Thermal Well Completion Method And Apparatus

Abstract

A method and apparatus for attaching subsurface cementing devices to the periphery of free moving casing for use in thermal wells. Centralizers, wall scratchers, cement baskets, and the like are secured to the free-moving casing by means of a fusible material which is rigid at normal subsurface temperatures and which is deformable at thermal treatment temperatures.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to subsurface cementing equipment. In one aspect it relates to equipment particularly adapted for use in thermal wells completed by a technique known as free casing movement.

2. Description of the Prior Art

Thermal treatment of oil-bearing formations has proven to be an effective method for stimulating formation productivity. The high temperatures commonly encountered in thermally treated wells, particularly in steam injection wells, present serious operational problems. In early steam injection wells, no provisions were made for thermal expansion of the casing. Consequently, the high stresses associated with high treatment temperatures frequently exceeded the compressive yield of the casing, causing it to fail. An improved thermal completion technique providing the maximum free casing movement and thereby precluding development of the excessive compressive stresses was subsequently developed. Briefly, this technique involves anchoring the base of the pipe string in a high bonding thermally competent cement and providing an upper free-moving section sheathed, but unbonded, in the cement. A coating of thermal sensitive material is provided on the free-moving section and serves to inhibit cement bonding thereto. It also acts as a lubricant to reduce frictional drag. Since this improved technique contemplates vertical movement of the free-moving section, conventional subsurface cementing devices such as centralizers, wall scratchers, cement baskets, and the like cannot be used. Such devices are generally permanently secured to the pipe so that when cemented in place, they cannot accommodate relative pipe movement. The use of centralizers and wall scratchers greatly improves the chances of obtaining a competent cement sheath about the casing. The function of the centralizer is to keep the pipe centered in the well bore so that a uniform cement sheath is formed thereabout. Without centralizers, the pipe tends to lie against the wall of the well bore causing cement to channel up the annulus. In the free casing type of completion, the centralizers provide the additional function of protecting the lubricant coating during pipe running operations.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a method and apparatus for attaching subsurface-cementing devices particularly adapted for casing strings to be run in steam injection wells. to a free-moving casing section in a thermal well.

Another object of this invention is to provide subsurface cementing devices particularly adapted for casing strings to be run in steam injection wells.

Briefly, the invention contemplates the use of a heat-sensitive material for attaching subsurface-cementing devices to a free-moving section of the pipe string. The heat-sensitive material can be a fusible alloy or a fusible plastic material, the principal requirements being that the material selected be rigid at normal subsurface temperatures and deformable at a temperature above the normal subsurface temperature but substantially below the heat treatment temperature of the well, and that the material have sufficient physical strength for securely bonding the devices to the casing.

In one aspect, the invention can be viewed as a method for cementing pipe in a well bore which comprises the steps of placing in the well bore the pipe having a cementing device bonded to a free-moving section thereof by a heat-sensitive material; forming a cement sheath about a longitudinal portion of the pipe including the cementing device; and melting the heat-sensitive material.

The invention is particularly applicable for attaching collared centralizers to the casing, but can also be used for attaching other devices such as wall scratchers or cement baskets to the casing. In practice, the string may be equipped with all three types of equipment arranged in the desired array. For purpose of illustration, the invention will be described primarily in connection with centralizers and briefly in connection with the other types of subsurface cementing equipment.

The construction of the centralizer and means for bonding it to the casing may take a variety of forms. For example, a conventional slip-on centralizer may be bonded by applying a molten material between the centralizer and collar and permitting the molten material to set. Another embodiment provides for the construction of the centralizer collars of the thermal sensitive material, the collar including means for frictionally gripping the casing. In still another embodiment, a centralizer stop collar may be composed of the thermally fusible material. Regardless of the physical embodiment of this invention, the thermally fusible material is rigid at normal subsurface temperatures and become deformable at a temperature above the normal subsurface temperature but substantially below the heat-treating temperature of the well, the effect being to eliminate the bond between the centralizer and casing. Elimination of the bond permits relative movement of the centralizer and the casing, thereby relieving compressive stresses in the casing.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal sectional view of a portion of a casing string equipped with subsurface cementing equipment secured according to the present invention;

FIG. 2 is an enlarged sectional view of a centralizer secured to a free-moving section of casing according to one embodiment of the present invention;

FIG. 3 is a transverse sectional view of a centralizer collar and sleeve illustrating another embodiment of the invention; and

FIGS. 4 and 5 are longitudinal views of centralizers secured to a free moving section of casing by still other embodiments of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIG. 1, a pipe string 10 is shown suspended in a mud-filled well bore 11 preparatory for commencement of the cementing operations. The pipe string 10 includes a lower base section 12 free of any lubricant coating and an upper free-moving section 13. The lower base section 12 extends from its lower end to an elevation above the oil-bearing formation 14 and can be equipped with conventional cementing devices such as a centralizer 15. The free-moving section 13 of string 10 is provided with a lubricant coating 16 which serves to inhibit cement bonding to the outer periphery of the pipe and to reduce frictional resistance between the pipe and cement. Typically the coating 16 is composed of bituminous material such as asphalt, tar, grease, etc. Other suitable materials include thermoplastic resins, waxes, and the like.

