Insulating a wellbore in permafrost

[57] ABSTRACT

A method and apparatus for maintaining at least a portion of the interior region of a wellbore thermally insulated from the wall of the wellbore wherein a single zone in the wellbore is utilized for the introduction, circulation, and return of cooling

Claims, 3 Drawing Figures

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PATENTED MAY 16 1972

3,662,832

SHEET 1 OF 2

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FIG. I

INVENTORS

WILLIAM A. KEELER FRANK J. SCHUH

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ATTORNEY

PATENTEDMAY16 f97? 3,662,832

SHEET 2 OF 2

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INVENTORS

WILLIAM A. KEELER FRANK J. SCHUH

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ATTORNEY

3,662,832

INSULATING A WELLBORE IN PERMAFROST

BACKGROUND OF THE INVENTION

In Northern Alaska, Canada, and other frigid areas of the world, a larger and larger number of oil and/or gas wells are being drilled through permafrost. It is recognized that the production of relatively warm fluids through the wellbore over the production life of a well could cause melting of the permafrost and this in turn could cause failure of the casing and therefore, the well. ’

Accordingly, various proposals have been made for protecting the permafrost adjacent the wellbore from being thawed by warm fluids produced through the wellbore.

One such proposal is fully and completely disclosed in World Oil, Jan. 1970, page 85, the disclosure of which is incorporated herein by reference. This proposal utilizes a 20-inch diameter outer casing and a 13 % inch diameter inner casing to establish an area which is split into two zones by inserting a 16-inch casing between the 13 % inch and 20-inch casings. Thus, two cooling zones are established, one on either side of the 16-inch casing, and liquid refrigerant is pumped down into the wellbore in one cooling zone annulus and is returned, as warm liquid, up the other cooling zone annulus to the earth’s surface.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a portion of a wellbore which embodies one form of the single annulus cooling zone concept of this invention.

FIG. 2 shows surface cooling apparatus that can be used with the apparatus of FIG. 1.

FIG. 3 shows an alternative embodiment of surface cooling apparatus that can be employed with the apparatus of FIG. 1.

More specifically, FIG. 1 shows a wellbore wall 1 which has inwardly extending steps 2 and 3 as smaller diameter casing is employed.

For example, casing 4, which can be 20-inch diameter casing, extends to step 2 and any void between the outer surface of casing 4 and wellbore wall 1 is filled in a conventional manner with cement 5. Casing 6, which can be 13 %-inch diameter casing, is extended downward to step 3 and the space between the outer surface of casing 6 and wellbore wall 1 is filled with cement 7 up to and slightly above the lower end of casing 4. Casing 8, which can be 9 %-inch diameter casing, is set in the wellbore in a conventional manner with cement 9.

In the interior of casing 8 is disposed a tubing string 10 which can be, for example, 7-inch diameter tubing. Tubing 10 rests on packer 12 and provides an opening 13 through which oil, gas, and other fluid is produced from the bottom of the well up to the earth's surface.

In the annulus between tubing 10 and casing 8 is provided thermal insulation material 11 such as polyurethane foam and 30 the like.

Annulus 14 between casings 8 and 6 is the single annulus means (single cooling zone) of this invention.

Annulus 15 between casings 6 and 4 can be filled with a packer fluid which itself can act as an insulating material

Annulus 14 is not subdivided into two separate annuli (represented by dotted line 18) in the manner suggested by the World Oil reference. Rather, annulus 14 is left open so that it is a single annulus or zone in which descending and rising ₍ cooling fluid circulates at will between casings 8 and 6. In the prior art, an additional casing string 18 established two separate annuli or cooling zones in which liquid refrigerant was circulated only downwardly in one, arrow 16, and only upwardly in the other, arrow 17. The warmed but still liquid refrigerant was then recovered at the earth’s surface and cooled for reintroduction into only one of the two separate cooling zones.

Annulus 14 can have insulation 11 extend the full length thereof or have the lower uninsulated portion contain a liquid 19 such as drilling fluid up to and preferably overlapping the lower end of insulation 11.

