Abstract

Matrix acidizing of carbonate formations has been carried out for many years using HCl acid in various strengths. However, in some high temperature applications, HCl does not produce acceptable stimulation results due to lack of penetration or surface reactions. Organic acids, like formic acid and acetic acid, were introduced to offer a slower reacting a thus deeper stimulating acid. These "retarded" acids also had short-comings due to solubility limitations of acetate or formate salts. In recent years, several alternatives have been developed, including aminocarboxylic acids and long-chained carboxylic acids. These long-chained carboxylic acids offer low corrosion rates, good dissolving power at high temperature, high biodegradability, and easier and safer to handle.

Many experimental and theoretical studies in carbonate acidizing have confirmed the existence of an optimal acid injection rate at which major wormholes are formed, and the benefit from stimulation is maximized. This optimal rate depends on reservoir conditions, rock properties and chemical reaction rate of the acid being used. In our previous study, a theoretical model showed that under the same conditions, the optimal injection rate for weaker acids is lower than that for stronger acids. This paper presents a comparison of the efficiency of stimulation in carbonate acidizing of three different kinds of high temperature stimulation fluids. A chelating agent, EDTA, acetic acid, and a mixture of long-chained carboxylic acids were used to acidize carbonate cores at high temperatures. The effectiveness of the process and the optimal injection rate were studied by measuring the acid volume needed to propagate wormholes through 4-inch cores. The dendritic nature of the acid penetration was also determined by making castings of the wormhole structures after acidizing. The experimental results from this study showed that the optimal injection rate of long-chained carboxylic acids is lower than that for acetic acid and the EDTA. This increase in efficiency then determines that a deeper and more efficient stimulation per gallon of acid mixture used is obtained with the long-chained carboxylic acids.

Introduction

Matrix acidizing of carbonate formations has been carried out for many years using hydrochloric acid acid in various strengths. However, in some high temperature applications, hydrochloric acid does not produce acceptable stimulation results due to lack of penetration or surface reactions1,2. The success of conventional matrix acidizing in carbonate reservoirs with hydrochloric acid is often limited because the optimal pumping rate would exceed the fracture gradient of the formation3,4. The HCl-based acid fluids also pose problems such as high corrosivity and sludging tendencies when the acid contact crude oils, and the HCl sensitivity of some formations. These problems are intensified by high temperature and high pressure. Some corrosion problems may be alleviated by the use of a corrosion inhibitor, but the adsorption of corrosion inhibitors on the inside pipe surface may remove inhibitor and reduce the protection to corrosion caused by the live acid on the downhole tubulars. The adsorption of the inhibitors on the rock may block the pore space, reducing water wettability and therefore reduce the relative permeability to oil or gas16. Organic acids, like formic acid and acetic acid, were introduced to offer a slower reacting, and thus, deeper stimulating acids. These "retarded" acids also had short-comings due to solubility limitations of acetate or formate salts at high acid concentrations17 and corrosion problems at high temperatures12,13. In recent years, several alternatives have been developed in high temperature applications, including aminocarboxylic acids2,12 and long-chained carboxylic acids(LCA)13,14. Chelating agent-based fluids have been investigated for high temperature matrix acidizing15.

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