A centralizer 17 and wall scratcher 18 constructed according to the present invention are secured to the free-moving section 13. The number, spacing, and relative arrangements of the centralizers 17 and wall scratchers 18 along section 13 may be according to present cementing practices. Now when the annulus between the casing string 10 and well bore 11 is filled with a cement slurry, the slurry is in direct contact with the exposed portion of the string 10 (base section 12). Thus, the set cement opposite the base section 12 is bonded to the outer periphery of the uncoated pipe 10 and the wall of the well bore 11 providing a competent fluid seal in the well annulus and securely anchoring the string 10 at the bottom of the well bore 11. The cement slurry above the base section 12, while bonding to the wall of the well bore 11, is prevented from bonding to the free-moving section 13 by the lubricant coating 16. After the cement has set and the pipe perforated in the formation interval, the well is ready for steam injection. The application of heat has the effect of softening the lubricant coating 16 permitting vertical movement of the free-moving section 13. However, the centralizer 17 and wall scratcher 18 are firmly imbedded in the cement sheath surrounding the free-moving section 13. Therefore, if there is to be vertical movement of the section 13 in relation to the centralizer 17 and wall scratcher 18, the bond securing these parts to the pipe must be broken. Since vertical movement of the section 13 need be accommodated only at elevated treatment temperatures, it is desirable that the bond be temperature sensitive. At normal subsurface temperatures, the bond must securely maintain the equipment in place to withstand the rigors of running the pipe in the hole, but at elevated temperatures must yield to permit relative movement of the pipe.

The present invention is concerned with method and apparatus for attaching subsurface cementing devices to the pipe 10 by a thermal sensitive bonding material. Four specific embodiments are described herein but it should be understood that these are presented for purposes of illustration only and that variations and modifications can be made therein.

In one embodiment of the invention (see (FIG. 2), the centralizer 17 comprises a pair of spaced collars 19 and 20 interconnected by a plurality of outwardly bowed, circumferentially spaced spring members 21. The internal diameter of each collar 19 and 20 is slightly greater than the outside diameter of the pipe 10 so that when the former is mounted concentrically around the latter, an annular space 22 is formed. The lower collar 20 is bonded to the pipe 10 by a sleeve 23 disposed in the annular space 22. The sleeve 23 is composed of heat-sensitive material which is rigid at normal subsurface temperatures and is deformable at a predetermined heat-treatment temperature which is above the normal subsurface temperature but substantially below the heat-treating temperature to which the well is to be subjected. The heat-sensitive material must meet the requirements of being rigid at normal subsurface temperatures, deformable at heat-treatment temperatures, and have sufficient structural strength to maintain the centralizer-to-pipe bond during pipe running operations. The composition of such material can vary within a wide range. Because of their high physical strength and low melting temperatures, bismuth alloys make particularly good bonding sleeves. Other suitable materials include rigid thermoplastic resins such as acrylonitrile-butadiene-styrene, acetal, acrylic, nylon, polyethylene, polypropylene, polystyrene, vinyl polymers and copolymers, to name but a few. The selection of a specific material, whether alloy or thermoplastic resin, will depend upon the normal subsurface temperature and the steam injection temperature. For example, if the normal subsurface temperature at the setting location of the centralizer 17 is 140.degree. F., an alloy containing seven to eight parts bismuth, four parts lead, two parts tin, and one to two parts cadmium would make a suitable bonding sleeve. This material melts at about 160.degree. F. A rigid material which is deformable in a temperature range from about 100.degree. F. to about 300.degree. F. will be satisfactory for most applications.

The low-melting alloys, e.g., Woods metal, because of their low melting temperature (below 212.degree. F.) and rapid solidification time can be conveniently applied in the annular space 22 in the molten state. After the centralizer 17 is properly positioned on the pipe 10 at the surface, a flexible annular seal 24 is inserted between the collar 20 and the pipe 10 so as to close the bottom of annular space 22. The molten alloy is then poured in the annular space 22, completely filling it. In order to provide a sufficiently large annular space 22, oversized collars can be used. Now when the material solidifies, which requires only a few minutes, collar 20 is securely bonded to the pipe 10. The upper collar 19 remains loosely fit about the pipe 10 so that spring members 21 can expand and contract in accordance with hole conditions during pipe-running operations. Projections or internal grooves such as that shown at 25 may be used to increase the physical bonding strength of the material. The composition of the bismuth alloy can be selected to provide melting temperatures from as low as 100.degree. F. to as high as 300.degree. F. if necessary. The lubricant coating 16 is applied to the casing joint to which centralizer 17 is attached prior to running that joint in the hole.

