Method of sustaining fractures radiating from a well bore and compositions for use in the method

Description

Description of Invention

[0001] The present invention relates to a method of and means for sustaining fractures radiating from a well bore in a geological strata. In order to stimulate production of oil and gas from subterranean wells, it has become standard practice to fracture the oil or gas bearing strata hydraulically so that fractures radiate from the well bore. The fractures provide channels for the flow of oil and/or gas to the well bore and enhance the rate of production from the well. The pressures prevailing in the fractured strata will tend to close the fractures, unless they are sustained by introducing material into the fractures.

[0002] It is known to sustain fractures radiating from a well bore by introducing resin-coated proppant grains into the fractures. The grains are suspended in a gell for transfer down the well bore and into the fractures. Breakdown of the gell occurs in the fractures, producing a liquid which can flow away from the grains and leaving the grains deposited in the fractures. The conditions of temperature and pressure to which the grains are subjected in the fractures results in bonding of the grains into a coherent mass by means of the resin.

[0003] In order to promote breakdown of the gell within the fractures, there is incorporated in the gell a source of free-radicals or other suitable agent. A persulphate is a suitable breaker for the gell. At a temperature in the range 120 to 200 °F, a persulphate breaker produces free radicals. However, these free radicals are reactive towards the resin coating of the proppant grains so that the efficiency of the breaker for the purpose of causing the breakdown of the gell is impaired. To mitigate this problem, it is known to incorporate a relatively high proportion of the breaker in the gell but this leads to risk of the gell breakdown occuring before the gell has reached the required position in fractures. Premature breakdown of the gell can, for example, lead to the formation of a coherent body of resin-bonded grains in the well bore.

[0004] In US 4 888 240, Graham et al, there is disclosed a proppant for hydraulic fracturing, the proppant comprising a core on which there is a cured resin coating. An outer coating is present outside the curent coating, the outer coating being a heat-curable phenolic resin with which there is incorporated a small amount of a polyvinyl acetal and a small amount of a mineral oil. The curable resin of the outer coating would be reactive towards a free-radical breaker and would thus impair the efficiency of the breaker for the purpose of causing the breakdown of a gell in which the proppant is suspended. According to a first aspect of the present invention, there is provided a method of sustaining fractures radiating from a well bore in geological strata wherein a resin composition is applied to relatively inert grains and a protective coating is applied to the grains over the resin composition, the coated grains are suspended in a gell containing a breaker for the gell, the suspended grains are introduced down the bore into said fractures, when in the fractures, the gell breaks down under the action of the breaker, the protective coating breaks down and the resin composition binds the grains into a coherent, porous mass.

[0005] According to a second aspect of the invention, there is provided a composition comprising proppant grains suspended in a gell wherein the gell includes a breaker for the gell, wherein the grains comprise cores, an intermediate layer on the cores and a coating on the intermediate layer, wherein the intermediate layer comprises a resin and wherein the coating is less reactive with respect to the breaker than is said layer.

[0006] According to a third aspect of the invention, there are provided grains suitable for use as the proppant grains of a composition according to the second aspect of the invention, which grains comprise cores, an intermediate layer on the cores and a coating on the intermediate layer, wherein the intermediate layer comprises a resin and wherein the coating is less reactive with respect to the breaker than is said layer.

[0007] Naturally-occuring and synthetic mineral grains are suitable for use as the cores of the proppant. The proppant cores may be silica sand or synthetic alumino silicates.

[0008] The resin composition applied to the cores of the proppant is selected for its ability to cure, when subjected to elevated temperature and to pressure in the fractures radiating from a well bore. Suitable phenolic resins are available commercially. The resin may be applied as an aqueous dispersion and then dried under turbulent conditions, so that the grains do not become bound into a coherent mass. Alternatively the resin may be applied by a solvent, or by direct coating of the cores with the resin in a melted condition.

