US3695351A - Suspending casing through permafrost - Google Patents

Abstract

Failure of casing set in wells drilled through permafrost zones can be anticipated if the casing is set in the usual manner. As the warm formation fluid is produced up through the permafrost zone, the permafrost adjacent the well thaws. In many instances the casing could be damaged by buckling when it loses its lateral support, while the upper termination of the casing moves downward due to subsidence. This problem is relieved by cementing the casing only below the permafrost zone and suspending the weight of the uncemented portion of the casing from a means at the surface which would not be affected subsidence-wise by the thawed region. A constant tension device can be used to adjust the tension on the casing.

Description

United States Patent Lubinski [54] SUSPENDING CASING THROUGH PERMAFROST [72] Inventor: Arthur Lubinski, Tulsa, Okla.

[73] Assignee: Pan American Petroleum Corporation, Tulsa, Okla.

[22] Filed: June 3, 1970 1211 Appl. No.: 43,073

[' Oct. 3, 1972 OTHER PUBLICATIONS Primary Examiner-James A. Leppink Attorney-Paul F. Hawley and John D. Gassett [571 7 ABSTRACT Failure of easing set in wells drilled through peri" 395 mafrost zones can be anticipated if the casing is set in r the usual manner. As the warm formation fluid is [58] Field of Searchmg produced up through the permafrost zone, the perr mafrost adjacent the well thaws. In many instances the casing could be damaged by buckling when it loses its [56] References (med lateral support, while the upper termination of the casuNl D STATES'PATENTS ing moves downward due to subsidence-This problem is relieved by cementing the casing only below the 2,262,490 11/ 1941 Crake 166/96 afr t zone and 4 suspending the weight of the 3,561,531 2/1971 Miller ..166/DlG. 1 uncememed portion of the casing from a means at the 3,166,125 H1965 Hubby ..l66/75 7 surface which would not be affected Subsidence wise 1,81 1,76l 6/ 1931 Roberts ..6l/ by the thawed regionv A constant tension device can 1,894,039 1933 l'llll .;...l/l7l be used to adjust the tension on the casing 2,126,933 8/1938 Stone et al ..l75/l7l 2,716,018 8/1955 Williams /171 6 Claims, 4 Drawing Figures 0F PERMAFROST BASE OF PERMAFROST MELT BOUNDARY 2 I PATENIEnnm 3 I972 sum 1 or 4 2 V/A\Y/AV/MY/M g a 5 a QNORMAL THAW LINE g b; S 6 E Z s P 2 5 A MELT BOUNDARY 20 k g OF PERMAFROST BASE OF PERMAFROST s INVENTOR. ARTHUR LUBINSKI ATTORNEY PATENTEDnma m2 3.695.351

' SHEET 3 OF 4 v \7 v v g] g 2 5 4 l2 i 2 MELT BOUNDARY 2 A v OF PERMAFROST v BASE OF PERMAFROST\ INVENTOR. ARTHUR LUBINSKI BY ATTORNEY i and the support means.

- DETAILED DESCRIPTION OF THE INVENTION 1 SUSPENDING CASING THROUGH PERMAFROST BACKGROUND OF THE INVENTION Field of the Invention This invention relates to the setting of casing in a well bore drilled through a permafrost zone. It relates especially to a system of preventing damage to the casing when the permafrost thaws adjacent the casing.

Setting of the Invention It is well known that the search for oil and gas has gone to practically every area of the earth. One'of the more recent areas to which such a search has gone is the Arctic North; such as the North Slope of Alaska. Here, the upper layer or portion of the earth includes a frozen section known as permafrost. Permafrost consists of both a frozen matrix of water and soil and lenses of ice without any soil. This zone extends in thickness from a few feet to a thousand feet or more. When this permafrost section is frozen it is a rigid mass. However, upon thawing, the thawed portion becomes rather fluid. Ordinarily this thawing under natural conditions only takes place to a depth of not over about five feet in the summer and then it refreezes in the winter. However, when wells have been drilled through the per-- mafrost and then produced, the warmer produced fluid tends to melt the permafrost adjacent the well bore throughout its entire thickness of several hundred feet.

