US3362475A - Method of gravel packing a well and product formed thereby - Google Patents

Description

Jan. 9, 1968 .1. l.. Hul'r'r ETAL 3,362,475

l METHOD OF GRAVEL PACKING A WELL AND PRODUCT FORMED THEREBY Filed Jan. 1l, 1967 2 Sheets-Sheet 1 INVENTORS .//MM/E 1.. ,uu/7'7- BRUCE a. M.- aoroz//V Jan. 9, 1968 J. l.. HUITT ETAL 3,362,475

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United States Patent O 3,362,475 METHOD F GRAVEL PACKING A WELL AND PRODUCT FORMED THEREBY `limrriie L. Huitt, Glenshaw, and Bruce B. McGlothlin, OHara Township, Allegheny County, Pa., assignors to Gulf Research & Development Company, Pittsburgh,

Pa., a corporation of Delaware Filed `lan. 11, 1967, Ser. No. 613,718 Claims. (Cl. 166-20) ABSCT GF THE DHSCLOSURE A method of gravel packing a well to prevent flow of formation sands into the Well in which substantially spherical glass particles are packed in -a cavity surrounding the well bore in the producing formations penetrated by the well. The size of the glass particles in the glass pack decreases as the distance from the borehole increases. The density of the liquid used to carry glass particles into place in the gravel pack is controlled to aid in deposition of the particles against the wall of the cavity.

This application is a continuation-in-part of Ser. No. 413,460 of Jimmie L. Huitt and Bruce B. McGlothlin, entitled Method and Product, led Nov. 2.4, 1964, now abandoned.

This invention relates to a method for controlling oil producing sand formations penetrated by a well bore and it particularly relates to a novel gravel pack of high permeability and high integrity and to a method of forming this new gravel pack.

Oil bearing sand formations frequently disintegrate and produce sand in the well bore as a result of insuthcient cohesion between adjacent sand particles under the conditions prevailing in the well. These formations may be so weakly consolidate that disintegration begins during the drilling or well completion operations as the internal stresses created by the weight of overburden are relieved. On the other hand a formation may be suciently consolidated to withstand drilling and well completion operations but will begin to produce sand after the well is brought into production as the stress of the flowing reservoir fluid adds to the natural reservoir stresses to overcome the weak binding forces between the sand grains. This disintegration of weak formations is a serious problem. The production of sand with reservoir oil may severely erode the production equipment including tubing, valves and pumps and may eventually plug up the well. The oil and sand mixture produced from these wells requires separation above ground which represents an economic burden. On occasion massive cave-ins of subsurface formations occur; thereby damaging the well and equipment and necessitating the abandonment of the well.

Gravel packing of loosely consolidated sand formations is one approach which has been used with varying success for their support to prevent the sanding-up of wells and the possible destruction of the well. In this general method a cavity in the formation adjacent the production casing is formed. Gravel is then packed into this cavity. The gravel particles are usually selected to have a relatively uniform diameter several times the diameter of the sand particles at a specific point, usually the 10 percentile point (the size of Athe smallest particles in the coarsest 10 percent by weight of formation sand), in the sieve analysis. Since the openings in the gravel pack are larger than most of the sand particles, reliance is placed upon the tendency of the sand grains to bridge across the openings t-o stabilize the formation. Heretofore, it has not been possible to place a gravel pack of predictable permeability and flow characteristics. Newly installed packs ordinarily are interlaced with voids which collapse during oil production. In addition, failure in bridging occurs with substantial migration of particles into the pack accompanied by plugging of the pack and a substantial reduction in its permeability. It is believed that these pack defects in part result from the initial inclusion in the pack of irregular sized and shaped particles.

Gravel entrained in a carrier liquid is substantially fractured and broken up in the process of pumping it under pressure into a gravel pack. Thus, an ordinary gravel of uniform size above ground will enter the pack intermixed with irregular fragments or lines from broken gravel to form a mixed pack of low permeability and interlaced with voids resulting from bridging of the jagged fragments. The void zones present no support to the formation and permit the production of formation sand in the well while the remainder of the pack of initial low permeability is easily plugged with formation lines for a further reduction of permeability.

