Abstract:
A wellbore fluid comprising an aqueous carrier liquid, hydrophobic fibers suspended therein, hydrophobic particulate material also suspended in the carrier liquid, and a gas to wet the surfaces of the particles and fibers and bind them together as agglomerates. The wellbore fluid may be a slickwater fracturing fluid and may be used for fracturing a tight gas reservoir. 
     Using a combination of hydrophobic particulate material, hydrophobic fibers and gas inhibits settling out of the particulate material from an aqueous liquid. Because the gas acts to wet the surfaces of both materials and agglomerates them, the particulate material is made to adhere to the fibers; the fibers form a network which hinders settling of the particulate material adhering to them, and the agglomerates contain gas and so have a bulk density which is less than the specific gravity of the solids contained in the agglomerates.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to delivery of particulate material to a location below ground. A significant application is as part of a method of hydraulic fracturing of a subterranean reservoir formation, placing proppant in the fracture so as to keep the fracture open as a flow path. However, the invention also extends to other applications where placing of particulate material underground, notably within subterranean reservoirs, is required. It is envisaged that the invention will be used in connection with exploration for, and production of, oil and gas. 
       BACKGROUND OF THE INVENTION 
       [0002]    Placing particulate material at a location below ground is a very significant part of a hydraulic fracturing operation. It may also be done in the context of various other operations carried out on underground wells, including plugging, diversion, control of lost circulation and zonal isolation. 
         [0003]    Hydraulic fracturing is a well established technique for reservoir stimulation. Fluid is pumped under pressure into a subterranean formation, forcing portions of the formation apart and creating a thin cavity between them. When pumping is discontinued the natural pressure in the subterranean formation tends to force the fracture to close. To prevent the fracture from closing completely it is normal to mix a solid particulate material (termed a proppant) with the fracturing fluid at the surface and use the fluid to carry the proppant into the fracture. When the fracture is allowed to close, it closes onto the proppant and a flow path to the wellbore between the proppant particles remains open. The proppant is then under considerable pressure from the formation rock pressing on it. 
         [0004]    When the proppant is mixed with the fracturing fluid at the surface and pumped into the wellbore it is subjected to very high shear. The proppant-laden fluid then flows down the wellbore under conditions of lower shear. Subsequently it turns and flows out of the wellbore and into the fracture in the formation. Entry to the fracture may be associated with an increase in shear, in particular if the wellbore is cased and the fluid passes through perforations in the wellbore casing to enter the fracture. Once the fluid enters the fracture, the fluid is subjected to much less shear and suspended solid begins to settle out. Subsequently pumping is discontinued, allowing the fracture to close onto the proppant packed in the fracture. 
         [0005]    In order that the fluid can convey particulate material in suspension and place it across the fracture face, it is conventional to include a viscosity-enhancing thickening agent in the fluid. Typically the fluid is then formulated so as to achieve a viscosity of at least 100 centipoise at 100 sec −1 . Guar is widely used for this purpose. Guar derivatives and viscoelastic surfactants may also be used. However, for some fracturing operations, especially where the rock has low permeability so that leak off into the rock is not a significant issue, it is preferred to pump a fluid, often called “slickwater”, which is water or salt solution containing a small percentage of friction reducing polymer which does not enhance viscosity as much as a thickening agent such as guar. The fluid then has low viscosity. This considerably reduces the energy required in pumping but keeping particulate material in suspension becomes much more difficult and a higher pump flow rate is commonly used. 
         [0006]    As recognized in Society of Petroleum Engineers Papers SPE98005, SPE102956 and SPE1125068, conventional proppant particles suspended in slickwater pumped into a large fracture will settle out more quickly than is desired and form a so-called “bank” or “dune” close to the wellbore. Because of this premature settling, proppant may not be carried along the fracture to prop the full length of the fracture and proppant may not be placed over the full vertical height of the fracture. When pumping is stopped and the fracture is allowed to close, parts of the fracture further from the wellbore may not contain enough proppant to keep them sufficiently open to achieve the flow which would be desirable. As a result, the propped and effective fracture size may be less than the size created during fracturing. 
         [0007]    One approach to improving the transport of particulate proppant has been to use a material of lower specific gravity in place of the conventional material which is sand or other relatively heavy mineral (sand has a specific gravity of approximately 2.65). SPE84308 describes a lightweight proppant having a specific gravity of only 1.75 which is a porous ceramic material coated with resin so that pores of the ceramic material remain air-filled. This paper also describes an even lighter proppant of specific gravity 1.25 which is based on ground walnut hulls. This is stated to be a “resin impregnated and coated chemically modified walnut hull”. 
         [0008]    These lightweight proppants are more easily suspended and transported by slickwater and their use is further discussed in SPE90838 and SPE98005, the latter paper demonstrating that settling out is reduced compared to sand, although not entirely avoided. There have been a number of other disclosures of proppants lighter than sand. Examples are found in U.S. Pat. Nos. 4,493,875 and 7,491,444 and in US patent applications 2005/096207, 2006/016598 and 2008/277115. 
         [0009]    A recognized issue with lightweight proppants is that they are frequently not as strong as sand and are at risk of becoming partially crushed when a hydraulic fracture is allowed to close on the proppant placed within it. An approach to the suspension of particulate proppant which seeks to avoid this issue is disclosed in US2007/015669, also in WO2009/009886 and in “Lightening the Load” New Technology Magazine, January/February 2010 pages 43 and 44. According to the teachings of these documents, a conventional proppant such as sand is treated to render its surface hydrophobic and is added to the slurry of proppant and water. Bubbles adsorb to the hydrophobic solid particles so that the adsorbed gas gives the particles a lower effective density. The literature describing this approach advocates it on grounds that the conventional sand is both cheaper and stronger than lightweight proppant. 
