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Diagenetic Flow and Transport Modeling to Predict Reservoir Quality in Carbonate Formations

Abstract

Diagenetic modifications remain a challenge for predicting reservoir quality in carbonate reservoirs. Conventional study approaches (petrography, geochemistry, etc) describe products of diagenesis, their paragenesis, the chemical and physical environment in which they formed, and define the types of diagenetic modification to expect in a reservoir but not its spatial distribution. Subsurface data offers a spotty view on the spatial distribution of petrophysical properties but no way to discern whether they are the result of depositional or diagenetic processes, or both. Numerical simulations of physical and chemical interactions between rocks and diagenetic fluids along flowpaths allows us to unravel the mechanisms controling diagenesis and to model the size, shape and connectivity of diagenetic bodies and trends. We illustrate such process-based approach by a numerical model of reflux dolomitization based on outcrops of the Permian San Andres formation in west Texas. Transient simulations of flow and solute transport during the accumulation of this carbonate platform are accompanied of a magnesium mass-balance to evaluate the growth and evolution of dolomite bodies in time and space. A parallel reactive transport modeling study evaluates transient changes in mineralogy, porosity and permeability. Our work shows that the location of active dolomitization changes over the accumulation of the platform, as do the mechanisms of growth and development of dolomite bodies. The principal controls of this variations are (1) expansion and shrinking of brine source, controlled by sealevel, accumulation rates, shifting environments of deposition, and other environmental factors, (2) flow paths and rates controlled by permeability pathways, also linked to environments of deposition and their stacking patterns (3) presence of dolomite bodies that act as substrate for further dolomite precipitation, (4) kinetics of dissolution and precipitation of carbonate minerals, and (5) feedbacks with evolving permeability field. In this manner, we observe dolomite bodies that develop near the surface and the brine source. Other bodies grow in the subsurface, following propitious permeability settings and/or precursor dolomite bodies. Additionally, dolomite bodies can continue to grow during times of no flow, by a process known as “latent reflux”. This approach can be applied to other diagenetic systems in carbonate and siliciclastic, conventional and unconventional reservoirs.