Sandstone matrix acidizing is a challenge in most cases. The conventional mud acid system involving hydrochloric acid and hydrofluoric acid (HCl:HF) is widely used in stimulating sandstone formations. The disadvantage of using HCl:HF is in its rapid reaction rate, resulting in insoluble by-products from secondary and tertiary reactions that limit the treatment effectiveness. The system can also limit the acid's performance capability when deep formation stimulation is targeted.
Enhancing HCL:HF acid system has been developed by utilizing of a protective ion-complexing agent. The agent forms a temporary protective film on clay surfaces allowing the acid to react with the remaining parts of the formation. The stimulation treatment fluids can be placed within the open hole reservoir section using two methods. The first utilizes 2-in. by 1-in. concentric coil tubing (CCT) with side-jetting nozzles to achieve uniform placement across the horizontal section and deep penetration inside the matrix. The second method uses a conventional bull heading treatment.
Laboratory testing was performed to qualify the treatment. Different minerals were used to evaluate and understand the retardation of different acid systems. The solubility of clays was lower comparing to normal HF acid systems, indicating the slow reaction of the acid on clays and a sign that improved retardation was achieved. Further testing confirmed the formation of a protective layer on clays, enabling the acid to react more efficiently with the remaining minerals. After successful laboratory testing, field trials were conducted on four oil-producing wells in southern Oman that experienced fines migration issues. The main acid system was squeezed into the matrix, followed by brine post flush, and then flowed back. Treatments with the protective acid systems in the treated wells showed a net oil gain of 30% to 80% sustained for longer than six months.
This paper outlines the technical challenges in the design and application of the proposed acid system and presents the successful lab results indicating the formation of the protective film on the clays minerals and the deep penetration phenomenon.