US3384177A - Treating reservoir matrix - Google Patents

Description

May 21, 1968 1. J. DAY ETAL TREATING RESERVOIR MATRIX Filed July 21, 1966 INVEN fo/:ww J. DAYTORS United States Patent C) 3,384,177 TREATING RESERVOIR MATRIX l Johnny J. Day, Pittsburgh, `limmie L. Huitt, Glenshaw, and Bruce B. McGlothlin, Pittsburgh, Pa., assignors to Gulf Research & Development Company, Pittsburgh, Pa., a corporation of Delaware Fiied July 21, 1966, Ser. No. 566,957 12 Claims. (Cl. 166--42) This invention relates to a method of conditioning reservoir formations to reduce the embedment of fracture proppants.

One of the primary methods of stimulating production in the oil industry is to hydraulically fracture the formation. Gwing to the great overburden pressures, these fractures must be mechanically held in an open psition to prevent the 4closure of the fractures. Proppants are injected into the fractures to maintain an open fracture.

At a time subsequent to the initiation of a successful hydraulic fracture treatment, the flow capacity may seriously decrease to the extent that `a further remedial Workover of the well is advisable. One of the primary methods utilized in the oil industry to increase the ow capacity of a well which has previously been fractured is to hydrothermally treat the formation, Owing to the various expandable minerals contained in hydrocarboncontaining formations, such hydrothermal treatment will often soften the reservoir matrix to the extent that proppants will embed in the fracture face thereby permitting closure of the fracture.

Methods heretofore employed to prevent fracture closure during subsequent hydrothermal treatments have been to anticipate such closure and inject a sufficient -amount of proppant to pack the fracture. Such a multilayered packed fracture will reduce the flow capacity of ythe fracture in a like manner as encountered when proppants embed into the fracture faces. In addition, the utilization of a multilayer of proppants will create a well condition favoring possible screen out of the proppant during injection. Dangerous excessive pressures may be applied to the well bore when a sand screens out during fracture treatment. The attempt to form a multilayer of proppants in the fracture is also associated with increased sand clean out after the treatment is completed.

Owing to the reduction of flow capacity when fractures `are packed with a multilayer of proppants, the oil industry has found that fracture proppant placement approaching a monolayer arrangement is .the most desirable. The tendency for the proppants to embed will however be increased as the operator reduces the amount of proppant placed within the fracture. To maintain a high capacity fracture after subsequent hydrothermal treatments it therefore becomes necessary to condition, to a greater hardness, the fracture faces of the reservoir prior to the hydrothermal treatment.

This invention resides in la method of altering a reservoir martix t-o decrease fracture propp-ant embedment by injecting a heated mixture of ammonia hydrothermal treating fluid down the well bore and into a fr-acture, and -continuing the injection until the formation is heated to a minimum of 300 F. to a desired radial distance from the well bore and for a desired duration.

In the drawing:

The drawing represents a diagrammatic view of a partial vertical section of a well bore and the adjacent formation.

This drawing represents a well bore 2 extend-ing downward from the surface of the earth, through a hydrocarbon-containing formation 4. Casing 6 is run into the 3,384,177 Patented May 21, 1968 well bore 2 with cement 8 placed in the annulus formed between the face of the well bore Z Iand the casing 6. The Well may be completed as an open-hole completion with the casing 6 terminating at a point above the hydrocarbon-containing formation 4 or as a cased-through complet-ion with the casing 6 terminating, as preferred in this invention and las illustrated in the drawing, at a point bel-ow the hydrocarbon-containing formation 4. The upper end of the well will be equipped with n-ormal well head equipment, to effect a closure of the well.

In this cased-through completion, opening 10, known as perforations, notches or windows, will be cut ythrough the casing 6 and the encompassing cement sheath 8. IFractures 12 will be produced through the formation 4 by various fracturing methods known to the oil industry. These fractures 4 will be propped in ian open position with proppants 14.

To harden the matrix in the vicinity of the fracture faces a mixture of heated .ammonia and a suitable hydrothermal treat-ing uid is injec-ted down the Well and into the hydrocarbon-containing formation 4 through the formation fractures 12. Hy'drothermal treating uid as used in this invention refers to any aqueous liquid utilized to heat the formation. It is preferred in this treatment to install tubing 16 through the casing 6 and to seal the 4annulus created by the casing 6 and the tubing 16 with a packer 18 positioned above the openings 10 to the hydrocarbon-containing form-ation 4.

