Insulated Casing And Tubing String In An Oil Well For A Hot Fluid Drive

Abstract

Tubing and casing strings in a well penetrating an oil stratum to be produced by hot fluid drive, such as with steam, are coated on the walls thereof forming the annulus by packing off the annulus adjacent or near the stratum, filling the annulus with an aqueous solution of a water-soluble inorganic salt, such as borax, sodium carbonate, sodium sulfate, and mixtures thereof, preferably containing a binder, and injecting a hot fluid through the tubing string into the stratum to evaporate water from the solution in the annulus while venting steam therefrom at the wellhead and deposit a substantial coat of the salt in solid form on the walls of the annulus.

Parent Case Text

This application is a division of my copending application Ser. No. 645,442, filed June 12, 1967, now U.S. Pat. No. 3,451,479.

Description

This invention relates to a method for insulating the walls of a casing and tubing string forming the annulus of a well penetrating an oil stratum to be used in a hot fluid drive process and to the resulting insulated tubing and casing strings.

The production of oil from underground oil-bearing strata by hot fluid drive such as steam, hot water, and in situ combustion is conventional in the petroleum industry. In hot steam drive at temperatures above 400.degree. F. and as high as 750.degree. F., the casing is subjected to these extreme temperatures, resulting in casing buckling and severe damage to the cement around the casing. Similar deterioration and damage to the casing often results from reverse burning in situ combustion wherein the hot gases from the combustion zone pass through hot sand or rock in passing to the production well and are at temperatures up to 1,000.degree. F. and higher. In addition to the damage to the downhole casing, there is also a substantial and expensive heat loss to the strata surrounding the casing generally termed the "overburden." Slip-on and wrap-on insulation of various types have been proposed and it is estimated that such insulation will reduce thermal losses by more than 80 percent and provide adequate protection for the downhole casing.

This invention is concerned with a method of insulating the walls of the casing and tubing strings forming the annulus of a well which is simple, practical, and economically feasible and with the resulting insulated tubing and casing strings.

Accordingly, it is an object of the invention to provide a simple and effective method for insulating the inner wall of a casing string and the outer wall of a tubing string forming the annulus of a well, while the strings are in place in the well. Another object is to provide a tubing string in a well coated on its outer wall with a layer of insulating material comprising solid inorganic salt. A further object is to provide a casing string in a well having a corresponding insulating layer of salt on its inner wall in combination with the insulated tubing string. Other objects of the invention will become apparent to one skilled in the art upon consideration of the accompanying disclosure.

A broad aspect of the invention comprises setting a packer downhole on a tubing string in a well leading to a stratum containing oil to seal off the annulus between the tubing string and the surrounding casing, substantially filling the annulus above the packer with a concentrated aqueous solution of a water-soluble inorganic salt, injecting a hot fluid through the tubing string into the stratum so as to heat and boil said solution while venting steam from the annulus at the wellhead, and continuing the injection of hot fluid through the tubing string so as to deposit said salt in solid form on the outside wall of the tubing string and the inside wall of the casing string substantially to the packer. The salt solution can be saturated and contain undissolved salt. The salt solution may also contain a suitable binder for binding the particles of salt together and for binding the salt to the walls of the tubing and casing strings. One or more salts of the group borax, sodium carbonate, and sodium sulfate are generally utilized. Concentrated solutions of these salts are effective in forming porous insulating coatings on the strings. Sodium silicate and metal stearate soaps provide suitable binders for the salt particles and for binding the salt particles to the metal in the well. It is also feasible to incorporate in the salt solution solid polymers in particulate form, such as in short fiber form, as well as fibers of other materials which deposit on the walls of the casing and tubing strings in admixture with the solid salt with or without a binder. The concentration of binder or filler is generated in the range of about 5 to 20 weight percent of the salt in the solution.

A more complete understanding of the invention may be had by reference to the accompanying schematic drawing of which FIG. 1 is an elevation in partial section through an oil-bearing stratum penetrated by a well and FIG. 2 is an enlarged partial section of the insulated tubing and casing of FIG. 1.

Referring to the drawing, and particularly to FIG. 1, a stratum 10 containing oil is penetrated by a well 12 which is provided with a casing string 14 and a tubing string 16. Casing 14 is cemented at 18 and is perforated at 20. Valved conduit 22 connects with the annulus 24 between the casing and tubing strings at the wellhead. A packer 26 seals off annulus 24 just above stratum 10. The inner wall of the casing string is provided with a layer 28 of insulating material formed of solid inorganic salt including a binder and/or a filler. A similar insulating layer 30 is provided on the outer wall of tubing string 16.