The thawed region at the surface will subside as the saturated soil begins to consolidate. Furthermore, the thawed media adjacent the casing will not be able to provide the lateral support required by the casing over its entire length. In such cases, the casing can be damaged due to buckling under its own weight.

BRIEF DESCRIPTION OF THE INVENTION DESCRIPTION OF THE DRAWINGS Various objects and a better understanding of the invention can be had from the following description taken in conjunction with the drawings.

FIG. 1 is a side elevation view, partly in section, of a welldrilled through the permafrost zone and means of supporting the casing at the surface.

FIG. 2 is a perspective view of the top of a casing extending above the permafrost and of means for supporting it.

FIG. 3 is similar to FIG. I except that constant tensioning units are used in the connection between the casing and the supporting means.

FIG. 4 is similar to FIG. 1 except that weight indicators and jack means are inserted between the casing Attention is first directed to FIG. 1 which shows well bore drilled through a permafrost zone 12 and into a zone 14 below the permafrost zone. Zone 14- may be shale, sand or other type formation. Well 10 has a casing 16- placed therein. The lower end of casing 16 which is in zone 14 has beencemented by placing cement 18 between the casing wall and the borehole wall.

Inside casing I6 is a producing string of tubing 20. The lower end of casing 16 extends downwardly to a producing zone (not shown) at some depth which may be anywhere from a few thousand to l0,000 or 15,000 feet or more. The upper end of producing string 20 extends through well head 22 on the upper end of casing 16 wherein it is connected to a gathering system (not shown). Ordinarily the fluid produced through production string 20 is at a temperature above the thawing temperature of the permafrost. This eventually causes the permafrost adjacent casing 16 to melt. I

If the walls along the borehole of the permafrost zone are weakened as by the thawing of the permafrost, and the upper termination of the casing 'moves downward due to subsidence, then the casing will tend to buckle.

If this thawing of the permafrost adjacent the casing results in an enlargement of the hole, thenthe subsidence may also cause the casing to buckle. This is true if the casing is hung in the ordinary manner because subsidence of its ordinary support will lower the top of the casing. This puts the casing under compression and results in its buckling. I propose to eliminate or reduce this problem by providing a unique support for the top of the casing so that the top will remain at a fixed or selected level. Attention is directed to FIGS. I and 2 which show such unique support. This includes four piles 24 which are set in the permafrost.

They are set a sufficient lateral distance from borehole 10 so that the permafrost around the piles will either not subside at all, or subside very little. A distance of 50 feet is probably sufficient. A common practice of placing piles in the permafrost is to have about twice as much of the piles extend into the part which does not thaw as is in the top layer which does thaw.

As shown in FIG. 2 each pair of diagonally opposite piles 24 are laterally braced at the top by a frame comprised of cross members 26 and 28 and angle members 32 and 33. A support ring 30 is secured from piles 24 by angle members 32 and from the approximate center of lateral braces 26 and 28 by bracing 33. The support ring 30 is thus supported in a relatively fixed position above casing 16. Casing 16 is connected to support ring 30 by a plurality of support members 34.

If there is no subsidence of the piles 24 then the structure of FIGS. 1 and 2 will hold the top of the casing at a reasonably fixed level. The top of the casing is fixed at a level so that there is not sufficient compression therein to cause buckling in the event the casing 1 becomes unsupported. This should prevent the casing side, then under the problem con-ditions described above the top of casing 16 will be lowered. This could place the casing in compression and cause buckling. (2) The casing could buckle due to its being heated. If the casing were, for example, set at -20 F and then was heated to 150 F by the produced fluid, this differential in temperature could cause appreciable compression stresses, namely 207 psi per degree Fahrenheit. This could put the casing in compression and cause buckling. (3) If the casing is filled with a fluid denser than the fluid on the outside of the casing, buckling could become a problem. The explanation of this phenomenon is given in an article entitled Influence of Tension and Compression on Straightness and Buckling of Tubular Goods in Oil Wells by Arthur Lubinski and published in the Proceedings of the Thirty-First Annual Meeting of the American Petroleum Institute, Section 4, November 5-8, I95 I.

