Modeling the Effect of Long-term Diffusive Infiltration of Reactive Water on Seal Integrity: Case Study of Lower Paleozoic Strata, Illinois Basin, Kentucky

Sealing formations overlying CO2 sequestration targets will be variably affected by geochemical interactions and mechanical forces. Chemically, the rocks are subject to acidified water produced from the dissolution of injected CO2 in the formation water. Mechanically, the rocks are subject to increased fluid pressure that could induce fracturing. Both of these processes affect seal properties. The driving forces and the time scales of these geochemical and mechanical processes are interdependent, i.e., coupled nonlinear processes: chemical diagensis can change rock mechanical properties and peremeability, and fractures can induce rapid change in the water composition as water and solutes are expelled from the rock. However, the two processes are sufficiently distinct and can be addressed individually to assess their effects on long-term integrity of seals. In this study the effect of 1D vertical solute diffusion and the attendant reactions in a carbon storage reservoir and overlying confining layers are modeled using Sym.8, a reactive-transport and water-rock interaction simulator. The open-system simulator uses elemental mass-conservation equations and thermodynamic equilibrium reaction expressions to solve mass-transfer processes, speciation among solutes, and kinetic reactions among solutes and minerals. The simulator uses a composite porous media model to describe textural and compositional evolution of rocks as water-rock interactions progress. Water-saturated pore space and isothermal configurations are used in this study. Inputs for the model are based on a robust subsurface geologic data set collected from the Cambrian-Ordovician Knox Group dolostones, a potential sequestration target in Kentucky, which are overlain by Ordovician limestone and shale of the Black River (secondary seal) and Maquoketa Formations (primary seal), respectively. The data set includes stratigraphic, petrographic, and water data collected from Blan No. 1 well, in Hancock County, Kentucky. Two scenarios were modeled to predict the effects of CO2 infiltration across contacts between (1) Knox dolostone overlain by Black River limestone, and (2) Black River limestone overlain by Maquoketa shale. Simulations of up to 10,000 years suggest that minor compositional and petrophysical property changes can be expected in the rocks. Most extensive alterations occur immediately above lithologic contacts due to strong mineralogical contrasts. Between the Knox and Black River dolomitization of the Black River limestone is the most prominent process with dissolution of dolomite in the Knox, providing the Mg. In the Maquoketa shale dolomitization of calcite and kaolinite precipitation from dissolving chlorite are the most prominent reactions. Small quantities of feldspar in the shale contribute a minor amount of solutes to produce kaolinite. Infiltration of carbonate and bicarbonate solutes in both Black River and Maquoketa are limited to the lower regions of the sealing formation (within 50 meters of the contact) after 10,000 years.