Fracture acidizing has been a dominant practice in the industry to enhance well productivity in low permeability carbonate reservoirs. Many acid systems have been developed to improve this stimulation process. The most desirable characteristics for an acid system to be suitable for fracture acidizing are leakoff control and retarded reaction rate. These characteristics are required for deep acid penetration so that when fracture closes, long flow channels are etched on the fracture surfaces. Leakoff control is normally achieved by a pad containing viscosifying or solid bridging agents to plug wormholes generated by acid dissolution. Reaction retardation is normally attempted by lowering the effective diffusivity of hydrogen ion.
It is well known that during an acid fracturing operation the overall reaction rate of hydrochloric acid with limestone is mass transfer limited. Designing the treatment requires knowing the effective diffusivity of the acid fluid system. This parameter does not exist for viscoelastic surfactant gelled acids. Due to its combined leakoff control and retardation capabilities, surfactant-based acids have been used in acid fracturing treatments. As more carbonate reservoirs to be treated by the surfactant-based acid, it is important to obtain the effective diffusivity of H+.
The rotating disk device has been used to investigate the reaction kinetics between a reactive solution and carbonate rocks because the thickness of the boundary layer is uniform throughout the disk surface. This paper discusses the reaction rate data recently generated for surfactant-based acid using a rotating disk apparatus and presents the methodology used to quantitatively extract the effective diffusivity from the measurements.
The results obtained indicated that the viscoelastic surfactant examined (Betaine-type) reduced the dissolution rate of calcite with HCl acid. The surfactant reduced the diffusion coefficient for H+. The effect of temperature on the diffusion coefficient did not follow Arrhenius law.