We have now discovered, contrary to prior experiences, that gravel packs can be produced having a good and predictable permeability and characterized by a uniformity without voids or plugs. In addition, we have discovered that these packs will maintain their integrity while in production over long periods of time. Furthermore, these gravel packs will maintain their integrity when subjected to pressures approximating the overburden pressure and support the formation against movement into the cavity under this large hydrostatic pressure. As used herein high integrity of the gravel pack refers to its substantially complete fulfillment of the characteristics and functions projected for it. It is characterized by a controlled uniformity of gravel size without fragments, voids, plugs or other nonuniformities which characterize ordinary gravel packs. It is further characterized by a predetermined, good permeability which remains substantially constant over long use without plugging or physical breakdown regardless of the pressure to which it is subjected.

Our gravel pack is formed of graded special hardened glass pellets deposited in layers concentric with the well axis. The initial layer adjacent the cavity wall is formed with the smallest of the uniformly sized pellets. Each successive layer in a direction towards the well bore axis is formed from glass pellets of increasing size. By this arrangement we are able to combine the high filtering advantages of the small pellets contacting the formation with the high permeability of the relatively large pellets adjacent the production tubing with its higher flow rates to form a body of relatively high overallpermeability. When our hardened glass spheres are pumped with a carrier liquid into the well in accordance with our invention they enter the pack in their original form without any significant breakage. Thus, the optimum, a pack of spherically shaped, uniformly sized pellets is now possible. In addition by forming the pack in layers of increasing particle size from the cavity wall to the well bore, a pack is produced having an increasing permeability in the zones of increasing ow rate of reservoir oil. An increase in size of the pore spaces of our pack from layer to layer towards the well bore is a characteristic or our pack. As a result, any formation fines which by chance migrate into the pack will be carried through to the well without being trapped within the pack to form plugs. A further advantage of using these hardened glass pellets is that a pressure approximately the overburden pressure may be applied radially t0 the pack for complete support of the formation against inward movement and cave-in without fracturing the individual glass pellets thereby avoiding a mixed pack with the ultimate destruction of the packs effectiveness. In order to produce the superior pack described herein, the pellets should have a sphericity and roundness of at least 0.8 and preferably as close to 1.0 as is possible, in accordance with the definitions of Krumbein and Sloss at pages 78 through 83 in Stratigraphy and Sedimentation, published by W. H. Freeman Company, 1951 edition. Glass pellets having a roundness and sphericity exceeding 0.9 can readily be manufactured.

A further aspect of our invention is the method of forming this novel pack. In order to form the pack in layers in a direction from the formation to the production tubing we utilize a carrier liquid which will leak off into the formation and deposit the glass pellets in suc` cessive layers on the wall of the cavity. Suitable leak-off liquids are oil, water or weighted water. In order to insure that the glass particles deposit onto the wall of the cavity without settling out we prefer that their density approximates or equals the density of the leak-off liquid. This is accomplished by utilizing low density glass formulations, by incorporating minute gas bubbles in the glass during production, by using high density liquids, or by a combination of these factors. This equalization of density is particularly desirable when only a relatively low flow of leak-off liquid into the formation is possible due to the occurrence of a low permeability formation.

A more detailed explanation of our invention disclosing further features, objects and advantages follows. Only so much of the well and apparatus necessary to illustrate our invention to those skilled in the art is shown in the accompanying drawings.

FIGURE 1 is a vertical sectional view of an open hole completed oil well penetrating an oil bearing formation showing a cavity in the oil bearing formation in readiness for gravel packing;

FIGURE 2 is a vertical sectional view showing the first layer of glass pellets on the cavity wall and the second layer being deposited on the first layer;

FIGURE 3 is a diagrammatic view partially in vertical section showing the gravel pack after all of the special glass pellets have been introduced; and

FIGURE 4 is a vertical sectional view of the completed gravel pack ready for oil production with a production liner inserted through the pack.

In FIGURE l, l0 represents the well bore penetrating overburden 11 and extending down through the oil sands 12 into the underlying formation 13 a short distance. `Casing 14 extends down to the pay zone I2 and is cemented in place within the well bore using standard cementing practices. Cavity 15 for insertion of the gravel pack may have resulted from caving in of the formation or preferably it is intentionally formed by a suitable technique once the need for the gravel pack has been established. For example, the well bore may be enlarged into a cavity by jetting an abrasive-laden fluid against the formation. A jetting tool with horizontally directed jet nozzles is reciprocated through the height of the formation to form a good cavity. An advantage of producing the cavity with a jetting tool is that the weaker portions of the formation are selectively removed. The jetting tool is removed and the cavity is ready for packing.

