GB2037727A - Sintered spherical ceramic pellets for gas and oil well proppants - Google Patents

Abstract

In a process for manufacturing spherical sintered ceramic pellets e.g. of bauxite, a sinterable ceramic powder composition having an average particle size up to 5 microns is added to a rotatable table mixer provided with a rotatable impacting impeller; the table is rotated at 10 - 60 rpm and the impeller is rotated with a tip speed of 25 - 50 meters per second. Sufficient water is added to cause spherical ceramic pellets to form and 5 to 15 per cent of additional ceramic powder is added. The impeller is then rotated with a tip speed of 5 - 20 meters per second for 1 to 6 minutes while the table is rotated at 10 - 60 rpm. The pellets are dried at 100 - 300 DEG C and subsequently furnaced to maximum density. The sintered pellets may be used as propping agents for propping open fractures in gas and oil wells.

Description

SPECIFICATION Sintered high density spherical ceramic pellets for gas and oil well proppants and their process of manufacture This invention relates to oil and gas well proppants and more particularly relates to sintered ceramic proppants and a method for maintaining a fracture in a subterreanean formation in a propped condition by utilizing the proppant.

Oil and natural gas are produced from wells having porous and permeable subterranean formation. The porosity of the formation permits the formation to store oil or gas and the permeability of the formation permits the oil or gas fluid to move through the formation. Permeability of the formation is essential to permit oil and gas to flow to a location where it can be pumped from the well. Sometimes, the permeability of the formation holding the gas or oil is insufficient for economic recovery of oil and gas. In other cases during operation of the well, the permeability of the formation drops to the extent that further recovery becomes uneconomical. In such cases, it is necessary to fracture the formation and prop the fractures in an open condition by means of a proppant material or propping agent.Such fracturing is usually accompished by hydraulic pressure and the proppant material or propping agent is a particulate material such as sand, glass beads or ceramic particles which are carried into the fracture by means of a fluid.

Spherical particles of uniform size are generally acknowledged to be the most effective proppants due to maximized permeability. For this reason, assuming other properties to be equal, spherical or essentially spherical proppants such as rounded sand grains, metallic shot, glass beads and fused tabular alumina are preferred.

Unfortunately, in deep wells, where high pressures are encountered; e.g., above about 5000 psi, the foregoing specifically mentioned proppants are either entirely ineffective or have greatly reduced permeability. The best of the foregoing specifically mentioned propants at high pressures, as disclosed in U.S. Patent 3,976,138 to Colpoys, Jr. et al, is fused alumina. However, even fused alumina, as disclosed in U.S. Patent 3,976,138, has dramatically reduced permeability at pressures in excess of 5000 psi.

As disclosed in U.S. Patent 4,068,718 to Cook, Jr. et al, it has recently been discovered that sintered bauxite unexpectedly has a permeability which is superior to the previously mentioned proppant materials at pressures as high as 10,000 psi or higher.

Unfortunately, the sintered bauxite material actually used in making the measurements disclosed in U.S.

Patent 4,068,718, does not have the most desired spherical shape for highest permeability since prior to the present invention, it was not possible to commercially manufacture spherical sintered bauxite particles having a specific gravity in excess of about 3.5 which is required to have sufficient compression strength.

The prior art sintered bauxite particles were elongated pellets which were tumbled to form rounded edges in order to increase permeability. In addition, the yields of such pellets were low compared to the amount of material processed. Rolling the particles prior to sintering, as disclosed in column 4 of U.S. Patent 4,068,718, was entirely ineffective since particles having insufficient density were invariably obtained.

There is therefore provided, in accordance with the present invention, spherical ceramic pellets or particles having densities in excess of 95 percent of the theoretical density of the ceramic material, which spherical particle is useful as an oil and gas well proppant and which may be additionally useful, in certain circumstances, as a lubricant, abrasive, filter media, catalyst support material or bearing material. When the particle is used as a proppant, the ceramic material is preferably sintered bauxite, although other ceramics such as aluminium silicate clays containing aluminium and iron may be used.

The invention further includes a process for propping fractures in oil and gas wells utilizing the particles of the invention by introducing the particle of the invention was proppant or propping agent into a fluid such as oil or water and introducing the propping agent containing fluid into a fracture in the subterranean formation containing the well, the compaction pressure upon the fracture being at least 4,000 psi and usually 10,000 psi or higher, said propping agent having an average particle size between 0.1 and 2 millimeters. It has been found that permeability at 4,000 psi or greater is superior to the permeability of prior art essentially spherical sintered bauxite material.

