US3277963A - Completing wells - Google Patents

Description

Oct 11, 1966 D. H. FLICKINGER 3,277,963 I COMPLETING WELLS Filed May 27, 1964 DON H. FLICKINGER INVENTOR.

ATTORNEY.

United States Patent 3,277,963 COMPLETING WELLS Don H. Flickinger, Tulsa, Okla, assiguor to Pan American Petroleum Corporation, Tulsa, Okla., a corporation of Delaware Filed May 27, 1964, Ser. No. 370,735 13 Claims. (Cl. 16621) This invention relates to completing wells. More par ticularly it relates to completing oil wells in which a metal casing is cemented through the producing zones and this casing is then perforated.

Two problems have persisted in this type of completion. First, a poor seal is sometimes formed between the casing and cement. This results in a flow passage through which fluids above or below the producing formation can flow down or up the well outside the casing to the perforation through which the desired fluid enters the casing. Second, the casing perforations may be so plugged with debris that fluids can flow through the perforations only with difficulty, if at all. The second problem is particularly important in locations where the formation is not fractured after the perforations are formed.

An object of my invention is to provide a method for forming a better bond between the casing and the cement outside the casing. Another object is to provide a method for forming casing perforations with higher flow capacities. Still other objects will be apparent to those skilled in the art from the following description and claims.

In general I accomplish the objects of my invention by running empty tubing into the well, the tubing having a means for closing the bottom of the tubing to prevent entry of liquids. A packer is attached to the lower part of the tubing string. The tubing is run into the well until the packer is a short distance above the top producing zone in the well. The packer is then set and the closing means at the bottom of the tubing is opened to release the pressure in the bottom portion of the casing.

In a cementing operation these steps are taken immediately after cement slurry has been placed behind the casing. Release of pressure inside the casing in this case allows the pressure outside the casing to compress the casing and reduce its circumference. The cement then sets around the casing in this condition. Thus, when the mud or other liquid is removed from the well, the casing does not shrink away from the cement due to the decrease of pressure inside the casing to provide a space between the casing and the cement outside the casing.

If the technique is used in perforating, the decrease of pressure has been shown to decrease the amount of debris in the perforations. The exact reason is not certain. It is probably due to the lower peak pressure caused by the explosive used in the perforator. Any decrease in this peak pressure should also decrease the rapid flow of liquids and debris into the perforations. It is also possible that the rapid back flow of fluids through the perforations from the high pressure formation to the low pressure well may simply remove much of the debris leaving a clean perforation.

In the drawing, FIGURE 1 is a view in cross section of the apparatus used in the preferred embodiment of my in vention. FIGURE 2 is another cross-sectional view of an alternate sealing mechanism for ports in the tubing.

Considering the drawing in more detail: in FIGURE 1 well casing extends into a well penetrating oil producing formation 11 and nonprodueing formations 12 and 13. On the bottom of casing 10 is casing shoe 14- with check valve ball 15. The casing has been cemented by displacing a cement slurry 16, shown surrounding the casing, down the casing by means of drilling fluid, water or the like. The cement slurry is separate from the following liquid by cementing plug 17.

Tubing 18 extends into the casing to a level near the top of the cement column behind the casing. The bottom of the tubing is closed by frangible disc 28 {which is held in place by cap 19. A packer 20 seals the space between the tubing and casing.

In the method of my invention the casing is run into the well as usual and the cement slurry is displaced down the inside of the casing and up around the outside. Preferably, solids-free water is used for this operation if the casing is to be perforated before removal of the liquid. Tubing 18 with packer 20 is then run into the well as quickly as possible. The tubing is empty. Packer 20 is set to form a seal between the tubing and casing at or near the elevation of the cement outside the casing or at least above the level of the top producing zone in the formations penetrated by the well. A go-devil (not shown) is then dropped down the inside of the tubing to break frangible disc 28. When the disc is broken, the pressure inside the casing below the packer drops to the level of the pressure in the tubing. This pressure is substantially atmospheric. Therefore, the pressure within the casing below packer 20 alsobecomes substantially atmospheric, except for the hydraulic head of liquid below the packer. The pressure outside the casing will be approximately the pressure of fluids in the formation. The higher pressure outside the casing will cause the casing to contract. That is, the circumference and diameter of the casing will diminish. If the operations leading to the reduction of pressure below the packer are carried out quickly enough, the cement slurry outside the casing will not have taken a final set. Therefore, as imposition of the differential pressure causes the casing to shrink, cement slurry follows it and completely fills the space between the contracted casing and the well wall. The cement is then permitted to set while maintaining the reduced pressure inside the casing.

