3D reactive transport modeling of porosity evolution in a carbonate reservoir through dolomitization

Dolomitization of carbonate reservoirs is considered a major process for porosity enhancement and is therefore of great relevance in hydrocarbon exploration. Despite the ubiquitous occurrence of massive dolostones, the question of whether reasonable amounts of dolomitizing fluid and Mg supply are capable of substantial porosity generation remains. In this work, a model of hydrothermal dolomitization of a carbonate reservoir is assessed by means of 3D reactive transport simulations implemented in iCP, an interface between Comsol Multiphysics and PHREEQC. The model is based on a currently producing carbonate reservoir with large regions of high porosity values deduced through seismic inversion. The model geometry and parameterization are based on the present-day configuration of the basin. Using an embedded model approach, the geochemical reactions in the reservoir have been coupled to the thermo-hydrogeological behavior of the entire basin. The results indicate that, after 100 kyr, dolomitization of limestone by fluids with high Mg/Ca ratio is restricted to areas around faults. Predicted volumes affected by dolomitization are consistent with dolomite bodies observed in nature. However, the intensity of dolomitization (up to 20% of limestone replacement) and the corresponding porosity enhancement (up to 3%) is not in agreement with the observed porosity values in the studied area. Even considering uncertainties in the fluid chemistry and in the hydrological conditions, numerical predictions suggest that times much longer than 100 kyr of recurrent water-rock interaction are needed to account for the generation of massive dolostones formed from limestone replacement. However, longer periods of time are not consistent with the tectonic evolution of the basin. Therefore, additional or alternative porosity generation mechanisms must be involved, such as compaction and late diagenetic corrosion.