Assessment of the chemical stability of clay materials with different reactive surface areas under varying pH environments: A numerical modeling approach

Abstract This study examined the fluid-rock interactions to assess the stability of clay minerals under a subsurface heterogeneous reservoir. Three-dimensional reactive transport simulations were run up to 100 years using a grid representing the Morrow B Sandstone of Farnsworth Unit and parts of the adjacent Morrow Shale layers in the western Anadarko basin, Texas. Six model scenarios were tested where the minimum and maximum reactive surface areas of clay minerals were repeated with the systematically altered formation water pH level from neutral (pH = 7) to highly acidic states (pH = 3). A relative increase in total aqueous concentrations of Na⁺ and K⁺ and a lowering of SiO₂ (aq) were predicted under a higher acidic environment. The models predicted a continuous dissolution of illite and secondary precipitation of smectite minerals. In contrast the stability of kaolinite temporally varied depending on the size of the reactive surface areas considered for the simulations. This resulted in the largest increase in the net volumetric abundance of smectite compared to the kaolinite and illite minerals over the model domain. The sensitivity to the variations of the formation water pH level and the interaction effects of pH and surface area for the precipitation and dissolution of clay minerals was found to be relatively insignificant.