After the cement has set, packer 20 is released and tubing 18 is withdrawn from the well. The remainder of the completion of the well can then be in any usual manner. When packer 20 is released, the full hydraulic head of liquid in the casing will again be applied inside the casing below the packer. The casing will not be able to expand, however, since it is supported on the outside by the hardened cement which is, in turn, supported by the formation. During the depletion of the formation in which the well is completed, the pressure inside the casing decreases. It is this decrease in pressure which in the past has often caused the casing to contract and pull away from the set cement sheath outside the casing. The result has been a flow path between the casing and the cement. In my method, however, the casing has been caused to contract before rather than after the cement sets. Therefore, no space can be left between the casing and cement when the pressure inside the casing is reduced during the life of the well.

In FIGURE 2 tubing 18 is the same as in FIGURE 1, In this case, however, ports 21 are provided in the wall of the tubing below packer 20. The bottom of the tubing can be closed in this case by a permanent plug, cap olr the like. The bottom of the tubing can also carry a perforator if desired as described later in more detail. Ports 21 are sealed by sealing elements 22 such as O-rings around an open-ended tubular body 23. Preferably a small shear pin 24 is provided through. the wall of the tubing and extended into body 23. This pin holds the body with its seals in positicin over ports 21 while the tubing is being lowered into the well. Retrieving head 25 is provided on the top of the tubular body.

When the apparatus of FIGURE 2 is used, ports 21 are opened by lowering a wire line into! the well with a retrieving tool to latch around head 25. An upward pull on the wire line then shears pin 24 and permits removal of body 23 from the tubing. To facilitate removal of body 23 from the well, that portion of the tubing a short distance on each side of ports 21 may be of slightly smaller internal diameter than the regular internal diameter of the tubing. Thus, the O-rings can seal in the small-diameter portion but not contact the inside wall of the regular tubing. Opening ports 21 has the same effects as described for breaking frangible disc 28 in connection with FIG- URE 1.

If the apparatus of FIGURE 1 is used, a perforator can be lowered through the tubing after the cement is set and the casing can be perforated opposite the producing formation. The pressure in the casing below the packer is still low and the perforation step takes advantage of this fact to produce perforations with higher flolw capacities than those which would be made with high pressures inside the casing.

If the apparatus of FIGURE 2 is used and a perforator is placed on the bottom of the tubing opposite a producing zone, this perforator can be fired by any of the well known means after the cement has set.

It will be apparent that the technique can be applied to improving cementing even if the techinque is not used for perforating the casing. For example, if the formation is to be subjected to hydraulic fracturing, the flow capacities of the perforations in the formation are not particularly important. The flow capacities of the holes through the casing wall will, of course, be important. The flow capacity of the fracture in the formation will be so much greater than the flow capacities of any perforations in the formation that these flow capacities become unimportant. Thus, after using my technique in cementing, a horizontal casing-cutting shaped charge, as shown in US. Patent 2,939,532, for example, can be used to form a large cut through the casing through which hydraulic fracturing operations can be carried out. Such an explosive charge would not, on the other hand, be used in areas where wells ordinarily are not fractured. In these cases, the flow capacities of the perforations are very important. The casing cutting charge sometimes does not even penetrate the cement sheath. Thus, penetration into the formation is much too small to provide a high flow capacity into the well. Where the perforating step is not to be followed by further formation treatment, it will be obvious that :a bullet or linear jet perforator should be used with a low pressure inside the casing.

The cementing technique can also be used to set casing above a producing formation before drilling into the producing formation for an open-hole completion. In this case, no perforation is usually considered necessary.

It will also be apparent that the perforating step can be carried out at reduced pressure by use of my technique even though the casing has been cemented by other methods. Thus, the perforating technique can be applied to new or old wells in which standard cementing processes have been used. It is preferred, however, to combine the cementing and perforating operations as described above.

Many types of specific apparatus may be used as long as they include a packer which can be set in the casing to isolate a portion of the casing below the packer, and a means for decreasing the pressure inside the casing below the packer. For example, apparatus used in well testing can be used. Such apparatus frequently includes a packer which can be set in casing and a chamber which can be opened to take a sample of fluids from inside the casing below the packer. Opening of the sample chamber allows expansion of the liquid in the casing into the chamber thus reducing the pressure. Since the liquid has a low coefficient of expansion, a small volume, a gallon or two for example, will provide adequate expansion volume to drop the pressure to a point near the original pressure in the sample chamber.