[0009] The coating applied to the proppant grains over the resin is required to protect the resin against reaction with the breaker or with free radicals or other products derived from the breaker and is therefor required to be less reactive towards the breaker or towards free-radicals derived from the breaker than is the resin layer. The composition of the coating may be such that the coating will break down under the action of heat and pressure and permit bonding of the proppant grains into a coherent mass by curing of the resin. Composition comprising natural and/or synthetic waxes may be used to form the coating. Naturally-occuring and/or synthetic resins may comprise or be incorporated in the coating composition.

[0010] Examples of coatings which rupture under pressure, to permit bonding of the proppant grains by curing of the resin, are polyethylene, polypropylene, and polyvinylidene chloride.

[0011] In a case where the gell is aqueous, the coating composition may incorporate water-soluble components, for example polyacrylates, polyvinyl alcohols and polyvinyl acetates. Silicates of alkali metals may also be used in the coating composition.

[0012] The gell may be an aqueous gell and may comprise naturally-occuring gums and modified gums. The gell further comprises a breaker suitable for promoting break-down of the gell under the conditions to which the gell is subjected in the fractures radiating from the well bore. A source of free-radicals, for example a persulphate, e.g. sodium or ammonium persulphate, is a suitable breaker for use with a gell comprising an aqueous dispersion of gums.

[0013] The coated proppant grains may be prepared, transported and stored separately from the gell. Prior to use, the proppant grains are dispersed in the gell and the resulting dispersion is then pumped down a well bore and into fractures radiating from the bore. The gell containing coated proppant grains as herein described may be preceded by gell containing uncoated proppant grains which will be carried into parts of the fractures remote from the well bore. The gell containing coated proppant grains will occupy at least those parts of the fractures adjacent to the well bore.

[0014] In the fractures, the gell is subjected to pressure and to an elevated temperature. The temperature may be within the range 120 to 200 °F. At the elevated temperature, the breaker promotes breakdown of the gell to a relatively low viscosity aqueous mixture. For example, free radicals released by a persulphate at the elevated temperature will attack the carbon chain of a gum and thereby cause the breakdown of the gell. The aqueous mixture resulting from breakdown of the gell flows to the well bore and is drawn up the bore to the surface, leaving the coated grains in the fractures.

[0015] The protective coating on the proppant grains also will break down in the fractures. In a case where the coating is water-soluble, the coating may, at least in part, be carried away from the proppant grains by the aqueous mixture which results from break-down of the gell. The effect of the temperature and pressure to which the coated grains are subjected in the fractures also contributes to break-down of the protective coating over a period of time which is generally longer than that required for the breaker to act on the gell and cause break-down of the gell to a relatively low-viscosity mixture. Break-down of the protective coating is sufficient to enable the resin layer on the cores of the proppant grains to bind the grains into a coherent mass as the resin cures under the temperature and pressure to which the proppant is subjected in the fractures.

[0016] The proportion of the resin in the coated proppant grains is sufficiently high to facilitate bonding of the cores into a coherent mass but is sufficiently low to ensure that the resulting mass is permeable to gas and/or to oil. The proportions of resin and mineral will, in part, be determined in accordance with the size of the mineral grains. Typically, the mineral grains will constitute at least 95% by weight of the coated proppant grains and the resin composition will be present in the range 1.5 to 5%, by weight, of the coated proppant grains. The weight of the protective coating will be substantially less than that of the resin layer.

[0017] We have found that, when using coated proppant grains as herein described, satisfactory breaking of the gell can be achieved by incorporating a persulphate at the rate of 0.5 to 2lbs, e.g. 1 or 1.5lbs, per thousand US gallons of gell (approximately 250 grams to 1000 grams, e.g. 500 or 750 grams, per 3785 litres of gell). In a comparable composition using proppant grains coated only with a resin, it has been found necessary to incorporate the persulphate at a rate of greater than 2, e.g. between 3 and 5lbs per 1 thousand US gallons (greater than approximately 1000 grams, e.g. from 1500 to 2500 grams, per 3785 litres).

