Abstract

The development of a highly efficient calcium sulfate (CaSO4) scale removal fluid containing a first-of-its class dissolver system is presented. Common CaSO4 scale dissolvers require chelant(s) along with scale converters. A drawback of this type of dissolver is that removal of anhydrite represents a problem if extended shut-in periods are necessary. To help minimize the nonproductive time (NPT) necessary to dissolve CaSO4, a solution has been identified that can restore production within two hours or less and is field-proven.

Mineral anhydrite and gypsum samples were exposed to dissolver formulations at 200°F under static conditions, from one hour up to 24 hours. Dissolution kinetics investigation indicates that the limiting step is a substitution surface process that catalyzes the reaction. The formulation was generated based on the discrete chemical activities of individual components with respect to anhydrite, and then refined against a performance-based design of experiments (DoE). Various chemical agents were tested for activity as a function of the chemical functionality (i.e., carboxylate, halide, etc.). Combining the results from multiple design spaces allowed determination of the most desirable formulation.

A customized performance methodology was implemented to rapidly identify formulations having a dissolution efficiency exceeding 70% within one hour, followed by a targeted design space aiming for 90% dissolution of actual wellbore anhydrite scale. All tests were conducted with low surface area samples cut from mined substrates for the purpose of experimental designs while wellbore scale was used to validate the formulation efficacy. Elemental analysis of gypsum and anhydrite dissolution products over time was conducted to elucidate kinetic parameters at 150 and 200°F from 1 to 24 hours. The dissolution of anhydrite as a function of time, at 150 and 200°F, showed limiting effect dependency as a function of scale converter concentration. Mass of scale to volume of dissolver fluid effects indicate a dependence on the converter concentration while temperature had the most significant effect. Formulations showed 60-80% dissolution in one hour using anhydrite. This signifies that wellbore and equipment clean out can be accomplished with a simple, non-corrosive treatment fluid when the mineral is of the CaSO4 type. Field trials have been conducted, demonstrating the efficacy of the fluid.

The anhydrite dissolver formulation provides mineral dissolution within one hour under conditions representative of a wellbore, such as static fluid column and temperatures of 200°F. The treatment is delivered as a one-fluid system, or single treatment design, whereas a majority of existing treatments require multiple fluids providing protection to downhole equipment such as electro submersible pumps (ESP). The new system functions without the need for prolonged well shut-ins or the use of complicated treatment fluid designs.

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