US3431977A - Forming fractures in the desired direction in earth formations - Google Patents

Description

United States Patent 3,431,977 FOG FRACTURES IN THE DESIRED DIRECTION IN EARTH FORMATIONS Clarence Robert East and Morton A. Mallinger, Tulsa,

Okla, assignors to Pan American Petroleum Corporation, Tulsa, Okla, a corporation of Delaware No Drawing. Filed July 24, 1967, Ser. No. 655,293 U.S. Cl. 166-281 6 Claims Int. Cl. EZlb 33/13, 43/26 ABSTRACT OF THE DISCLOSURE A vertical fracture is formed extending into an earth formation in a desired direction from a well penetrating the formation. In the process, the usual steps are first followed of forming a vertical slot through a liner in the well and forming a notch in the formation behind the slot, the notch extending in the direction of the desired fracture. Pressure is applied as usual to form or extend a fracture. The improvement comprises sealing this first fracture by filling it with a sealing liquid, re-notching the formation in the plane of the desired fracture and again applying a fracturing pressure.

It is frequently desired to form vertical fractures from wells in earth formations with fractures being oriented in a particular direction. Examples include waterfiooding operations, as described in U.S. Patent 3,199,586, Henderson et al., or extraction processes, as described in U.S. Patent 3,270,816, Staadt, for example. It has been suggested that fractures in the desired direction be created bynotching a formation vertically in cased or open hole in the desired direction, and then applying a hydraulic fracturing pressure to the well to form a fracture in the plane of the notch. Such an operation is described in U.S. Patent 2,758,653, Desbrow, for example.

As explained by Desbrow, a vertical notch decreases the pressure required to fracture the formation in the plane of the notch. Unfortunately, however, we have found that even less pressure is often required to form a fracture in a natural plane of weakness in some other direction. For example, in shallow wells, a horizontal fracture may form before the pressure reaches a level necessary to fracture the formation in the plane of the notch. In some cases, a formation has a vertical plane of weakness, which can cause a vertical fracture to form in this plane, rather than in the plane of the notch. In many cases, fractures already exist. Obviously, these fractures will re-open before a new fracture forms in the plane of the notch.

The problems are, of course, particularly severe in open holes. In such cases, the difficulties can be reduced to at least some extent by setting a liner in the well and placing the notch through the liner. In the case of horizontal weak zones, however, it will be apparent that the vertical notch intersecting the horizontal weak zones exposes the horizontal weak zone to the hydraulic pressure. As a result, a horizontal fracture may form before the pressure can reach a level necessary to form a vertical fracture in the plane of the notch. Even if a vertical fracture forms in the desired direction through the slot in the casing in a formation with a vertical plane of natural weakness, the fracture may not extend very far before changing direction to lie in the plane of natural weakness.

With the above problems in mind, an object of this invention is to provide an improved method for forming vertical fractures extending in the desired direction from wells penetrating earth formations. Still other objects will be apparent from the following description and claims.

In general, we accomplish the objects of our invention 3,431,977 Patented Mar. 11, 1969 by placing a liner in the well, if it is not already lined, forming a slot in the liner and a notch in the formation behind the liner, forming a fracture through the slotted liner, filling the fracture with a permanent sealing agent, such as portland cement, and then repeating the notching and fracturing technique without the sealing material so the second fracture remains open.

We have found that, by establishing a permanent seal within the first fracture which forms, the pressure can be increased to a higher level before another fracture forms. This, of course, greatly increases the chance for the second fracture to be formed and to continue in theplane of the notch.

We also suggest that props, such as sand grains, metal shot, granular nutshells, or the like, be included, not only in the second fracturing liquid, but also in the permanent sealing material. The object is to prop the fracture open and thus compress the formation in a direction perpendicular to the first fracture. This is particularly important in case a vertical plane of weakness is present in the formation. If the first fracture turns to lie in the natural plane of weakness, and if props are placed in this fracture, the formation is then held in compression, so a second fracture forms much less readily in the former natural plane of weakness. As a result, a second fracture can [be extended much farther from the well before changing directions. This is not to say that the second fracture will not also turn eventually, but the distance which it extends from the well in the desired direction is greatly increased. A fracture extending still farther in the desisred direction may sometimes be obtained by propping and sealing the second fracture and forming a third.

