Predictions of permeability, surface area and average dissolution rate during reactive transport in multi-mineral rocks

The interactions between minerals and reactive fluids have great impact on the pore structure and hydrologic properties of rocks. A 3D pore-scale reactive transport model is applied to predict multi-mineral reactions in porous rocks. Eight mineral phases are distinguished in 2D slices of a reservoir sandstone by using Quantitative Evaluation of Minerals by SCANning electron microscopy (QEMSCAN) imaging technique. The 2D image is then registered into the corresponding cross-section of the 3D rock image obtained from micro-computed tomography (CT) scanning. Reactive transport with chemical dissolution is simulated directly on the 3D image of the sandstone. In the simulations, each mineral is assigned with a specific intrinsic reaction rate constant. The dissolution-induced changes in the pore structure are measured. The pore size distribution of the rock is analysed to investigate the evolution of the pore space. The changes of surface area and permeability are compared with single mineral simulations to illustrate the importance of mineralogical heterogeneity on reactive transport. The results show that the 3D pore-scale mineralogical heterogeneity causes overestimation of the permeability variations during reactive transport. The average effective dissolution rates for the rock are also measured. The simulations with single mineral assumptions present an overall reaction rate which is 10 times higher than the predictions with multi-mineral dissolution at the start of reactions. Relationships between average reaction rate and time are proposed to characterise the evolution of dissolution rate. A correction is also suggested to reduce the errors in predictions of reaction rates with single mineral assumptions.