         [0010]    A further approach to retarding the settling out of proppant within a fracture is to incorporate fibrous material in the composition. SPE102956 teaches that incorporation of fibers can create a fiber-based network within the fracturing fluid which entangles proppant and reduces proppant settling. 
         [0011]    U.S. Pat. No. 7,665,522 discloses fracturing fluids which contained viscosifying agent, proppant, gas to form a foam (so that the fluid is a so-called “energized fluid”) and fibers to improve proppant suspension and transport. The possibility of hydrophobic fibers is mentioned in passing but it is stated that hydrophilic fibers are preferred. 
       SUMMARY OF THE INVENTION 
       [0012]    According to one aspect of the present invention there is provided a wellbore fluid comprising an aqueous carrier liquid, hydrophobic fibers suspended therein, hydrophobic particulate material also suspended in the carrier liquid, and a gas to wet the surfaces of the particles and fibers and bind them together as agglomerates. 
         [0013]    In a second aspect the invention provides a method of delivering particulate material below ground, comprising supplying, underground, a fluid composition comprising an aqueous carrier liquid in which there are suspended hydrophobic fibers and hydrophobic particulate material, the fluid also comprising a gas wetting the surfaces of the particles and fibers and binding them together such that agglomerates of the particulate material and fibers held together by the gas are present below ground. 
         [0014]    Using a combination of hydrophobic particulate material, hydrophobic fibers and gas inhibits settling out of the particulate material from an aqueous liquid. Several effects work together: 
         [0015]    because the gas acts to wet the surfaces of both materials and agglomerates them, the particulate material is made to adhere to the fibers; 
         [0016]    the fibers form a network which hinders settling of the particulate material adhering to them, and 
         [0017]    the agglomerates contain gas and so have a bulk density which is less than the specific gravity of the solids contained in the agglomerates. 
         [0018]    In practice it is likely that the particulate material will be smaller than the fiber length. To put that more precisely: it is likely that the median particle size of the particulate material will be less than the median length of the fibers. Indeed 90% by volume of the particulate material may have a largest particle dimension which is less than half possibly less than one fifth, of the median length of the fibers. 
         [0019]    The wellbore fluid is preferably envisaged to be a fracturing fluid in which the hydrophobic particulate material serves as a proppant. This hydrophobic particulate material may have a particle size and size distribution such that more than 90% of the particles have a particle size below 1 mm. The hydrophobic fibers are likely to have a mean length greater than 1 mm, perhaps greater than 2 mm, with a fiber diameter of not more than 100 micron. 
         [0020]    For embodiments of this invention used for fracturing a formation, the formation may be a gas reservoir and the fracturing step may be followed by producing gas, gas condensate or a combination of them from the formation through the fracture and into a production conduit in fluid communication therewith. 
         [0021]    Agglomeration may be prevented or reversed when the composition is subjected to shear. As already mentioned a composition which is pumped downhole is subjected to varying amounts of shear in the course of the journey downhole. Consequently agglomeration may take place at the subterranean location to which the particulate material is delivered. However, it is possible that agglomeration may take place or commence in the course of flow towards the subterranean location where the material will serve its purpose. 
         [0022]    We have observed that hydrophobic particulate material remains suspended whereas in a comparison experiment using hydrophilic particulate material of similar size, the particulate material did not remain suspended amongst the hydrophobic fibers but instead settled out. 
         [0023]    Inhibiting the settling out of particulate solid allows it to be transported more effectively to its intended destination. In the context of hydraulic fracturing the length and/or vertical height of fracture (i.e. the fracture area) which remains propped open after pumping has ceased is greater than would be the case if the particulate material was used alone, without fibers. 
         [0024]    The particulate material and the fibers must have hydrophobic surfaces in order that they can be agglomerated. They may be formed of materials which are inherently hydrophobic or may be formed of materials which are hydrophilic but have a hydrophobic coating on their surface. For instance, ordinary silica sand which is commonly used as a proppant is hydrophilic and is not agglomerated by oil or gas in the presence of water. By contrast, we have found that sand which has been surface treated to make it more hydrophobic will spontaneously agglomerate in the presence of oil, air or nitrogen gas. The particulate material used for this invention may be such hydrophobically modified sand. Likewise glass fibers are hydrophilic and are not agglomerated by oil or gas but they can be surface treated so as to be hydrophobic. 
         [0025]    A quantitative indication of the surface polarity of a solid (prepared with a smooth, flat surface) is the concept of critical surface tension pioneered by Zisman (see Fox and Zisman J. Colloid Science Vol 5 (1950) pp 514-531 at page 529). It is a value of surface tension such that liquids having a surface tension against air which is lower than or equal to this value will spread on the surface of the solid whereas those of higher surface tension will remain as droplets on the surface, having a contact angle which is greater than zero. A strongly hydrophobic solid has a low critical surface tension. For instance the literature quotes a critical surface tension for polytetrafluoroethylene (PTFE) of 18.5 mN/m and for a solid coated with heptadecafluoro-1,1,2,2-tetra-hydro-decyl-trichlorosilane the literature value of critical surface tension is 12 mN/m. By contrast the literature values of critical surface tension for soda-lime glass and for silica are 47 and 78 mN/m respectively. 
         [0026]    We have found that an analogous measurement of the hydrophobicity of the surface of a particulate solid can be made by shaking the solid with a very hydrophobic oil (preferably a silicone oil) having a low surface tension and mixtures of ethanol and water with a progressively increasing proportion of ethanol. This may be done at a room temperature of 20° C. The surface tensions of a number of ethanol and water mixtures are tabulated in CRC Handbook of Chemistry and Physics, 86 th  edition, section 6 page 131. This measurement could also be carried out on fibers. 
