GB2175579A - Process for the manufacture of proppant material from bauxite fine fractions - Google Patents

Abstract

A superior proppant having very high permeability at formation pressures up to 20,000 psi, very high compressive strength, low specific gravity, and good acid resistance, is produced by a process comprising: (a) separating a fine fraction from a naturally occurring bauxite containing same; (b) pelletizing the separated fine fraction in the presence of water; (c) treating the pellets produced in step (b) to remove water therefrom; and (d) sintering the product of step (c) to produce a low density material suitable for use as a proppant.

Description

SPECIFICATION Process for the manufacture of proppant material and material made by the process This invention relates to a new and improved propping agent (proppant) derived from bauxite.

Bauxite - based proppants are known but are subjectto considerable drawbacks as will be discussed inthe survey ofthe prior art below.

In one aspectthe present invention is distinguished from the prior art by use as a raw material or fine fraction separated from naturally occurring bauxite containing same. This fine fraction when treated in the manner described in more detail below, unexpectedly provides a proppant having superior properties aswell as significant economic advantages compared to the proppants ofthe priorart.

The accompanying drawing relates to Example 1 below and is described in more detail in that Example.

Survey ofthepriorart The use of propping agents in the hydraulic fracturing of subterranean formations to improve the recovery of oil and gas is a procedure which has been used for nearlyfortyyears. Thetechnique is well described inthe literature and the application of the technique is common practice.

The original particulate material used forthis purpose was silica sand chosen for its particle shape and freedom from internal weaknesses. This material continues to be preferred product for shallow wells because of its cheapness and ease of availability. In recent years it has been necessary to seek alternative proppants because of the increasing depth at which oil and gas is being produced and the aggressive environments encountered. This has led to the use of materials such as glass beads and resin coated sands. One ofthe more effective proppants which has been developed has been sintered bauxite, the use ofwhich is described in US patent 068 718. This patent describes procedures for manufacture of propping agents and typical cases are quoted where the raw material is a calcined bauxite ore.This ore aftergrinding to a very fine powdertypically less than twelve micrometres in size is formed into spherical particles or green pellets by compaction or rolling priorto being sintered. The objective ofsintering isto causethe individual particles comprising the green pellets to bond together at temperatures below the melting pointofthe minerals present. In the above mentioned patent it is stated that the end product should have a specific gravity greater than about 3.4 with a preferred range from 3.5 to 3.8.The comment is made "Sintered bauxite, having a specific gravity below 3.4 is difficultto produce and because ofthe high void concentration would likely result in a product having low compressive strength." It is fu rther stated that high density, fine grain particles have higher compressive strength than sintered large grain particles. Evidence is also presented to show a very broad correlation between the specific gravity of the final product and permeability at an applied stress of 10,000 psi. High permeability and compressive strength are essential for proppant to be used at depths greater than those for which sand is satisfactory.

Australian patent 521930 g ranted to the Norton Company again stresses the need to produce sintered bauxite proppantfrom finely ground pre-calcined material. In this case the use of coarser particles up to 50 micrometres in diameteras a minorcomponentofthe mixture was claimed as being satisfactory. This patent also refers two the benefits of using a temporary binderto retain stabilityofthe green pellets priortofiring. It specifically refers to a density of at least 3.6 as the desirable level to achieve satisfactory crush resistance ofthe proppant.

It has been recognised that the high density of proppants made of sintered bauxite creates a problem in the placement ofthe material. This arises from the fact that the fluid used to transport the proppant requires modification from that used to transportfor example, silica sand because of the substantially greatertendency for particles of sintered bauxite to settle more rapidly from the suspension. This increases the cost of placing the proppant but an even greater cost penalty results from the higher density by comparison with that ofsilica sand. Itwill be immediately obvious that a proppant with density of 3.7, which is typical of sintered bauxite presently used, will require a greater mass of material to fill a fracture of given volumethan proppantwith density of 2.65.In factthechange ofdensityquoted requires 40% more by weight ofthe higher density proppantto fill the samevolumethan the lower density proppanttofill the samevolumethanthe lower density proppant wouid require. Proppants are purchased on a weight basis and a sintered bauxite proppant may have a price 20 times that of silica sand or more. When the density factor is taken into accountthecostof changing from a silica sand proppantto sintered bauxite proppant may result in a more than 30 fold increase in cost of this component. Many attempts havetherefore been madeto produce proppants of high strength relative to sand but with a lower density than that of sintered bauxite.In this context European patent application 0 087 852 by Dresser Industries, Inc is relevant. This is based on the use of Eufala aluminous ore which after calcination contains 40-60% Al203 and 36.5-51% SiO2. A density of less than 3 is claimed forthis product with a minimum permeability at 10,000 psi of 100,000 Millidarcies at ambienttemperature.