Prior to initiation of the gravel packing operation formation characteristics have been ascertained, preferably from a core sample, and the gravel pack designed, that is the quantity and sizes of glass pellets has been estimated, `and the pellets are on hand for the packing operation. As indicated it is essential that the leak-off liquid and pellets be so adjusted in density that the rate of ow of leak-off liquid into the formation, as determined by formation characteristics, will be sufficient to hold the pellets in suspension and deposit them `against the cavity wall. The pellets should be within about 50 percent of the density of the leak-off liquid and it is preferred to insure reliability of the pack forming process that the density difference be between about 0 to 20 percent of the liquid density.

This packing is accomplished by entraining the pellets in the liquid as it is pumped to the well head and pumping the mixture down the well. As the mixture reaches the cavity, the liquid will flow through the cavity wall 16 as indicated by the arrows in FIGURE 2 and disappear into the formation depositing and retaining the pellets firmly against the cavity wall. The smallest pellets are entrained in the liquid first and are deposited against the cavity wall to form the layer 17 in FIGURE 2. This is followed as shown in FIGURE 3 by the deposition of a larger size glass pellet 18 which is packed against the first batch 17. The final and largest size pellets are entrained in the leak-off fluid and are packed against the previous layer 1S to fill up the entire cavity as indicated by 19.

In FIGURE 4 slotted liner 20 is washed through the pack by conventional means until it contacts the bottom of the borehole. This washing operation is conducted under overall uid pressure to hold the pack in position. The well is now put into production. The pack as produced possesses good permeability and support for the cavity wall. A cavity and pack may also be formed in a cased-hole completed well behind the casing by washing out the cavity in a conventional manner and injecting the liquid-pellet mixture through the perforations.

As illustrated the gravel pack is formed in three layers of differently sized pellets with the smallest adjacent the cavity wall and the largest adjacent the production liner. It is possible to utilize two or more layers of differently sized pellets, the number of layers depending in part upon the size of the formation particles. The smaller the formation particles, the smaller will be the glass pellets adjacent the formation for effective control. We believe that one of the reasons that prior packing efforts have so fre- -quently proven to be less effective than desired is that the stable lbridging of small particles across large openings is not possible in an environment of shifting pressures and varying flow rates. We do not rely on the bridging phenomenon but instead size the pellets of our first layer so that the openings will be smaller than the formation particles. Thus, it is preferred that the pellet size of the first layer be from about 2 to 4 times the sand size at the percentile point on the formation sieve analysis (US. Standard Sieve Series referred to throughout). At this size it is not possible for any significant quantity of formation sand to enter the pack and the only particles that can be carried through the pack are some of the minute fines and particles of clay and silt. The pellets in the first or outer layer maybe as small as about mesh. Ordinarily pellets having a size in the range of 20-40 mesh or 40-6() mesh are used. For production of an effective pack overall, it is preferred that the pellet size in each succeeding layer be from about 2 to 6 times larger than the preceding layer with the lower ratios producing the more effective pack. A final layer of 4 to 6 mesh size pellets will be found to be very satisfactory for many packs, with the final size depending in part on the particular production equipment utilized. The most effective packs result if the pellets in each layer have a high degree of sphericity and roundness and are substantially equal in size. For example, if analysis suggests a 60 mesh gravel for one of the layers, it is preferred to use a gravel that passes 50 mesh but is retained on 60 mesh 4rather than a gravel that is less rigidly graded.

The leak-off liquid is preferably a hydrocarbon or water solution. If water is used it is possible to control its density by dissolving a suitable weighting agent in the water. It is important to avoid fine particles in the leakoff liquid that would deposit onto the cavity wall and render it impermeable. In order to prevent the glass pellets from settling out to the bottom of the cavity rather than depositing onto the cavity wall, it is desirable to adjust the pellets and liquid densities to be similar or equal. If

of glass on an inclined the permeability of the formation to the leak-olf liquid is low, the ow of leak-off liquid into the formation will be low. In this -case it is essential that the relative densities be closely adjusted to prevent the segregation of the glass pellets in the well. The density of the glass can be adjusted down to about l.5 by formation control. The density of the glass can also be adjusted by entraining minute gas bubbles in the glass in its manufacture. It is important in this instance that the pellets are formed without surface bubbles since these introduce a structural weakness into the pellets. The more useful hydrocarbons are lighter than water. If a heavier leak-off liquid is desired, water itself or preferably a weighted water is used. Sodium chloride, calcium chloride, zinc chloride and other wellknown additives are useful for increasing the density of water. In this manner densities up to about twice the density of water itself are attainable. In addition, treating acids or other chemicals may be incorporated into the liquid for concurrent acidiz'ing or other formation treatment. The selection of the density-modifying additive is dependent both on cost, on well characteristics and on the density of the solution desired. Thus, by adjusting the densities it is readily possible to produce a nonsegregating mixture.

