Abstract

Acidization of sandstones to improve permeability is a complex process. Linear coreflood acidization in a laboratory environment is useful in developing an experimental understanding of the process. Numerical simulation which incorporates the basic scientific principles of fluid mechanics in porous media and the principles of fluid mechanics in porous media and the reaction rate kinetics of sandstone acidization can provide a valuable interpretive aid for understanding provide a valuable interpretive aid for understanding the total process. Adjustment of model parameters so that laboratory experiments may be emulated is analogous to pressure history matching models to time-dependent field pressure response. once model parameters are correctly adjusted, the model provides a parameters are correctly adjusted, the model provides a predictive tool for estimating acidization results at predictive tool for estimating acidization results at other flooding rates and acid strengths. The purpose of this discussion is threefold:

  1. to describe the development of a numerical model containing variable porosity (with time and distance),

  2. to describe constraints other than the acid response coefficient (ARC) curve for insuring concert between the model and experimental results, and

  3. to describe laboratory measurements of the emerging core effluent and the corresponding pressure drop across the core with injection time.

In conjunction with these three objectives, we will present some experimental results of our own. present some experimental results of our own

Introduction

A popular method for estimating the effect of acid stimulation of a permeable sandstone matrix is to subject a linear core to a floodwater containing mud acid (hydrochloric-hydrofluoric acid mixture) and record the resulting diminution of pressure drop across the core as a function of time. This particular type of core-flood is performed at a constant injection rate so that after the flood is completed one can convert the pressure drop to permeability by calculation using Darcy's pressure drop to permeability by calculation using Darcy's Law. Dividing the calculated permeability at various points in time by the initial permeability provides a points in time by the initial permeability provides a measure of the acid's effectiveness. This effectiveness is commonly referred to as the acid response coefficient (ARC). The ARC curve is the gross response of the porous reservoir rock to the flowing acid. The scientific basis for this response is a complex combination of chemical reactions and their equilibria superimposed on fluid flow through porous media.

In addition to the simple mechanics of developing an ARC curve, there are other items of general importance which have a bearing on the reliability of this measurement. To initiate the flood, several pore volumes of hydrochloric acid are usually injected as a "spearhead." The hydrochloric acid dissolves all exposed carbonate and dolomite intergranular cements and prevents later formation of calcium fluoride. Calcium prevents later formation of calcium fluoride. Calcium fluoride is an insoluble precipitate which can plug the core. The hydrochloric acid forms an in situ environment high in hydrogen ions which is advantageous to the reaction kinetics of hydrofluoric acid with clays and feldspars. A third advantage of the hydrochloric acid "spearhead" and continued simultaneous injection with the hydrofluoric acid is that the solubility of sodium and potassium fluorosilicates is increased. These chemical species are reaction products which may also cause core plugging. products which may also cause core plugging. Within the last several years, acidizing technology has become more sophisticated. The scientific bases of chemistry and fluid mechanics have been combined to provide an analytic model. Much of this work was performed at Chevron Oilfield Research in collaboration with engineers from the University of Michigan. It is this fundamental work which provides the basis for the numerical simulation model described here. Lund and Fogler provide an analytical solution to a coupled set of partial differential equations, assuming a constant porosity for the porous matrix during the acidization process. The numerical model developed here allows extension of the analytic model so that a time and distance variant porosity may be included. Once the basis for the model is derives, the development of concert between the model and the measurements will be discussed. Finally, the important features of coreflood effluent measurements will be discussed along with experimental measurements performed on a sandstone reservoir core. performed on a sandstone reservoir core.

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