Data presented in this patent indicates that permeability begins to decrease sharply beyond a closure stress of 700kg/cm2 (10,000psi).

US patent4427 068 issued to Kennecott Corporation teaches a process of manufacturing sintered spherical composite pellets or particles comprising one or more clays as the majorcomponenttogetherwith bauxite, alumina ormixturesthereof. The product is claimed to have an apparent specific gravity oflessthan 3.40 and the mixture should contain atleast40% on a dry weight basis of clay. The five clays which may be usedforthe manufacture ofthe material are specifically described. Calcination ofthe clays prior to formation ofthe pellets is a prerequisite.

Australian patent application no. 16568/83 bythe Norton Company is entitled "Low Density Proppant For Oil and Gas Wells". This document discusses a method of producing a low density sintered ceramic proppant containing chemically combined aluminium and silicon. The aluminium oxidecontentclaimedis in the range 60-85% with densities of less than 3.4 grams per cubic centimetre, although such claim cannot besubstantiated from the evidence quoted in examples. The raw materials used forth is are calcined bauxite and clay togetherwith a binder to give an acceptable green strength ofthe pellet.

With the exception of the Dresser application all documents refer to a preferred particle size of the feed material and typically specify average particle sizes of less than 10-15 micrometreswith the exception of Australian patent 521930 which permits the limited use of material up to 50 micrometers. US Patent4 068718 refers to a preferred average particle size of 4-5 micrometres or less than 7 micrometres.

Fitzgibbon in US Patent 4427068 specifi es that a small particle size is required to obtain the desired density and states that the average particle size should be less than about 15 micrometres, and, preferably lessthan about 10 micrometres and, most preferably, less than about 5 micrometres. Elsewhere he states that an average particle size smallerthan 5 micrometres is desirable and the average particle size is most preferably below 3 micrometres and usually above 0.5 micrometres. He states further thatthe dry homogeneous particulate mixture has an average particle size of less than about 15 micrometres before pelletising.It will be immediately obvioustothose skilled in the artthatgrinding calcined bauxites orclaysto such fine particlesize is expensive in terms of equipment, grinding media and power usage, particularly when average particle sizes below 3 micrometres are specified. Large scale grinding ofthistype is likely to make the entire process uneconomic and this fact explains the use of only4to 8 8 micrometre material in the example quoted by Fitzgibbon.

Studies of proppants which are essentially based upon alumina and silica in various proportions demons trate thatthe major minerals present are corundum (alpha-phase Al203 and mullite 3Al203- 25i02). The phase diagrams representing the formation ofthese minerals are well known and their presence has been widely confirmed by mineragraphic and X-ray diffraction studies oftypical sintered clay/bauxite products.

Similar mineralogy is observed in products such as high temperature calcined bauxite used for refractory purposes and calcined bauxites ofthetype used as the initial feed stockfor production of sintered bauxite.

Depending upon the nature of impurities present other minerals such as spinels or hematite may also occur.

Studies of polished sections of a typical sintered bauxite of the type referred to in U.S. 4068718 using a high power optical microscope showsthe presence of irregularcystals of corundum with interstitial mullite. The proportion of corundum to mullite is a function ofthe proportions of Al203 and SiO2 in the original feed material. It is probable thatthe sintered bauxite owes its strength to the interlocking mass of corundum crystals while the interstitial mullite allows elastic deformation to take place under load.