Alternatively, the low melting alloy or thermoplastic resin can be applied as preformed split inserts 26 which in combination constitute the bonding sleeve 23 (see FIG. 3). The centralizer collar 20, as illustrated, is of the slip-on type and is provided with a clamping connector shown generally as 27. The inserts 26,26 are configurated to fit snugly about the outer periphery of the pipe 10 and mate with the centralizer collar 20 so that upon tightening of the connector 27 a compressive force is applied on the inserts 26 sandwiched between collar 20 and pipe 10. The compressive force applied must be sufficient to frictionally hold the inserts 26,26 securely to the pipe 10 during pipe running operations.

In another embodiment of the invention, the collar 20 is composed of the thermal sensitive material such as the low-melting alloy or rigid thermoplastic material described above. As shown in FIG. 4, the collar 20 includes a clamping connector shown generally as 28 and a plurality of mechanical fasteners 29 for attaching the spring members 21 to the collar. The collar 20 may be hinge-type or slip-on type and once placed in position is compressively secured to the pipe by actuation of the clamping connector 28.

In still another embodiment of the invention, the centralizer 17 is maintained in position on the pipe 10 by means of a stop collar 30 as illustrated in FIG. 5. The stop collar 30 may be similar to that illustrated in FIG. 3 wherein inserts are sandwiched between the collar 30 and the pipe 10, or itself may be composed of the thermal sensitive material. A single collar 30 can be clamped to the pipe 10 intermediate the centralizer collars 19 and 20, or a pair of stop collars 30 may be clamped to the casing on either side of the centralizer 17.

Basically then the thermal sensitive material in all the embodiments is in the form of a sleeve which is either applied in the molten state or as an insert, or is an integral part of the centralizer. Reciprocating wall scratchers 18 of the type shown in FIG. 1 are generally provided with collars for attachment to the pipe 10. Thus it will be appreciated that wall scratchers 18 and cement baskets can be secured to the pipe 10 by any one of the embodiments described above for attaching the centralizer 17. Furthermore rotating wall scratchers can be attached to the pipe 10 by a pair of the stop collars 30 arranged to secure opposite ends of the wall scratcher.

In operation, the base section 12 of pipe string 10 is run in the well bore 11 using conventional equipment and techniques. The subsurface cementing equipment (centralizer 17 and wall scratcher 18 can be secured to the pipe 10 comprising the free-moving section 13 while the pipe is still on the pipe rack or during running operations. After the cementing equipment is secured, the lubricant coating 16 is applied to the outer periphery of each joint making up the free-moving section 13. When the string 10 is located at the desired elevation, a cement slurry is pumped internally down the pipe and up the annulus forming an encircling sheath about the outer periphery of the string 10. Upon setting, the cement is securely bonded to the base section 12 thereby anchoring the string 10. The cement opposite section 13 is lightly bonded, or not bonded at all to the pipe 10. However, the centralizer 17 and wall scratcher 18, imbedded in the cement sheath, are permanently located in the well bore 11. Next, the well is perforated by conventional techniques providing communication between the lower end of string 10 and the formation 14. Now when steam or hot water is pumped through the pipe 10, the increased temperature causes the free-moving section 13 of the casing 10 to expand and at the same time decomposes the wax coating 16. At this point the only forces resisting upward movement of the free-moving section 13 are the centralizer-to-casing and scratcher-to-casing bonds. When the subsurface temperature reaches the yield temperature of the bonding sleeve 23 or slip collar 30, the bond is broken thereby permitting vertical movement of the pipe 10 past the centralizer 17 and collar 18.

The following example illustrates the strength of the bonding arrangement constructed according to the present invention and the effects of temperatures applied thereto.

EXAMPLE

A 3-inch ID steel collar was bonded to a 2-inch OD steel pipe as follows: the collar was positioned concentrically about the pipe and the annulus separating them was filled with a molten fusible alloy composed of 50 percent bismuth, 26.7 percent lead, 13.3 percent tin, and 10 percent cadmium. The melting temperature of the alloy is 170.degree. F.

After the alloy solidified, the exposed pipe on either side of the pipe was coated with a microcrystalline wax. A pressure test of the resulting bond indicated a shear strength of 2,070 p.s.i.

The pipe with the collar bonded thereto was then inserted into a chamber containing a cement slurry. The slurry was allowed to set forming a cement sheath about the periphery of the pipe and collar. The sheath was 7 inches long and 13/8 inches thick.

