A Low-Viscosity Retarded Acid System for Stimulation of High-Temperature Deep Wells

Abstract To enhance the efficiency of acid stimulation treatments in high-temperature carbonate reservoirs, the industry requires a superior retarded acid system that inherently displays high thermal stability, controlled reaction rate, and acceptable corrosion losses. One of the earliest means of acid retardation was through the addition of polymer gelling agents; however, these sytems commonly lead to formation damage. Alternatively, a commonly used approach to slow down the acid/rock reaction rate is through the use of acid-in-oil emulsions (i.e., emulsified acids). The viscous nature of these fluids, however, leads to unfavorably high friction pressures during the pumping process. Additionally, the thermal stability of emulsified acids is questionable at temperatures exceeding 300°F. Herein, we introduce a novel engineered acid system for stimulation of high-temperature carbonate reservoirs. The proposed technology reduces the free water in the system and consequently prevents full dissociation of the acid. Reducing the free water in the acid system was successfully achieved by engineering the acid formulation in the presence of another organic compound that is readily soluble in the acid. A series of benchtop carbonate dissolution experiments were performed under static conditions at room temperature to identify the optimal engineered formulation needed to achieve the desired retardation effect. The retardation behavior of this system was further supported by reaction rate studies, coreflood measurements and corrosion loss data. The proposed acid system was evaluated at temperatures and pore pressures up to 350°F and 3000 psi, respectively. Experimental results confirmed that the engineered acid system requires only 0.28 PV to achieve a breakthrough (12 inch core), which represents a significant reduction as compared to some reported emulsified acid systems. Importantly, the fluid formulation was systematically fine-tuned in order to impart the desired fluid-property characteristics: low-viscosity (est. 4 cp), high dissolving power (4.16 lb acid/gal), fast wormhole propagation rate, low corrosion loss (<0.05 lb/ft2, 275°F) and high thermal stability (>300°F). The acid system can be readily mixed on-the-fly with existing equipment. This is a desirable feature particularly when it comes to offshore operations as no special preparation methods are required. It is the opinion of the authors that this novel acid system provides the industry with a suitable fluid alternative that may offer the potential to enhance oil and gas production in carbonate reservoirs. The fluid is stable at high temperatures and may have the potential to be used in a broad range of stimulation applications. Importantly, this low-viscosity retarded acid system presents the industry with a novel acid alternative where acid retardation by polymer addition or emulsification in diesel is no longer required.