In summarising the foregoing, it may be said that prior art has taught usthatthe most desirable properties for propping agents in the hydraulic fracturing of subterranean formations at considerable depth are: - high permeability at high formation pressures - high compressive strength - high acid resistance - low specific gravity Until now, proppants made from feed material containing bauxite have possessed all of these properties, except for low specific gravity. in orderto achieve the requisite low specific gravity, low density additives such as clays, must be added to the bauxite. This is disadvantageous since the clays are normally first calcined, an energy intensive step. Likewise, the bauxite feed material also must be pre-calcined, adding to the costs of production.Further, binders may have to be added at the pelletising stage to give acceptable green strength to the pellets. It may also be noted here that excessive addition of siliceous material, such as clay, can lead to a decrease in acid resistance ofthe proppant.

We are also taught bythe prior artthatthe strength ofthe proppant pellets is strongly related to the size of the constituent grains. Grain size of the feed material of less than about 5 micrometres, even as low as 3 micrometres, is considered preferable. Large scale grinding to achieve these grain sizes is expensive.

To date, workers in the art have been unableto reconcile the two most desirable properties of proppants, namely very high strength and low specific gravity.

The present invention relates to a process for manufacture of an improved material suitable for use as a proppant; to a process for manufacture of green pellets from which the improved material can be made and to material made by the process.

Thus, according to one aspectofthis invention, we provide a process for manufacture of a material suitable for use as a proppantcharacterised by the following steps; (a) separating a fine fraction from a naturally occurring bauxite containing same; (b) pelletising the separated fine fraction in the presence of water; (c) treating the pellets produced in step (b) to remove watertherefrom; (d) sintering the product of step (c) to produce a low density material suitable for use as a proppant.

According to a second aspectofthe invention we provide a process for manufacture of green pellets which compnses:- (a) separating a fine fraction from a naturally occurring bauxite containing same, said fine fraction being composed largelyofmonominerallic particles ofgibbsite, boehmite and kaolinite, the kaolinite representing no more than 25% of the total, said particles having a typical size range from 0.02 to 0.3 micrometres and an associated specific surface area of typically 30 square metres per gram; and (b) pelletising the said fine fraction without binder or other additive in the presence water; furthercharacterised in that the said green pellets upon subsequent drying and calcination can produce a material suitablefor use as a proppant having a density of less than 3.4g/cm3.

According to a third aspect ofthe invention, we provide a process for manufacture of a material suitablefor use as a proppant, characterised by the following steps: (a) separating a fine fraction from a naturally occurring bauxite containing same; said fine fraction being composed largely of monominerallic particles ofgibbsite, boehmite and kaolinite, the kaolinite representing no more than 25% of the total, said particles having a typical size range from 0.02 to 0.3 micrometres and an associated specific surface area oftypically30 square metres per gram; (b) pelletising the separated fine fraction in waterwithout binder or other additiveto produce green pellets, said pellets having sufficient strength in both wet and dry states to retain their integrity during handling and thermal processing operations;; (c) treati ng th e pellets produced in step (b) to remove water therefrom; (d) sintering the product of step (c) at a temperature in the range of 1350 to 1 500"C to produce a material suitablefor use as a proppant, said material having a density oflessthan 3.4g/cm3.

The invention also consists in material made by the process in accordance with the first aspect ofthe invention and further characterised in that it is in the form of spherical particles in the size range 2 mm to 0.3 mm suitable for use as proppant in hydraulicfracturing offormations exhibiting pressures of unto 20,000 psi, in which the particles are predominantly composed of microcrystallites of corundum and mullite.

The invention further consists in material made by the process in accordance with the first or third aspects of the invention further characterised in that the permeability of a 20/40 mesh (850/425 micrometre) fraction at 10,000 psi exceeds 50% ofthe value attained at 2,000 psi and further retains a permeability at 15,000 psi of about30% ofthe value at 2,000 psi.

The proppant made in accordance with the various aspects ofthe invention is superiorto previous proppants made from bauxite particularly in respectto: - very high permeability at formation pressures up to 20,000 psi.

