Predicting the Flow and Reaction of HCl/HF Acid Mixtures in Porous Sandstone Cores
Authors: Fogler H.S. , Lund K. , McCune C.C. 
Published: December 2007

Abstract
An extensive theoretical and experimental analysis of the acidization of linear sandstone cores has been carried out. The relative rates of reaction of common minerals found in sandstone, which were determined from rotating-disk-experiments, are presented and discussed along with the underlying presented and discussed along with the underlying reaction mechanisms. A lumped-parameter mathematical model developed from this information predicts the movements of acid and permeability predicts the movements of acid and permeability changes through the porous sandstone cores as a function of acid concentration and flow rate. Two acid core flood experiments are needed to determine the characteristic parameters of the model (the Damkohler number and the acid capacity number). However, this paper also shows how, in the absence of these experiments, one can extend the model to make predictive calculations for various sandstones from a porosity measurement and an X-ray analysis of the sandstone. In carrying out this latter analysis, a comparison of the experimental results of other investigators with the model shows the theory and experiment to be in excellent agreement.

Introduction
The stimulation of petroleum wells by acid injection is accomplished by selective dissolution of part of the formation rock to reduce the resistance to fluid flow in the vicinity of the wellbore. Because of the radial flow geometry, the productivity of the well may be increased greatly. Field and laboratory data have been gathered through years of applying the process to allow the design of an acid treatment for a particular formation.
The complexity of the porous media and the reactions occurring during the acidization have made it difficult to predict a priori the exact results of the stimulation. A large step forward in the technology was made by the introduction of permeameters to permit the testing of cores at permeameters to permit the testing of cores at conditions similar to those existing in the reservoir. A series of experiments then can be carried out using different acid strengths, volumes, and flow rates to determine the best conditions for acidization.
In recent years, the acidization process has been investigated in detail experimentally and theoretically. This has led to a better understanding of acidization and the development of mathematical models of the process. The application of a model allows a successful acidization treatment to be designed on the basis of few or no laboratory experiments. Even though differences exist in the manner by which the treatment is carried out in practice, the basic processes occurring during practice, the basic processes occurring during acidization are the same: homogeneous reactions in the fluid phase, heterogeneous reactions between the fluid and the solid phase, mass transfer, fluid flow, and changes in the pore structure of the porous media. porous media. This paper is primarily concerned with matrix acidization of sandstones, but the results will be of interest for other forms of acidization. Although emphasis is placed on the results of studies by the authors, it is seen that by comparing these results with the work of other investigators a uniform theory of acidization may be proposed.

HOMOGENEOUS REACTIONS
In the acidization of sandstones, and also of carbonates, a large number of product species are released to the acid solution. Since these may react with each other and with the acid, it is necessary to understand the equilibria between the different chemical species to determine the reaction stoichiometry. Carbonic acid formed by the dissolution of carbonates may participate in several reactions. However, since strong acids or acid mixtures are generally used, these side reactions usually may be neglected.
SPEJ
P. 248