In order to make an effective pack of high integrity according to our invention it is necessary to utilize uniformly sized pellets and get them into the pack witho-ut significant breakage. The glass pellets are withdrawn from a storage unit at the well head 'by a screw conveyer and injected into the desired liquid in `a mixing tank. The liquid with the pellets entrained therein then passes through one or more centrifugal pumps to a final high pressure pump all with associated valves to be forced through the well bore under pressure. We have discovered that this multiple pumping operation involves significant breakage of conventional gravel packing materials. A special glass pellet whose production will now be described is used to prevent significant breakage in this pumping operation and permit the placement of our multilayer pack of high integrity.

The special gravel packing agents required in this invention can be prepared from any of the usual types of glass such as soda-lime, lead, boro-silicate, and high silica glasses or they can be prepared from slags and other low density formulations. Particles of soda-lime glass are heated near its softening temperature in excess of 1800o F. such that any bubbling of gas in the glass either has not begun or has ceased, and then the particles are rapidly quench in a tluid at a temperature below about 900 F.

T he quenching fluid may be a gas or a liquid having a viscosity greater than water; however, water alone is not suitable. The temperature from which the glass is quenched will have a strong influence on the ability of the glass particles to resist crushing and deformation under compressive loads. For example, ordinary bottle glass spheres quenched in accordance with this invention from a temperature of 2200 F. in a uid to a temperature below 900 F. have a higher strength than similar particles quenched from 1800 F., and it is preferred that the packing agent be prepared by quenching glass particles from a temperature above 2000 F. One of the advantages of the rapid quenching process is that it allows the production of glass particles -capable of withstanding higher compressive loads than ordinary soda-linie bottle glass.

The particles are formed by dropping molten globules heated surface of a material, such as graphite, which is not wetted by the glass. The globules of the glass roll along the surface of the graphite to form spheres and then roll from the surface into the quenching medium. Suitable quenching media are hydrocarbon oils such as SAE l0, SAE 20, and SA-E 30 motor oils, viscous aqueous solutions such as aqueous water glass solutions, starch solutions, soap solutions or aqueous solutions of ethylene glycol, and mixtures of oils and melted greases and fats. The quenching medium is at a temperature below about 400 F., and is preferably at room temperature. After the temperature of the glass particles has reached a temperature below 900 F., further cooling may be accomplished in any convenient manner. It is not necessary that the cooling from 900 F. to lower temperatures be rapid. The particles are then Sorted and graded as to size, density, sphericity and roundness. The particles are highly rounded, ordinarily having a roundness and sphericity well about 0.9.

The novel packing agents are strong and hard, but brittle in that they fail in tension when subjected to a compressive load. A convenient measure of the properties of the particles of the packing agent is given by the ratio of L/D2, where L is the maximum compressive load in pounds a single particle can carry and D is the diameter of the particle in inches. The ratio of L/D2 is determined by placing a measured single particle of packing agent between hard steel plates and pressing the plates together with a known force that is increased until the particle ruptures. These packing agents are in general characterized Iby an L/D2 ratio exceeding 50,000 when the particles are tested between steel plates having a hardness of 35 Rockwell C. The lower density packing agents prepared from slags and other low density formulations have an L/D2 ratio of 30,000 p.s.i. or more. An excellent grade of packing sand in 10-20 mesh size has an L/D2 ratio of approximately 10,000. Ordinary sand has an L/D2 ratio of approximately 4,000. Ordinary glass spheres have an L/D2 ratio approximately the same as the excellent grade of packing sand. These special glass packing agents are further characterized by being unyielding or substantially non-deformable when subjected to compressive loads; hence, they retain substantially their initial dimensions until they fail. Other suitable processes including chemical tempering of glass pellets may be used for producing special gravel packing agents, useful hereunder, having a suitably high L/D2 ratio and the other desirable attributes.