Steam was applied internally of the pipe increasing the temperature of the assembly to 400.degree. F. At the elevated temperature the pipe was free to move relative to the collar indicating that the collar-to-pipe bond had been broken.

The above example demonstrates that the collar-to-pipe bond provides sufficient structural strength for maintaining the centralizer or other equipment in fixed relation on the pipe during running operations. Furthermore, it demonstrates that the bond is yieldable at elevated thermal treatment temperatures to permit relative movement of the parts.

While the description of the preferred embodiments of the present invention has been specific for purposes of illustration, it again should be emphasized that the physical form of the subsurface cementing equipment and the type of fusible material can vary within wide limits without departing from the scope and spirit of the invention as set forth in the appended claims.

Claims

I claim:

1. Apparatus for attaching a subsurface cementing device to the outer periphery of a free-moving section of a pipe to be subjected to an elevated heat-treating temperature in a thermal well, said cementing device having at least one collar adapted to be slidably mounted around said pipe prior to introducing said pipe into the well, said apparatus comprising: a sleeve adapted to fit circumferentially around said pipe and composed of heat-sensitive material rigid at normal subsurface temperatures and deformable at a temperature above said normal subsurface temperature but substantially below said elevated heat-treating temperature; and means for securing said sleeve to said pipe in engaging relation to said collar whereby said device and said pipe are maintained in fixed relation at normal subsurface temperatures and are relatively movable at temperatures equal to and greater than said temperature above said normal subsurface temperature but substantially below said elevated heat-treating temperature.

2. The invention as recited in claim 1 wherein said sleeve is composed of a low-melting alloy.

3. The invention as recited in claim 2 wherein said alloy is a bismuth alloy fusible at temperatures from about 100.degree. F. to about 300.degree. F.

4. The invention as recited in claim 2 wherein said bismuth alloy contains seven to eight parts bismuth, four parts lead, two parts tin, and one to two parts cadmium.

5. The invention as recited in claim 1 wherein said heat-sensitive material is a thermoplastic resin.

6. The invention as recited in claim 1 wherein said sleeve is disposed intermediate said collar and said pipe, and said means for securing said sleeve includes clamping means mounted on said collar for applying a compressive force to said sleeve.

7. The invention as recited in claim 1 wherein said sleeve is mounted on said pipe in end-to-end abutting relation to said collar, and said means for securing said sleeve includes clamping means mounted on said sleeve for grippingly securing said sleeve about said pipe.

8. A centralizer for attachment to the outer periphery of pipe to be subjected to an elevated heat-treating temperature in a thermal well, said centralizer comprising: a pair of spaced collars particularly sized to fit about the periphery of said pipe at a longitudinally spaced interval, at least one of said collars being composed of heat-sensitive material rigid at normal subsurface temperatures and deformable at a temperature above said normal subsurface temperatures but substantially below said elevated heat-treating temperature; means for securing said one collar to said pipe; and outwardly bowed spring members interconnecting said collars.

9. The invention as recited in claim 8 wherein said one collar is composed of a bismuth alloy having a melting temperature in the range from about 100.degree. F. to about 300.degree. F.

10. Apparatus for positioning devices on the outer periphery of pipe to be subjected to an elevated heat-treating temperature in a thermal well, said apparatus comprising: a collar sized to fit about the periphery of said pipe and adapted to clampingly engage a portion of said device maintaining said device in fixed relation on said pipe, said collar being composed of heat-sensitive material rigid at normal subsurface temperatures and deformable at a temperature above said normal subsurface temperature but substantially below said elevated heat-treating temperature; and means for securing said collar to said pipe.

11. A method for attaching a collared device to a free-moving section of pipe to be used in thermal wells, said method comprising the steps of: positioning said device on said pipe wherein said device collar is disposed circumferentially around said pipe; pouring a molten fusible material in the annulus separating said collar and said pipe, said material having a melting temperature above normal subsurface temperatures but substantially below the maximum heat-treating of the well temperature; and permitting said alloy to solidify whereby said device is firmly bonded to said pipe.

12. The invention as recited in claim 11 wherein said alloy is a bismuth alloy having a melting temperature below 200.degree. F.

13. The invention as recited in claim 12 wherein said alloy is composed of seven to eight parts bismuth, four parts lead, two parts tin, and one to two parts cadmium.

14. A method of cementing pipe in a well bore comprising: placing said pipe in the well bore, said pipe having an upper section peripherally coated with a lubricating material, a lower section free of said lubricating material, and a cementing device bonded to said pipe in said upper section by heat-sensitive material rigid at normal subsurface temperatures in the well bore and having sufficient mechanical strength to maintain said device in fixed relation on said pipe at said normal subsurface temperatures; forming a cement sheath about said lower section and a portion of said upper section including said cementing device, and melting said heat-sensitive material permitting relative movement of said pipe and said device.