An explanation seems appropriate of the difference between the fracture-sealing action which we propose and that which is obtained in so-called multiple fracturing operations. In the latter type of process, several high-flow capacity fractures in a formation are desired at different locations in a well. In this case, after a first fracture is formed and propped, it is bridged and sealed at the well bore face by a temporary bridging and sealing material. Then, upon applying a somewhat higher pressure, a second fracture forms at a different location and is propped open. The second fracture can be sealed and a third formed and so on. The process is described in U.S. Patents 2,734,861, Scott et al., and 2,873,250, Scott, for example. Upon removal of the bridge and seal at the face of the well bore, all fractures become high-flow capacity channels into the well bore.

In our process, on the other hand, the second fracture is formed at as nearly as possible the same location as the first, after the first fracture has been permanently sealed. In our process, the second fracture is not formed at a location other than the location of the first as in multifrac. In addition, in our process, the seal is not formed just at the well bore face, but extends into the entire fracture. A permanent seal is formed. The sealing material must be something like portland cement, phenolformaldehyde resin, silica gel, or the like, which forms a permanent solid or gelatinous material upon setting. Another material which has been found to be rather surprisingly effective is finely divided inert powder, such as nutshell flour, diatomaceous earth, or the like. If a solid pack of such flour is formed in a fracture, it has been found that the fracture becomes very effectively sealed. In U.S. patents application S.N. 586,484, entitled Formation Plugging, filed Oct. 13, 1966, by Flickinger et al., for example, an operation is described which used nutshell flour to plug a fracture extending from a waterinjection well in an oil field. In this operation, the parting pressure for a formation was increased by about pounds per square inch from about 360 to about 450 pounds per square inch by packing the fracture full of nutshell flour. This was an increase of about 25 percent.

In another example, a sodium silicate solution (6 percent SiO containing ammonium sulfate, was injected into a very shallow test well. A fracture opened at only about 50 pounds per square inch pressure. The silicate solution was injected into the fracture and allowed to stand until the ammonium sulfate caused the formation of a silica gel plug. The well was cleaned out and water pressure was again applied. This time, no pressure parting occurred until the pressure reached about 190 pounds per square inch.

A preferred sealing agent for the fractures is portland cement. Portland cement, like the nutshell flour, contains finely divided particles which will form a seal. The packed particles of cement also serve to keep the fracture faces held apart and the formation stressed while the cement attains a chemical set to develop compressive strength and hold the formation in compression.

The use of a low water-loss cement slurry and a high final-squeeze pressure at the end of the plugging operation is usually advisable. This insures that the fracture is packed tightly with solids while a high pressure is imposed on the fracture. In this way, the formation is stressed as much as possible. A high final-squeeze pressure also helps by filling any mud channels or other passages outside the casing or liner through which fracturing fluid might be lost. Of course, if a liquid such as a liquid settable resin is used, a high final-squeeze pressure cannot be imposed unless finely divided solids, such as nutshell flour, diatomaceous earth, or the like, are used in these liquids.

It may seem that a high water-loss cement slurry should be used since this slurry dehydrates rapidly, permitting quick buildup of a squeeze pressure. As a practical matter, however, it is usually advisable to use a cement slurry having a water-loss rate somewhat lower than that of portland cement. The reason is that even portland cement sometimes dehydrates so rapidly that dehydrated cement is deposited inside the casing between the packers, sticking the packers in the well. By use of a low water-loss slurry, the dehydration rate can be controlled by alternately starting and stopping the pumps. It is, thus, possible to fill the first fracture with cement at high pressure while depositing little if any dehydrated cement inside the well casing or liner. The packers can then be unseated and the cement slurry inside the casing or liner can be circulated out of the well.

