Compositional Reservoir-Flow Simulation for Organic-Rich Gas Shale

A new-generation compositional reservoir-flow-simulation model is presented for gas-bearing organic-rich source rocks, including convective/diffusive mass-balance equations for each hydrocarbon component in the organic (kerogen), inorganic, and fracture continua. The model accounts for the presence of dispersed kerogen with sorbed-gas corrected dynamic porosity. The Maxwell-Stefan theory is used to predict pressure-and composition-dependence of molecular diffusion in the formation. The equations are discretized and solved numerically by use of the control-volume finite-element method (CVFEM). The simulation is derived from a new multiscale conceptual flow model. We consider that kerogen is dispersed at a fine scale in the inorganic matrix and that it will be the discontinuous component of total porosity at the reservoir-simulation scale, which could be up to six orders of magnitude larger. A simple mass-balance equation is introduced to enable kerogen to transfer gas to the inorganic matrix that is collocated in the same gridblock. The convective/diffusive transport takes place between neighboring gridblocks only in the inorganic matrix. The simulation results show that the multiscale nature of the rock is important and should not be ignored because this could result in an overestimation of the contribution of the discontinuous kerogen. We also observe that although adsorbed fluid could contribute significantly to storage in the shale formation, its contribution to production could be severely limited by the lack of kerogen continuity at the reservoir scale and by a low degree of coupling between the organic and inorganic pores. The contribution of the Maxwell-Stefan diffusion to the overall transport in the shale formation increases as the inorganic matrix permeability is reduced because of pressure decline during production.