Injection of the heated ammonia mixture is continued until the formation is heated a desired radial distance from the Well bore 2, preferably 50-75 feet from the well bore. As the heated ammonia mixture is brought in contact with the hydrocarbon-containing formation 4 various minerals contained in the Iformation matrix 4 will undergo a conversion whereby certain of these minerals crystalline form will be altered. This zone of ammonia treated matrix 24 exists as an area harder than the virgin matrix prior to ammonia treatment. The hardened matrix 24 will extend a suicient depth into the formation to support the overburden pressure `and prevent embedment of the proppants after hydrothermal treatment. By so preventing embedment of the proppant the Width of the fracture 12 will thereby be maintained.

In the formation treatment of this invention the injected heated ammonia and hydrothermal treating fluid forms a 10.0- to 10.5 angstrom ammonium mica phase from the original reacting minerals of the formation matrix. In addition some analcite phases may be produced. These irreversible mineral phases are non-expandable and will not cause matrix softening when in contact with water or reservoir iiuids.

The ammonia utilized to form the ammonia mixture of this invention may be injected in .the form of either gas or liquid. The hydrothermal treating fluid which operates as the heating medium and transporting material of the ammonia may be any fluid which an operator would normally consider to use as a hydrothermal treating liquid. yExamples of such fluids are steam, water, oilwater emulsions, and other aqueous fluids.

The variables encountered in utilizing this invention are the temperature to which the matrix should be subjected, the ammonia concentration and the duration of treatment. lt has been found that the formation matrix to be hardened should be heated by the ammonia mixture to a minimum temperature of 300 F. The amount of ammonia necessary to achieve the hardening reaction is found to be at least one milliequivalent oframmonia per milliequivalent of montmorillonite which would be synthesized by hydrothermal treatment. This amount of ammonia in a treating fluid mixture must however be of sufficient concentration to raise the pH of the mixture to at least 11.0. The minimum time to achieve an effective matrix hardening by this invention is the subjection of the reservoir matrix to a minimum temperature of 300 F. at a minimum ammonia concentration for a minimum period of at least l2 hours.

In a majority of reservoirs the preferred range of these variables would ybe to subject the reservoir to a range of 0.02 to 0.05 pound of ammonia per pound of hydrothermal treating fluid at a temperature between 500 to 700 F. for a duration of 48 to 72 hours. These preferred ranges are not proposed to be limits for the variables of this treatment, but merely are disclosed as preferred ranges for the operation of this invention. It has been found that any excess in the duration, concentration or temperature, above the minimum limits prescribed, will enhance the treatment.

The utilization of `water as the hydrothermal treating fluid and mixing this water with a minimum amount of 0.013 pound of ammonia per pound of water will produce a fluid whose pH is in excess of 11.0. Any excess of ammonia above this amount will produce a treating mixture possessing a higher pH. A treating mixture whose pH is lower than 11.0 will seriously reduce the effectiveness.

The method of treating fracture faces of this invention is applicable where a hydrothermal `treatment is to be performed on a fractured well. This invention more specifically is applicable in sandstone formations.

The various minerals constituting the formation matrix are the constituents which give the matrix its hardness properties. This hardness results from the cementation of the minerals within the matrix, the orientation of .the minerals crystalline structure and each minerals own hardness property. Certain of these minerals will be caused to go into solution as the heat and liquid of a hydrothermal treatment comes in contact with this matrix material. The minerals in solution will then exist within the reservoir to form precipitants or aid in the synthesis of unwanted mineral structures.

When such mineral precipitants or unwanted mineral structures are produced by hydrothermal atmospheres, the reservoir matrix is softened to such an extent that the in place proppants embed in the fracture faces when overburden loads are placed upon the fracture. This proppant embedment drastically reduces the permeability and porosity of the fracture. The flow capacity of the fracture is thereby reduced. In more extreme softening conditions the fracture may completely close thereby severely restricting the productivity of the well.

In formations containing silica, non-expandable clay forms and traces of carbonates, a hydrothermal treatment will dissolve a portion of these minerals and convert a further portion into a new expandable form of clay mineral called montmorillonite. As the montmorillonite t is formed there will be a destruction of the clay or feldspar Imineral groups. Mineralogists term this reaction a hydrothermal synthesis of montmorillonite. This new synthesized montmorillonite, owing to its water adsorption and lswelling properties, is expandable and possesses such low structural hardness that proppants embed into the fracture faces with resultant closure of the fracture.

Since the majority of hydrocarbon-containing formations contain minerals that are capable of such montmorillonite synthesis, it is advisable for an operator to prevent softening of the formation fracture faces. The formations which will be softened by hydrothermal treatment may be hardened and prevented from softening by the application of this invention. The ammonia treatment of this invention will also be useful where it is desired to harden the matrix prior to placing proppants into a fracture. In the event one or more of the minerals which cause softening are not present in the formation, these missing minerals may be added by injecting them, in a colloidal suspension or solution, into .the formation.

