Organic-rich rocks have long been recognized as source rocks for clastic reservoirs, but more recently they have gained importance as reservoirs. However, the processes of kerogen maturation and hydrocarbon transport and storage are still poorly understood. Some empirical relations have been developed to relate the increase in elastic modulus with increasing maturity. A systematic study of the cause for this increase in elastic modulus is still lacking, and information about seismic and mechanical properties of kerogen and its alteration products is scarce. Consequently, any rock models must rely on anecdotal or extrapolated data about various types of kerogen. Our experiments address this paucity of data by grain-scale modulus measurements coupled with careful field emission scanning electron microscopy (FESEM) microstructural assessments on organic rich Bakken formation shale samples with a range of maturities. Carefully acquired and detailed FESEM images help to understand the microstructural controls on the reduced (nanoindentation) Young’s modulus of minerals, clay particles, and kerogen matter with maturity in naturally matured shales. Using hydrous pyrolysis, we further investigate the cause for change in modulus with maturity and the mobility of the pyrolized organic matter. In naturally matured shale samples, we find a direct relationship between the reduced Young’s modulus and the total organic content or hydrogen index. Significant lowering of Young’s modulus is observed after hydrous pyrolysis due to bitumen generation. We measured modulus of the extruded bitumen to be less than 2 GPa. The presence of bitumen comingled with the organic matter also reduces its modulus, in excess of 30%. These results are critical to help understand how organic-rich sediments evolve with burial and maturation. The modulus measurements can be used for modeling modulus variations during maturation.

Mobile colloids can play an important role in contaminant transport in soils: many contaminants exist in colloidal form, and colloids can facilitate transport of otherwise immobile contaminants. In unsaturated soils, colloid transport is, among other factors, affected by water content and flow rate. Our objective was to determine whether water content or flow rate is more important for colloid transport. We passed negatively charged polystyrene colloids (220 nm diameter) through unsaturated sand-filled columns under steady-state flow at different water contents (effective water saturations Se ranging from 0.1 to 1.0, with Se = (θ - θr)/(θs - θr)) and flow rates (pore water velocities v of 5 and 10 cm/min). Water content was the dominant factor in our experiments. Colloid transport decreased with decreasing water content, and below a critical water content (Se < 0.1), colloid transport was inhibited, and colloids were strained in water films. Pendular ring and water film thickness calculations indicated that colloids can move only when pendular rings are interconnected. The flow rate affected retention of colloids in the secondary energy minimum, with less colloids being trapped when the flow rate increased. These results confirm the importance of both water content and flow rate for colloid transport in unsaturated porous media and highlight the dominant role of water content.

Energy extraction from subsurface reservoirs is important for addressing the increasing energy demand and environmental concerns such as global warming. However, the characterization of subsurface reservoirs, particularly reservoirs dominated by fracture networks remains a challenge due to the lack of means to directly observe subsurface processes. This study explores the feasibility and efficacy of characterizing fracture flow and transport processes in an enhanced geothermal system (EGS) testbed through stochastic tracer modeling. There are two enabling factors that allow application of stochastic modeling to characterize a subsurface reservoir. First, an abundance of geological and geophysical measurements enables the development of a high-fidelity and well-constrained fracture network model. Second, high-performance computing (HPC) allows running massive realizations efficiently. Six conservative tracer tests were stochastically modeled and produced satisfactory realizations that successfully reproduce field tracer recovery data from each tracer test. The evolution of flow and transport processes in the fracture network was then analyzed from these satisfactory realizations. The present study demonstrates that stochastic tracer modeling on a high-fidelity fracture network model is feasible and can provide important insights regarding flow and transport characteristics in subsurface fractured reservoirs.

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A conceptual Enhanced Geothermal System (EGS) model, where water is circulated through a pair of parallel injection and production wells connected by a set of single large wing fractures, is designed, modeled, and evaluated in this work. The water circulation and heat extraction in the fractured reservoirs is modeled as a fully coupled process of fluid flow and heat transport. Using a newly developed, open-source, finite element based geothermal simulation code, FALCON, simulation results were obtained for a 30-year operation at a depth of 3km and geothermal gradient of C per km of depth. With a sensitivity study of the heat production to the design parameters, preferable fracture horizontal spacing, downward deviation angle of the parallel wells, and injection flow rate are recommended. Upscaling calculations of the developed EGS model have shown that, an industrial production-level system may be achievable if it consists of 40 equidistant fractures that connect two 1.2 km long parallel well sections with a well separation of 500 m; and if a system of these dimensions operates for 30 years at a flow rate of 0.1 m 3 /s, with an electric power output at least 5 MW and pumping power of less than 1 MW. In particular, the performance metrics demonstrated in this work match well with those suggested by others, thus indicating the general applicability of our conceptual models.