Patent Application
20150129200
This invention relates to gas and oil production, and more particularly to improvements in casing centralization.
To obtain pressure integrity and structural adequacy, typically after an oil or gas well is drilled the hole is lined with steel pipe and cement is inserted between the pipe and the formation. The centering of the casing within the hole is performed by casing centralizers of various types. Many casing centralizer types exist including fixed diameter solid cast or molded types with various types of thin-like extensions such as cords or spiral-shaped, bow-spring types that are self or double bow with welded and non-welded bows made of various material including zinc, steel, and plastic.
Casing centralization is of importance to oil and gas wells because proper centralization of the casing within the hole leads to improved cementing of the casing, and hence, pressure, integrity and safety. Centralizers are also important to allow use of slotted liners to avoid slot plugging, reduce drag during installation, and limit differential sticking of the casing to the formation during installation.
Historically, many different attempts were made to satisfy the multiple requirements for proper casing centralization; but these have failed because only one or two of the performance requirements were satisfied in previous designs. Requirements include the need to keep the casing in the center of the hole and allowing the cement to be evenly distributed around the casing. This centralization is difficult because of well bore configuration and common drilling problems. For example, in non-vertical wells, such as extended reach wells or horizontal wells, the casings weight forces the casing to the low side of the hole; without centralization, the casing will sit on the bottom side of the hole and prevent proper cementation. Further, certain drilling curvatures in the well bore trajectory caused by variations in rock hardness and orientation; these are commonly called “dog-legs,” and can result in the casing contacting the whole wall in a non-concentric manner.
Also, part of casing centralization is efficient passage of the cement past the centralizer towards the surface. If the centralizer fills a significant portion of the annulus between the casing and the well bore, the result is restriction of the cement flow, thus requiring greater pumping, but more incomplete cement coverage.
Another common problem occurs when running a smaller casing liner through a casing exit without a whipstock in place. For these applications, failure of the centralizers run on liners through casing exits can result in expensive time lost due to fishing (retrieving parts) and milling of pieces of centralizers in order to obtain proper well function. This significant problem is associated with the transition across the sharp edge of the casing and into open hole.
When drilling oil and gas wells in deep water locations such as in the Gulf of Mexico and offshore Brazil, contingency planning frequently involves the drilling string design with many concentric casing. Post Macondo accident in the Gulf of Mexico, various governmental regulatory agencies and industry associations have required more contingency planning that has resulted in increased number of casing strings that have outside diameters that allow less space between the formation and the casing for cement.
One of the methods to allow contingency planning for additional casing strings is to drill out from one casing with an under-reamer thus producing a drilled hole that is larger than the inside diameter of the casing the under-reamer passes through. For example, a centralizer installed on a 11¾ inch casing application may be required to pass through (i.e. collapse) from 12⅜ inch and expand to 14½ inch and support 1,000 pounds of side load. Consequently, a need exists for a centralizer that can run between two well casings having a relatively narrow annular space between casings. Such an application eliminates the use of fixed diameter centralizers because they cannot expand or contract through the restrictions. Further, existing bow-type centralizers do not provide enough side load support, and hence when used will collapse, thus not providing a centralized pipe and may result in a poor or even unsafe cement bond between the casing and the formation. Hence, there exists a need for a centralizer with limited expansion and collapse capability but with high amounts of side load support capability.
The present invention is directed to a slim-line casing centralizer having limited expansion and collapse capability but with high amounts of side load support capability. The slim-line casing centralizer of the present invention includes three components including a top and a bottom stop collar positioned above and below a centralizer sleeve. The centralizer body or sleeve is typically made of steel that is quenched and tempered to produce a high strength spring-steel-like behavior. Alternatively, the centralizer body or sleeve can be made of a low modulus high-strength metal such as titanium or a specially designed fiber-epoxy composite. The centralizer body can have a coating of low friction materials. The centralizer body consists of a multiplicity of support fingers, each finger is formed into a collapsible spring. Each finger is shaped to support a predetermined load at its apex. The shape is approximated by the performance of a beam and consists of a leading forward angle (downhole), a following aft angle (uphole). Typically the forward and downward angle range from 15 to 85 degrees, but most typically are forty-five degrees. The forward and aft angles are not necessarily the same. The end of the support fingers consist of a landing tab. When the centralizer experiences radial loads, the support finger collapses into a finger pocket. The pocket is sized to allow the complete collapse of the support finger. The landing tab guides the spring finger into its collapse without interference with the body of the centralizer.
These and other features and advantages of the present invention will be more fully understood by reference to the following detailed description in connection with the drawings.
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The centralizer body can include a coating of low friction materials to prevent wear on the casing. The coating may be urethane or an epoxy filled with ultra-high molecular weight polyethylene. These coatings reduce wear as well as rotating friction. The casing itself can also be coated. The coating can also include Teflon® (PTFE polytetrafluoroethylene) or PFA (perfluoroalkoxy alkanes).