         [0027]    Increasing the proportion of ethanol in the aqueous phase (i.e. the ethanol and water mixture) reduces its surface tension. Eventually a point is reached when the surface tension of the aqueous phase is so low that the solid can no longer be agglomerated by the oil. The boundary value at which agglomeration by the oil ceases to occur is a measure of the hydrophobicity of the solid and will be referred to as its “agglomeration limit surface tension” or ALST. 
         [0028]    We have observed that particulate solids which can undergo spontaneous aggregation from suspension in deionised water on contact with oil always display an ALST value of approximately 40 mN/m or less. This ALST test covers a range of values of practical interest, but it should be appreciated that if no agglomeration takes place, this test does not give a numerical ALST value, but demonstrates that the surface does not have an ALST value of 40 mN/m or less. Moreover, if the surface has an ALST value below the surface tension of pure ethanol (22.4 mN/m at 20° C.), this test will not give a numerical ALST value but will show that the ALST value is not above 22.4 mN/m. 
         [0029]    When particulate materials or fibers are not inherently hydrophobic, a range of different methods can be used to modify the surface of the particles or fibers to become more hydrophobic—these include the following, in which the first three methods provide covalent bonding of the coating to the substrate. 
         [0030]    Organo-silanes can be used to attach hydrophobic organo-groups to hydroxyl-functionalized mineral substrates such as proppants composed of silica, silicates and alumino-silicates. The use of organosilanes with one or more functional groups (for example amino, epoxy, acyloxy, methoxy, ethoxy or chloro) to apply a hydrophobic organic layer to silica is well known. The reaction may be carried out in an organic solvent or in the vapour phase (see for example Duchet et al, Langmuir (1997) vol 13 pp 2271-78). 
         [0031]    Organo-titanates and organo-zirconates such as disclosed in U.S. Pat. No. 4,623,783 can also be used. The literature indicates that organo-titanates can be used to modify minerals without surface hydroxyl groups, which could extend the range of materials to undergo surface modification, for instance to include carbonates and sulphates. 
         [0032]    A polycondensation process can be used to apply a polysiloxane coating containing organo-functionalized ligand groups of general formula P—(CH 2 ) 3 —X where P is a three-dimensional silica-like network and X is an organo-functional group. The process involves hydrolytic polycondensation of a tetraalkoxysilane Si(OR) 4  and a trialkoxy silane (RO) 3 Si(CH 2 ) 3 X. Such coatings have the advantage that they can be prepared with different molar ratios of Si(OR) 4  and (RO) 3 Si(CH 2 ) 3 X providing “tunable” control of the hydrophobicity of the treated surface. 
         [0033]    A fluidized bed coating process can be used to apply a hydrophobic coating to a particulate solid substrate. The coating material would typically be applied as a solution in an organic solvent and the solvent then evaporated within the fluidized bed. 
         [0034]    Adsorption methods can be used to attach a hydrophobic coating on a mineral substrate. A surfactant monolayer can be used to change the wettability of a mineral surface from water-wet to oil-wet. Hydrophobically modified polymers can also be attached by adsorption. 
         [0035]    The surface modification processes above may be carried out as a separate chemical process before the wellbore fluid is formulated. Such pretreatment of material to make it hydrophobic would not necessarily be carried out at the well site; indeed it may be done at an industrial facility elsewhere and the pretreated material shipped to the well site. However, it is also possible that some of the above processes, especially an adsorption process, could be carried out at the well site as part of the mixing procedure in which the wellbore fluid is made. 
         [0036]    The solid particulate material used in this invention may vary considerably in shape and size. The particles may have shapes typical of sand grains which can be loosely described as “more spherical than elongate” where the aspect ratio between the longest dimension and the shortest dimension orthogonal to it might be 5 or less or even 2 or less. Other shapes such as cylinders or cubes are possible, notably if the particles are a manufactured ceramic product. A further possibility is that solid particles can have a platy form as is the case with mica particles. In general, median particle sizes are unlikely to be larger than 5 mm and the aspect ratio is unlikely to exceed 25. Median particle sizes are more likely to be 3 mm or less and preferably are 1.6 mm or less. Embodiments of this invention may use mixtures of solid particles where the median particle size is less than 1 mm. 
         [0037]    Particle sizes may conveniently be specified by reference to sieve sizes, as is traditional for proppant materials. American Petroleum Institute Recommended Practices (API RP) standards 56 and 60 specify a number of proppant sizes by stating upper and lower US Sieve sizes. 90 wt % of a sample should pass the larger sieve but be retained on the smaller sieve. Thus ‘20/40 sand’ specifies sand having a particle size distribution such that 90 wt % of it passes 20 mesh (840 micron) but is retained on 40 mesh (420 micron). Correspondingly 90 wt % of a sample of 70/140 sand, which is the smallest size, recognized by these standards, passes a 70 mesh (210 micron) sieve but is retained on a 140 mesh (105 micron) sieve. It will be appreciated that for any proppant specified by upper and lower sieve sizes, the median and mean particle sizes fall somewhere between the upper and lower sieve sizes. 
         [0038]    Another method for determining size of particles is the commonly used technique of low angle laser light scattering, more commonly known as laser diffraction. Instruments for carrying out this technique are available from a number of suppliers including Malvern Instruments Ltd., Malvern, UK. The Malvern Mastersizer is a well known instrument which determines the volumes of individual particles, from which mean and median particle size can be calculated using computer software which accompanies the instrument. When determining particle sizes using such an instrument, the size of an individual particle is reported as the diameter of a spherical particle of the same volume, the so-called “equivalent sphere”. Volume median diameter denoted as D[v,05] or d 50  is a value of particle size such that 50% (by volume) of the particles have a volume larger than the volume of a sphere of diameter d 50  and 50% of the particles have a volume smaller than the volume of a sphere of diameter d 50 . Particle sizes determined by low angle laser light scattering are similar to particle sizes determined by sieving if the particles are approximately spherical. 