I modify the support means of my invention described in FIGS. 1 and 2 to compensate for such problems. Such modification is illustrated in FIG. 4. FIG. 4 is similar to FIG. 1 except for the means connecting the upper end of casing 16 to support ring 30. Here I provide weight indicators 50 and jack means 52. Jack means 52 can be a simple turn buckle as indicated in FIG. 4 or it can be any of the more elaborate type jacks. One can determine the desired weight to be supported and can tighten or loosen jacks 52 as necessary to obtain the correct reading on weight indicator 50.

By observing weight indicator 50 and adjusting jack 52 accordingly, it is possible to main a proper tension on casing 16 so as to eliminate buckling.

Sometimes it may be desirable to provide a constant tensioning device on the upper end of casing 16. This can conveniently be accomplished by the modification shown in FIG. 3. There the upper end of casing 16 is connected to support ring 30 by constant tensioning device 36. Many suitable constant tensioning devices are commercially available such as the Rucker System manufactured and sold by the Rucker Company of Oakland, California. One can decide how much tension needs to be applied to the casing and set the constant tensioning device 36 accordingly. Then, even though there is a change in length of any metallic element due to temperature change or there is minor settlement of the support piles 24, the tension on casing 16 will nevertheless remain constant.

Ordinarily, most wells, sooner or later, need to be worked over, e.g., have their tubing removed, etc. The device shown in the drawing is arranged so that there is a vertical opening between lateral brace members 26 and 28 directly above casing 16. This is indicated by center line 44. This permits workover while maintaining tension on the casing.

While the above embodiments of the invention have been described with considerable detail, it is to be understood that various modifications of the system can be made without departing from the spirit or scope of the invention.

Iclaim:

l. A system for supporting a structure which comprises:

an outer casing permanently placed in a hole drilled in the earth, said earth comprising permafrost;

a frame ab ve the borehole' means disti nct from any casing in said hole and any associated well head for supporting said frame;

means connecting the upper end of said casing to said frame such that said frame supports said cas- 2. A system as defined in claim 1 in which said supporting means includes a plurality of piles set in the earth and spaced from said well bore a sufficient distance so that the piles are subjected to essentially no subsidence.

3. An apparatus as defined in claim 1 in which said connecting means includes a constant tensioning device.

4. A system as defined in claim 1 in which said connecting means includes an adjustable jack. I

5. A system as defined in claim 4 in which said connecting means includes a weight indicator to give information on the weight of the suspended casing.

6. A system as defined in claim 1 including cement between said casing and said hole only below said permafrost.

Claims (6)

1. A system for supporting a structure which comprises: an outer casing permanently placed in a hole drilled in the earth, said earth comprising permafrost; a frame above the borehole; means distinct from any casing in said hole and any associated well head for supporting said frame; means connecting the upper end of said casing to said frame such that said frame supports said casing.

2. A system as defined in claim 1 in which said supporting means includes a plurality of piles set in the earth and spaced from said well bore a sufficient distance so that the piles are subjected to essentially no subsidence.

3. An apparatus as defined in claim 1 in which said connecting means includes a constant tensioning device.

4. A systeM as defined in claim 1 in which said connecting means includes an adjustable jack.

5. A system as defined in claim 4 in which said connecting means includes a weight indicator to give information on the weight of the suspended casing.

6. A system as defined in claim 1 including cement between said casing and said hole only below said permafrost.

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