The centralizer sleeve is made into a continuous cylindrical body 30 by rolling the sheet metal and welding the ends 32 together. A plurality of finger support elements 34 are cut into the body 30 uniformly along the body. The finger support elements are cut into the body to include gaps for the finger elements to function. Each support finger 34 is formed into a collapsible spring. Each finger is shaped to support a pre-determined load at its apex 36. The shape of the finger is approximated by the performance of a beam and consists of a leading forward angle 38 and a following aft support angle 40. Typically, the forward and aft angles range from about 15 to about 85 degrees, but most typically are 45 degrees. The forward and aft angles are not necessarily the same. The end of the support fingers consists of a landing tab 42. When the centralizer experiences radial loads, the support fingers collapse with the landing tab 42 extending into a finger pocket 44. The finger pocket is sized to allow the complete collapse of the support finger. The landing tab guides the support finger into its collapse without interference with the body of the centralizer which prevents the support fingers from snagging on surfaces during sliding into the hole.
Although manufacturing convenience would indicate that the support fingers were the same size, it is not necessary. Specifically, the support fingers can be of different sizes to facilitate running in the hole. In one embodiment, the support fingers are of increasing height moving to the center of the sleeve from either end. This embodiment helps reduce snagging during running. Further, a variable amount of side load support may be useful in applications where dogleg severity changes abruptly.
The number and dimensions of the support fingers are adjusted according to the anticipated side loads on the centralizer at the time of installation. Because of the multiplicity of support fingers, which could range from about 10 to 100, each finger contributes to the side load capacity of the centralizer. For example, in a 14 inch diameter configuration, made from 0.05 inch thick steel with yield strength of 150,000 psi, a forward angle of 45 degrees, an aft angle of 30 degrees, a 0.5 inch wide finger with a 0.5 inch space between fingers, a 3 inch collapsed length, can support approximately 22 pounds side force per finger that is normally loaded. As fingers that are not readily under the side load, a percentage of the normal load is proportional to the angle of the load to the transfer axis of the casing. For an application that requires 1500 pound side load will require a centralizer that is approximately 68 inches long.
Table 1 provides dimensional requirements for the slim-line casing centralizer examples of the present invention for various casing sizes.
Using American Petroleum Institute API (API) Specification 10D/ISO 10427-1-2001 for centralizers, the above method and modifying the materials, material thickness, width and length of fingers, finger forward and aft angles, number of fingers on the circumference, number of rings of fingers can be changed to meet any size of casing centralizer side load requirement. This process can be done in a closed form calculation or with iterative finite element analyses.
For example, in Table 1 of the API-Specification 10D for a 5½ inch casing the minimum restoring force (a radial force) at 67% standoff (67% of the cross-sectional area of the hole is filled by the centralizer) is 620 pound and a maximum 620 pound starting force (axial sliding). For a 5½ inch slim line centralizer, the restoring radial force is approximately 800 pounds and the maximum axial sliding starting force in formation is approximately 325 pounds. Lower starting forces advantageous for starting to run casing and a higher standoff force is advantageous, especially for inclined or horizontal wells, which the slim line casing centralizer of the present invention achieves.
The number of centralizers placed on a casing string can vary from about 5 to about 200 depending upon the well inclination and casing size and weight, dog-leg severity, and other well completion parameters. The side loads can be estimated by current commercially available software packages. Typically, each well is analyzed and specific recommendations per well are made.
Benefits of the slim-line casing centralizer of the present invention include centralization in narrow openings between individual casing. This allows for centralization between casing when the annular space is limited such as when running contingency casing strings in offshore applications. Utilizing a high forward angle or aft angle (from about 15 to about 85 degrees) in the support fingers allows the largest amount of support per finger and when combined with a large number of fingers within sheet metal construction, the result is an extremely slim structure with high collapse resistance and low drag resistance. A large number of support fingers that have wide range of expansion and collapse configurations allows the centralizer to be flexible to anticipated side load conditions. The centralizer of the present invention is scalable for casing between 4½ inches and 16 inches in outside diameter. Because there are a large number of support fingers, if any single one is snagged during the running process, the loss of an individual or severed finger does not significantly inhibit running the casing nor does it significantly affect the drag during installation. By changes to the forward angle and the aft angle of the support fingers, the expansion from full expanded to collapsed can be adjusted. By changing the expanded height of the support fingers a gradient of height facilitates running of the centralizer with fewer hang ups. The slim-line casing centralizer of the present invention typically exceeds the API 10D minimum radial standoff force and is typically less than the maximum starting force; from filling both of these requirements facilitates centralization in inclined holes and ease of running.
Although the present invention has been described with respect to a preferred embodiment thereof, it is to be understood that changes and modifications can be made therein which are within the full intended scope of the invention as hereinafter claimed.
This application claims the benefit of and priority to U.S. Provisional Application No. 61/902,091, filed Nov. 8, 2013, the contents of which are incorporated herein in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61902091 | Nov 2013 | US |