-very high compressive strength - low specific gravity - good acid resistance These desirable features arise primarily from the choice of the feed material, namely a fine fraction of a naturally occurring bauxite. The fine fraction as separated from the natural bauxite has a grain size typically from about 0.02 to 0.3 micrometres with an associated specific surface area of about 30 square metres per gram. This is considerably finerthan bauxite material previously used in proppant manufacture, and lendsthe following advantages to the manufacturing process.

- the need for expensive preg rinding is eliminated -the need for expensive pre-calcination is eliminated - high strength of the green pellets is obtained without the addition of binder - the very high surface area of the feed material makes it highly reactive. This leads to a reduction in sintering time, and hence reduced energy consumption - an exceptionally high degree of uniformity in the composition of the pellets The advantages in manufacturing and superior properties ofthefinished proppant are discussed in detail below.

1. Theveryfine particle size allows an exceptionally high degree ofuniformity of blend in the manufacture of the pellet.

2. The said pellet comprising a multitude of very fine particles has numerous points of contact between the ultrafine particles and it is at these points of contact that sintering will be initiated, resulting in the rapid production of a very strong proppant. Kingery (1976) estimates that sintering rate is roughly proportional to the inverse ofthe particle size which suggests that particles with an average size of 0.2 micrometres or less such as are used in the work described herein will sinter at a rate at least 20 times faster than 4 micrometre particles provided the minimum temperature required to initiate sintering is achieved.

3. It has been found that the extremely intimate dispersion of minerals and thus of constituent elements is conducive to a high degree of reactivity with each pellet as it is heated, leading to the production of a wide range oftemperature dependent crystal sizes and structures. The proppant particles retain very high strength throughout these internal changes, considerably in excess ofthat normally associated with other proppants of similar chemical composition. This behaviour is discussed in greater detail below.

4. The density of the fired particle is a function ofthe mineralogy and the volume of closed voidswithinthe particle. The presence of voids has generally been regarded as undesirable in that they lead to a reduction in strength.

It is this factor which has led previous investigators to lay stress upon the need to achieve densities of the sintered bauxite particle in excess of 3.4 and preferably between 3.5 and 3.8. It has now unexpectedly been found that the ultrafine nature of the feed material used to make the high strength proppant which isthe subject of discovery in this patentcausesthe closed pores to be so distributed afterfiring that a density of less that 3.4 can be achieved while maintaining strengths adequate to permit high levels of permeability at formation pressures upto 20,000 psi.

5. Yet another benefit associated with using such finely divided materials obtained from natural bauxite relates to the avoidance of any need for a binderto maintain a high degree of strength in the green pellet.The inter-particulateforces within a packed pellet madefrom such ultrafine material are believed to be very high and the pellets are able to resist rupture by the forces which develop during drying and calcination, so they can be handled without special precautions.

6. Afinal andvery obvious benefit arising from the ability to use natural uncalcined bauxite of the particle size considered is cost. Pre-calcination of bauxite or other aluminous material is expensive and energy intensive.

The same is also true of any intensive grinding procedure.

Disclosure ofpreferred embodiments ofthe invention Finely divided materials suitable for the production of proppants according to the invention are readily obtained from bauxite deposits such as for example those ofthe type occurring atWeipa in Northern Queensland. In these and similar bauxites a fine fraction exists which can be easily separated, for example by slurrying the bauxite in water in the presence of a dispersant. A variety of dispersants may be used such as those based upon phosphate compounds but in the Weipa case adjustment of the pH to a value of approximately 11 by the use of caustic soda is adequate to provide the required dispersion.

A simple classification step results in recovery ofthe fine fraction as a dilute slurry. The contained solids are separated by settling orfiltration and itwill be recognised by those experienced in the artthatflocculants may be used, if appropriate, to assist in carrying outthis dewatering operation more rapidly.

To facilitate the preparation of the feed for pelletising, drying may be necessary and a variety of equipment can be used for this purpose. The preferred equipment is a spray dryer or a nozzle dryer, since the dried material will then be in a divided state suitable for pelletising in an intensive mixer such as that manufactured bythe Eirich Company of West Germany, butthe use of other dryers orfilters is acceptable, particularly ifthey decrease the energy required to evaporate water.