By the method of this invention, liquid cooling fluid is introduced into single cooling zone 14, allowed to travel freely down that zone, absorb heat from the region on the interior of casing 8 and tubing 10 which carries the relatively warm oil, gas and the like to the earth’s surface, vaporize due to this heat absorption, and rise as a gas countercurrently with the downcoming liquid cooling fluid. The vaporized cooling fluid is removed at the earth’s surface, condensed, cooled, and returned to zone 14.

FIG. 2 shows only zone 14 for sake of simplicity. Vaporized cooling fluid is removed from zone 14 through conduit means 20 to compressor means 21. Compressor 21 compresses the vaporized cooling fluid to a pressure substantially above its recovery pressure from zone 14, e.g. the pressure in recovery conduit 20 adjacent casing 6.

The compressed cooling fluid then passes by way of conduit means 22 to an ambient air fan cooler 23 which effects an indirect heat exchange relationship between the compressed cooling fluid and ambient air to cool the compressed cooling fluid to a temperature approaching that of the ambient air temperature. This substantially cools the compressed cooling fluid for return by way of conduit means 24 through back pressure valve 27 to zone 14. Thus, gaseous cooling fluid

SUMMARY OF THE INVENTION

According to this invention, cooling of the wellbore in the area of the permafrost is obtained by a method and apparatus employing only a single cooling zone.

By this invention a single annulus for handling all of the introduction, circulation, and removal of cooling fluid from the wellbore is achieved, for example, by using only 13 %-inch and 9 %-inch casing. This invention, therefore, eliminates the need for the 16-inch casing of the World Oil reference and further eliminates the need for using the 20-inch casing for the outer wall of the cooling zone.

Accordingly, by this invention substantial economic savings are enjoyed not only be eliminating the 16-inch casing without eliminating the function thereof, but also by allowing the use of a narrower diameter wellbore since the outer wall of the cooling zone can be 13 %-inch casing instead of the 20-inch casing of the World Oil reference.

This invention provides apparatus for thermally insulating a region in a wellbore utilizing a single annulus means for transporting cooling fluid into and out of the wellbore, means for introducing and removing cooling fluid from the single annulus means, and cooling means for cooling the cooling fluid after it has been removed from the single annulus means.

There is also provided a method of thermally insulating a region in a wellbore wherein the single annulus means (single cooling zone) has added thereto a substantially liquid cooling fluid which will vaporize at or below the maximum pressure in the single cooling zone, vaporizing the cooling fluid by allowing same to absorb heat from the region to be cooled, recovering the vaporized cooling fluid, and condensing the vaporized cooling fluid for return to the cooling zone.

Accordingly, it is an object of this invention to provide a new and improved method and apparatus for producing a well in a permafrost area without substantial thawing of the permafrost. It is another object to provide a new and improved method and apparatus for cooling at least a portion of a wellbore. It is another object to provide a new and improved 65 method and apparatus which will cool a wellbore without requiring the use of a larger diameter wellbore and/or the use of an additional casing string to establish separate annuli to handle the introduction, circulation, and recovery of refrigerant. It is another object to provide a new and improved 70 method and apparatus which will render the wellbore cooling system automatically self-refluxing.

Other aspects, objects, and advantages of this invention will be apparent to those skilled in the art from this disclosure and the appended claims.

«»73

3,662,832 passes in the direction of arrow 25 while liquid cooling fluid passes in the direction of arrow 26.

As an alternative to or an addition to ambient air fan cooler 23, an artificially refrigerated heat exhanger can be used to primarily or supplementarily cool the cooling liquid that is to be returned to zone 14. The artificially refrigerated heat exhanger can use any conventional refrigeration system, e.g. a separate refrigerant which is passed in indirect heat exchange relationship with the cooling fluid, the separate refrigerant being separately compressed, cooled, and expanded through a conventional expansion valve in the well-known refrigeration cycle.

FIG. 3 shows cooling zone 14 by itself, for sake of simplicity, with cooling apparatus modified so as to render the system automatically self-refluxing. In this system conduits 30 and 32 slope upwardly from zone 14 to cooling means 31.