The invention additionally includes a novel efficient process for manufacturing the sperical sintered ceramic pellet in accordance with the invention which comprises adding a sinterable ceramic powder composition, which is most desirably bauxite when the pellet is to be used as a proppant material, having an average particle size of between 0 and about 5 microns to a rotatable table mixer provided with a rotatable impacting impeller which is the same or similar to the one described in U.S. Patent 3,690,622.

The table is rotated at from about 10 to about 60 rpm and the impacting impeller is rotated to obtain an impeller tip speed of from about 25 to about 50 meters per second. Sufficient water is added to cause essentially spherical ceramic pellets to form and after such pellets have formed, from about 5 to about 15 percent of additional ceramic powder by weight of pellets is added and the impeller is rotated at a tip speed of between about 5 and about 20 meters per second for from about 1 to about 6 minutes while rotating the table at from about 10 to about 60 rpm.

The resulting pellets are then dried at between about 100 and about 300 degrees centigrade and furnaced at sintering temperature until maxium density is obtained.

The sintered ceramic pellets, in accordance with the invention, have a density in excess of 95 percent of the theoretical density of the ceramic and are spherical in shape, meaning that the average ratio of the minimum diameter to maximum diameter of pellets (sphericity) of repeated random samples of pellets manufactured in accordance with the present invention is greater than 0.82 almost always greater than 0.85 and frequently greater than 0.9 with 95 percent confidence limits.

In contrast, the average ratio of the minimum diameter to maximum diameter of extruded and tumbled sintered bauxite pellets in accordance with the prior art is generally less than about 0.80.

"Essentially spherical", as used herein, is intended to mean an average ratio of minimum diameter to maximum diameter of between 0.7 and 0.82.

"Spherical", as used herein, is intended to mean an average ratio of minimum diameter to maximum diameter of greater than 0.82.

The spherical pellets, in accordance with the present invention, are manufactured by sintering a ceramic powder composition. The spherical pellets are not manufactured, as in prior art, by fusion of ceramic material followed by solidification. The ceramic powder may be any sinterable ceramic powder such as powders of bauxite and silicon carbide. If desired, sintering aids may in incorporated as a part of the sinterable ceramic powder. For example, when bauxite is used, bentonite clay or iron oxide aids sintering, when silicon carbide is used, boron, boron cabide, aluminium diboride, boron nitride, boron phosphide and other boron compounds aid sintering and when aluminium silicate type clays are used, fluxes such as iron oxide aid sintering.Between 0 and 30 weight percent of such sintering aids may be used. the most desirable range of sintering aid can be readily determined by those skilled in the art depending upon the particular ceramic and aid used. For example, from 0 to 8, preferably 0 to 3 percent and most preferably between 0.2 and 1 percent by weight of bentonite aids the sintering of bauxite, from 0.4 to 5 percent of a sintering aid such as a boron containing compound is required for sintering of silicon carbide and up to 30 weight percent of a flux material such as sodium carbonate, lithium carbonate, feldspar, manganese oxide, titania,iron oxide and sodium silicates aid sintering of aluminium silicate clays.

The process comprises adding a sinterable ceramic powder composition having an average particle size of from between 0 and about 5 microns to a rotatable table mixer provided with a rotatable impacting impeller.

The table may be somwhat inclined from the horizontal. The small particled size is required in order to obtain a finished spherical sintered ceramic pellet having sufficient density. A ceramic powder average particle size of even smaller than four microns is desirable and the average particle size is preferably below 3 microns and usually above 0.5 microns.

The rotatable table mixer provided with the rotatable impacting impeller can be any such device such as the device obtainable from Eirich Machines Inc. known as the Eirich Mixer. Such a device is provided with a flat or inclined circular table which can be made to rotate at a speed of from about 10 to about 60 revolutions per minute (rpm) and is provided with a rotatable impacting impeller which can be made to rotate at a tip speed of from about 5 to about 50 meters per second. The central axis of the impacting impeller is generally located within the mixer at a position off center from the central axis of the rotatable table. The table may be in a horizontal or inclined position wherein the incline, if any, is between 0 and 35 degrees from the horziontal.

After the sinterable ceramic powder composition is added to the mixer, the table is rotated at from about 10 to about 60 rpm and preferably from about 20 to about 40 rpm and the impacting impeller is rotated to obtain a tip speed of from about 25 to about 50, preferably 25 to about 35, meters per second and sufficient water is added to cause essentially spherical ceramic pellets of the desired size to form. If desired, the impeller may be initially rotated at from about 5 to about 20 meters per second during addition of one-half of the sufficient water and subsequently rotated at the higher tip speed of 25 to about 50 meters per second during the addition of the balance of the water.

