Analyzing the dynamics of mineral dissolution during acid fracturing by pore-scale modeling of acid-rock interaction

Hydrochloric acid is commonly used in acid fracturing. Given that the interaction between acid and rock affects multiphase flow behaviors, it's important to thoroughly understand the relevant phenomena. Darcy-Brinkman-Stokes (DBS) method is most effective to describe the matrix-fracture system among the proposed models. The objective of this study is to analyze the impact of acid-rock interaction on multiphase flow behavior, by developing a pore-scale numerical model applying DBS method. The new pore-scale model is developed based on OpenFOAM, which is an open source platform for the prototyping of diverse flow mechanisms. The developed simulation model describes the fully-coupled mass balance equation and the chemical reaction of carbonate acidization in an advection-diffusion regime. Volume of Fluid (VOF) method is employed to track liquid and gas phase interface on fixed Eulerian grids. Here, penalization method is applied to describe the wettability condition on immersed boundaries. To compute the numerical solutions of discretized equations, finite volume method is applied, where the equations of saturation, concentration, and diffusion are solved successively, and momentum equation is solved by using Pressure-Implicit with Splitting of Operators (PISO) method, respectively. The simulation results computed by this numerical model have been validated by experimental results. Different injection velocities and the second Damkohler numbers have been simulated to investigate their effects on the evolving porosity and rock surface area. The newly developed pore-scale model in this research provides the fundamental knowledge of physical-and-chemical phenomena of acid-rock interaction and their impact on acid transport. The modelling results describing mineral aci dization will help us to implement an effective fracturing project while reducing environment impacts.