The same general type of equipment can be used if the method is to include a perforating step. The only difference is that a perforator must be provided in the zone below the packer or provisions must be made to introduce such a perforator. One class of apparatus known generally as perforate and test equipment is particularly useful. Such apparatus is shown, for example, in US. Patent 2,535,342, Ahlgren, 2,601,122, McKinley, and 2,760,541 Johnston et al. It is only necessary to modify such equipment so the sample chamber is opened below the packer before the perforator is fired rather than after the perforator is fired, as proposed in the patents.

In all the methods described above, a packer is set in the casing and the pressure inside the casing below the packer is decreased. No limits have been set with regard to the amount of pressure reduction. In most cases the pressure is decreased as much as conveniently possible. In the preferred embodiment, for example, the tubing is run into the well empty and the packer is set only a few feet, for example, a few hundred feet, above the top producing zone. Thus, when the inside of the tubing is connected to the space inside the casing below the packer, the pressure at this point is reduced to substantially atmospheric pressure. If the cementing operation is to be followed by perforation of the casing at low pressure and the formation is known to be at high pressure, however, it may be advisable to fill the tubing partly with liquid to help control the well after the perforators are fired. A higher pressure can also be maintained below the packer by simply setting the packer a considerable distance, such as a thousand feet or so, up the well from the top producing formation,

The use of higher pressure, but still below the pressure outside the casing, is particularly desirable in the cementing operation if it is known that the inside of the casing is later to be subjected to high internal pressures as in a hydraulic fracturing operation. In any case the packer should be set a substantial distance, at least several hundred feet, below the top of the well. The tubing should be at least partly empty leaving at least several hundred feet empty at the top.

In general, in the perforating operation, the greater the pressure reduction the greater the advantage. A small reduction in pressure below the packer will provide some benefits. The major improvement is provided, however, by reducing the pressure inside the casing to a value less than that outside the casing at the time of the perforation.

In the perforating operation timing is not important as long as the cement is allowed to set sufficiently before the perforating takes place. In the cementing operation, however, it is essential that the packer be set and the pressure be reduced before the cement slurry has taken its final set. The hydraulic head of liquids outside the casing is ordinarily great so as large force is available to cause the cement slurry to deform and fill the complete space between the casing and well wall as the casing contracts. If a final set has not been reached when the pressure inside the casing is decreased, there is little question that sufiicient deformation of the cement slurry will take place. Preferably, the pressure is to be reduced as quickly as possible to provide the greatest assurance of success.

The above-described methods and apparatus are given by way of example only. I do not wish to be limited to the specific methods and apparatus but only by the following claims.

I claim:

1. A method for completing a well comprising lowering a casing string into the well, said casing string having a check valve on the bottom to prevent backflow, introducing cement slurry down said casing and forcing said slurry up around the outside of said casing by introducing a liquid into said casing after said cement slurry, lowering a packer a substantial distance into said casing to a level above the highest producing formation of interest penetrated by the well, setting said packer in said casing to isolate a zone inside the casing below said packer, opening a low pressure expansion chamber to the Zone inside the casing below said packer to reduce the pressure within said casing below said packer before said cement slurry takes a final set, and maintaining a reduced pressure within said casing below said packer until said cement slurry takes a final set.

2. The method of claim 1 in which said casing is perforated with a linear perforator opposite at least one producing formation after said cement slurry has set but while asid reduced pressure is maintained in said casing below said packer.

3. The method of claim 2 in which said liquid introduced after said cement slurry is solids-free water.

4. The method of completing a well in which a casing string is cemented through a producing formation comprising lowering a packer into said casing to a level substantially below the top of said well but above the top of said producing formation, setting said packer to isolate a zone inside the casing below said packer, opening a low pressure expansion chamber to the Zone inside the casing below said packer to reduce the pressure in the easing below said packer, and firing a linear perforator opposite said formation at said reduced pressure, whereby high flow capacity perforations are formed through said casing and into said formation.

5. The method of claim 4 in which the pressure inside said casing below said packer is decreased to a value below the pressure outside said casing before firing said perforator.