As an example of the hardening produced `by treating a reservoir matrix by this invention the following penetrometer hardness tests were conducted on various sandstone cores which previously had been recovered during drilling operations.

These `tests were conducted by cutting a sample of the core perpendicular to the bedding plane. Formation brine conditions were simulated by saturating the core with a five percent sodium chloride solution. A 0.072 inch ball point penetrometer was utilized to measure the depth of penetration perpendicular to the bedding plane. The penetration depth of this ball joint penetrometer was directly recorded.

From the recorded penetration depth the diameter of the impression made into the core sample was calculated. A plot was then made on log log paper of the diameter of impression divided by the diameter of the ball point penetrometer versus the load divided by the square of the diameter of the ball point penctrometer. The hardness of the sample therefore is the value of the load divided by the square of the diameter of the ball point pentrometer corresponding to the point at which the diameter of impression divided by the diameter of the ball point pentrometer equals one, this representing total embedment.

Stream treatment of the cores was accomplished by stream heating each core in a pressure bomb to 575 F. for a duration of 96 hours.

Ammonia treatment mixture of this invention was accomplished by treating each core in a pressure bomb. The cores were heated to 575 F. for 96 hours by a steam mixture consisting of ammonia and water having a pH in excess of 11.0.

In all tests it will be noted that the hardness of each treated core, after subsequent steam treatment, was greater than the hardness of the original Virgin matrix.

Other tests have been conducted on cores using sodium and potassium hydroxide solutions. It was noted in these tests that the rock was hardened by both sodium treatments and potassium treatments. The hardness achieved however was not retained as in the ammonia tests and both the sodium and the potassium had upper concentration limits at which the rock was made softer than the virgin matrix.

I'IARDNESS TESTS ON SNDSTONI CORES The treatment of a fractured hydrocarbon-containing formation to a minimum temperature of 300 F. with an ammonia mixture having a pH of at least 11.0 for a minimum duration of 12 hours will harden the formation matrix in the vicinity of the fracture faces to an extent that proppant embedment will be reduced. By maintaining the fracture width, after a subsequent hydrothermal treatment, the operator may thereby maintain the flow capacity of the fracture without experiencing the detrimental effects of reduced permeability or porosity which are generally associated with hydrothermal treatment.

Therefore we claim:

l. A method for altering the matrix of a hydrocarboncontaining formation to decrease further fracture proppant. embedment in a fracture having a proppant deposited therein, said fracture extending into the reservoir from a well penetrating the reservoir, said method cornprising injecting a heated mixture of ammonia and aqueous fluid containing at least 0.013 pound of ammonia per pound of aqueous transporting fluid down the well bore and into the formation and continuing injection until the formation matrix adjacent the fracture is heated to a minimum of 300 F. to a desired radial distance from the well bore and for a period whereby the formation is hardened.

2. A method as set forth in claim 1 wherein the injection of the heated ammonia mixture is continued until the formation matrix adjacent the fracture is heated to a temperature of at least 300 F. to the radial extent of each fracture.

3. A method as set forth in claim 1 wherein the injection of the heated ammonia mixture is continued until the formation matrix adjacent the fracture is heated to a radial distance of 50 to 75 feet from the well bore.

4. A method as set forth in claim 1 wherein the injection of the heated ammonia mixture is followed by a hydrothermal treatment, said injection of ammonia mixture, and hydrothermal treating being continued to extend substantially the same distance from the well bore.

S. A method for hardening a reservoir matrix penetrated by a well bore extending from the surface of the earth downward through said matrix, comprising injecting a heated mixture of ammonia containing at least 0.013 pound of ammonia per pound of aqueous transporting fluid down the well bore and into the matrix, and continuing injection until the formatiion matrix desired to be hardened is heated to a minimum temperature of 300 F. for a desired duration.

6. A method as set forth in claim 5 wherein the ammonia mixture possesses a pH of at least 11.0.

7. A method for hardening, to minimize fracture proppant embedment, a reservoir matrix penetrated by a well bore extending from the surface of the earth downward through said reservoir matrix comprising fracturing the matrix, placing proppants within the fracture, and heating the matrix to a minimum temperature of 300 F. for a desired duration with a heated mixture of ammonia and aqueous fluid containing at least 0.013 pound of ammonia per pound of aqueous transporting uid.

