Assessment of the chemical stability of clay materials with different reactive surface areas under varying pH environments: evidence from the Morrow B sandstone reservoir in the Farnsworth unit, Texas

This study examined fluid-rock interactions to assess the stability of clay minerals in a subsurface heterogeneous reservoir. Three-dimensional reactive transport simulations were run for up to 100 years using a grid representing the Morrow B Sandstone of the 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 the total aqueous concentrations of Na+ and K+ and a decrease in the total aqueous concentration of SiO2 (aq) were predicted under the higher acidic environment. The models predicted a continuous dissolution of illite and secondary precipitation of smectite minerals within the reservoir. 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 kaolinite and illite minerals in the model domain. The sensitivity to variations in the formation water pH level and the interaction effects of pH and surface area on the precipitation and dissolution of clay minerals were relatively insignificant.