         [0039]    Particle size distribution is then conveniently indicated by the values of d 10  and d 90  measured in the same way. 10% by volume of the particles in a sample have an equivalent diameter smaller than d 10 . 90% by volume are smaller than d 90  and so 10% by volume are larger than d 90 . The closer together the values of d 10  and d 90 , the narrower is the particle size distribution. 
         [0040]    In forms of this invention where the first particulate material is a proppant for hydraulic fractures, the first particles may have a d 90  upper size similar to that of conventional proppant, such as 10 mesh (2 mm) or 20 mesh (840 microns). Their particle size properties may be such that 
         [0041]    d 10 &gt;110 micron, possibly &gt;120 micron or &gt;150 micron 
         [0042]    d 50 &lt;1 mm, possibly &lt;800 micron 
         [0043]    d 90 &lt;3 mm, possibly &lt;2 mm or &lt;1 mm 
         [0044]    The particle size distribution may be sufficiently wide that d 90  is more than 5 times d 10 , possibly more than 10 times d 10 . These particle size properties may also apply to other forms of this invention, such as those where the method of the invention is applied to preventing lost circulation, or achieving isolation of one zone from another. 
         [0045]    However, this invention can be implemented with a fine particulate material having a d 50  of 105 microns or less. A fine mesh proppant may have d 90  below 150 microns, d 50  below 105 micron and d 10  of at least 10 microns. We have found that use of a fine particulate material may be advantageous for fracturing shale, notably for fracturing of a gas-bearing shale. Using a fine mesh proppant, that is to say a proppant of small particle size, may be expected to lead to fractures with lower permeability and conductivity than would be achieved with a proppant of larger size. Nevertheless, such fractures can carry gas with acceptable flow rates. The benefit of conveying proppant further into a fracture, so that the fracture has a greater effective size after it closes onto the proppant outweighs the lower conductivity. Overall, the stimulation of the formation is greater. 
         [0046]    Numerous materials which are hydrophobic or which have a hydrophobic surface could be employed as the particulate material. Lightweight proppant materials with a specific gravity of 1.8 or less might possibly be used, if treated to have a hydrophobic surface. Denser materials such as hydrophobically modified sand, which has good strength when used as a proppant, may be preferred and may have a specific gravity of above 1.8, possibly at least 2.0 or 2.5. Specific gravity of a particle is the weight of a particle relative to the weight of an equal volume of water, without including any other material in interstitial spaces between particles. Sand is almost entirely silica and has a specific gravity of 2.65. As an alternative to sand or other mineral, the particulate material could be a manufactured ceramic proppant, treated to give it a hydrophobically modified surface. Another possible material to be hydrophobically modified and used in this invention is flyash recovered from the flue gas of coal fired power plants. This is a small particle size material with a high silica content. It typically has d 90  below 100 micron and specific gravity in the range 1.9 to 2.4. 
         [0047]    The fibers used for this invention may also have a specific gravity of 1.8 or above, possibly at least 2.0 or 2.5. One possibility is that the fibers are glass fibers (having a specific gravity above 2.5) treated to have a hydrophobic surface. Alternatively the fibers may be inherently hydrophobic. Available hydrophobic materials tend to have lower density than sand or glass. Thus, one possibility within this invention is that the particulate material is a solid mineral material such as sand with a hydrophobic coating on the exterior of the particles and having a specific gravity of 1.8 or more while the fibers are formed of an inherently hydrophobic material with specific gravity below 1.8, possibly in a range from 0.8 to 1.5. Possible materials include polyethylene and polypropylene. 
         [0048]    The amounts of the particulate material and the fibers may be such that they have a volume ratio in a range from 30:1 to 1:5, possibly 20:1 to 1:3. The volume of the particulate material may exceed the volume of the fibers. Thus the ratio may be from 30:1 or 20:1 to 3:1 or 1.5:1. Of course if the particulate material and fibers have similar specific gravities, their weight ratio and volume ratio will be similar. 
         [0049]    Both the particulate solid and the fibers must of course form a separate solid phase when the agglomeration takes place. At this time they must therefore be insoluble in the carrier liquid, or at least be of low solubility. For many applications of this invention it will be desirable that both the fibers and the particulate material remain insoluble after agglomeration has taken place. However, it is within the scope of some forms of this invention that the agglomerates might not have a permanent existence. For instance, the fibers might be formed of a material with a limited lifetime. After the fracture has closed onto a proppant, trapping that proppant in place, the agglomerates and the fibers will have served their purpose and the fibers could be allowed to decompose. Fibers are likely to have a length which is more than 5 times, possibly more than 50 times, their transverse width or diameter. Thus in this invention the fibers may have a diameter in a range from 5 to 100 micron and a median length of 3 mm or more, probably 5 mm or more. 
         [0050]    The agglomerating agent which binds the particles together as agglomerates is a gas. This gas must be sufficiently hydrophobic to form a phase which does not dissolve in the aqueous carrier liquid, although it is possible for it to have some limited water solubility, as is the case with air and with nitrogen. 
         [0051]    We have observed that the amount of gas incorporated in agglomerates has an upper limit which is governed by geometry of packing and properties of the materials. It should be appreciated that when agglomerates are formed in accordance with this invention the amount of gas in the agglomerates may not be the maximum amount which the agglomerates are capable of retaining, although it must be sufficient that it agglomerates the particles and fibers at downhole pressure. 
         [0052]    We have found that agglomeration by gas may be assisted and improved if a small amount of hydrophobic oil is present. However, the amount should be small, and desirably will be less than 10% or even less than 5% or 2% by volume of the amount of gas downhole. If the amount of oil is larger, agglomeration occurs but the oil displaces gas from the agglomerates and so the amount of gas which can be held by agglomerates is reduced. 
