1. Field of the Invention
This invention relates to methods and compositions for treating subterranean formations. More particularly it relates to methods and compositions for treating a subterranean formation penetrated by a wellbore utilizing a crosslinked gel and thereafter reducing the viscosity of the crosslinked gels in the presence of phenolic resin-coated proppants.
2. Description of the Prior Art
In the completion and operation of oil, gas and water wells and similar boreholes, it frequently is desirable to alter the producing characteristics of the formation by utilizing various treating fluids. Many such treating fluids contain gelled or viscosified fluids and or solid particles, commonly referred to as "proppants." These gelled fluids further include linear gels and crosslinked gels.
Examples of subterranean treatments employing gelled fluids in combination with proppants include hydraulic fracturing and sand control treatments. In the case of hydraulic fracturing, proppants, suspended in the gelled fluid, are pumped downhole under pressure sufficient to fracture the target formation. The proppants, carried by the fluid, are positioned between the parted faces of the formation. In this way, when the pressure against the formation is relaxed, the proppants maintain the fracture in an open or propped condition.
In the case of sand control, particulate materials may be placed in the well to prevent the influx or incursion of formation sand or fine particles. In other instances, the gelled fluid, without particulate, may be used for example as a "pad" or other precursor treatment to contacting the formation with the gel/proppant system.
Upon completion of the treatment, it is generally desirable to remove the gelled treating fluid from the formation of the well. To effectively remove the fluid, the viscosity of the fluid is reduced. The reduction of the gelled-fluid viscosity is referred to as "breaking" the gel. The agent responsible for breaking the gel is referred to as a "breaker" or "gel breaker."
Polysaccharide polymers are well known gelling or viscosifying agents useful in treating subterranean formations. Suitable polysaccharide polymers include cellulose derivatives and glactomannan gums. Crosslinking agents, such as for example boron, titanium zirconium, and aluminum, when added to a quantity of hydrated polysaccharide polymer generally increase the viscosity thereof.
Particulate materials suitable for use in gelled fluids may be selected from both organic and inorganic materials. Common organic materials include for example, wood chips, nut shells, crushed coke, and coal. Inorganic materials include for example crushed rock, sand, spherical pellets of glass, sintered bauxite and various synthetic ceramics.
In some instances, the particulate material may be coated with natural or synthetic film-forming materials. The coating on the surface of the particulate assists in controlling fragmentation and dispersion of particulate fragments. Particulate fragmentation can result from closure pressures exerted by the formation on the particulate. Controlling the migration of these fragments enhances formation conductivity by preventing free fragments from plugging formation flow passages.
In addition to the above mentioned improvements achieved by coated particulates, coating with a curable material also reduces particulate flow-back and improves overall bed strength. The uncured resin coated particulate, upon exposure to curing conditions, such as a curing agent or elevated temperatures, forms a cured consolidated matrix.
Traditional gel breakers include enzymes and oxidizing breakers. Examples of such oxidizing breakers include ammonium, sodium or potassium persulfate; sodium peroxide; sodium chlorite; sodium lithium or calcium hypochlorite; chlorinated lime; potassium perphosphate; sodium perborate; magnesium monoperoxyphthalate hexahydrate; and several organic chlorine derivatives and/or salts thereof.
At formation temperatures of between about 75.degree. F. to about 120.degree. F. and pH range of generally between about 4 to 9, enzyme breakers are suitable. At formation temperatures above about 140.degree. F. enzyme breakers are inadequate and oxidizing breakers are required. Generally, depending upon the temperature of the gelled carrier fluid, between about 0.5 and 5.0 pounds of oxidizing breaker, such as a persulfate breaker, per 1000 gallon of aqueous gel is sufficient to break the carrier fluid in a non-resin coated proppant/aqueous gelled carrier fluid system.
Curable adhesive proppant coatings, such as phenolic and furan resins, in an uncured state, exhibit some compatibility with aqueous gel carrier fluids and enzyme gel breakers. However, when the pH and temperature of the gelled carrier fluid preclude the use of enzyme breakers, it is not uncommon to employ exceedingly high concentrations of oxidizing breakers to reduce the viscosity of these carrier fluids when such carrier fluids are in contact with uncured resins. Generally, the concentration of oxidizing breaker required in an uncured resin/aqueous gel system can be as high as 4 to 40 times the amount required for non-resin or cured resin/aqueous gel systems.
Solutions to the oxidizing breaker problem presented by the uncured resin-aqueous gel system have been as straight forward as adding increased amounts of oxidizing breaker to as complex as encapsulating the oxidizing breaker. The first alternative can result in uncontrolled breaks or limited breaking. Uncontrolled breaking can result in a "sand out" of the proppant prior to optimal placement in the target formation. Limited gelled fluid breaking can reduce formation conductivity by leaving unbroken gel in the formation and the proppant bed.
Breaker encapsulation, while somewhat more successful than the former method, in many instances also requires the addition of excessive quantities of oxidizing breaker. In addition, the process of encapsulation increase the cost of the breaker and may result in uneven distribution of the breaker in the curing proppant matrix or proppant pack. Thus there exist the need for improved gel breaker systems, and particularly gel breakers systems capable of predictably reducing the viscosity of the gelled fluid when such gelled fluid is admixed with an uncured or curing resin.