The use of the Eirich mixer allows the fine powderto be pelletised in the presence of appropriate amountsof water,typically in the range from 14-18% by weight Proper operational control ofthe mixer equipment allows the production of uniformly spherical particles with a particle size ranging typically from 0.15 millimetres to 4.0 millimeters. It is possible to exercise control over particle size distribution by controlling the conditions of operation ofthe mixer. Furthermore if any particular particle size is in excess of that required for later processing such material can be readily recycled in the process after size reduction or dispersion if necessary.

It has been found thatthe particularfraction proposed in this document as a source of proppant has in the Weipa deposits an alumina content of about 60%with a typical rangefrom 57-63%. Similarlythe silica content will average about 9% with a range oftypically 7-11%. Substantially all ofthis silica will be present asthe mineral kaolinite, although small amounts of quartz may be present. Typicallythe mineralogy will comprise 30-50% gibbsite with 22-45% boehmite, 16-24% kaolinite and less than 1% quartz. Oxides of iron and titanium will total about9-12 /O.

Electron microscope studies have confirmed thatthe mineral particles are typically from about 0.02 to 0.3 micrometres in diameter and free minerals such as boehmite, kaolinite and hematite are commonly present. In other words, the particles are frequently mono-minerallic in nature and because of the very large total surface area rapid reaction between particles is facilitated at elevated temperatures producing primarily corundum and mullite.

In the course of heating such a product to the typical temperature range of 1350-1500"C required to produce strength beyond that generally associated with intermediate grade proppants a series of changes occurs in these pellets. Before firing is actually commenced, drying to remove free water is desirable to provide stability ofthe pellets when they are introduced to the kiln. The alumina minerals which are present, namely, gibbsite, boehmite and kaolinite progressively losetheircombined water asthetemperature is increased to about 600"C.

The crystal lattices become disordered and as the temperature is further increased the aluminium oxides undergo a series of phase changes. It is to be expected thatthe sequence of phase changes may include as transition states gamma, delta, theta, chi and kappa forms of alumina. At a temperature of approximately 950 Ctheformation of mullite commences and at about 1050tC the alumina begins to convertto the alpha-phase mineral which is known as corundum. The minerals corundum and mullite are the final major phases in the end product and as the temperature is increased the particles gain in compressive strength due to the development of an extremelyfine intergrowth or network ofthese minerals.

During calcination, the pellets may shrink in diameter by morethan 20%.

The high reactivity of the uncalcined bauxite and its very fine particle size facilitates the formation ofthe required phases.

The initial crystallite size of the proppant subjected to temperatures below 1400"C is so fine that it is difficult to resolve by an optical microscope. The exception to this is the presence of occasional grains of spinel. As the temperature is increased, significant grain growth does occur and it becomes possible to resolvethe indi- vidual minerals optically. Confirmation of the mineralogy as corundum and mullite with subsidiary iron titanate has been obtained by X-ray diffraction. From the XRD data of a sample fired at 1400"C in a laboratory muffle it has been estimated that the average crystallite size of corundum is about 880 Angstroms and of mullite 740 Angstroms (0,088 and 0,074 micrometres respectively).

As the temperature of calcination is increased, very rapid growth of crystallite size can be detected, and this is thoughtto be associated with the extreme intimacy of the constituent minerals. Atthe high temperatures studied (1450-1 500'C) relatively coarse crystals particularlyofspinel like minerals of 20 micrometre length and even up to 100 micrometres can be seen. The proppant patent literature teaches that proppant particles containing such coarse crystals or grains should be of low strength. The high strength continuing to be observed is related to the development of a "basket -weave" structure of randomly matted crystal.

This is analogousto a reinforcing fibre system conferring the desirable properties oftoughnessand resistance to fracturing. Once again, this property appears to be dependent upon the extremely fine nature of the feed which in turn generates initially very fine intimately dispersed grains ofmulliteand corundum and these latterfine minerals can easily develop into larger grains while retaining the required strength.

Hence, the feedstock used is capable of producing the unexpected property of yielding a proppantwith continued high strength despite the variation in crystal size texture and fabric resulting from exposure to different temperatures.