         [0053]    The aqueous carrier liquid which is used to transport the particles may be a non-viscous or slickwater formulation. Such a formulation is typically water or a salt solution containing at least one polymer which acts as a friction reducer. A combination of polymers may be used for this purpose. Polymers which are frequently used and referred to as “polyacrylamide” are homopolymers or copolymers of acrylamide. Incorporation of a copolymer can serve to give the “modified” polyacrylamide some ionic character. A polyacrylamide may considered a copolymer if it contains more than 0.1% by weight of other comonomers. Mixtures of homopolymers and copolymers may be used. Copolymers may include two or more different comonomers and may be random or block copolymers. The comonomers may include, for example, sodium acrylate. The polyacrylamide polymers and copolymers useful as friction reducers may include those having an average molecular weight of from about 1000 up to about 20 million, or possibly above, with from about 1 million to about 5 million being typical. Other suitable friction reducers may be used as well; for example vinyl sulfonates included in poly(2-acrylamido-2-methyl-1-propanesulfonic acid) also referred to as polyAMPS. 
         [0054]    The polyacrylamide may be used in the treatment fluid an amount of from about 0.001% to about 5% by weight of the treatment fluid but the amount is frequently not over 1% or even 0.5% by weight by weight. In many applications, the polyacrylamide is used in an amount of from about 0.01% to about 0.3% by weight of the fluid. The polyacrylamide may be initially dissolved or dispersed as a concentrate in mineral oil or other liquid carrier to enhance the delivery or mixability prior to its addition to water or a salt solution to make the carrier liquid. 
         [0055]    A slickwater formulation may be substantially free of viscosity-enhancing polymeric thickener and have a viscosity which is not much greater than water, for instance not more than 15 centipoise which is about 15 times the viscosity of water, when viscosity is measured at 20° C. and a shear rate of 100 sec −1 . 
         [0056]    It is particularly envisaged that this invention will be used when fracturing a reservoir formation which has low permeability, so that slick water is the fracturing fluid of choice. As mentioned above, this has considerable advantage when pumping into the formation but makes suspension of proppant much more difficult. 
         [0057]    A reservoir formation of low permeability may well be a gas reservoir, although this is not inevitably so. It may be a gas-bearing shale. A formation of low permeability may have a permeability of no more than 10 millidarcies (10 mD), possibly no more than 1 millidarcy. Its permeability may be even lower, such as less than 100 microdarcy, or even less than 1 microdarcy. This invention may also be used where the carrier liquid is a more traditional fracturing fluid incorporating a thickening agent to increase the viscosity of the fluid. Such a thickening agent may be a polymer. It may be a polysaccharide such as guar, xanthan or diutan or a chemically modified polysaccharide derivative such as hydroxyalkylcellulose or a hydroxyalkylguar. These polysaccharide thickeners may be used without cross linking or may be cross-linked to raise viscosity further. Viscoelastic surfactants are another possible thickening agent which may be used to increase viscosity. We have observed that some thickening of the carrier liquid does not prevent agglomeration, although it may be preferred that the viscosity is not allowed to become too high before agglomeration takes place. 
         [0058]    Distributing proppant throughout a fracture, but with some non-uniformity in the distribution of proppant (sometimes referred to as heterogeneous proppant placement) may be helpful in enhancing fracture conductivity when the fracture is allowed to close onto the proppant. In some embodiments of this invention, localized non-unity may be deliberately enhanced. 
         [0059]    One known method for heterogenous proppant placement which may be used in this invention is to pump a fluid containing suspended proppant alternately with a fluid containing less of the suspended proppant or none at all. This approach is the subject of U.S. Pat. No. 6,776,235. Another known method which may be employed is to pump the proppant together with a removable material, referred to as a ‘channelant’. After pumping has ceased and the fracture has closed onto proppant in the fracture, removal of the channelant leaves open pathways between islands or pillars of the proppant. This approach is the subject of WO2008/068645, the disclosure of which is incorporated herein by reference. 
         [0060]    A degradable channelant material may be selected from substituted and unsubstituted lactide, glycolide, polylactic acid, polyglycolic acid, copolymers of polylactic acid and polyglycolic acid, copolymers of glycolic acid with other hydroxy-, carboxylic acid-, or hydroxycarboxylic acid-containing moieties, copolymers of lactic acid with other hydroxy-, carboxylic acid-, or hydroxycarboxylic acid-containing moieties, and mixtures of such materials. Representative examples are polyglycolic acid or PGA, and polylactic acid or PLA. These materials function as solid-acid precursors, and undergo hydrolytic degradation in the fracture. 
         [0061]    Apart from application in hydraulic fracturing as discussed above the invention may also be used when the objective is to close rather than create a flow path. Agglomeration of particles with gas to improve transport of the particles may be utilised when transporting particles to form a plug to divert the flow or block off one zone of a formation from another. 
         [0062]    Agglomerates of hydrophobic particles and a gas as agglomerating agent will form spontaneously in an aqueous carrier liquid when the materials are mixed together. One possibility is that the particulate materials, carrier liquid and agglomerating gas are all mixed together at the surface and then pumped down a wellbore. In this case the particles may agglomerate before passing through the pumps. If so, they may be sheared apart by the pumps, but spontaneously reform downstream of the pumps as they pass down the wellbore. 
         [0063]    A possibility to avoid passing the agglomerates through the pumps is that the gas is compressed at the surface and then admitted to the high pressure flowline downstream of the surface pumps which are driving the carrier liquid and the particulate materials into the wellbore. As a variant of this, the gas could be transported down the wellbore in a separate pipe so as to travel to a considerable depth underground before mixing with the particulate materials. 
