Effect of produced carbon dioxide on multiphase fluid flow modeling of carbonate acidizing

Abstract

A two-scale continuum (TSC) numerical model is used at pore and Darcy scales to model and optimize the matrix acidizing dissolution process. During matrix acidizing process in carbonate reservoir, the solubility of carbon dioxide, as one of the hydrochloric acid/calcite reaction products, is limited, which leads to the formation of a separate gas phase in the reservoir based on pressure and temperature. The presence of this free gas has nonlinear effect on the fluid flow and phase distribution in porous media due to the alteration in the relative permeability of the injected/spent acid. Density and viscosity of the spent acid will also be changed. Ignoring these effects on TSC model can reduce the accuracy of the final results compared to similar laboratory studies. The significance of our study is to examine and apply the nonlinear effects of produced carbon dioxide on wormhole propagation, dissolution patterns, and breakthrough curves at different injection rates of the acid. The innovation presented in this study is the evaluation of the effect of the presence of the gaseous carbon dioxide produced during the hydrochloric acid and calcite reaction on the relative permeability of the injected acid. This effect has been numerically investigated and solved along with the governing equations of the matrix acidizing at different time steps. In our work, The TSC model is coupled with a nonlinear relative permeability model to consider the effect of free carbon dioxide. Density, viscosity, and solubility correlations are also coupled with TSC model to update fluid properties during acid propagation and acid/rock interactions. The final results show that the neglecting of free gas affects the accuracy of wormhole propagation estimation. The model shows better agreement with the experimental data to predict the rock dissolution patterns and wormhole breakthrough. By considering free gas, the model can track thicker wormholes due to the free gas blockage and the slower acid/rock reaction rate. Our approach showed that isothermal models are not accurate enough to predict the change in carbon dioxide solubility at higher temperatures. Reduction in solubility affects the acid's relative permeability and increases the volume of acid required for the breakthrough. Hence, the developed model improves our knowledge of wormhole propagation during carbonate acidizing.