M.E. Politov Rosneft, Moscow, Russian Federation
Keywords: carbonate formations, acid stimulation, self-diverting-acid based on viscoelastic surfactant, experiments, mathematical modeling.
A self-diverting-acid based on viscoelastic surfactant (SDVA) has been used recently on stimulation treatments of carbonate formations. The decrease of acid concentration during the spending process viscosifies the fluid by the transformation from spherical micelles to an entangled wormlike micellar structure while penetrating the carbonate rock. The highly viscous fluid acts as a temporary barrier and diverts the fluid into the remaining lower-permeability treating zones. After treatment, the SDVA barrier breaks when contacted either by formation hydrocarbons or pre-and post-flush fluids. The objective of the present work is to examine in detail the effect of a viscoelastic surfactant on the reaction of HCl acid with calcite. Rheological measurements were conducted on rotational viscometers at temperatures 25°C. We develop the semi-empirical rheological model describing the dependence the apparent viscosity on the shear rate and of the degree of reduction of the concentration of HCl. The core flood experiments were carried out in a specialized testing unit simulating the downhole conditions, while controlling the corresponding pressure and temperature, as well as the injection volume of the acid agents flowing across the core sample. Results demonstrate the pressure drop as a function of pore volume of fluid pumped. The steady increase in the pressure drop as the fluid enters the core is a good indication that the viscosity increase indeed occurs in the porous rock during acid spending. The experiments were conducted with different flow rates. Core flow tests indicated that the surfactant delayed acid breakthrough in carbonate cores. We present a two-scale continuum model to describe reactive dissolution of carbonates with SDVA and use it to analyze wormhole formation in single core set-ups.