Abstract

This work reports on the application of a complex kinetic geochemical model to explore key issues in sandstone acidizing. Issues include the importance of the minerals initially present, the consequences of mineral precipitation and the effect of acid formulation and injection rate. The model's capabilities exceed those of its predecessors that have greatly simplified the reaction chemistry or have ignored kinetics altogether. No assumptions were made as to the nature of precipitation that could ensue—all potential precipitates were considered.

An important advance is the use of a new permeability prediction model taken from the work of Panda and Lake. This model relates the permeability of a permeable medium to the porosity, grain size distribution and the amounts and identities of all detrital minerals present. Coupled with the reaction chemistry, this model allows predictions that are relevant for specific treatments. The use of this model also gives quite realistic productivity improvement predictions.

We study three cases of generic mineral assemblages: high quartz, high clay, and high feldspar. Model results indicate that the precipitation of reaction products is both inevitable and substantial. Hence, the optimal matrix stimulation is a compromise between maximizing the dissolution of the damaging minerals and minimizing secondary precipitation. Given such competitive effects, our results indicate that high rate stimulations tend to give the largest productivity improvements even though these usually do not cause the largest removal of damaging minerals.

Introduction

The success of sandstone acidization is based on the unique quality of hydrofluoric acid to attack silicates and aluminosilicates. While the injected hydrofluoric acid will dissolve most of the minerals present, the primary target is the damaging solids that are introduced to the near-wellbore region by drilling, completion, or production. Stimulation is principally achieved through restoration of the original reservoir permeability.

Acid treatments do not always improve the well performance. As various minerals dissolve, others may precipitate that can significantly reduce or negate the benefits of acidization. Treatment design becomes a matter of optimization: efficient removal of damaging material balanced against the minimization of secondary precipitation.

A complex kinetic model was used to explore several key issues in well treatment design: the importance of the initial mineral assemblage, the consequences of precipitation of reaction products, and the effect of acid formulation and injection rate. This geochemical simulator, KGEOFLOW, is based on the partial local equilibrium assumption. This formulation provides the capability to model an arbitrary combination of equilibrium and kinetic reactions involving an arbitrary number of chemical species. KGEOFLOW is a one-dimensional, single-phase, finite-difference model that assumes a constant volumetric injection rate.

Unique kinetic rate expressions are used to describe the dissolution of each reservoir mineral; these include terms for the catalytic effect of HCl on certain minerals. No assumptions are made as to the nature of precipitation that may ensue—all potential precipitates are considered. This work also incorporates a new permeability estimator that translates specific changes in mineral volumes into a theoretical productivity improvement factor, allowing the dissolution of the matrix (quartz and feldspars), the cement (clays and calcite), and the damaging clay to have unique impacts on the permeability.

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