Furthermore, the internal structures are such that strength is enhanced beyond that which would be expected from a proppant of similar chemical composition made by previously described procedures. At the same time, the density of the pellets, which in all the samples tested was less than 3.40 and averaged 3.35 g/cm3, is substantially below that of the sintered bauxite proppants.

The excellent values apparentforstrength and toughness are accompanied by good resistance to corrosion in acid or alkaline environments.

It will be apparentfrom the foregoing disclosure that in a principal aspect the invention provides a process for manufacture of a material suitablefor use as a proppant,characterised by the following steps: (a) separating a fine fraction from a naturally occurring bauxite containing same; (b) pelletizing the separated fine fraction in the presence of water; (c)treating the pellets produced in step (b)to removewatertherefrom; (d) sintering the product of step (c) to produce a low density material suitable for use as a proppant.

The final product is preferably in the form of spherical particles in the size range 2mm to 0.3mm suitablefor use as proppant in hydraulic fracturing offormations exhibiting pressures ofupto 20,000 psi, in which the particles are predominantly composed of microcrystallites of corundum and mullite.

The raw material is preferably uncalcined natural bauxite fraction composed largelyofmonominerallic particles ofgibbsite, boehmite and kaolinite,the kaolinite representing no more than 25%ofthetotal,said particles having a typical size range from 0.02 to 0.3 micrometres and an associated specific surface area of typically 30 square metres per gram.

The invention includes a processforthe manufacture ofgreen pellets from the material described in the preceding paragraph without the requirement to use any binder or other additive, said pellets having sufficient strength in both wet and dry states to retain their integrity during handling and thermal processing operations.

The density of the calcined proppant particles is less than 3.4 after calcination in the range 1380 to 1500 C.

In the preferred final product, the permeability of a 20/40 mesh (850/425 micrometre) fraction at 10,000 psi exceeds 50% ofthe value attained at 2000 psi and further retains a permeability at 15,000 psi of about 30% of the value at 2000 psi.

The invention will be further illustrated by the following non-limiting example.

Example 1 A bauxite slurry sample produced from the Weipa bauxite deposit was dispersed using caustic soda to achieve a pH level of about 11. The coarse fraction was allowed to settle and the supernate collected by decantation. The silica content of 10.5% was equivalent to a kaolinite content of 23%. The decanted fraction was examined by transmitted electron microscopy and found to comprise particles generally from 0.02 to 0.3 micrometres in size. Afterflocculation and decantation of clearwater a parcel of thethickened slurry offine particles containing in excess ofthree tonnes of solid was dried in a spray dryer.

The dried solids were blended with another portion ofthe thickened slurry and pelletised in an Eirich mixer-pelletiser at about 17%free moisture level. No binderwas used. A highly spherical product resultedwith a typical particle size range from about 0.5 to 4 millimetres, although it was demonstrated that this sizing distribution could be modified. The spherical product was then dried to about 2% free moisture in a rotary dryer priorto being fed to a continuous rotary kiln.

The pellets were calcined to a maximum temperature of 1500 C.The resulting fired pellets retained their highly spherical shape and did not bond to one another during firing. A high kiln throughput was achieved and the high quality ofall samples taken, even attemperatures below 1400'C confirmed the high sintering rate of the material.

Several samples were taken ofthe product during calcination. The 0.85/0.425mm fraction (20/40 mesh) of each was recovered and tested. the following table gives the arithmetic means and standard deviations of important properties.

Mean S.D.

Particle density 3.35 g/cm3 0.035 Bulkdensity 1.91 g/cm3 0.05 American Petroleum Institute 2.3% 0.14 Acid solubilitytest American Petroleum Institute 5.5% 0.9 Crush test (10,000 psi) Permeability (Darcies) Mean S.D.

1000 psi 483 18 3000 psi 432 13 6000 psi 360 19 8000 psi 303 17 10000 psi 247 22 12000 psi 200 24 15000 psi 133 19 20000 psi 70 12 The properties shown in the above tabulation conform to American Petroleum Institute specifications where these are available.

