| Abstract |
A number of interrelated properties control the amount of heat resource in the earth’s crust that can be extracted from a target reservoir rock including temperature gradient, natural porosity and permeability, rock physical properties (e.g., elastic; thermal), the stress regime, indigenous stored water and susceptibility to seismicity. These factors, taken together, not only control the physical processes of extracting heat, but also play a major role in determining the economics of producing energy. While solid rock is excellent for storing heat, the rate of heat removal by conduction is slow, due to its low thermal conductivity. Hence, only that fraction of the rock volume accessible via natural fractures and pores or made accessible by Enhanced Geothermal Systems (EGS) stimulation can be considered part of the ‘active’ reservoir where heat extraction occurs. In addition, the interface between the circulating geothermal fluid and potentially reactive mineral phases is defined by the relative distribution of solution filled pores and fractures. Coupled processes occurring at this interface transport thermal and mechanical energy, fluid mass, momentum, and individual chemical components through porous rock that comprises the geothermal system. While fracture-dominated flow is no-doubt critical in shallow crustal settings, there is a continuum of coupled reaction/transport phenomena from this scale down to the finest nanopores that influence energy- and mass-transfer in evolving geological reservoirs. The cost of investment capital for geothermal development is influenced significantly by reservoir risk, including fundamental uncertainties in the fluid mass in place and the permeability of the rock matrix. The storage capacity of the reservoir rock is strongly affected by capillarity and adsorption that, in turn, affects the economics of water injection. This paper describes an analysis of changes in the multiscale pore structure of granites in the Coso geothermal system with depth and location within the reservoir. Small angle, very-small angle and ultra-small angle neutron scattering measurements have been used, in combination with autocorrelation analysis of SEM/BSE imagery. Neutron scattering techniques have many distinctive advantages over other methods for studying the multiscale pore structure of rock materials, permitting statistically meaningful results for relatively large rock volumes (compared, for instance, to transmission electron microscopy). These have been used to determine variations in total and cumulative porosity, fractal behavior and surface area to volume ratio of samples from four drill holes in and near the reservoir, outlining significant variations in the Coso complex. |