In general, the total quantity of water which is sufficient to cause essentially spherical ceramic pellets to form is from about 17 to about 20 percent by weight of the ceramic powder and usually between about 18 and about 20 percent by weight of the ceramic powder. The total mixing time after addition of an initial quantity of the sufficient water to the formation of essentially spherical pellets of the desired size is from about 2 to about 6 minutes.

From about 5 to about 15 percent and preferably from about 8 to about 10 percent of additional ceramic powder by weight of pellets is then added, followed by rotating the impeller at a tip speed of between about 5 and about 20 meters per second, preferably between about 10 and about 20 meters per second for from about 1 to about 6 minutes while continuing to rotate the table at from about 10 to about 60 rpm and preferably from about 20 to about 40 rpm.

If desired, the rotation of the impeller may then be stopped while the table continues to rotate for between about 1 and about 5 minutes.

The impacting impeller is preferably a disc provided with peripheral rods or bars attached to the disc. The longitudinal axis of the rods or bars is desirably essentially parallel with the axis of rotation of the impeller, which is usually a vertical axis. The diameter of the impeller is measured from the axis of rotation to the center of the most distant rod or bar. Tip speed is the speed of the most distant rod or bar.

The diameter of the impeller depends upon the size of the mixer but is usually less than 25 percent of the diameter of the mixer. The impeller in most applications is between 10 and 100 centimeters in diameter and usually rotates at from 200 to 3750 rpm at the lower tip speeds of 10 to 20 meters per second depending upon impeller diameter and at from 500 to 6500 rpm at the higher tip speeds of 25 to 35 meters per second depending upon impeller diameter.

The mixer may also be provided with a deflector plate to deflect ceramic material from the mixer wall and preferably to the impeller.

The resulting pellets are dried at a temperature of between about 100 and about 300"C until preferably less than 3 percent and most preferably less than 1 percent moisture remains in the pellets. The most preferred drying temperature is between about 175 and 275"C and the drying time is usually between about 30 and about 60 minutes. The pellets are then furnaced at sintering temperature until maximum density is obtained.

In the case of bauxite having less than about 90 percent alumina with substantial quantities of Fe2O3, SiO2 and TiD2, the furnacing usually occurs at a temperature of between about 1.450 C and 1,550 C for from about 1 to about 10 minutes and preferably occurs at from about,1485"C to about 1,515"C for from about 2 to about 4 minutes.

The density which is obtained is in excess of about 95 percent of the theoretical density of the ceramic material used. The theoretical density of bauxite varies somewhat due to the somewhat different bauxite compositions occurring in nature; however, the theorectical density of bauxite is usually about 3.72 and the maximum density obtained as a result of the process of the invention exceeds 3.57 and usually is between about 3.60 and 3.68. Since tumbling for 10 minutes to 1 hour substantially enhances such smoothness, the finished pellets can be tumbled if desired.

When the pellets are used as a propping agent for increasing permeability in a subterranean earth formation penetrated by a well, the spherical pellets are introduced into a fluid and the fluid containing the propping agent is introduced into a fracture which has a compaction pressure of at least 4,000 psi, to deposit a propping distribution of the propping agent in the fracture. The propping distribution is usually, but not necessarily, a multilayer pack and the overall particle size of the propping agent is between 0.1 and 2 millimeters.

The following examples serve to illustrate and not limit the invention. Unless otherwise indicated, parts and percentages are be weight.

Example 1 About 135 kilograms of Surinam bauxite powder having an average particle size of less than 4 microns were added with about 1.35 kilograms of bentonite clay powder to an Eirick mixer having a pan diameter of about 115 centimeters, an operating capacity of about 160 kilograms and an impacting impeller diameter of about 27 centimeters.

The pan was rotated at about 35 rpm and the impeller was rotated at about 1090 rpm and about 82 kilograms of water was added. Rotation of the pan and impeller continued for about 1 minute followed by increasing the impeller speed to about 2175 rpm and an additional 82 kilograms of water was added. The pan and impeller continue to rotate until seed pellets are formed which contain less than 5 percent pellets of a size smaller than 35 mesh. (about 3 minutes) The impeller is then reduced to about 1090 rpm and about 9 pounds of the foregoing bauxite powder containing 0.5 percent bentonite clay is added. Rotation of the pan and impellerthen continues for an additional 2 minutes to form spherical pellets.

