A comprehensive simulation model of kerogen pyrolysis for the in-situ upgrading of oil shales
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Copyright 2015, Society of Petroleum Engineers. Oil shale, which comprises abundant organic matter called kerogen, is a vast energy source. Pyrolysis of kerogen in oil shales releases recoverable hydrocarbons. Here we describe the pyrolysis of kerogen using an in-situ upgrading process, which is applicable to the majority of oil shales. The pyrolysis is represented by six kinetic reactions resulting in 10 components and four phases. Expanding Texas A&M Flow and Transport Simulator (TAMU FTSim), which is a variant of TOUGH + simulator (Moridis and Freeman 2014), we develop a fully functional capability that describes kerogen pyrolysis and accompanying system changes with a minimum of simplifications and assumptions. The simulator describes coupled process of mass transport and heat flow through porous and fractured media and includes all known physics and chemistry of reservoir systems. The simulator involves a total of 15 thermophysical states and all transitions between them and computes a simultaneous solution of 11 mass and energy balance equations per element. The simulator solves the equations in a fully implicit manner by solving Jacobian matrix equations using Newton-Raphson iteration method. To conduct a realistic simulation, we account for geological structure of oil shale reservoirs and physical properties of bulk oil shale rock. In addition, we consider interaction between fluids and porous media, diverse equations of state for computation of fluid properties, and numerical modeling of fractured media. We intensively validate the simulator by reproducing the field production data from Shell In-situ Conversion Process implemented in Green River Formation. We conduct sensitivity analyses of diverse reservoir parameters, such as presence or absence of a pre-existing fracture system, oil shale grade, permeability of the pre-existing fracture system, and thermal conductivity of a reservoir formation. We analyze the effects of the reservoir parameters on productivity and find a model that shows a similar production rate curve to the field production. The simulator is successfully validated and provides a powerful tool to evaluate effectiveness of in-situ upgrading processes and corresponding amount of recoverable hydrocarbons.