightened and received within the stroke of the trailing extendable collar as it retracts to a shorter length. Upon passage of the bow springs of the centralizer through the borehole restriction, the resiliency of the bow springs restore the bow springs to their radially outwardly deployed condition and both the leading and the trailing extendable collars are restored to their extended condition, unless the centralizer continues to be shaped by some outside force such as frictional contact between the deployed bow springs and the interior wall of the borehole.
The low-clearance centralizer of the present invention achieves its low-clearance design as a result of the inventive method of making the centralizer from a tube. Preferably, a laser is used to cut a tube into three interlocking pieces comprising two stop collars at the ends, and a center assembly, comprising two moving collars with a plurality of bow springs, intermediate the two moving collars. Alternately, a high pressure water nozzle may be used to create a water jet to cut the tube wall. The centralizer formed in this manner from a single tube in accordance with the present invention comprises two extendable collars, each extendable collar comprising one of the stop collars movably interlocked with the adjacent moving collar of the center assembly. The movement between a stop collar and the adjacent moving collar is provided by cutting the tube into an interlocking pattern and by strategically cutting and removing coupons from the interlocked wall of the tube to facilitate axial movement, but not rotation, between the stop collar and the adjacent moving collar. The cutting and removal method of the present invention results in protrusions extending from one of either the moving collar or the stop collar, or both, being slidably captured within a chamber cut into the other.
The present invention provides a centralizer and a method of forming a centralizer. The centralizer of the present invention comprises three members: a cage comprising a plurality of bow springs intermediate a first extendable collar and a second extendable collar. The centralizer of the present invention is cut from a tube using a laser or some other device for precision cutting the wall of a tube.
In one embodiment of the method of the present invention, the tube is cut, preferably using a laser, along a pre-programmed pattern to remove generally elongate material coupons to form an open-ended and generally tubular cage having a plurality of generally parallel ribs. The ribs are preferably equi-angularly distributed about the axis of the tube. At each end of the cage, and after the ribs of the cage are formed into bow springs, the remaining portions of the tube are cut to form a pair of opposed extendable collars, each comprising a stop collar and a moving collar. The stop collar and moving collar of each extendable collar are permanently interlocked one with the other unless one or both are deformed from their generally tubular shape to be separated.
The stop collar and the moving collar are formed, one adjacent to each end of the cage, by cutting the tube wall in a circumferentially interlocked configuration, and by strategic removal of material coupons from the wall of the tube. The stop collar and the moving collar formed thereby are generally rotatably locked, but axially movable, one relative to the other. The range of axial movement between the stop collar and the moving collar is determined by the axial length of the removed material coupons and the configuration of the portions of the pattern that extend along the axis of the tube.
The interlocked configuration cut into the tubular wall in forming each extendable collar may vary in geometrical shape. Generally, the interlocked configuration comprises two interlocked tubular members, a stop collar and a moving collar. Each interlocked tubular member of the extendable collar includes a plurality of circumferentially distributed heads, each head integrally formed on the end of an extension that extends axially from the member. Each head is captured within a circumferential chamber formed intermediate adjacent extensions from the opposite interlocked member. The axial extensions from the stop collar, which are shaped from the wall of the tube, are integrally formed with heads that are slidably captured within chambers that are cut into the wall of the tube from which the moving collar is formed. Also, the axial extensions from the moving collar, which are shaped from the wall of the tube, are integrally formed with heads that are slidably captured within chambers that are cut into the wall of the tube from which the stop collar is formed. The heads connected to the extensions may have a variety of shapes, such as generally rectangular, arrow-shaped or bulbous or teardrop-shaped, but all are generally curved with the radius of the wall of the tube from which the extendable collars/extension/heads are cut.
Each head is integrally formed with a generally central axially-oriented extension intermediate the head and the body of the tubular member (i.e., the stop collar or the moving collar). Each head is axially movably captured within one of a plurality of chambers formed within the tubular member. Consecutive, angularly distributed extensions of the first tubular member define the side walls of a chamber in which a head of the opposing second tubular member is movably captured (the “captured head”), and vice-versa. The body of the first tubular member may provide an end wall of a chamber within the first tubular member for limiting movement of the captured head extending from the second tubular member in the axial direction. Each extension from a tubular member is slidably received within the space between adjacent heads of the other tubular member. The heads integrally formed on consecutive extensions of the first tubular member limit axial movement of the captured head extending from the second tubular member. The first and second tubular members are, thereby, rotatably locked on relative to the other, and axially movable one relative to the other between a retracted configuration corresponding to the shorter configuration of the extendable collar and an extended configuration corresponding to the extended configuration of the extendable collar.
In the extended configuration, each captured head of one tubular member abuts the heads on the interlocked tubular member that, in part, define a portion of the chamber. In the retracted configuration, the captured heads may, but do not necessarily, abut the end walls of the respective chamber (see discussion of allowance for debris accumulation below). Thus, the first and second tubular members are “slidably interlocked” within a defined range of axial movement between the extended and retracted configurations.
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The bow springs 34 are shown in their radially outwardly deployed configuration to provide stand-off from an interior wall of the borehole during installation of the casing 70 into a borehole. Each of the upper and lower extendable collars 8 are shown in the extended configuration as the deployed bow springs 34 pull the moving collars 20 toward the center portion of the centralizer 6 and away from the stop collars 10 that are secured to the exterior of the casing 70.
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.
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.