By utilizing the apparatus of FIG. 3, vaporized cooling fluid rises primarily in conduit 32 as shown by arrow 36 toward cooling means 31, although some vapor also rises through conduit 30. Due to the cooling effect encountered in conduit 32, some of the vaporized cooling fluid is condensed in conduit 32 and gravity feeds, without the use of an external pump, back in the direction of arrow 37 and into zone 14.

The vaporized cooling fluid which does reach cooling means 31 is condensed and cooled and gravity feeds back to zone 14 by way of downwardly inclined conduit 30.

The system of FIG. 3 can be utilized during periods when the ambient air temperature is relatively low, e.g. less than about 20° F. When the ambient air temperature exceeds about 20° F., it can be helpful to obtain greater condensation and cooling of the cooling fluid to compress same before it reaches cooling means 31. This can be accomplished by an optional use of conduit 33 for passing at least part of the gaseous fluid therein through compressor 34 and then by way of conduit 35 back to conduit 32 and on to cooling means 31.

As with cooling means 23 of FIG. 2, cooling means 31 can be any cooling means or a combination of cooling means such as an ambient air fan cooler or an artificially refrigerated heat exchanger, and the like.

In the method of this invention, single cooling zone 14 is provided around at least a portion of the region to be cooled and is also disposed intermediate the region to be cooled and the wellbore wall. Thereafter, substantially liquid cooling fluid which will vaporize at or below the maximum pressure in the cooling zone is added to that zone. The liquid cooling fluid is allowed to remain in the cooling zone until it absorbs sufficient heat to vaporize same. The vaporized cooling fluid rises to the top of the cooling zone and is recovered at a recovery pressure, i.e. the pressure in conduit means 20 of FIG. 2 adjacent to casing 6. The vaporized cooling fluid is then condensed by cooling or a combination of pressurization and cooling and thereafter is returned as cool liquid to the cooling zone.

If the vaporized cooling fluid is cooled sufficiently by an artificial refrigeration system or the use of quite cold ambient air, e.g. less than 20° F. ambient air, sufficient condensation and cooling can be achieved without compression. However, if relatively stringent cooling is not achieved, e.g. when only ambient air is used and the ambient air temperature is at least about 70° F., compression of the vaporized cooling fluid to a pressure substantially above that of its recovery pressure can be employed before heat exchange with ambient air. This achieves greater condensation and cooling of the cooling liquid for return to the cooling zone.

Generally, any cooling fluid can be employed so long as the fluid can be made to vaporize at less than 32° F. under pressures of less than 1,000 psia. The cooling fluid is preferably a hydrocarbon or a halogenated hydrocarbon, each having from one to eight, inclusive, carbon atoms per molecule or mixtures of two or more of such hydrocarbons and/or halogenated hydrocarbons. The hydrocarbons are preferably straight- or branch-chained, saturated or unsaturated materials and the halogenated hydrocarbons can be mono- or poly- halogenated '

NH_3&gt; methyl dichlorotetrafluoroethane, ethyl chloride, with one or more of chlorine, bromine, iodine, fluorine, or combinations of two or more such elements. Suitable specific materials include natural gas, methane, ethane, propane, butane, pentane, mixtures of two or more of these materials, dichloromethane, difluoromethane, CO, chloride, isobutane, SO₂, dichloromonofluoromethane, trichloromonofluoromethane, methyl formate, methylene chloride, trichlorotrifluoroethane, dichloroethylene, ¹ trichloroethylene, and the like.

When the vaporized cooling fluid is compressed, it should be compressed to a pressure of at least about 50 psia greater than its recovery pressure above mentioned. Generally, the recovery pressure is less than about 400 psia and the vaporized cooling fluid is then compressed to a pressure of at least about 450 psia. Preferably, the recovery pressure is from about 100 to about 400 psia and is compressed to a pressure of from about 450 to about 1,000 psia.

The amount of cooling carried out on the vaporized cooling fluid, compressed or uncompressed, is that which is sufficient to substantially condense same to a liquid and cool same to a temperature below 32° F. for reintroduction into zone 14. The degree of cooling before re-entry into zone 14 can vary widely as desired, depending upon the cooling means employed. Generally, this temperature will be in a range of from about —25° F. to no greater than 30° F.

