Hydrogeochemical Modeling of Illite-forming Mechanisms in the Unayzah Formation, Saudi Arabia

Illite is considered to represent one of the most important clay minerals affecting reservoir properties and even causing formation damage. The Late Carboniferous to Early Permian Unayzah Formation in Central Arabia was selected as a case study to improve understanding of illite-forming processes. A combination of petrographic-mineralogical analysis of well cores and cuttings, hydrochemical composition of formation water, and reservoir conditions from three selected wells are input for developing a hydrogeochemical model. Based on thermodynamic equilibrium, numerical modeling of fluid-rock interaction is used to identify secondary alteration processes affecting reservoir quality. The outcome of various scenarios suggests minor effects from increasing temperature (due to burial) or formation water salinity on illite formation with a maximum measured content of 7 wt%. The present study indicates that CO2 - as a major component in subsurface systems - may be responsible for the formation of several wt% of illite, coupled with dissolution of K-feldspar. As the modeling of the Unayzah Formation demonstrated the stability of newly formed illite over a wide pH range, removal of diagenetic illite to enhance the porosity and permeability properties on near-wellbore sites by formation acidification may be problematic.