         [0064]    Another approach is to allow the materials to mix, but inhibit agglomeration for at least part of the journey of the carrier liquid and entrained materials to the subterranean location where the agglomerates are required. Some possibilities for this are as follows: 
         [0065]    Encapsulation or coating. The particulate materials are coated with a hydrophilic material which dissolves slowly or undergoes chemical degradation under conditions encountered at the subterranean location, thereby exposing the hydrophobic surface within. Degradation may in particular be hydrolysis which de-polymerises an encapsulating polymer. While such hydrolytic degradation may commence before the overall composition has travelled down the wellbore to the reservoir, it will provide a delay before contact between agglomerating gas and exposed hydrophobic surface becomes significant. 
         [0066]    A number of technologies are known for the encapsulation of one material within another material. Polymeric materials have frequently been used as the encapsulating material. Some examples of documents which describe encapsulation procedures are U.S. Pat. No. 4,986,354, WO 93/22537, and WO 03/106809. Encapsulation can lead to particles in which the encapsulated substance is distributed as a plurality of small islands surrounded by a continuous matrix of the encapsulating material. Alternatively encapsulation can lead to core-shell type particles in which a core of the encapsulated substance is enclosed within a shell of the encapsulating material. Both core-shell and islands-in-matrix type encapsulation may be used. 
         [0067]    An encapsulating organic polymer which undergoes chemical degradation may have a polymer chain which incorporates chemical bonds which are labile to reaction, especially hydrolysis, leading to cleavage of the polymer chain. A number of chemical groups have been proposed as providing bonds which can be broken, including ester, acetal and amide groups. Cleavable groups which are particularly envisaged are ester and amide groups both of which provide bonds which can be broken by a hydrolysis reaction. 
         [0068]    Generally, their rate of cleavage in aqueous solution is dependent upon the pH of the solution and its temperature. The hydrolysis rate of an ester group is faster under acid or alkaline conditions than neutral conditions. For an amide group, the decomposition rate is at a maximum under low pH (acidic) conditions. Low pH, that is to say acidic, conditions can also be used to cleave acetal groups. 
         [0069]    Thus, choice of encapsulating polymer in relation to the pH which will be encountered after the particles have been placed at intended subterranean location may provide a control over the delay before the encapsulated material is released. Polymers which are envisaged for use in encapsulation include polymers of hydroxyacids, such as polylactic acid and polyglycolic acid. Hydrolysis liberates carboxylic acid groups, making the composition more acidic. This lowers the pH which in turn accelerates the rate of hydrolysis. Thus the hydrolytic degradation of these polymers begins somewhat slowly but then accelerates towards completion and release of the encapsulated material. Another possibility is that a polymer containing hydrolytically cleavable bonds may be a block copolymer with the blocks joined through ester or amide bonds. 
         [0070]    Sensitivity to temperature. A development of the use of a hydrophilic coating makes use of the difference between surface temperatures and temperatures below ground, which are almost always higher than at the surface. During transit to the subterranean location, the carrier liquid and everything suspended in it will pass through a wellbore exposed to subterranean temperatures and will begin to heat up, but if the flow rate is substantial, the flowing composition will reach the subterranean location at a temperature well below the natural temperature at that location. In particular, in the case of hydraulic fracturing the fracturing fluid will leave the wellbore and enter the fracture at a temperature significantly below the reservoir temperature. A possibility, therefore, would be to coat the hydrophobic particles with a coating of a hydrophilic material which remains intact at surface temperatures, but melts or dissolves in the carrier liquid at the temperature encountered below ground. 
         [0071]    Generating gas below ground. Another possible approach for delaying agglomeration during at least part of the journey of the materials to the subterranean location where the agglomerates are required is to generate the agglomerating gas chemically, for example by including aluminium powder in the composition and formulating the carrier liquid to be alkaline so that hydrogen is generated by reaction of aluminium and the aqueous alkaline carrier liquid. Conversely, iron or zinc particles could be incorporated in a fluid with pH below 7 to generate hydrogen. A further possibility for generating gas below ground would be to pump a neutral slickwater fluid containing suspended calcium carbonate particles, followed by an acidic slickwater fluid containing hydrophobic fibers and hydrophobic particulate proppant. Carbon dioxide would then be liberated below ground on contact between the acidic slickwater and the carbonate previously placed below ground. In the methods above for generating gas below ground, the solid material could be encapsulated in or coated with a material which dissolves or melts at the reservoir temperature, thus delaying the start of the chemical generation of gas. Another way to generate carbon dioxide would be to incorporate nanoparticulate polycarbonates which decompose, liberating carbon dioxide, at a temperature of around 150° C. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0072]      FIGS. 1 to 3  diagrammatically illustrate samples made in Examples below; 
           [0073]      FIG. 4  schematically illustrates the use of the invention in fracturing, and 
           [0074]      FIG. 5  illustrates fracturing from a horizontal wellbore. 
       
    
    
     DETAILED DESCRIPTION 
     Example 1 
     Hydrophobic Modification of Sand 
       [0075]    Sand, having particle size between 20 and 40 US mesh (840 micron and 400 micron), i.e. 20/40 sand, was washed by mixing with ethanol at ambient temperature, then filtering, washing with deionised water and drying overnight at 80° C. 
         [0076]    Quantities of this pre-washed sand were hydrophobically modified by treatment with various reactive organosilanes, using the following procedure. 75 gm pre-washed sand was added to a mixture of 200 ml toluene, 4 ml organo-silane and 2 ml triethylamine in 500 ml round bottomed flask. The mixture was refluxed under a nitrogen atmosphere for 4 to 6 hours. After cooling, the hydrophobically modified sand (hm-sand) was filtered off (on a Whatman glass microfiber GF-A filter) and then washed, first with 200 ml toluene, then 200 ml ethanol and then 800 ml deionised water. The hm-sand was then dried overnight at 80° C. 
         [0077]    The above procedure was carried out using each of the following four reactive organo-silanes: 
         [0078]    5.64 gm Heptadecafluoro-1,1,2,2-tetrahydro-decyl-triethoxysilane (&gt;95% purity, specific gravity=1.41 gm/ml). 