As previously indicated it is not only necessary to get uniformly sized pellets into the pack but in addition it is necessary that they do not break up in the pack over a long period of time. The formation may be subjected to a large overburden pressure which is translated into a horizontal component when the borehole and cavity have been produced. This horizontal component of the overburden pressure together with the pressure of the oil being produced will cause a substantial, continuous pressure inwardly towards the well bore. In some instances it is desirable to counteract this pressure by exerting an opposing positive pressure against the pack. An expandable liner such as described by Hildebrandt in U.S. Patent No. 2,998,065 can be used to exert a pressure on the pack and support the cavity wall. The pressure placed on the pack to support the formation will be from about 0.5 to 1.1 times the overburden pressure. In order to withstand the very substantial underground pressures that may exist and still retain pack integrity and permeability, it is desirable to use these special glass pellets. Otherwise the pellets may gradually fracture to form a dense, pluggedup, substantially non-permeable pack. When the gravel pack is located in shallow formations not subject to higr overburden pressures, glass pellets of less strength, such as the low density pellets having an L/D2 of 30,000 p.s.i. or more can be used.

A further advantage of using these hardened glass pellets is their ability to withstand elevated temperatures and their maintenance of strength and corrosion resistance in the presence of acids, brine, caustic and other chemicals. Ordinary glass is generally considered to be about half as strong in the presence Iof water as it is in the dry state. Our special glass pellets are not affected by the presence of water. Since water in the well is almost a certainty, this advantage is of substantial signicance in maintaining pack integrity. Furthermore, chemical or heat treatment of the formation is possible without affecting the pack. For example, corrosive acids and brine have 7 ngliglixble effect on the pellets at temperatures as high as 3 0 In a specific example of my invention a borehole is drilled and an oil bearing zone discovered between 3476 feet and 3501 feet. A core sample reveals that the formation is a weakly consolidated sandstone having a permeability of about 200 millidarcies. Ninety percent of its particles are retained on a 270 mesh sieve, U.S. Standard Sieve Series. It is decided to use our gravel pack as a protective measure against formation disintegration and production of sand with the reservoir oil.

A casing is lowered into the well to a depth of 3470 feet and is cemented in place. The cement is then drilled out through a depth of 3510 feet, the bottom nine feet being in consolidated rock. A cavity approximately three feet in diameter is formed in the pay zone by jetting a sand-laden water mixture from a jetting tool reciprocated through the productive interval. An aqueous leak-off solution having a specific gravity of 1.9 is prepared from Water, calcium chloride and zinc chloride. Seventy to eighty mesh hardened slag-type glass pellets having a specific gravity of approximately 2.3 are entrained in the leak-olf liquid in an amount of five pounds per gallon and the mixture is pumped down the tubing at a pressure not exceeding 2500 p.s.i. After 7000l pounds of these 70 to 80 mesh pellets have been deposited against the cavity wall, 5000 pounds of to 25 mesh pellets are entrained in the leak-off liquid in like manner and pumped into the cavity. This is followed up by a final 6000 pound batch of 6 to 7 mesh pellets to till the cavity. A fluid pressure of about 1600 p.s.i. is maintained in the well at the formation to hold the pack in place as a six-inch diameter slotted liner is washed into position through the 6 to 7 mesh pellet portion of the pack. Inspection of the pellets that are washed to the well head during the placement of the liner reveals that there is no significant quantity of crushed pellets in the pack. The pack consists of an outer four inch thick layer of 70 to 80 mesh pellets, a middle layer four inches thick of the 20 to 25 mesh pellets and an interior layer seven inches thick of the 6 to 7 mesh pellets. The well is then put into production in the conventional manner. It is ascertained from pressure build-up data that pack permeability exceeds 100 darcies which indicates that the pack possesses good integrity with no crushed pellets, voids or plugs.

By this invention a gravel pack of high permeability and integrity may be produced. The pack will support poorly consolidated formations against pressures as great as the overburden pressure or greater and maintain its high permeability for an extended period of time. lt may be used in a great variety of situations to protect weakly consolidated formations against disintegration, provided that the formation possesses sufficient permeability to place the pack in the manner required by this process.

We claim:

1. In the production of oil from an oil bearing formation containing a packed cavity between the production tubing and formation to prevent the migration of formation particles into the well, the method of forming a pack in the cavity which comprises entraining in a leak-off liquid a rst batch of discrete, rigid, substantially spherical glass pellets essentially uniform in size and specially produced to have an L/D2 ratio of at least about 50,000 Where L is the maximum compressive load in pounds that the pellet can carry and D is the diameter of the particle in inches, pumping the leak-off fluid and glass pellet mixture into the well under pressure to cause the leak-off uid to enter the formation through the cavity wall and deposit the glass pellets as a layer on the wall of the cavity, entraining in a further portion of the leak-off liquid at least one more batch of glass pellets identical to the preceding batch -but of a uniform larger size than the preceding batch, and depositing them in the cavity against the preceding batch of glass pellets to form a pack of high integrity of graded spherical glass pellets of increasing f8 size in -a direction from the cavity wall to the borehole.

