Modeling Study of Acid Fracture Fluid System Performance

Abstract Acid fracturing stimulation can be an effective means to improve well performance in carbonate formations. In general, a treatment consists of multiple stage injections alternating between acid and non-reacting fluid to better place acid, and therefore, to create sustainable conductivity for enhanced well productivity. Selecting appropriate fluid systems is critical in success of acid fracturing. To optimize acid fracturing design, an integrated approach is needed to model the fluid behavior inside of the fracture, the conductivity after fracturing, and the productivity of fractured wells. The integrated approach includes an acid transport model and a fracture propagation model. Theoretical models were developed to obtain the conductivity distribution and acid penetration distance inside fractures. Continuity and momentum balance equations are solved in three-dimensional space for pressure and velocity profiles. Once the velocity profile is generated, the acid balance equation can be solved for the acid concentration profile. Obtaining a concentration profile helps in calculating the amount of rock etched through diffusion and convection. Conductivity distribution is calculated using a correlation where statistical parameters are used to account for fracture heterogeneity. Finally, a reservoir simulator is used to predict the production performance from acid fractured wells. Straight, emulsified, and gelled acid systems are examined using the integrated approach to capture the effect of each on fracture conductivity and acid penetration distance. Optimizing these two parameters will improve the production performance significantly. Straight acid reacts aggressively with carbonate formations and leaks off significantly more near the entrance of the fracture when compared with the other two acid systems. The apparent viscosities of gelled or emulsified acids are substantially higher than that of straight acid, and the diffusion coefficients of these viscous fluids are substantially lower than that of straight acid. These properties result in deeper penetration down the fracture with gelled or emulsified acids, but also less fracture etching by acid near the well. The tradeoff between deeper penetration with viscosified acid systems versus more near well etching with less viscous systems means that the acid formulation can be optimized depending primarily on the formation permeability. Production analysis of these three acid systems suggests that emulsified acid is better used in tight formations. Gelled acid, on the other hand, results in the highest production rate when used in relatively high to medium permeability formations. Straight acid is the preferred system only when short, highly conductive fractures are desired.