Obviously, it is not as important to use fracture props with portland cement as with some other materials such as silica gel. It should be noted in this regard, however, that in the silica gel example given above no fracture props were used. The high fracturing pressure was due simply to filling the most easily formed fracture with a gel which prevented entrance of fracturing liquid. Thus, while use of fracture props in the sealing liquid is preferred, some benefit can be obtained without the props.

When reference is made to a sealing liquid, it will be understood that this term is intended to include not only solutions, such as phenol-formaldehyde resins, or the solutions which form silica gel or the like, but also slurries, such as portland cement in water, nutshell flour in oil, water, or other liquid, or settable solutions, such as silicate solutions containing fillers such as diatomaceous earth.

After the first fracture has been sealed, a new vertical notch is formed in the desired fracture plane. Preferably, the same slot through the liner is used and the notch is in the same location as before. It is possible, however, that the location of the second slot and notch may be slightly different from that of the first. An ordinary hydraulic fracturing operation is then performed as described in U.S. Reissue Patent 23,733, Farris; U.S. Patent 2,699,212, Dismukes; U.S. Patent 2,758,653, Desbrow, or the like. The notch should extend at least about 3 or 4 inches into the formation and preferably at least about 6 inches.

The slot in the liner and the notch in the formation may be made by mechanical means, as suggested in U.S. Patent 2,699,212, Dismukes, by explosives, as described in The Oil and Gas Journal for Apr. 12, 1947, page 86, or by a jet liquid with or without abrasive particles. The liner slot may be cut by one means and the formation notch may be formed by another. Preferably, however, both are formed by a jet of liquid containing abrasive particles.

Our process will be better understood from the following example. A well is near the edge of an oil field which is to be waterflooded. It is desired to use the well as a water input well. It is also desired that the injected water be directed principally in the direction of the field rather than away from it. The formation to be flooded is 40 feet thick and extends from a depth of 1,800 feet to 1,840 feet. The well is uncased through this interval.

First, a string of tubing is run to the bottom of the well and water is circulated to clean the well wall. Next, three liners of glass fibers and resin are pressed against the walls of the well, as described in U.S. Patent 3,028,- 915, Jennings, for example, and the resin is allowed to set to a hardened state. Each patch is 20 feet long and ends of the patches overlap about a foot to get an overall length of about 58 feet. The patch extends about 9 feet above and below the 40-foot formation to be flooded. Next, a vertical slot about 30 feet long is cut in the patch in the direction of the desired fracture. The slotting is done by means of a sand-laden jet of water, as described in U.S. Patent 2,758,653, Desbrow. The jet is operated until not only is the plastic liner slotted, but a considerable vertical notch about 6 inches deep is formed in the formation behind the liner. The 30-foot slot and notch are located at about the center of the 40-foot formation.

After circulating abrasive particles and pieces of the cut liner and formation out of the well, a string of tubing is run into the well with two packers about 45 feet apart. The packers are set in the plastic liner, one above and one below the slot and notch. An opening is provided in the tubing wall between the packers. The bottom of the tubing string is closed. Water is pumped down the tubing, through the slot in the liner, and into the notched formation. At a bottomhole pressure of about 1,300 pounds per square inch, the formation parts, so pumping at higher rates causes little increase in pressure. A portland cement slurry, containing sand props passing a No. 8 and retained on a No. 20 U.S. standard sieve, is then introduced down the tubing and into the fracture. After displacing about 500 gallons of cement slurry into the fracture, the pumps are alternately stopped and started to permit the slurry in the fracture to dehydrate by loss of water through the fracture faces. Upon dehydration of the slurry, the bottomhole injection pressure at a given pumping rate begins to rise and finally reaches about 2,500 pounds per square inch. At this time, little fiow of cement slurry into the formation is occurring even at the elevated pressure. The packers are released and water is circulated through the well to remove cement slurry from the well while holding a pressure of about 1,500 pounds per square inch on the formation to prevent back flow of cement out of the fracture. Pressure is held on the well for about two hours to permit the cement to set with the fracture faces held apart. Pressure is then released but the cement is allowed to set for about another 24 hours to insure development of a high strength before additional operations are attempted.

