Reaction-Transport Simulation of Matrix Acidizing and Optimal Acidizing Strategies
Authors: Chen Yueting , Fambrough John , Bartko Kirk , Li Ying , Montgomery Carl , Ortoleva Peter 
Published: December 2007

Abstract
A new type of geochemical model, CIRF.A, is used to analyze matrix acidizing procedures. The model considers the transport of pore-fluid solutes and the chemical reactions, which are either kinetic or equilibrium controlled. One of its new, distinct features is that it can predict the mineral dissolution and precipitation processes as well as their chemical natures and their consequences on porosity and permeability changes. After calibrating the model with flow-through acidizing experimental results, a series of simulations was carried out under different acidizing conditions. Those simulations show an important phenomenon: the permeability improvement does not always increase with acidizing time. Rather, there exists a maximum value of permeability improvement (MAXPI) that is a function of injection velocity and the mineralogy of formations. The time to reach the MAXPI is the optimal acidizing time (OPAT) that also depends on injection velocity and mineralogy. Further acidizing beyond OPAT lead to permeability decrease and unnecessary costs. The simulations also show that 1) high injection velocity can generate better stimulation results with equal or even less amount of acid; 2) MAXPI is in inverse proportion to the amount of clay; and 3) for fixed total content of illite and kaolinite, MAXPI is in proportion to the ratio of illite/kaolinite. These results demonstrated that reaction-transport simulation can be used to predict MAXPI and OPAT and thus is a valuable tool for optimal design of matrix acidizing strategies.

Introduction
Improper acidizing strategies can generate, rather than reduce, formation damage. One source of formation damage during acidizing is the precipitation of minerals and gels, resulting from reactions between injected acids and reservoir rocks.
Geochemical modeling and computer simulation methods were applied to understand the damage caused by the precipitation of secondary phases. They can predict the chemical nature of these byproducts based on formation mineralogy and treatment compositions or pressure, temperature, and injection velocity and time. Unfortunately, as pointed out by Piot and Lietard, "these models cannot predict the damaging potential of these products because they do not include any physical description of the way they are precipitated."
However, that situation changed and new types of geochemical models - Reaction-Transport model - appeared. One of the features of Reaction-Transport models is that transport processes are incorporated into traditional geochemical models so that the full impact of mineral dissolution and precipitation, as well as their chemical natures, are accounted for.
This paper presents the application of such a new geochemical model to address some of the fundamental issues in designing acidizing strategies. The objectives are to help understand the geochemical processes of matrix acidizing and to show how various acidizing conditions affect the outcomes. Our study is based on the general reaction-transport code CIRF.A. Numerical Simulation Technique: the Reaction-Transport Code CIRF.A
The general reaction-transport code CIRF. A was developed at Indiana University to analyze the spatial-temporal behavior of porosity-permeability resulting from reactions between pore fluids and reservoir rocks. By solving a set of coupled nonlinear partial differential equations describing the time and spatial changes of the concentrations of solutes and the dissolution/precipitation rates of solids, CIRF. A computes the porosity and permeability distributions over time and space.
CIRF. A uses a fully coupled finite-difference method to solve the conservation equations for solutes. The CIRF. A method has the advantage over the popular operator-splitting method of rapid convergence and robustness.