The present invention relates to a process for preparing particulate matter coated with a solid curable resin and utilizing the coated particulate material as a proppant or for sand control by consolidating and curing the coated particulate material underground.
Hydraulic fracturing is a technique for stimulating the production of subterranean formations. The technique normally involves (1) injecting a viscous liquid through a well and into a formation at a sufficient rate and pressure to overcome the earth stresses and form a crack or fracture in the formation; and (2) placing a particulate material, referred to as a "propping agent" or "proppant" in the formation to maintain the fracture in a propped condition when the injection pressure is released.
The propped fracture thus provides a highly conductive channel in the formation. The degree of stimulation afforded by the hydraulic fracture treatment is largely dependent upon the permeability and width of the propped fracture.
In hydraulic fracturing applications where proppants are used, proppant flow back can be a problem when the well is put into operation. Some of the proppant can be transported out of the fractured zones and into the well stem by fluids produced from the well.
This backflow is undesirable and has been controlled to an extent in some instances by the use of a proppant that has been coated with a solid curable resin which will consolidate and cure underground. Phenolic resin coated proppants have been commercially available for some time and are used for this purpose.
These curable phenolic resin coated proppants work best in environments where temperatures are sufficiently high to consolidate and cure the phenolic resins. However, at underground temperatures below about 130.degree. F., these curable phenolic resin coated proppants are not useful because they do not consolidate and cure adequately to obtain sufficient compressive strength to prevent flowback.
Many shallow wells often have underground temperatures of less than about 130.degree. F., and in some cases of less than about 100.degree. F. At the present time there are no commercially available curable free-flowing resin coated proppants which satisfactorily cure at these temperatures.
Curable free-flowing phenolic resin coated particulate material can also be used for sand control. Wells that are placed in formations that contain poorly consolidated sand can produce sand along with the fluid. This is undesirable for a number of reasons. This sand production can be controlled by placing curable phenolic resin coated particulate material around the well stem, then curing it to form a consolidated sand filter to prevent loose formation sand or other debris from flowing into the well stem. As in fracturing the problem is that the phenolic resin coated sands do not operate satisfactorily at about below 130.degree. F.
An attempt to deal with this sand control problem has been with the use of liquid curable resins. In this approach, particles, usually coarse sand, are placed around the well bore and a liquid resin such as an epoxy or furan resin is pumped through the sand in an effort to coat the liquid resin onto the sand. This is followed by contacting the liquid resin coated proppant with an overflush fluid containing a suitable catalyst, which is pumped into the proppant and retained there until the resin cures and consolidates the proppant.
In many cases it would be preferable to use a proppant with a solid curable resin coating which could consolidate and cure at temperatures in the range of 60.degree. to about 130.degree. F., rather than a liquid resin. The disadvantages of using liquid resins in these conditions is that it is difficult to control the amount of resin that coats the proppant. If too little resin is used, flowback could occur. If too much resin is used, the permeability could be too low. In addition, the liquid resin could enter the formation and seal it off. There is no way to assure that the liquid resin completely and uniformly coats and bonds the sand around the well bore.
The patent literature dealing with this technology includes U.S. Pat. No. 3,625,287 to Young which discloses the use of specific silane or organosilicon compounds added to a liquid resin system can be used to consolidate loose or incompetent sands to produce a stronger, stable product. The resin systems include furan resins, phenolic resins, urea formaldehyde resins, and epoxy resins.
U.S. Pat. No. 3,419,073 to Brooks discloses the use of normal hexanol or a similar aliphatic alcohol containing from 5 to 10 carbon atoms, injected into an unconsolidated formation. The well is then shut for about one-half to about 120 hours, and a resin solution or mixture of resinous materials is thereafter injected, resulting in improved strength and reduction of permeability losses. Resins useful in this process include epoxy resins, furfuryl alcohol resins and urea formaldehyde resins.
U.S. Pat. No. 3,404,735 to Young et al discloses a method for consolidating loose solids introduced into a well by dispersing a predetermined amount of resin or consolidating fluid in an oil base liquid hydrocarbon. Subsequently, a quantity of particulate material is introduced into the resin-oil dispersion thereby coating the solids with resin. The oil-resin-solids mixture is then introduced through a well bore to a fracturing formation, with injection continuing until a sand out or pack out occurs and the desired amount of resin coated solids is deposited in the well bore. Excess solids are removed by reverse circulation.
U.S. Pat. No. 4,073,343 to Harnsberger discloses a method for treating an incompetent sand containing underground formation by introducing specific amounts of furfuryl alcohol, surfactant, water, silane coupling agent, catalyst and aromatic distillate in the formation sand to be consolidated.
U.S. Pat. No. 3,393,736 to Goodwin discloses a method for controlling movement of sand in a well by pumping particulate matter coated with a resin-forming liquid or semi-liquid material, into the well. The liquid or semi-liquid resinous material also contains a catalyst which is capable of curing the resin. Suitable resins include furfuryl alcohol resins, urea formaldehyde resins, epoxy resins, phenol-formaldehyde resins and alkyd resins.
U.S. Pat. No. 4,443,347 to Underdown et al discloses a method for propping a fracture in a subterranean well formation which comprises injecting a proppant composed of individual substrate particles having a thermoset coating. The coating produces a charge wherein the conductivity ratio throughout a given closure stress range is greater than that of a charge of uncoated particles having substantially the same particle size distribution.
U.S. Pat. No. 4,413,931 to McDonald discloses the treatment of subterranean geological formations such as those surrounding oil well bore holes by placing particulate material in or adjacent to the formation. The particulate material is coated with a two-step, curable, novolac-type phenolic resin, which is thereafter cured in situ to bond the particulate matter together. In order to achieve the desired compressive strength, the resin must have an insolubility parameter of greater than about 1. This type of resin is not useful below about 130.degree. F.
U.S. Pat. No. 4,336,842 to Graham et al, discloses curing a packed resole resin coated sand in a solution of a resin softening agent comprising alcohols such as isopropanol, methanol or ethanol and nonionic surfactants. It has been found that this resin system becomes tacky, but does not satisfactorily cure at low temperatures.