After the cement has set and developed strength, the slot and a notch about 6 inches deep are again formed through the plastic patch in the desired direction. The packers are set again above and below the solt and notch and water presure is again applied to fracture the formation. This time, the formation parts only after the bottomhole pressure has reached a little over 1,500 pounds per square inch. Sand passing a No. 12 US. standard sieve and retained on a No. 20 US. standard sieve is injected into the fracture with the water to prop the fracture open. The packers are then unset and withdrawn from the well, which is then ready for use to inject Water principally through the oriented fracture extending toward the oil field rather than in other directions.

Various w-ays have been described above for performing our process. Many alternates are possible. For example, instead of forming a single oriented fracture extending in one direction, two fractures extending in approximately opposite directions, as shown in US. Patent 3,199,- 586, Henderson et al., may be formed simultaneously by notching the formation in two directions. Two fractures in directions other than 180 apart may also be formed, if desired. In open hole, the liner may sometime be a steel liner, as shown in US. Patent 3,191,680, Vincent, or may be a conventional metallic liner cemented in place in the conventional manner. Instead of fracturing with water or some other fracturing fluid and then injecting a sealing agent, the sealing agent may be used as the fracturing liquid.

Still other variations will be apparent to those skilled in the art. Therefore, we do not wish to be limited to the examples given above, but only by the following claims.

We claim:

1. A method for forming a fracture in an earth formation, said fracture extending in the desired direction from a well penetrating said formation, said method comprising,

forming a vertical slot through a liner in the well opposite said formation and forming a vertical notch extending into said formation behind said slot, said notch being formed in the direction of the desired fracture,

applying sufiicient pressure to said notch to part said formation and extend a fracture into said formation, sealing said first fracture by means of an impermeable solid,

again forming a vertical notch extending into said formation behind a slot in said liner in substantially the same location as the first slot, said notch being formed in the direction of the desired fracture, and

applying sufficient pressure to the second notch to fracture said formation.

2. The method of claim 1 in which said sealing liquid includes fracture-propping particles to hold the fracture apart and compress the formation in a direction perpendicular to the plane of the first fracture.

3. The method of claim 1 in which said notches are formed by a jet of liquid containing abrasive particles.

4. The method of claim 1 in which the Well has an unlined portion opposite said formation and a liner is set in, said unlined portion before the method of claim 1 is performed.

5. The method of claim 4 in which said liner is formed by pressing a liner of glass fibers and resin against the Well wall opposite said formation and allowing the resin to set to a hardened state.

6. The method of claim 1 in which said sealing liquid is a slurry of portland cement.

References Cited UNITED STATES PATENTS 2,734,861 2/1956 Scott et a1 166-42 X 2,758,653 8/1956 Desbrow 166-42 X 2,811,207 10/1957 Clark 166-22 2,952,319 9/1960 Po-pham 166-35 3,180,414 4/1965 Parker 166-42 X 3,249,158 5/1966 Kieschnick et a1. 166-42 X 3,323,594 6/1967 Huitt et a1. 166-42 CHARLES E. OCONNELL, Primary Examiner.

I. A. CALVERT, Assistant Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,431,977 March 11, 196,

Clarence Robert Fast et al.

It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 4, "East" should read Fast line 58, "zones" should read zone Column 2, line 30, "desisred" should read desired line 65, "patents" should read patent Column 4, line 7, after "jet" insert of line 73, "solt" should read slot Signed and sealed this 14th day of April 1970.

(SEAL) Attest:

Edward M. Fletcher, Jr. WILLIAM E. SCHUYLER, JR.

Attesting Officer Commissioner of Patents