In applying the insulation to the tubing and casing strings, the selected solution or slurry of salt with or without a binder and/or filler is injected through conduit 22 into annulus 24 so as to substantially fill the annulus from packer 26 to the wellhead. Steam or other hot fluid is injected through tubing string 16 and through perforations 20 into stratum 10 at such a rate as to raise the temperature of the aqueous solution in the annulus to its boiling temperature and evaporate water therefrom, the same being vented through conduit 22. As the level of the solution in annulus 24 is progressively lowered due to evaporation, solid salt is deposited on the outer wall of tubing string 16 and the inner wall of casing 14 to form substantial layers of insulating material on these strings. Even a one-sixteenth inch or a one-eight inch layer of porous salt on the walls of the strings substantially reduces heat losses from the injected steam during steam flooding of stratum 10 or, in the event that well 12 is a production well of a reverse in situ combustion operation, during venting of hot produced gases and vapors through tubing string 16. However, it is feasible to deposit substantially thicker layers of insulating material on the walls of the annulus by the method of the invention. It is also feasible to substantially fill the annulus with water-soluble insulating salt. The water-soluble nature of the insulating material facilitates the removal of the insulating material by passing water through the annulus at any time that it is desired to remove the insulation. In order to do this, packer 26 may be released or unsealed so that flushing water may be injected down the tubing string and up the annulus to remove the insulating salt layer. It is also feasible to lower a waterline or hose down the annulus substantially to packer 26 and flush water up the annulus and out through conduit 22 to dissolve and remove the salt layer.

To illustrate the invention, 400 g. of sodium carbonate were mixed with 250 cc. of water, using "Osterizer" on high speed for about 10 minutes. This produced a viscous slurry of sodium carbonate in water. A steel pipe nipple 4 inches long and 1/4-inch ID was cleaned by grinding off dirt and rust and one end thereof was plugged and the pipe was dipped into the sodium carbonate slurry. The plug was removed from the end of the pipe and the pipe was clamped at a 45.degree. angle on a ringstand. The pipe was then heated by directing a flame of an oxygen-natural gas torch through the pipe for about 5 minutes. A substantial layer of sodium carbonate insulation was produced on the outer surface of the pipe. It is quite obvious that a substantially heavier layer of insulating salt can be coated onto the pipe strings in the well by filling the annulus with a concentrated aqueous solution of the salt and evaporating the water therefrom. Layers up to 3/8-inch in thickness and even thicker layers can be produced by the method of the invention.

It is desirable to clean the walls of the annulus to be coated with insulating material by flushing with water or cleaning solution prior to the filling of the annulus with the concentrated salt solution. This enhances the adherence of the salt to the wall of the iron pipe.

When producing the stratum with hot steam, the injected steam utilized in evaporating the water from the annulus can be a part of the steam injection operation, the steam and condensate from the injection operation passing from the well below packer 26 directly into the stratum through perforations 20.

It is also feasible to start a reverse burning in situ combustion process around well 12 with the annulus filled with salt solution and effect the evaporation and deposition of salt with resulting hot produced gases passing up the tubing string as the combustion is continued.

Certain modifications of the invention will become apparent to those skilled in the art and the illustrative details disclosed are not to be construed as imposing unnecessary limitations on the invention.

Claims

I claim:

1. A string of metal tubing in a well extending from the wellhead to an oil stratum, said string comprising a plurality of tubing lengths connected by collars, and an insulating layer of solid, water-soluble, inorganic salt covering the outside wall of said tubing string, including said collars, said insulating layer having an average thickness of at least one-sixteenth inch.

2. The string of tubing of claim 1 wherein said salt is selected from the group consisting of borax, sodium carbonate, sodium sulfate, and mixtures thereof.

3. The string of tubing of claim 1 including in said insulating layer a binder in minor but effective concentration.

4. The string of tubing of claim 1 in combination in said well with a surrounding casing string extending from the wellhead into said stratum, the inside wall of said casing string being covered with an insulating layer corresponding to that on said tubing string.

5. The strings of claim 4 wherein said salt is selected from the group consisting of borax, sodium carbonate, sodium sulfate, and mixtures thereof.

6. The strings of claim 4 including in said layer on both said tubing and said casing strings a binder in minor but effective concentration.

7. The strings of claim 4 including a packer sealing off the annulus between said tubing and said casing adjacent the upper level of said stratum, the upper surface of said packer being coated with a corresponding insulating layer.

8. The strings of claim 4 including a binder in said insulating layer on both said tubing and casing strings comprising sodium silicate.

9. The string of tubing of claim 1 wherein said salt is borax.