Analysis and simulation of rheological behavior and diverting mechanism of In Situ Self-Diverting acid

Abstract

SDVA acid is one kind of In Situ Self-Diverting acid fluids that are widely used for diverting acid into a low-permeability zone in carbonate reservoir treatment, thus stimulating the production of the heterogeneous reservoir effectively. However, the rheological behavior and diverting mechanism of In Situ Self-Diverting acid (SDVA acid) is so complicated that no models reported can perfectly simulate the acidizing process of SDVA acid, especially in radial flow. In this paper, we study the factors influencing rheological behavior such as pH value, calcium ion concentration and SDVA concentration. An empirical rheological model is developed to describe the relationship between the apparent viscosity of SDVA acid and the coefficient of pH, calcium ion and SDVA. Based on the two-scale continuum models, a numerical model is developed to describe reaction, transportation and diversion of SDVA acid in carbonate rocks for linear flow and radial flow. From the simulation results, the gel region is observed around the wormholes and branches, and this high-resistance region limits the propagation of wormholes more seriously in the radial direction compared to the length direction of linear core. When the acidizing is performed in a single linear core or in a single radial core, the injection pressure drop decreases monotonically versus the injected pore volume of HCl acid. But when SDVA acid is injected, the pressure drop increases initially and decreases afterward. The pressure drop curves from simulations are consistent with experimental results. Finally, we analyze the diversion results in parallel core set-up with different permeability, where the wormhole propagates much longer in the low-permeability core when SDVA acid is injected compared to HCl acid systems and wormhole breaks in high-permeability core firstly no matter which acid systems are used. The curves of diversion flow rate in the parallel core simulation further confirms that the diverting mechanism is simulated successfully for SDVA acid by models developed in this paper.