[0018] Suitably coated proppant grains can be prepared as follows:

[0019] High quality silica sand, or synthetic proppant with particle sizes in the range 420-840 microns (ASTM sieve no's: 20-40) is heated to 160°C. 3.0% by weight based on the weight of sand or other proppant of a phenol/formaldehyde solid novolac resin in granule, pastille, flake or needle form, having a molar phenol to formaldehyde ratio of 1 to 0.8 in its initial kettle charge, is then charged onto the sand in a known sand mixer. The sand and resin are mixed until the resin has melted and coated the sand evenly.

[0020] A hexamine solution (44% w/w) in water is added to the sand mixture. The amount of hexamine solution is such that the weight of solid hexamine added is 13% of the weight of the resin mixed with the sand. When all the water has evaporated, a second protective overcoating is then put onto the sand, which is still at an elevated temperature in the mixer. There is used for the overcoating a wax which is solid at ambient temperature, a phenolic resole resin which is solid at ambient temperatures but has a low melting point, an emulsion of polyvinylidene chloride, a solution of a phenolic resole resin, a solution of natural wood resin products, for example, colophony, or a solution of a gum. The solution may be in water or some other solvent. In a case where there is used for the overcoating a resin, this resin is selected to have low reactivity with gell breakers.

[0021] The composition which is to form the overcoating is added to the mixture in the mixer while the temperature of that mixture is still elevated. The amount of this composition depends upon the surface area and type of substrate. For example, in the case of Chelford 20/40 sand, use of the composition which forms the overcoating in an amount which is 0.9% by weight of solid material, based on the weight of the sand, achieves the required protective coat. In the case of the synthetic proppant Carbolite 20/40, use to form the overcoating of an amount which provides a weight of the solid overcoating material which is 0.8% of the weight of synthetic proppant provides a satisfactory protective coating. Proppants which have larger surface area require greater amounts of coating composition and proppants which have lower surface area require less coating composition. When the coating material has melted and coated the substrate, in the case of a low-melting point, solid material, or when the solvent has been driven off by the residual heat of the substrate, in the case of a coating composition which is a solution, and the temperature has fallen sufficiently to ensure that the overcoating is not molten, the mixture is discharged from the mixer and is passed through a vibrating screen to ensure that there are no agglomerations. The coated proppant is then cooled in a fluidised bed.

[0022] The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, or a class or group of substances or compositions, as appropriate may, separately or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims

1. A method of sustaining fractures radiating from a well bore in geological strata, wherein a resin composition is applied to relatively inert grains and a protective coating is applied to the grains over the resin composition, the coated grains are suspended in a gell containing a breaker for the gell, the suspended grains are introduced down the bore into said fractures and when in the fractures the gell breaks down under the action of the breaker, the protective coating breaks down, and the resin composition binds the grains into a coherent, porous mass.

2. A method according to Claim 1 wherein said protective coating breaks down under the action of heat and pressure.

3. A method according to Claim 1 wherein the protective coating breaks down by dissolution of water-soluble components in an aqueous gell.

4. A composition comprising proppant grains suspended in a gell which includes a breaker for the gell; wherein the grains comprise cores, an intermediate layer on the cores, and a coating on the intermediate layer; the intermediate layer comprising a resin and the coating being less reactive with respect to the breaker than is said layer.

5. A composition according to Claim 4 wherein the proppant grains are transported unstored separately from the gell and dispersed into the gell prior to use thereof.

6. A composition according to Claim 4 or Claim 5 comprising 0.5 to 2lbs persulphate per thousand US gallons of gell (approximately 250 grams - 1 kg per 3785 litres).

7. Grains suitable for use as the proppant grains of a composition according to any one of Claims 4 to 6, said grains comprising cores, an intermediate layer on the cores, and a coating on the intermediate layer, wherein the intermediate layer comprises a resin and the coating is less reactive with respect to the breaker than is said intermediate layer.

8. Grains according to Claim 7 wherein said coating comprises natural and/or synthetic waxes.

9. Grains according to Claim 7 wherein said coating comprises polyethylen, polypropylene or polyvinylidene chloride.

10. Grains according to any one of Claim 7 wherein the gell is aqueous and the coating incorporates water-soluble components.