Evolution of Porosity, Permeability, and Fluid Saturations During Thermal Conversion of Oil Shale

Abstract

Abstract The In-situ Conversion Process, called ICP, is a thermal recovery process that was developed to produce oil and gas from kerogen-bearing oil shale. In-situ conductive heating of the oil-shale reservoir converts its kerogen (hydrogen-rich solids) into oil and gas (fluids) plus coke (hydrogen-deficient solids). While coke is left behind on the rock, hot vapor and heavy oil flow through the thermally altered oil shale to producer wells, and predicting ICP recovery performance depends on having an accurate characterization of the flow and storage behavior of the oil shale. However the rock fabric changes radically during ICP - the rock's porosity and permeability change with time in a complex way. While changing pore geometry (and pore-wall composition) also results in changing relative-permeability (kr) and capillary-pressure (Pc) curves, this work focuses on the evolution of oil and gas saturations (which dictate how kr's and Pc's influence ICP performance), as well as the change in porosity and its resulting alteration of oil-shale permeability. Numerous pyrolysis experiments were performed in the laboratory, where crushed oil shale was heated and effluents were monitored. Furthermore, through a series of otherwise identical experiments that were stopped at different degrees of conversion, changes in the oil-shale solids were determined by analyzing the spent rock. Through these experiments, a reservoir simulation model was developed that, for a variety of oil shales and time scales, captures the chemical-reaction kinetics for the solids and fluids, and the phase behavior of the fluids. Sensitivity studies with this model identified the changing oil-shale fluid-transport properties as a key driver for ICP performance. In this work, the ICP simulation model is used to illustrate ICP reservoir physics for different oil shales. Simulations reveal how changing in-situ fluid compositions (due to chemical reactions, temperature increase, etc.) not only result in changing fluid properties, but also complex behavior with time for oil and gas saturations, and this saturation history governs in part the flow of oil and vapor to producers and thus ICP performance. The early time following kerogen conversion is a period of higher oil saturation and thus higher oil mobility, illustrating how ICP production can include the flow of both vapor and heavy oil to producer wells. Another key to ICP success is the permeability history that results from the evolving porosity. As kerogen converts to fluids, porosity first increases (owing to reactive grain loss), then decreases (from the deposition of coke). While porosity behavior was determined from grain volumes measured in crushed-rock experiments, some complementary tests on pyrolyzed intact rock reveal increasing oil-shale permeability during ICP, which enables production from rock whose original permeability is quite low. Also simulations show that 90% of the pattern pyrolyzes after there are already permeable pathways developed connecting the pyrolyzing regions to the producer well.