The pellets are then dried for about 30 minutes at about 225"C in a rotating tray dryer and are then fired at about 1 500"C for about three minutes. The yield of useful pellets having a size between 14 and 60 grit is greater than 90 percent. The resulting pellets have a density of about 3.64 and a sphericity of about 0.9.

Example 2 About 450 kilograms of Surinan bauxite powder having an average particle size of less than 4 microns were blended with about 4.5 kilograms of Bentonite clay and about 164 kilograms of water in a mix muller.

The resulting putty-like material is then extruded through an eight inch extruder at room temperature with a water cooled barrel. The die size corresponds to the diameter of a 10 to 12 grit particle. The resulting extruded material is dried for one hour at 260or and granulation is accomplished in a Stokes reciprocating granulator followed by a four minute pass through a Blunger. Dust is removed by sifting and rounded shapes are generated by tumbling. The resulting pellets are sintered as in Example 1.

The yield of useful pellets having a size between 14 and 60 grit is less than 80 percent. The resulting pellets have a density of about 3.64 and have a sphericity of less than about 0.8.

Example 3 Bauxite powder containing 3 weight percent bentonite was pelletized by rolling in a Ferro-Tech pelletizer and fired in a rotary kiln at 1 525"C for 3 minutes. 1525"C in a rotary kiln was insufficient for densification as indicated by the white colour.

Pellets were then fired at a gas kiln for one hour at 1525"C. This firing treatment resulted in apparent specific gravity of 3.30.

Example 4 The permeability in darcies of the pellets manufactured in accordance with Example 1 was compared with the permeability in darcies of the pellets manufactured in accordance with Example 2 in 2% KCL solution at 200"F at various applied pressures. The results are shown in Table A.

TABLE A Pressure Permeability Permeability psi Example 1 Example 2 2000 285 304 4000 274 251 6000 248 242 8000 233 222 10000 222 213 This example clearly shows that the pellets in accordance with the invention show between about 4 and above 9% better flow at pressures of 4000 psi to 10,000 psi.

EXAMPLES The pellets of Example 1 were compared for compressibility with the pellets of Example 2 at 16,000 psi under a piston in a confined tube. A bed of about 0.25 inch of the pellets of the invention in the test showed a compressibility of between 40 and 56 thousandths of an inch; whereas, the pellets of Example 2 under the same conditions and thickness showed a compressibility of between 52 and 60 thousandths of an inch.

Claims (31)

1. A process for manufacturing spherical sintered ceramic pellets comprising: (a) adding a sinterable ceramic powder composition having an average particle size of between 0 and about 5 microns to a rotatable table mixer provided with a rotatable impacting impeller; (b) rotating said table at from about 10 to about 60rpm and rotating said impacting impeller to attain a tip speed of from about 25 to about 50 meters per second; (c) adding sufficient water to cause spherical ceramic pellets to form; (d) adding from aout 5 to about 15 percent of additional ceramic powder by weight of pellets; (e) rotating said impeller at a tip speed of between about 5 and about 20 meters per second forfrom about 1 to about 6 minutes while rotating said table at from about 10 to about 60 rpm; (f) drying said pellets at between about 100 and about300 C;; and (g) furnacing said pellets at sintering temperature until maximum density is obtained.

2. A process as claimed in claim 1 wherein prior to drying the pellets, the rotation of the impeller is stopped while the table continues to rotate for between 1 and about 5 minutes.

3. A process as claimed in claim 1 or 2 wherein said mixer is also provided with at least one deflector plate to deflect ceramic material from the mixer wall.

4. A process as claimed in any of claims 1 to 3 wherein between about 0 and about 30 weight percent of a sintering aid is added to the ceramic material.

5. A process as claimed in any one of claims 1 to 4 wherein the sinterable ceramic powder is selected from bauxite, silicon carbide and aluminium silicate clay.

6. A process as claimed in any one of claims 1 to 5 wherein said table is rotated at from about 20 to 60 rpm.

7. A process as claimed in any one of claims 1 to 6 wherein said impeller is rotated at a tip speed of from about 25 to about 35 meters per second prior to the adding of said additional ceramic powder and from about 10 to about 20 meters per second after the adding of said additional ceramic powder.

8. A process as claimed in claim 7 wherein the diameter of said impeller is from about 10 to about 100 centimeters and prior to the adding of said additional ceramic powder, rotates at from about 500 to about 6500 rpm and subsequent to the adding of said additional ceramic powder rotates at from about 200 to about 3750 rpm.

