Core/Shell Systems for Delayed Delivery of Concentrated Mineral Acid
Acid stimulation, for both oil and gas wells, greatly supports the industry as a versatile means of enhancing production. Although acids enhance carbonate reservoir permeability to hydrocarbons, the reaction rates of the acid [e.g., hydrochloric acid (HCl)] with the rock often are too rapid at high temperatures, leading to a reduction in acid penetration. Several methods exist to improve the effectiveness of acidizing in high-temperature reservoirs (e.g., greater than 120°C), including the use of conventional emulsified-acid systems, mixtures of HCl and organic acids, and gelled acids. Many of the aforementioned techniques are effective forms of treatment; however, they hold significant limitations such as reduction in acid efficiency, poor control over penetration depth, and the requirement of corrosion inhibitors. Accordingly, encapsulated HCl holds potential as an attractive alternative to address these shortcomings because its prolonged release profile would permit transport of acid deep within the reservoir. In addition, when successfully encapsulated, this technology could eliminate, or at the very least minimize, the use of corrosion inhibitors. Herein, we demonstrate the design and preparation of highly modular core/shell particles comprising concentrated HCl encapsulated within an acrylate-based thermoset polymer shell. We show that the shell-generation mechanism (i.e., photopolymerization of acrylate monomers) is compatible with concentrated HCl and further detail the encapsulation process. Our results demonstrate that the acid-release profiles are dictated by the properties of the shell material, enabling a prolonged delivery of HCl in laboratory studies. This is a first step toward the design of particle-shell systems that can tolerate harsh reservoir conditions, including high temperatures, pressures, and salinity of mixing water. A tunable core/shell delivery system that encompasses a sufficient amount of strong mineral acid is well-poised to address the unmet need of deeper penetration of HCl into the reservoir, enabling greater stimulation efficiency.