Novel Retarded Acid System Improves Acid Fracturing in High-Temperature Gas Wells

Abstract

Abstract The oil and gas industry continue to rely on chemical methods for well stimulation of carbonate-rich formations, particularly hydrochloric acid (HCl). The selection is primarily driven by the fact that it is cost-effective and readily reacts with calcium and magnesium-based carbonates to yield soluble products. However, enhancing the fracture half-length using conventional HCl treatments in acid fracturing operations in high-temperature reservoirs remains a challenge. This is because the fast reaction rate between HCl and the rock matrix causes the acid to be rapidly spent and thus hinders deeper placement into the formation. These limitations were tackled through the development of a novel retarded acid system, referred to as LVAS-1 (Low Viscosity Acid System-1). The new acid system is prepared by blending a strong mineral acid with a suitable strong organic acid while minimizing the amount of free water in the formulation as a mechanism to control dissociation of the acid and afford the desired retardation properties. This new single-phase low-viscosity acid system has proven its effectiveness in acid fracturing applications. The technology was trial tested in high temperature/high pressure conditions to fracture a conventional gas well in the Middle East. The motivation for field testing this system was to significantly increase the etched fracture half-length and maximum attainable pump rates over the current standard emulsified acid system. This was achieved by controlling both the engineering parameters during the treatment and the acid retardation properties. A rigorous post-treatment analysis was conducted to evaluate the performance of this acid system and benchmark it to previously tested acid systems. This includes pressure transient analysis, post–frac flow back, and friction pressure calculations. Pressure measurements relied on installing downhole gauges for friction calculations and Pressure Build-up (PBU) analysis. We report here the successful development and deployment of a novel engineered hybrid acid system that overcomes the limitations associated with using emulsified acids. Specifically, the technology exhibits superior retardation properties, the ability to pump at higher rates due to lower friction pressure, and the formulation preparation method reduces operational complexity at the wellsite. Acid fracturing operations using the new acid system have proven successful, as evidenced by the prolonged gas production rate. Results of the PBU analysis show finite-conductive fractures in the reservoir with half-lengths higher than conventional acid systems and negative skin factor. This paper presents a success story, i.e., from laboratory-to-field scale, in the development and utilization a new retarded acid system for acid fracturing applications. It highlights essential design parameters specific to the new system and lists the advantages and limitations over conventional emulsified acid systems.