It should be noted that by varying the recovery pressure, the amount of liquid cooling fluid that can be tolerated in zone 14 and vaporization of the cooling fluid still obtained can be varied. Thus, at a lower recovery pressure more liquid cooling fluid can be introduced into zone 14 and better heat transfer obtained in zone 14 since the zone is closer to liquid full. However, with this lower recovery pressure, the cooling fluid has a lower maximum temperature of reintroduction into zone 14. With various cooling fluids this lower maximum pressure can be substantially below 0° F. and can even approach the brittle temperature for the steel from which casings 8 and 6 are formed. Further, it is not desirable to super-cool permafrost because there may be free water in permafrost that can be subject to freezing.

Thus, too cold a cooling fluid might be damaging just as well as too warm a cooling fluid so that it is preferable in most cases to maintain the recovery pressure so that the cooling fluid is in the range of from about —25° F. to about 30° F. as it re-enters zone 14. It should also be noted that the amount of liquid cooling fluid present in the zone 14 should be limited so that the pressure at the bottom of that cooling zone is not so great that the liquid cooling fluid will not vaporize below 32° F.

It should also be noted that the apparatus of FIGS. 2 and 3 can be operated in accordance with the method of this invention in a manner such that the compression step is employed only when this is necessary to cause liquefaction of the vaporized cooling fluid at the prevailing ambient air temperature and that the compression step can be eliminated at any time when the prevailing ambient air temperature is sufficiently low to achieve a liquefaction and cooling of the cooling fluid into the temperature range above described.

For example, when ethane is used as a cooling fluid in the apparatus of FIG. 3 and the vaporized ethane recovery pressure in conduit 32 is 250 psia, automatic self-refluxing of liquid ethane in conduit 32 is achieved. Adequate condensation and cooling of the remaining vapor is then achieved by cooling means 31 when the prevailing ambient air temperature is at least 10° F. below the 8° F. boiling point of liquid ethane. In this situation, the use of compressor 34 would be eliminated and only cooling means 31 operated as an ambient air fan cooler, liquid ethane being returned to zone 14 by way of conduit 32 due to condensation by cooling in that conduit and also by way of conduit 30 due to cooling means 31.

Thus, in the embodiment of FIG. 3, liquid cooling fluid is returned to the cooling zone at two points, i.e. the connection points of conduits 32 and 30, rather than the one point, e.g.

5174

3,662,832 the connection point of conduit 24 in FIG. 2. The return of liquid cooling fluid at two points is advantageous in that larger surface areas of casings 8 and 6 are contacted with liquid cooling fluid than when the cooling fluid is introduced at one point.

EXAMPLE

The method of this invention is carried out in apparatus substantially the same as that shown in FIGS. 1 and 2 using casing of the diameter dimensions set forth hereinabove with respect to the initial description of FIG. 1.

Ethane is used as the cooling fluid and its recovery pressure in conduit 20 is 150 psia. At this pressure the maximum allowable liquid ethane charge to zone 14, so that the ethane at the bottom of the zone will still vaporize at less than 32° F., is 9,400 pounds. This gives a temperature range of maximum ethane charge to zone 14 of from —25° to 30° F.

Taking a prevailing ambient air temperature of 70° F., compressor 21 (at a capacity of 1,500 pounds/hour) compresses vaporized ethane in conduit 20 from the 150 psia recovery pressure to 700 psia. The compressed ethane then passes through a conventional fin-fan cooler 23 where it is brought into indirect heat exchange relationship with the ambient air by means of a fan and radiating fins. The compressed ethane is thereby cooled into the temperature range of —25° to 30° F. The now liquefied and cooled ethane is then returned by way of conduit 24 to zone 14.

By way of comparison, if in the above example the recovery pressure was 250 psia instead of 150 psia, the maximum allowable liquid ethane charge in zone 14 would be limited to 4,700 pounds, a 4,700 pound reduction in order to achieve vaporization of the liquid ethane at the bottom of zone 14 at less than —32° F. Also, the temperature range of maximum ethane charge would then change from the range of —25° to 30° F. to the range of 8° to 30° F.

Reasonable variations and modifications are possible within the scope of this disclosure without departing from the spirit and scope of this invention.