6. A method of completing a well comprising lowering a casing string into the well, said casing string having a check valve on the bottom to prevent backflow, introducing cement slurry down said casing and forcing said slurry up around the outside of said casing by introducing a liquid into said casing after said cement slurry, lowering a packer a substantial distance into said casing to a level above the highest producing formation of interest penetrated by the well, said packer being lowered on a tubing string closed at its lower end and at least partly empty, setting said packer between said casing and tub ing, connecting the inside of said tubing to the inside of said casing below said packer to decrease the pressure inside said casing below said packer before said cement slurry takes a final set, and maintaining a reduced pres sure inside said casing below said packer until said cement slurry takes a final set.

7. The method of claim 6 in which said casing is perforated opposite at least one producing formation after said cement slurry has set but while said reduced pressure is maintained in said casing below said packer.

8. The method of completing a well in which a casing string is cemented through a producing formation comprising lowering a packer into said casing to a level substantially below the top of said well but above the top of said producing formation, said packer being lowered on a tubing string closed at its lower end and at least partly empty, setting said packer between said casing and tubing, connecting the inside of said tubing to the inside of said casing below said packer to decrease the pressure inside said casing below said packer and firing a linear 6 perforator opposite said formation at said reduced pressure, whereby high flow capacity perforations are formed through said casing and into said formation.

9. A method for completing a well comprising lowering a casing string into the well, said casing string having a check valve on the bottom to prevent backfiow, introducing cement slurry down said casing and forcing said slurry up around the outside of said casing by introducing a liquid into said casing after said cement slurry, lowering a packer a substantial distance into said casing to a level above the highest producing formation of interest penetrated by the well, said packer being lowered on a tubing string which is at least partly empty, said tubing string having below said packer an opening closed by a removable member, setting said packer between said casing and tubing, removing said removable member from said opening to decrease the pressure inside said casing below said packer before said cement slurry takes a final set, and maintaining a reduced pressure inside said casing below said packer until said cement slurry takes a final set.

it The method of claim 9 in which said casing is preforated opposite at least one producing formation after said cement slurry has set but while said reduced pressure is maintained in said casing below said packer.

11. The method of claim 9 in which said opening is the bottom end of the tubing and said removable member is a frangible disc.

12. The method of claim 11 in which a linear perforator is lowered through said tubing and is fired opposite at least one producing formation after said cement slurry has set, while the pressure inside said casing below said packer remains at a reduced value.

13. The method of completing a well in which a casing string is cemented through a producing formation comprising lowering a packer into said casing to a level substantially below the top of said well but above the top of said producing formation, said packer being lowered on a tubing string which is at least partly empty, said tubing string having below said packer an opening closed by a removable member, setting said packer between said casing and tubing, removing said removable member from said opening to decrease the pressure inside said casing below said packer, and firing a linear perforator opposite said formation at said reduced pressure, whereby high fiow capacity perforations are formed through said casing and into said formation.

References Cited by the Examiner UNITED STATES PATENTS 2,798,558 7/1957 McCulloch 166115 2,805,722 9/1957 Morgan et al. 166-35 2,833,352 5/1958 Lloyd 16635 X 2,895,554 7/1959 Krueger et al. 166-35 3,183,971 5/1965 McEver et al. 16621 3,195,631 7/1965 Smith 166-35 JACOB L. NACKENOFF, Primary Examiner.

CHARLES E. OCONNELL, Examiner.

S. J. NOVOSAD, Assistant Examiner.

Claims (1)

1. A METHOD FOR COMPLETING A WELL COMPRISING LOWERING A CASING STRING INTO THE WELL, SAID CASING STRING HAVING A CHECK VALVE ON THE BOTTOM OF PREVENT BACKFLOW, INTRODUCING CEMENT SLURRY DOWN SAID CASING AND FORCING SAID SLURRY UP AROUND THE OUTSIDE OF SAID CASING BY INTRODUCING A LIQUID INTO SAID CASING AFTER SAID CEMENT SLURRY, LOWERING A PACKER A SUBSTANTIAL DISTANCE INTO SAID CASING TO A LEVEL ABOVE THE HIGHEST PRODUCING FORMATION OF INTEREST PENETRATED BY THE WELL, SETTING SAID PACKER IN SAID CASING TO ISOLATE A ZONE INSIDE THE CASING BELOW SAID PACKER, OPENING A LOWER PRESSURE EXPANSION CHAMBER TO THE ZONE INSIDE THE CASING BELOW SAID PACKER TO REDUCE THE PRESSURE WITH-

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