Method and apparatus for fracing earth formations surrounding a wellbore

Abstract

A dry powder product comprised of an inorganic silicate-based compound and an organic compound are mixed with fresh water as a catalyst, such that the mixture enters into hydration reaction, and when injected into existing fractures in low permeability formations surrounding earth boreholes, the resulting increase in volume of the mixture from the hydration reaction causes the existing fractures to have increased dimensions. Because the hydrationreactionbetweenwater and the drypowderwill commence almost as soon as the water is mixed with the dry powder, there are various means and methods described to inhibit the hydration reaction from occurring until after predetermined times, for example, such as by using water soluble coatings for encapsulating the dry powder which dissolve after predetermined times, and by using water insoluble coatings over the powder and by causing the capsules to be mechanically disintegrated to allow water to come into contact with the dry powder, and by providing means for transporting the dry powder from the earth's surface in the tubular string down to the formation of interest surrounding the wellbore, the hydration reaction can be inhibited for predetermined periods of time.

Description

BACKGROUND OF THE INVENTION

[0001] This invention relates, generally, to the art of fracing low permeability formations surrounding an earth borehole drilled for the purpose of producing oil and gas. In particular, the invention relates to the fracing of such low permeability formations using a frac fluid which generates an expansive stress through hydration reaction to fracture such earth formations.

PRIOR ART

[0002] It is well known in the oil and gas industry to be able to produce more oil and gas from low permeability formations surrounding and earth borehole by using fracturing of such formations, sometimes referred to as fracing, to allow oil and natural gas to move more freely from the rock pores where such pores are trapped, to a producing well they can bring the oil or gas to the surface.

[0003] After a well is drilled into a reservoir rock that contains oil, natural gas, and water, every effort is generally made to generally maximize the production of oil and gas. One way to improve or maximize the fluids to the well is to connect many preexisting fractures and then to optimize the production of fluids from the reservoir rock with a larger fracture. This larger, man-made fracture starts out at the well and extends out into the reservoir rock for as much as several hundred feet. The man-made or hydraulic fracture is formed when a fluid is pumped down the well at high pressures for short periods of times, usually hours. The high pressure fluid (usually water with some specialty high viscosity fluid added) exceeds the rock strength and opens a fracture in the rock. A propping agent, usually sand, carried by the high viscosity additives, is propped into the fractures, to keep them from closing when the pumping pressure is released. The high viscosity fluid becomes a lower viscosity fluid after a short period of time. Both the injector water and the now low viscosity fluid travel back through the man-made fracture to the well and up to the surface. If it is desired that the man-made fracture be kept open for longer periods of time, well-known propping agents such as, for example, sand, bauxite, glass beads, etc., can be injected into the fracture for an indefinite period of time.

[0004] The prior art includes many patents and patent publications describing the hydraulic fracturing of a subterranean formation, for example, as illustrated and described in U.S. Pat. No. 4,186,802 to William Perlman.

[0005] It is also known in the prior art to practice a sonic fracing process in which sonic waves are used to fracture the formation, for example, as is illustrated and described in U.S. Pat. No. 4,537,256 to Franklin Beard.

[0006] It is also known in the art of fracturing of formations surrounding a wellbore to use the well known explosive fracturing processes, for example, as described in U.S. Pat. No. 2,790,388 to N. A. MacLeod which uses the explosive process to generate acoustic waves which are then in turn used to fracture the earth formation.

[0007] In U.S. Pat. No. 2,696,259, propellant charges are fired into the earth formations surrounding an earth borehole.

[0008] While the dry powder used in the present invention has been hydrated and used to fracture and even demolish concrete and rock structures such as walls and buildings on or near the earth's surface, the hydrated powder according to the present invention has not, to the knowledge of the applicant, previously been used to fracture low permeability oil and gas formations, usually thousands of feet beneath the earth's surface.

OBJECTS OF THE INVENTION

[0009] It is therefore the primary object of the present invention to provide new and improved fracing fluids and methods for fracturing an earth formation surrounding a borehole.

