Thermodynamic and Kinetic Aspects of Argillaceous Sandstone Acidizing
Authors: Labrid Jean Claude 
Published: December 2007

Abstract
The thermodynamic aspect of sandstone acidizing by hydrofluoric acid (HF) is examined. It is shown that silica dissolution, with a first order in HF concentration leads almost exclusively to the formation of fluosilicic acid. Clay and feldspar dissolution is much more complex; after a uniform alteration of the crystalline lattice, partial precipitation of silicic species occurs when the precipitation of silicic species occurs when the acid is spent.
An approach to the kinetic aspect is made by defining, for a naturally complex medium, a reactivity profile that is a characteristic of the medium instead of a single reaction-rate constant. Experimental data enable a correlation between permeability, porosity, and reactivity. Also, a permeability, porosity, and reactivity. Also, a qualitative interpretation of acid response curves is given. The numerical simulation of the acidizing process satisfactorily reproduces the experimental process satisfactorily reproduces the experimental results. When extended to radial flow, the model shows the influence of stimulation parameters, injection rate, concentration, and time.

Introduction
Use of acids for increasing well productivity has been well established since the description of the first acid treatment appeared at the end of the last century. However, commercial development of acidizing as a stimulation technique became widespread around 1930. Since then, extensive research has broadened the number of techniques and chemical additives that not only improve the operation, but also extend applications to more complex reservoirs under severe pressure and temperature conditions. These applications are extended by the use of special corrosion inhibitors and organic acids for deep well treatment (with or without the cool-down technique) by the handling of high-strength acids recommended in offshore operations where it is advisable to use small volumes, and by the development of acid mutual-solvent technique to prevent fine particles from migrating near the wellbore, improving stimulation response.
In As present state, the acidizing process largely involves empirical methods primarily because of insufficient understanding of the physical, chemical, and physicochemical phenomena involved. As a result, the anomalous behavior of some acid-response curves used for predicting treatment efficiency has not yet received a satisfactory explanation. The chemistry of the dissolution of detritic materials by acids is, in fact, not entirely understood because of the great number of equilibria between the reagent and the different species in solution derived from the native rock.
The thermodynamic aspect of the problem is examined first giving a qualitative and quantitative description of chemical phenomena and attaining the solubilization mechanism of silica, feldspars, and clays.
Conclusions of this study enable interpretation of the phenomena observed when an acid flows through a natural porous sandstone (dynamic experiment). The kinetic aspect is considered, along with the definition and the measurement of the matrix reactivity, a concept as characteristic of this matrix as porosity and permeability.

THERMODYNAMIC CONSIDERATIONS
Sandstone acidizing is based on the unique quality of HF to attack silica and alumina-silicates.
Although this property has long been known (since HF is used in the glass-engraving method), its application to well stimulation quickly revealed the complexity of the problem, especially the possibility of side reactions that may affect formation permeability. The thermodynamic aspect is permeability. The thermodynamic aspect is approached in the following manner. The system formed by HF in water solution and a mineral occuring in a sandstone, such as silica, clay, or feldspar, is considered. There is a chemical reaction; that is, a consumption of the reagents and a solubilization of the mineral constituents (silicon, aluminum, potassium, etc.), which are then involved in a great number of chemical equilibria.
SPEJ
P. 117