8. A method for altering, to decrease further fracture proppant embedment, a reservoir matrix penetrated by a well bore extending from the surface of the earth downward through said reservoir matrix, said matrix having a fracture extending through said reservoir matrix from the well and propping agents contained within the fracture maintaining the fracture in an open position, comprising injecting a mixture of ammonia and an aqueous transporting uid containing at least 0.013 pound of ammonia per lpound of transporting fluid, injecting the mixture down the well bore and into the formation at a temperature of at least 350 F., and continuing injection until the formation matrix adjacent the fracture is heated to a temperature of at least 300 F. to a desired radial distance from the well bore and for at least l2 hours.

9. A method for altering, to decrease further fracture proppant embedment, a reservoir matrix penetrated by a well bore extending from the surface of the earth downward through said reservoir matrix, said matrix having a fracture extending through the matrix from the well and propping agents contained within the fracture, maintaining the fracture in an open position, comprising injecting a heated mixture of ammonia and an aqueous transporting fluid containing at least 0.013 pound of ammonia per pound of transporting fluid, injecting the mixture down the well bore and into the formation, continuing injection until the formation matrix adjacent the fracture is heated to a temperature in the range of from 500 to 700 F. to a desired radial distance, and maintaining this temperature for a duration in excess of two days.

10. A method for altering, to decrease further fracture proppant embedment, a reservoir matrix penetrated by a well bore extending from the surface of the earth downward through said reservoir matrix, said matrix having a fracture extending through the matrix from the well and propping agents contained within the fracture, maintaining the fracture in an open position, comprising mixing an aqueous treating fluid with at least one milliequivalent of ammonia per milliequivalent of montmorillonite which would be synthesized by hydrothermal to produce an ammonia mixture having a pH of at least 11.0, heating the ammonia mixture, injecting the heated ammonia mixture down the well bore and into the formation, continuing injection until the formation matrix adjacent the fracture is heated to a temperature higher than 300 F. to a desired radial distance from the well bore, and maintaining this temperature for a duration longer than two days.

11. A method as set forth in claim 6 wherein the ammonia mixture contains 0.02 to 0.05 pound of ammonia per pound of transporting fluid.

12. A method for altering, to decrease further fracture proppant embedment, a reservoir matrix penetrated by a Well bol'e extending from the surface of the earth downward through said formation matrix, said matrix having a fracture extending through the matrix from the well and propping agents contained within the fracture, maintaining the fracture in an open position, comprising mixing an aqueous treating fluid which contains more than 0.013 pound of ammonia per pound of water, heating the ammonia mixture, injecting the heated ammonia mixture down the well bore and into the formation, continuing injection until the formation matrix adjacent the fracture is heated to a temperature higher than 300 F. to a radial distance at which any further hydrothermal treatments are expected to be extended, and maintaining this temperature for a duration longer than two days.

References Cited UNITED STATES PATENTS 3,302,718 2/1967 Prats 166--42 X 3,323,594 6/1967 Huitt 166-42 3,343,600 9/1967 Phansalkar l66-42 3,349,351 l0/l967 Huitt 166-42 CHARLES E. OCONNELL, Primary Examiner.

NILE C. BYERS, IR., Examiner.

Claims (1)

10. A METHOD FOR ALTERING, TO DECREASE FURTHER FRACTURE PROPPANT EMBEDMENT, A RESERVOIR MATRIX PENETRATED BY A WELL BORE EXTENDING FROM THE SURFACE OF THE EARTH DOWNWARD THROUGH SAID RESERVOIR MATRIX, SAID MATRIX HAVING A FRACTURE EXTENDING THROUGH THE MATRIX FROM THE WELL AND PROPPING AGENTS CONTAINED WITHIN THE FRACTURE, MAINTAINING THE FRACTURE IN AN OPEN POSITION, COMPRISING MIXING AN AQUEOUS TREATING FLUID WITH AT LEAST ONE MILLIEQUIVALENT OF AMMONIA PER MILLIEQUIVALENT OF MONTMORILLONITE WHICH WOULD BE SYNTHESIZED BY HYDROTHERMAL TO PRODUCE AN AMMONIA MIXTURE HAVING A PH AT LEAST 11.0, HEATING THE AMMONIA MIXTURE, INJECTING THE HEATED AMMONIA MIXUTRE DOWN THE WELL BORE AND INTO THE FORMATION, CONTINUING INJECTION UNTIL THE FORMATION MATRIX ADJACENT THE FRACTURE IS HEATED TO A TEMPERATURE HIGHER THAN 300*F. TO A DESIRED RADIAL DISTANCE FROM THE WELL BORE AND MAINTAINING THIS TEMPERATURE FOR A DURATION LONGER THAN TWO DAYS.

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