2. A method in accordance with claim 1 in which a pressure of between about 0.5 and about 1.1 times the overburden pressure is applied radially against the pack without breakage of the pellets to force the pack against the cavity wall and support the formation against inward movement.

3. A method in accordance with claim 1 in which the pellets in the first layer adjacent the cavity Wall are from 2 to 4 times the 90 percentile point on the formation sieve analysis and the pellets in each succeeding layer are from 2 to 6 times larger than the pellets in the immediately preceding layer.

4. A method in accordance with claim 1 in which the density difference between the glass pellets and the leakoff liquid is adjusted to be no greater than about 50 percent and the leak-off liquid is a weighted water solution.

5. A method in accordance with claim 4 in which the glass pellets are formed to contain a multiple of minute gas bubbles.

6. A method in accordance with claim 4 in which the glass pellets are a specially hardened, low density, slagtype glass.

7. A method in accordance with claim 4 in which the density of the glass pellets and the leak-oft liquid is substantially equal.

8. A gravel pack of high integrity in a cavity formed in the oil producing formation surrounding the production tubing in a borehole comprising a series of at least two layers of discrete, rigid, substantially spherical glass pellets essentially uniform in size in each layer and of increasing size from layer to layer in a direction from the cavity wall to the production tubing, and specially produced to have an L/.D2 ratio of at least about 50,000 where L is the maximum compressive load in pounds that the pellet can carry and D is the diameter of the particle in inches, said pack characterized by a high permeability and by the absence of voids, plugs and fragmented pellets.

9. A gravel pack in accordance with claim 8 in which said production tubing comprises an expandable liner which applies pressure laterally against the pack to force the pack against the cavity wall at a pressure which is from about 0.5 to 1.1 times the overburden pressure.

10. A gravel pack in accordance with claim 8 in which the pellets in the first layer adjacent the cavity wall are from 2 to 4 times the 90 percentile point on the formation sieve analysis and the pellets in each succeeding layer are from 2 to 6 times larger than the pellets in the immediately preceding layer.

11. In the production of oil from an oil bearing formation containing a packed cavity between the production tubing and formation to prevent the migration of formation particles into the well, the method of forming a pack in the cavity which comprises entraining in a leak-off liquid a iirst batch of discrete, rigid, substantially spherical glass pellets essentially uniform in size, pumping the leak-off fluid and glass pellet mixture into the well under pressure to cause the leak-off fluid to enter the formation through the cavity wall and deposit the glass pellets as a layer on the wall of the cavity, entraining in a further portion of the leak-ofi liquid at least one more batch of glass pellets of a uniform larger size than the preceding batch, and depositing them in the cavity against the preceding batch of glass pellets to form a pack of high integrity of graded spherical glass pellets of increasing size in a direction from the cavity wall to the borehole.

12. A method in accordance with claim 11 in which the density difference between the glass pellets and the leak-off liquid is adjusted to be no greater than about 50 percent and the leak-olf liquid is a weighted water solution.

13. A method in accordance with claim 11 in which the glass pellets are hardened, low-density, slag-type glass.

14. A method in accordance with claim 11 in which the pellets in the first layer adjacent the cavity wall are from 2 to 4 times the 90 percentile point on the formation sieve analysis and the pellets in each succeeding layer a-re from 2 to 6 times larger than the pellets in the irnmediately preceding layer.

15. A gravel pack of high integrity in a cavity formed in an oil-producing formation surrounding production tubing in a borehole lcomprising a series of at least two layers of discrete, rigid, substantially spherical glass pellets substantially uniform in size in each layer and of increasing size from layer to layer in a direction from the cavity Wall to the production tubing, said pack characterized by a high permeability and by the absence of voids, plugs and fragmented pellets, and said production tubing comprising an expandable liner which applies pressure laterally against the pack to force the pack against the cavity wall at a pressure from about 0.5 to 1.1 times the overburden pressure.

References Cited CHARLES E. OCONNELL, Primary Examiner.

DAVID H. BROWN, Examiner.

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