[0010] These and other objects, features and advantages of the present invention will be understood from a reading of the following detailed description of the invention, in conjunction with the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIG. 1A is an elevated, schematic view of a cased earth borehole, with associated equipment at the earth's surface, used in accord with the present invention, to generate fractures in low permeability oil and gas formations;

[0012] FIG. 1B is an elevated, enlarged, schematic view of perforations of the casing and the Pay Zone itself illustrated in FIG. 1A;

[0013] FIG. 2 illustrates an inner sphere of dry powder used according to the invention encapsulated within a water soluble shell;

[0014] FIG. 3 illustrates an elongated capsule of dry powder used according to the invention encapsulated within a water soluble shell;

[0015] FIG. 4 illustrates an inner sphere of dry powder used according to the invention encapsulated within a water insoluble shell;

[0016] FIG. 5A illustrates an elevated, schematic view of a sub which can be used to disintegrate, mechanically, the water insoluble capsule of FIG. 4.

[0017] FIG. 5B is a plane view of one of the plates illustrated in FIG. 5A; and

[0018] FIG. 6 is an elevated schematic view of a sub which can be used in the tubular string of FIG. 1A, as an alternative embodiment of the invention to transport the dry powder from the earth's surface to the Pay Zone.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0019] The present invention contemplates the use of a dry powder product comprised of an inorganic, silicate-based compound and an organic compound. The product uses fresh water as a catalyst, and in combination with the ambient temperature around the product causes the product to expand and act much like an explosive material, without the dangerous negatives associated with the use of TNT, dynamite or the like.

[0020] The starting material for the frac fluid according to the invention is the dry powder discussed above, markeded by Onoda Corporation, having its principal place ofbusiness at Toyo Central Building, No. 1-3, Toyo 4-chome, Koto-Ku, Tokyo 135-0016, Japan, under its BRISTAR trademark. The inorganic compounds in the dry powder comprise the following components, within the ranges of each component being with the noted percentages (%) by weight:

[0021] 1) silicon dioxide 2-11%

[0022] 2) aluminum oxide 0.3-6%

[0023] 3) ferric oxide 0.5-3%

[0024] 4) calcium oxide 77-96%

[0025] 5) magnesium oxide 0-2%

[0026] 6) sulfur trioxide 0.3-5%

[0027] The organic compound in the dry powder comprises either sulfonated melamine (CAS No. 64787-97-9) or Naphthalene - sulfonic acid polymer with formaldehyde, sodium salt (CAS No. 9084-06-4)--- approximately 1%.

[0028] The BRISTAR product enables the fracing of earth formations, because despite being non-explosive in nature, the product when hydrated with water, generates expansive stress through hydration reaction. The BRISTAR product, when hydrated, is sensitive to temperature, but is essentially benign prior to hydration.

[0029] The expansive stress generated by the hydration ofthe powder is believed to be maximum when the water/powder ratio is approximately 30%, by volume, i.e., when the mixed frac fluid is 30% fresh water and 70% powder, prior to mixing, by volume. This is approximately 1 ½ liter of fresh water per one 11 pound bag of dry powder. The expansive stress is said to decrease if the water ratio is increased or decreased from the 30% level. Moreover, if the water used has a significant saline content, the expansive stress may be lessened.

[0030] Referring now specifically to FIG. 1A of the drawing, there is illustrated an earth borehole 10 which runs from the earth's surface 12 down to a hydrocarbon Pay Zone 14 which is typically several thousand feet from the earth's surface 12. The borehole 10 is lined with a steel casing 16. A tubular string 18, typically production tubing, runs from above the earth's surface 12 down to the proximity of the Pay Zone 14, and in addition to transporting a pair of isolation packers 20 and 22 which straddle the Pay Zone 14, has a conventional tubing conveyed perforator 24 at or near its lower end.

[0031] At the earth's surface 12, a fluid pump 30 has its output pumped through a conduit 32 into the interior of the tubular string 18. Fresh water from a source 34 is fed into the mixer 36 and the dry powder used in accordance with the invention is also fed into the mixer 34 from a dry powder hopper 38. If proppants are used in the process, such conventional proppants as sand, bauxite, glass beads, or the like, are also fed from a proppant hopper 40 into the mixer 36. As described hereinafter, the water may be chilled by a cooling tower or other refrigeration unit 35 to a point at or near freezing, or ice may be added to the water.

