From Matrix Acidizing to Acid Fracturing: A Laboratory Evaluation of Acid/Rock Interactions

Abstract

Summary Acid fracturing and matrix acidizing are used as well-stimulation processes for carbonated reservoirs. In matrix acidizing, a deep penetration of wormholes around the well is required; in the acid-fracturing process, fluid leak-off must be limited and wormholes are prohibited. Laboratory tests are necessary to improve the design of these operations. Specific laboratory equipment was designed to reproduce downhole flow conditions. Matrix acidizing is a constant flow rate process, whereas acid in fracture is forced from the fracture wall to the formation, a process occurring at a constant pressure drop. This process was represented in the laboratory and a new tangential cell, aiming at working with straight acids in reservoir temperature and pressure conditions, was specially designed to handle this application. The first part of the paper presents a methodology for the evaluation of acid/rock properties. Results of acid injections at constant flow rates are compared to results obtained from injections at constant pressure drop. Experiments have been conducted with limestone core samples of different petrophysical properties. Performance of different acid formulations including straight acids, emulsions, and gels was evaluated. Results are discussed in terms of acid propagation rates and dissolution patterns. They are analyzed using x-ray computed tomography. They are classified into wormholes, i.e., branched patterns of large extension, and compact ones. Finally, recommendations are made concerning operating conditions favorable for matrix acidizing. On the opposite, conditions are given to limit dissolution and to restrict fluid loss for the acid fracturing application.