Quantifying the role of microporosity in fluid flow within carbonate reservoirs

Abstract

Micropores can constitute up to 100% of the total porosity within carbonate hosted hydrocarbon reservoirs, usually existing within micritic fabrics. There is, however, only a rudimentary understanding of the contribution that these pores make to reservoir performance and hydrocarbon recovery. To further our understanding, a flexible, object-based algorithm has been developed to produce 3D computational representations of end-point micritic fabrics. By methodically altering model parameters, the state-space of microporous carbonates is explored. Flow properties are quantified using lattice-Boltzmann and network modelling methods. In purely micritic fabrics, it has been observed that average pore radius has a positive correlation with single-phase permeability and results in decreasing residual oil saturations under both water-wet and 50% fractionally oil-wet states. Similarly, permeability increases by an order of magnitude (from 0.6md to 7.5md) within fabrics of varying total matrix porosity (from 18% to 35%) due to increasing pore size, but this has minimal effect on multi-phase flow. Increased pore size due to micrite rounding notably increases permeability in comparison to original rhombic fabrics with the same porosity, but again, multi-phase flow properties are unaffected. The wetting state of these fabrics, however, can strongly influence multi-phase flow; residual oil saturations vary from 30% for a water-wet state and up to 50% for an 80% oil wet fraction. flow when directly connected. Otherwise, micropores control single-phase permeability magnitude. Importantly in these fabrics, recovery is dependent on both wetting scenario and pore-network homogeneity; under water-wet imbibition, increasing proportions of microporosity yield lower residual oil saturations. Finally, in grain-based fabrics where mesopores form an independently connected pore network, micropores do not affect permeability, even when they constitute up to 50% of the total porosity. Through examination of these three styles of microporous carbonates, it is apparent that micropores can have a significant impact on flow and sweep characteristics in such fabrics.