[0032] In operation, to frac the low permeability Pay Zone 14, if the casing 16 has either not been perforated, or needs additional perforations, the conventional tubing conveyed perforator 24 is activated to create a plurality of perforations 42, 44 and 46 in the casing 16 which also extend into the formation itself constituting the Pay Zone 14 as fractures 42a, 44a and 46a in FIG. 1B. The tubing conveyed perforation may use shaped charges or bullet perforators, both of which are well known in the art but bullet perforators will perhaps work better for this type of frac process. Conventional completion fluids, usually heavy brines, can be pumped down through the tubing 18 and back up to the earth's surface through a bypass valve in packer 20 to clean up the casing and the perforations, as it is well known in this art.

[0033] The water from the source 34 is then mixed with the dry powder from hopper 38, and with the proppants from hopper 40, if used, in the mixer 36, and then pumped through the tubing 18, and out of the screen holes 50 in the tubing conveyed perforator 24, into the borehole 10 between the packers 20 and 22. By continuing the pumping process, the combined frac fluid and proppants, if used, are pumped through the perforations 42, 44 and 46, and as illustrated in FIG. 1B, into the perforation extensions 42A, 44A and 46A in the formation itself.

[0034] If desired, to inhibit the hydration reaction of the dry powder from the hopper 38, the water from source 34 can be chilled to near or at the freezing point of water as discussed above.

[0035] Once the frac fluid according to the invention has been pumped into the perforation extensions 42a, 44a and 46a in the Pay Zone 14, the rock temperature of the Pay Zone 14 being typically in excess of 150-200° F., will commence the hydration reaction of the frac fluid in the perforation extensions, and because of the resulting expansive stress, will cause the desired additional fracturing of the formation. The added proppants, pumped into the Pay Zone 14 with the frac fluid, will keep the fractures open.

[0036] An important feature of this present invention is the ability to inhibit the hydration of the dry powder used as the basis for the frac fluid, primarily because of the distance between the dry powder in the hopper 38 and the Pay Zone 14, causing time delays. As but one example of the inhibition of the hydration reaction, there is discussed hereinabove the cooling of the water used in the hydration process, either by passing the water through a cooling tower 35 such as is illustrated in FIG. 1A or by placing ice within the water to lower the temperature of the water to some point at or near freezing of the water. The water can even be chilled to a point below 32° F. by adding various known chemicals to the water. Moreover, the point at which the water is chilled could be in any process step shown in FIG. 1A, for example, between the cooling tower and the water source 34 or in cooling the materials within the mixer 36, or even on the output side of the pump 30 immediately prior to pumping the mixed ingredients into the tubular 18.

[0037] There are several additional alternative solutions for inhibiting the hydration process. For example, as shown in FIG. 2, the dry powder 50 shown within a spherical capsule 52 can be encapsulated by technology well known in the art, sometimes referred to as miro-encapsulation, whereby the water soluble outer shell will prevent any water into which the capsule 50 has been placed from engaging the dry powder 50 for predetermined periods of time, as desired. For example, if the outer shell 54 of the capsule 52 requires contact with water for one hour before being dissolved, the plurality of the capsules 52 can be put into the mixer 36 of FIG. 1A, along with the requisite amount of water, and the proppants from the hopper 40 as desired, can be pumped down through the tubing 18 and through the perforations 42, 44 and 46 into the Pay Zone 14. With this type of process, the water pumped with the capsules and the proppants will cause the outer shell 54 of each of the capsules 52 to eventually dissolve and come into contact with the water, and thereafter commence the desired hydration reaction.

[0038] FIG. 3 illustrates another type of capsule similar to the capsule of FIG. 2 but which has the dry powder 60 encapsulated within a water soluble outer shell 64 for each of the elongated shaped capsules 62.

[0039] If desired, the capsules 52 and 62 of FIGS. 2 and 3, respectively, can be fabricated using time release technology which is commonly used within the pharmaceutical industry, to thereby inhibit for determined periods oftime the water from reaching the dry powder in accordance with the present invention to commence the hydration reaction process.