9. A process as claimed in any one of claims 1 to 8 wherein from about 8 to about 10 percent of additional ceramic powder by weight of pellets is added.

10. A process as claimed in any one of claims 1 to 9 wherein subsequent to the addition of said additional ceramic powder, said impeller is rotated for from about 2 to about 4 minutes before the rotation of the impeller is stopped.

11. A process as claimed in any one of claims 1 to 10 wherein the pellets are dried at a temperature of between about 175 and about 2750C in a rotating tray dryer for from about 30 to about 60 minutes.

12. A process as claimed in claim 1 wherein said ceramic composition is bauxite and said furnacing temperature is from about 1485"C to about 1515"C and thetime offurnacing is from about 2 to about 4 minutes.

13. A process as claimed in any one of claims 1 to 12 wherein the pellets are tumbled for 10 minutes to about one hour subsequent to firing.

14. A process for manufacturing spherical sintered bauxite pellets comprising; (a) adding bauzite powder having an average particle size of between 0 and about 4 microns to a rotatable inclined table mixer provided with a rotatable impacting impeller; (b) rotating said table at from about 10 to about 60 rpm and rotating said impacting impeller at a tip speed from about 25 to about 50 meters per second; (c) adding sufficient water to cause essentially spherical bauxite pellets to form; (d) adding from about 5 to about 15 percent of additional bauxite powder by weight of pellets; (e) rotating said impeller at a tip speed between about 5 and about 20 meters per second for from about 1 to about 6 minutes while rotating said table at from about 10 to about 60 rpm; (f) drying said pellets at between about 100 and 300"C;; and (g) firing said pellets at sintering temperature until maximum density is obtained.

15. A process as claimed in claim 14 wherein prior to drying the pellets, the rotation of the impeller is stopped while the table continues to rotate for between 1 and about 5 minutes.

16. A process as claimed in claim 14 or 15 wherein between about 0.2 and 1 percent of bentonite clay by weight of bauxite is blended with the dry bauxite prior to forming approximately spherical bauxite pellets.

17. A process as claimed in any one of claims 14 to 16 wherein said sufficient water comprises from about 17 to about 20 weight percent by weight of bauxite.

18. A process as claimed in any one of claims 14to 17 wherein said rotatable impacting impeller is located at a position other than at the center of the table.

19. A process as claimed in any one of claims 14to 18 wherein the surface of said table is inclined at an angle of from 10 to 35 from the horizontal.

20. Spherical sintered ceramic pellets having a density in excess of 95% of the maximum theoretical density of the ceramic wherein repeated random samples of such pellets indicate an average sphericity of greater than 0.82 with 95% confidence limits.

21. Spherical sintered bauxite pellets having a density in excess of about 3.57 grams per cubic centimeter wherein repeated random samples of such pellets indicate an average sphericity of greater than 0.82 with 95% confidence limits.

22. Spherical sintered ceramic pellets in accordance with claim 20 wherein repeated random samples of such pellets indicates an average sphericity of greater than 0.85 with 95% confidence limits.

23. Spherical sintered ceramic pellets in accordance with claim 22 wherein repeated random samples of such pellets indicates an average sphericity of greater than 0.9 with 95% confidence limits.

24. Sphercial sintered bauxite pellets in accordance with claim 21 wherein repeated random samples of such pellets indicates an average sphericity of greater than 0.85 with 95% confidence limits.

25. Spherical sintered bauxite pellets in accordance with claim 24 wherein repeated random samples of such pellets indicates an average sphericity of greater than 0.9 with 95% confidence limits.

26. A method for increasing permeability in a subterranean earth formation penetrated by a well wherein a fluid is pumped into the well to create a fracture therein, the improvement which comprises introducing a spherical sintered ceramic propping agent having a density in excess of 95% of the theoretical density of the ceramic into a fluid; introducing said propping agent containing fluid into a fracture, the compaction pressure of which is at least 4,000 psi, to deposit a propping distribution of said propping agent, said propping agent having an average particle size between 0.1 and 2 millimeters.

27. A method as claimed in claim 26 wherein the propping agent has been produced by the process of any one of claims 1 to 19.

28. A method as claimed in claim 26 wherein the propping agent is a spherical sintered ceramic pellet as claimed in any one of claims 20 to 25.

29. A process for manufacturing spherical sintered ceramic pellets substantially as hereinbefore described with reference to Example 1.

30. Spherical sintered ceramic pellets substantially as hereinbefore described.

31. A method for increasing permeability in a subterranean earth formation substantially as hereinbefore described.