Earth boring – well treating – and oil field chemistry – Well treating – Contains organic component
Reexamination Certificate
1998-03-27
2002-05-14
Tucker, Philip (Department: 1712)
Earth boring, well treating, and oil field chemistry
Well treating
Contains organic component
C507S209000, C507S212000, C507S214000, C507S217000, C507S922000, C507S230000, C536S108000, C536S110000, C536S063000, C536S123000, C536S123100, C525S058000, C525S061000
Reexamination Certificate
active
06387853
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to derivatization of polymer materials and, more specifically, to the use of derivatized polymers in well treatments. In particular, this invention relates to the derivatization of polymers, such as polysaccharides, in liquid slurries that are subsequently introduced into a well as part of a well treatment, such as a hydraulic fracturing treatment.
2. Description of Related Art
Hydraulic fracturing of oil and gas wells is a technique routinely used to improve or stimulate the recovery of hydrocarbons, and is typically employed to stimulate wells which produce from low permeability subterranean formations. In such wells, recovery efficiency is typically limited by the flow mechanisms associated with a low permeability formation. Hydraulic fracturing is usually accomplished by injecting a fluid, typically proppant-laden, into a producing interval at a pressure above the fracturing pressure of the subterranean formation. This fluid induces a fracture in the reservoir and transports proppant into the fracture before leaking off into the surrounding formation. After the treatment, proppant remains in the fracture in the form of a permeable pack that serves to prop the fracture open. In this way the proppant path forms a conductive pathway for hydrocarbons to flow into the wellbore. Typically, viscous gels are employed as fracturing fluids in order to provide a medium that will adequately suspend and transport solid proppant materials, as well as to impair loss of fracture fluid to the formation during the treatment. Fracture width and geometry are determined, in part, by the viscosity of a fracturing fluid. Viscosity of most water-based fracturing fluids is typically obtained from water-soluble polymers such as guar gums, guar derivatives, and cellulose derivatives.
Further enhancement of fracturing fluid viscosity may be obtained by treating polymeric solutions with cross-linking agents, typically selected from titanium, aluminum, boron and zirconium based compounds, or mixtures thereof. Most typically, boron and zirconium based additives are employed. Boron cross-linkers are typically used with galactomannan polysaccharides such as guar gum and its derivatives, including hydroxypropyl guar (“HPG”), carboxymethyl guar (“CMG”) and carboxymethylhydroxypropyl guar (“CMHPG”). Most typically, boron crosslinkers are employed with guar because it offers suitable performance at lower cost. Although gelation may be obtained by mixing zirconium-based additives with guar, the resulting gels are typically shear-sensitive and unstable. However, zirconium-based additives may be mixed with carboxymethylated guar derivatives such as CMG or CMHPG to form stable gels. Other suitable guar derivatives include alkylated carboxylated polymers such as methyl and ethyl carboxymethyl guar.
For a typical hydraulic fracturing operation derivatized polysaccharides, such as carboxymethylated guars, are usually commercially purchased from specialty polymer companies, such as Aqualon and Rhone-Poulenc. Polymers such as carboxymethylated guars are typically formed by derivatizing the guar seed endosperm, often referred to as a “split”, which is generally semi-spherical in shape (about ⅛″ long and {fraction (1/16)}″ in diameter) and from about 0.5 mm
3
to about 1 mm
3
in volume. The relatively large size of guar splits make them favored as a polymer feedstock because they form a derivatized polymer product that is more easily isolated and purified without the need of washing with organic solvents. Derivatized products formed from fine polymer powders often require organic solvents to disperse the powders without caking. These organic solvents are typically expensive and environmentally unsound.
Guar splits are typically treated with aqueous caustic to cause swelling, and then exposed to a derivatizing agent, such as sodium chloroacetate (“SCA”) in an amount necessary to provide a desired molar substitution (“M.S.”) value, which is the ratio of moles of derivative to total moles of sugar. Typical guar split derivatization yields using SCA are about 60%, with about 40% of the SCA being consumed to form undesirable byproducts such as glycolic acid. Following derivatization, the splits are typically cooled and washed to remove excess caustic and unwanted byproducts before being dried and ground to a powder.
Among the disadvantages associated with the use of commercially purchased derivatized polysaccharides is lack of control over the M.S. value. The M.S. value affects a polymer's hydration rate, solution viscosity and salt tolerance. Properties of zirconium and titanium crosslinked polymer gels are also affected by the polymer M.S., including gelation rate, gel strength as a function of temperature, proppant suspension, fluid loss control and controlled gel degradation. The optimum value of each such property is often achieved at different M.S. values. Therefore, the optimum M.S. value for a given fluid system typically reflects a compromise of the optimum M.S. values for each property. In the same way, the optimum M.S. value may also differ among fracturing fluids. The desired nature of the properties associated with the M.S. value varies from well treatment to well treatment based upon, among other things, individual well and formation characteristics. Therefore several commercially purchased polymers of the same class, but having different M.S. values, typically must be kept in inventory.
Another disadvantage of commercially available derivatized polysaccharides is their relatively high cost. This cost reflects the amount of post-derivatization processing required to produce derivatized polysaccharide powders. Post-derivatization processing typically includes isolation, washing (such as with organic solvents), and packaging of the derivatized polysaccharide product. Furthermore, derivatization of commercial polysaccharides is typically less uniform than desirable. Although not wishing to be bound by theory, it is believed that derivatization of relatively large polysaccharide materials such as guar splits yields a particle having a large percentage of derivatized molecules concentrated near the outer surface of the particle, and a substantial percentage of underivatized molecules located in the interior of the particle. This is undesirable because a portion of the polymers having lower than desired M.S. values may exhibit poor response to crosslinking while those have excess derivatization may be overly responsive. It is believed that this situation may result in non-uniform gelation.
Recently, continuous mixing processes have been employed in the performance of hydraulic fracturing treatments. In a continuous mixing process, the polymer is carefully metered into a flowing aqueous stream enroute to the wellbore. Because precise metering is best accomplished by handling the polymer as a liquid rather than as a solid, polymer slurries are typically prepared by dispersing the polymer in an organic solvent base which contains suspending agents. Examples of typical organic solvents include hydrocarbon solvents, such as kerosene, diesel, etc. Suspending agents are typically organophilic clays and surfactants such as naphthalene sulfonate resins. Nonionic surfactants such as ethoxylated fatty alcohols are typically added to improve wetting and hydration. During a typical well treatment employing continuous mixing, a liquid polymer slurry is measured into a flowing stream of aqueous treatment fluid, typically in a manner that allows the polymer from about one to about four minutes to hydrate before the aqueous stream enters a blender. In the blender the polymer-laden aqueous well treatment fluid may be mixed with other additives, such as cross-linking agents and other additives such as surfactants, gel stabilizers, clay control additives and proppants, before being injected into a wellbore.
SUMMARY OF THE INVENTION
In one respect, this invention is both a method for forming derivatized polymer material in a s
Dawson Jeffrey C.
Kesavan Subramanian
Le Hoang V.
BJ Services Company
O'Keefe Egan & Peterman, LLP
Tucker Philip
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