[0040] FIG. 4 illustrates an alternative embodiment of a process for inhibiting the hydration reaction process until the desired time, by using a water soluble outer shell 74 to encapsulate the dry powder 70 in accordance with the present invention. In use, the capsule 72, the water from the earth's surface and the proppants, if used, are pumped down through the interior ofthe tubing string 18 and are caused to pass through the apparatus illustrated in FIG. 5A, which includes a device 80 within the lower end of the tubing string 18 which pulverizes the capsules 72 and thus causes the dry powder 70 of each of the capsules 72 to come into immediate contact with any water which has been pumped down with the capsules. The apparatus 80 can take various forms but can be as simple as a screw thread run between two plates 82 and 84 by fluid pressure from the fluid being pumped down from the earth's surface to cause the thread to rotate and grind up the capsules 72 so as to no longer be protected by the outer shell 74. The rotating thread is designed to rotate freely on a shaft 85 having bushings 86 and 87 mounted in the center of plates 82 and 84 respectively, each of the plates 82 and 84 have fluid ports 90 mounted therein as shown in FIG. 5B.

[0041] As an additional process which can provide the inhibition of the hydration reaction, well known chemical retardants can be used to retard or inhibit the hydration process.

[0042] As an alternative embodiment for inhibiting the hydration process, the sub 100 of FIG. 6 can be used at or near the end of the tubing string 18 in place of the sub 25 illustrated in FIG. 5A, and includes a chamber 102 which has been filled with the dry powder used in accordance with the present invention, with the chamber 102 being used to hold the dry powder 100 and having an upper diaphragm 104 and a lower diaphragm 106. Each of the diaphragms 104 and 106 will rupture upon the pressure of the water being pumped down from the earth's surface exceeding a given pressure, for example, at 200 psi. In operation, the tubing string 18 is lowered down until the isolation packers of 20 and 22 of FIG. 1A are adjacent the Pay Zone 14. The water pump pressure at the earth's surface is increased to rupture the diaphragms 104 and 106, thus bringing the pumped down water into contact with the dry powder 100. The resulting mixture of the dry powder and water, and proppants if used, are then pumped through the perforations 42, 44 and 46 as referenced before in FIG. 1A and into the extensions 42a, 44a and 46a of FIG. 1B. This particular process will allow the inhibition ofthe hydration reaction merely because the entire process of mixing the water with the dry powder occurs immediately prior to the mixture being pumped into the Pay Zone 14.

[0043] Other forms of inhibition and retardation of the hydration process will occur to those skilled in this art from a reading of the foregoing various embodiments.

Claims

1. A fluid for the further fracturing of a low permeability formation surrounding an earth borehole, comprising:

a first silicate-based inorganic compound;

a second organic compound; and

water, said first and second compounds and said water, when mixedig the ability to enter into hydration reaction and to thereafter expand the volume of said mixture within an existing fracture in said formation to thereby increase the dimensions of said existing fracture.

2. The fluid according to claim 1, wherein the first compound and the second compound comprise a dry powder encapsulated with a water soluble shell prior to being mixed with the water.

3. The fluid according to claim 1, wherein the first compound and the second compound comprise a dry powder encapsulated with a water insoluble shell prior to being mixed with the water.

4. A process for the further fracturing of a low permeability formation surrounding an earth borehole comprising:

Mixing a dry powder comprised of a first silicate-based inorganic compound and a second organic compound with water to create a frac fluid at the earth's surface;

Pumping said frac fluid from the earth's surface down to said formation to be further fractured, and into existing fractures in said formation to thereby expand the volume of said frac fluid in said existing fractures by hydration reaction, thereby increasing the dimensions of said existing fractures.

5. The process according to claim 4, wherein said dry powder is encapsulated with a water soluble fluid.

6. The process according to claim 4, wherein said dry powder is encapsulated with a water insoluble shell.

7. The process according to claim 6, wherein said encapsulated dry powder is mechanically disintegrated prior to mixing with water.

8. The process according to claim 4, wherein said water is chilled at the earth's surface.

9. The process according to claim 4, wherein said dry powder is transported in its dry condition from the earth's surface to the proximity of the formation to be further fractured.