External casing anchor

Details External casing anchor External casing anchor

2000-12-14

2003-09-23

Bagnell, David (Department: 3672)

Wells

Guide for device or conduit

Surrounding existing device or tubing

C166S195000

Reexamination Certificate

active

06622788

ABSTRACT:

CROSS REFERENCE TO RELATED APPLICATIONS
This application claims priority under 35 U.S.C. §119 from Canadian Patent Application No. 2,292,170 filed on Dec. 14, 1999.
FIELD OF THE INVENTION
The present invention relates to a metal anchor joint for anchoring casing in a well and to tee process of making and using it. More particularly the anchor joint is a thick-walled steel tubular, such as a length of well casing, having outwardly protruding rings affixed thereto.
BACKGROUND OF THE INVENTION
Well structures installed in the earth to exploit geothermal or petroleum energy resources are typically lines with tubular steel casings, which in turn, are cemented in place within the well bore. Under certain conditions, such as significant temperature changes, the casing tends to displace axially relative to the adjacent earth material. The present invention provides a means to restrain such relative displacement.
Within the context of petroleum drilling and completion systems, the vast majority of casing systems need only accommodate the loads arising from installation prior to cementing, and non-thermal production methods after cementing. For these conventional production methods, casing designs typically only consider pressure containment, collapse resistance and hydraulic isolation requirements, and not axial load changes after cementing.
However, in thermal applications, or where ground movements induced by processes such as reservoir compaction may occur, it is often desirable to provide highly efficient axial load transfer over relatively short interval lengths to prevent casing movement and consequent damaging effects on adjoining or attached components of the completion system.
The present invention was conceived specifically as a means to restrain the axial movement of casing strings in well bores which will be used for production of heavy oil by means of the process of steam stimulation. When casing is heated, axial displacement resulting from thermal expansion tends to occur and be concentrated at locations coincident with changes in the axial strength of the tubulars.
These axial displacements are most obvious at ground surface, where the casing ends. Movement at this location typically cases the well head to rise and fall relative to ground surface, correlative with increases and decreases in temperature, respectively. Surface piping connected to the well head must therefore include provisions to accommodate this movement or risk failure. Such provisions and risk increase cost; therefore a cost effective and reliable means to reduce surface well head displacement by restraining or anchoring the casing is advantageous.
Less obviously, changes in axial strength may occur down hole at locations where there is a transition in size, grade or configuration of components in the casing string. For example, such changes occur at liner junctions, or where axially compliant devices such as corrugated tubulars are employed. At these locations, axial movement of the casing occurs relative to the adjacent formation; this tends to concentrate strain in the weakest member of the string, potentially causing it to fail with consequent loss of either structural or pressure integrity.
Because of the generally long string lengths employed to case wells, the magnitude of axial load transferred between the casing and surrounding earth materials through the cement sheath is usually very low, and for typical non-thermal applications, is largely static. Therefore, there has apparently been little interest in developing methods to improve the efficiency of axial load transfer between the casing and cement sheath, beyond what occurs ‘naturally’ by friction and interlocking at the upset surfaces at connection points.
Even where axial load transfer is considered, the conventional understanding of interaction between the pipe and cement as described by D. K. Smith in “Cementing,” SPE Monograph Vol. 4, Society of Petroleum Engineers Inc. January, 1990, anticipates that a cement bond exists, capable of transmitting shear between the casing and cement and hence transferring axial load. This reference reports measured ‘bond’ strengths ranging from 20 to over 200 psi. These values were derived from cemented tube-in-tube tests where the annular space between two lengths of pipe was cemented. Axial compressive load was then applied to one tube and reacted by the other. For these tests, the effective (radial) stress present across the cement to steel tubular interface is not reported or considered, and the total reported average ‘bond’ strength is considered adhesive. Hence, designs that do consider axial load transfer typically rely on the presence of this apparent bond mechanism that, if present, would provide substantial load transfer over a relatively short axial length. For example, given a bond strength of 100 psi (which is about mid range of the values reported) a 7 inch diameter pipe could develop a calculated axial load resistance of 500,000 lb over just 18.95 feet. However, as described by Schwall, G. H., Slack, M. W. and Kaiser, T. M. V. in “Reservoir Compaction Well Design for the Ekofisk Field”. SPE Paper 36821, 1996 SPE Annual Technical Conference and Exhibition, Denver, Oct. 6-9, 1996, the concept of significant adhesive cement bond was alleged to be erroneous. The interaction behavior between the cement and steel was explained as a frictional mechanism.
While significant frictional forces may be developed along the casing length at depth, this may not always be relied upon, particularly at shallow depths.
With this background in mind, it is the objective of the present invention to provide anchoring means, for incorporation in a casing string, which is intended to function to reduce relative movement between the string and the adjacent earth material.
SUMMARY OF THE INVENTION
In accordance with the invention, an anchor joint for incorporation in a casing string is provided. The anchor joint comprises a thick-walled metal tubular having means (e.g. threads) at its ends for connection with the casing string. The tubular has a plurality of outwardly projecting, abrupt diameter changes spaced along its length.
More particularly, one or more metal rings are crimped or shrink fitted onto the tubular. Preferably, in its instressed condition (that is, prior to crimping or shrink fitting), each ring has an inner diameter equal to or less than the original outside diameter of the tubular.
In a more preferred embodiment, at least one steel ring is crimped onto a steel joint of well casing. The ring has a yield strength less than that of the joint. Crimping may be carried out by hydroforming. As a result of crimping both the joint wall and the ring, a detent is formed in the joint side wall and the ring is trapped within the detent.
By locking the joint and rings together by crimping or shrink fitting, the resulting engagement is sufficient to enable the joint to transfer axial load from the casing string through the ring to the surrounding cement sheath of the well, to provide resistance to axial displacement of the anchor joint relative to the earth material.
As stated, the tubular is “thick-walled”. In a general sense, this word is intended to convey that the anchor joint tubular wall is sufficiently strong and thick so as to maintain the structural integrity of the casing string. More specifically, it means that the tubular has a diameter to thickness ratio (“D/t”) less than 100, preferably less than 50. Most preferably the tubular is a joint of the casing used in the casing string. By being thick-walled and having end connections, the tubular is compatible with the casing string.
By “abrupt” is meant that the diameter changes create shoulders that preferably are substantially perpendicular to the axis of the tubular or alternatively may be sloped with an angle of at least 20″, more preferably at least 45°, relative to the axis of the tubular.
Preferably the joint will have a length in the order of 40 feet, so that it conforms with the average length of casing joints.
It will be apparent that the

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