Coupled reaction-flow modeling of diagenetic changes in reservoir permeability, porosity and mineral compositions

Key reservoir properties within the same depositional facies such as permeability, can be strongly altered in time and space by water–rock interaction during diagenesis. A reaction–transport and flow code, DIAPHORE, was developed to address these effects at the reservoir scale, by coupling multi-dimensional mass transport and water–rock reactions. DIAPHORE is a three-dimensional (3D), two-phase, finite-volume, reaction–transport simulator, which couples kinetically-controlled dissolution and precipitation of minerals with equilibrium reactions of chemical species in the water phase. Mass transport of chemical species is coupled to geochemical reactions using an explicit method. DIAPHORE includes the feedback of chemical reaction upon texture (reactive surface area of minerals and porosity) and flow (permeability) properties. DIAPHORE is used to model the petrophysical effects of illitization on reservoir properties of sandstone in the Dunbar oil field (North Sea). This new approach to diagenetic modeling correctly predicts the amount of quartz and illite precipitated, along with the marked permeability damage observed in the Dunbar field. A simple one-dimensional (1D) case was used to highlight the fundamental diagenetic reactions taking place. The simulations correctly predict the sequence of illitization fronts (early thermodynamic instability of kaolinite and K-feldspar, followed by kinetically-controlled dissolution of albite and kaolinite) and the observed illite volume fraction. Hydrodynamic flow only influences the late illitization fronts resulting from albite and kaolinite dissolution. The 3D field-scale simulations fit the field data (permeability and illite volume fraction distributions).