         [0079]    5.40 gm Tridecafluoro-1,1,2,2-tetrahydro-octyl-triethoxysilane (&gt;95% purity, specific gravity=1.35 gm/ml). 
         [0080]    3.53 gm Octadecyl-trimethoxysilane (90% purity, specific gravity=0.883 gm/ml). 
         [0081]    5.93 gm Octadecyldimethyl 3-trimethoxysilylpropyl ammonium chloride (60% active solution in methanol, specific gravity=0.89 gm/ml). 
         [0000]    For convenience the hydrophobic groups introduced by these materials will be referred to hereafter as C 10 F 17 H 4 -silyl, C 8 F 13 H 4  silyl, C 18 H 37 -silyl and C 18 H 37 -aminopropylsilyl, respectively. 
         [0082]    It was appreciated that these quantities of organo-silane were far in excess of the stoichiometric amount required to react with all the hydroxyl groups on the surface of the sand particles. 20/40 sand has specific surface area 0.0092 m 2 /gm (calculated from particle size distribution determined by laser diffraction (Malvern Mastersizer) method). The theoretical maximum concentration of hydroxyl (—OH) groups per unit area of silica surface, is 4.5 hydroxyl groups per square nanometre. From these values it can be calculated that 75 gm sand has (at most) 3.1×10 18  hydroxyl groups exposed on its surface. Using Avogadro&#39;s number, 5.64 gm (0.00924 mol) heptadecafluoro-1,1,2,2-tetra-hydro-decyl-triethoxysilane contains 5.56×10 21  molecules. Therefore there is a very high ratio of organo-silane molecules in the reaction solution to surface hydroxyl groups. The calculated number ratio in the case of the C 10 F 17 H 4 -silyl example above was organo-silane (solution) /OH (surface) =1792. It should be noted that at least some excess organosilane is removed from the treated sand during the filtration and washing stages. 
       Example 2 
       [0083]    The procedure above was carried out with the following reduced quantities of organo-silane: 
         [0000]      0.27 gm Heptadecafluoro-1,1,2,2-tetra-hydro-decyl-triethoxysilane number ratio organo-silane (solution) /OH (surface) =85.8. 
         [0000]      0.02 gm Heptadecafluoro-1,1,2,2-tetra-hydro-decyl-triethoxysilane number ratio organo-silane (solution) /OH (surface) =6.4. 
         [0000]    It was found the smallest amount of organo-silane was insufficient to render the sand adequately hydrophobic to be agglomerated. 
       Example 3 
     Condensation Coating 
       [0084]    Pre-washed 20/40 sand, prewashed as in Example 1 above, was given a hydrophobic surface coating by the simultaneous condensation polymerization of tetraethylorthosilicate (TEOS) and tridecafluoro-1,1,2,2-tetrahydro-octyl-triethoxysilane in 3:1 molar ratio under basic conditions. 
         [0085]    200 gm pre-washed sand, 12 ml of aqueous ammonia (NH 4 OH, 28%), 57 ml of absolute ethanol and 3 ml deionized water were mixed and stirred vigorously (Heidolph mechanical stirrer at 300-400 RPM) for 30 min. Then 0.73 gm (3.53 mmol) of TEOS and 0.6 gm (1.17 mmol) tridecafluoro-1,1,2,2-tetrahydro-octyl-triethoxysilane were added and stirred for 3.5 hrs at room temperature. The resulting hm-sand was then filtered off, washed with ethanol and then with deionized water and dried at 120° C. overnight. 
         [0086]    This procedure was also carried out using pre-washed 70/140 sand with a mixture of tetraethylorthosilicate (TEOS) and heptadecafluoro-1,1,2,2-tetra-hydro-decyl-triethoxysilane. 
       Example 4 
     Condensation Coating of Glass Fibers 
       [0087]    The glass fibers used had a mean length of 20 mm and a diameter of 18 micron. 20 gm fibers, 12 ml of aqueous ammonia (NH 4 OH, 28%), 57 ml of absolute ethanol and 3 ml deionized water were mixed and stirred vigorously (Heidolph mechanical stirrer at 300-400 RPM) for 30 min. Then 0.73 gm (3.53 mmol) of TEOS and 0.6 gm (1.17 mmol) tridecafluoro-1,1,2,2-tetrahydro-octyl-triethoxysilane were added and stirred for 4 hrs at room temperature. The resulting hm-fibers were then filtered off, washed with ethanol and then with deionized water and dried at 120° C. overnight. 
       Example 5 
     Agglomeration of hm-Sand and hm-Glass Fibers 
       [0088]    A number of sample mixtures were prepared using 70/140 sand, hydrophobically modified with tridecafluoro-1,1,2,2-tetrahydro-octyl-triethoxysilane as in Example 3 together with glass fibers of 20 mm length which had been hydrophobically modified as in Example 4. Each sample was made with 80 ml of deionised water and 0.5 gm of the hm-fibers in a bottle of about 100 ml capacity, thus leaving an air-filled headspace of about 20 ml in the bottle. An amount of hm-sand was added to each bottle, then the bottle was closed and shaken vigorously so that the solids could be agglomerated with air from the headspace. 
         [0089]    After shaking samples where the amounts of hm-sand were 3.5 gm, 4.5 gm, 6.5 gm and 8.5 gm a fibrous network extended throughout the volume of liquid in the bottle and the hm-sand was distributed within this network. This is illustrated, in very diagrammatic form, by  FIG. 1  where the surface of the liquid in the bottle is denoted  10 , the network of fibers denoted  11  and sand agglomerated to fiber is indicated at  12 . 
         [0090]    Samples where the amount of hm-sand was 10.5 gm and 12.5 gm also contained a fibrous network  11  with the hm-sand  12  in it, but the fibrous network sank to the bottom half of the liquid in the bottle, as illustrated by  FIG. 2 . 