The permeability measurements were made using distilled water at ambient temperature. A graphical representation ofthe permeability behaviour with increasing closure stress is attached. The results oftwo intermediate proppants, one based on a mixture of calcined bauxite and clay and the other on an aluminous clay are included to demonstrate the comparative behaviour.

Relatively high levels of permeability were retained throughout the conductivity testing. For example it is common practice to compare permeabilities at elevated pressures with that achieved at 2000 psi. Fitzgibbon in US 4427068 claims a decrease in permeability from 2000 psi to 10,000 psi of not morethanthree-fourths (75%). In European patent application 0087852 the decrease is about 65%.

By comparison material ofthe present invention after loading to 10,000 psi loses only 46% of the conductivity achieved at 2000 psi and even at 15,000 psi still retains about 30% of its conductivity.

It will be clearly understood that the invention in its general aspects is not limited to the specific details referred to hereinabove.

Claims (9)

1. A process for manufacture of a material suitable for use as a proppant characterised bythefollowing steps; (a) separating a fine fraction from a naturally occurring bauxite containing same; (b) pelletizing the separated fine fraction in the presence of water; (c) treating the pellets produced in step (b) to remove water therefrom; (d) sintering the product of step (c) to produce a low density material suitable for use as a proppant.

2. A process according to claim 1 in which the fine fraction separated in step (a) is an uncalcined natural bauxite fraction composed largely of monominerallic particles ofgibbsite, boehmite and kaolinite, the kaolinite representing no morethan 25% of the total, said particles having a typical size range from 0.02 to 0.3 micrometres and an associated specific surface area of typically 30 square metres per gram.

3. A material suitable for use as a proppant, prepared by the process of claim 1 and further characterised in that it is in the form of spherical particles in the size range 2mm to 0.3mm suitable for use as proppantin hydraulic fracturing offormations exhibiting pressures of up to 20,000 psi, in which the particles are predomi nantly composed of microcrystallites of corundum and mullite.

4. A process for manufacture of green pellets which comprises; (a) separating a fine fraction from a naturally occurring bauxite containing same, said fine fraction being composed largely of monominerallic particles of gibbsite, boehmite and kaolinite,the kaolinite representing no more than 25% ofthetotal, said particles having a typical size range from 0.02 to 0.3 micrometres and an associated specific surface area oftypically 30 square metres per gram; and (b) pelletizing the said fine fraction without binder or other additive in the presence of water; further characterised in thatthe said green pellets upon subsequent drying and calcination can produce a material suitable for use as a proppant having a density of less than 3.4 g/cm.

5. A process for manufacture of a material suitable for use as a proppant, characterised bythefollowing steps: (a) separating a fine fraction from a naturally occurring bauxite containing same; said fine fraction being composed largely of monominerallic particles of gibbsite, boehmite and kaolinite, the kaolinite representing no morethan 25% ofthetotal, said particles having a typical size range from 0.02 to 0.3 micrometres and an associated specific surface area of typically 30 square metres per gram; (b) pelletizing the separated fine fraction in water without binder or other additive to produce green pellets, said pellets having sufficient strength in both wet and dry states to retain their integrity during handling and thermal processing operations;; (c) treating the pellets produced in step (b) to remove watertherefrom; (d) sintering the product of step (c) at a temperature in the range 1350 to 1500'Cto produce a material suitablefor use as a proppant, said material having a densityoflessthan 3.4g/cm3.

6. A material suitable for use as a proppant, produced by the process of Claim 1 or Claim 2 or Claim 5,and further characterised in that the permeability of a 20/40 mesh (850/425 micrometre) fraction at 10,000 psi exceeds 50% of the value attained at 2,000 psi and further retains a permeability at 15,000 psi of about 30% of the value at 2,000 psi.

7. A material according to Claim 6 fu rther characterised in that it is in the form of spherical particles inthe size range 2 mm to 0.3 mm suitable for use as proppant in hydra ulic fractu ring of formations exhibiting pressures of up to 20,000 psi, in which the particles are predominantly composed of microcrystallites of corundum amd mullite.

8. A process according to Claim 1, substantially as described in Example 1 herein.

9. Material according to Claim 3, substantially as described in Example 1 herein.