       Example 6 
     Comparative 
       [0091]    The previous example was repeated using hm fibers as before and 3.5 gm ordinary 70/140 sand. As shown in  FIG. 3 , after shaking, a fibrous network  11  extended throughout the volume of liquid in the bottle, but the unmodified sand was not suspended and settled as a layer  13  of sand on the bottom of the bottle. 
       Example 7 
       [0092]    The procedure of Example 5 was repeated, using 10 gm of 70/140 hm-sand in each sample and varying quantities of the hm-fibers. With 1 gm fibers and also with 0.5 gm fibers, after shaking, a fibrous network with the hm-sand distributed within it occupied about three quarters of the liquid volume. With 0.25 gm fibers the network occupied slightly less than half the liquid volume. With 0.1 gm fibers, the solids in the bottle were estimated to occupy between a quarter and a third of the liquid volume. Thus it was demonstrated that a sufficient quantity of the hm-fibers significantly enhances suspension of the hm-proppant. 
       Example 8 
       [0093]    Mica (muscovite of mean particle size 150 micron) was hydrophobically modified with heptadecafluoro-1,1,2,2-tetrahydro-decyl-triethoxy silane as in Example 1. A sample was prepared as in Examples 5 and 7, using 10 gm of hm-mica and 1 gm hm-fibers. After shaking a fibrous network with the mica absorbed in it was observed to extend throughout the entire volume of liquid in the bottle. Thus the suspension of solids in the liquid volume was even more efficient than the previous example&#39;s combination of hm sand and the same 1 gm quantity of hm-fibers. 
       Example 9 
     Comparative 
       [0094]    The previous example was repeated, again using 1 gm hm-fibers but with 10 gm of mica of the same size but which had not been hydrophobically modified. After shaking, a fibrous network  11  extended throughout the volume of liquid in the bottle similarly to  FIG. 3  (this volume of liquid appeared cloudy) but the unmodified mica settled as a layer  13  on the bottom of the bottle. 
       Example 10 
       [0095]    A sample was prepared as in Examples 5 and 7, using 10 gm of hm-sand, 0.5 gm hm-mica and 0.5 gm hm-fibers. After shaking, a fibrous network with the hm-sand and hm-mica distributed within it occupied about three quarters of the liquid volume and was very similar in appearance to the sample in Example 7 above with 10 gm hm-sand and 1 gm hm-fibers. 
       Example 11 
       [0096]    In order to demonstrate that agglomeration is dependent on gas, 0.4 gm hm-fibers and 3 gm hm-sand, both as in Example 5, were placed in a bottle filled to the brim with deionised water which had been de-gassed under vacuum. The bottle was closed and shaken. The fibers were seen to form a network in the bottle but the hm-sand settled to the bottom of the bottle. Despite repeated vigorous shaking the sand did not agglomerate with the fibers and settled to the bottom of the bottle. 
         [0097]    The bottle was then opened and about 20 ml water poured off, giving an air-filled headspace above the water in the bottle. The bottle was closed again and shaken. The fibers again appeared to form a network in the bottle, but the sand was held in the network, demonstrating that the air was enabling agglomeration of the sand with the fibers. 
       Example 12 
       [0098]    The procedure of Example 5 was repeated using hm-fibers of approx 5 mm length. The samples where the amounts of hm-sand were up to 6.5 gm contained a fibrous network throughout the volume of liquid in the bottle and the hm-sand was distributed within this network, as illustrated by  FIG. 1 . Samples where the amount of hm-sand was 8.5 gm or above also contained a fibrous network with the hm-sand in it, but the fibrous network sank to the bottom half of the bottle as illustrated by  FIG. 2 . 
       APPLICATION OF THE INVENTION 
       [0099]    To illustrate and exemplify use of some embodiments of the method of this invention,  FIG. 4  shows diagrammatically the arrangement when a fracturing job is carried out. A mixer  14  is supplied with water, fibers, particulate material and a small amount of viscosity reducing polymer as indicated by arrows W, F, P and V. The mixer  14  delivers a mixture of these materials to pumps  16  which pump the mixture under pressure down the production tubing  18  of a wellbore  20 . Nitrogen gas from a supply  22  pressurized by compressor  24  is driven down a tube  26  within the production tubing  18  and forms agglomerates of the fibers and the particulate material when it exits into the flow within tubing  18 . The aqueous carrier liquid and suspended agglomerates then pass through perforations  30  into the reservoir formation  32  as indicated by the arrows  34  at the foot of the well. 
         [0100]    In the early stages of the fracturing job, the liquid does not contain particulate solid nor fibers nor added nitrogen but its pressure is sufficiently great to initiate a fracture  36  in the formation  32 . Subsequently the particulate material and fibers are mixed with the liquid and nitrogen is supplied down tube  26  so as to mix downhole with the fluid which is being pumped in. Its pressure is sufficient to propagate the fracture  36  and as it does so it carries the suspended agglomerates  28  into the fracture  36 . Because the agglomerates have a low density they do not settle out at the entrance to the fracture, but are carried deep into the fracture. 
         [0101]      FIG. 5  illustrates the use of tubing  40 , which may be coiled tubing, to form fractures within a horizontal wellbore within reservoir formation  32 . As illustrated here, fracture  42  has already been formed and has been closed off by a temporary plug  44 . Fracture  46  is being formed. In a manner generally similar to the arrangement of  FIG. 4 , water, friction reducing polymer, fibers and the particulate material are supplied under pressure through tubing  40 . Pressurized nitrogen gas is supplied along smaller tubing  48 . Agglomerates form as nitrogen gas exits from the tubing  48 , and the flow of carrier liquid delivers these into the fracture  46  which extends both upwards and downwards from the wellbore.