WATER-ROCK INTERACTION IN THE COSO GEOTHERMAL SYSTEM

This study documents the result of water-rock interaction at the Coso geothermal area, where U.S. Department of Energy Enhanced Geothermal Systems (EGS) experiments are conducted to better understand the thermal evolution of this system. Mineralogy and micropore structure and pore connectivity are important factors for estimating a number of reservoir properties, including fluid characteristics, permeability, and geochemical and thermal feedbacks between the rock and the fluid. The relationships among the sizes, shapes and distribution of pores in minerals comprising rocks in the Coso system, as well as mineral abundances, are used as indicators of how fluids migrated through and reacted with the nano to micropore environment. This work summarizes the petrologic and geochemical characteristics of well cuttings from the Coso system to evaluate the hydrothermal alteration, its evolution, and how changes in mineralogy impacted porosity and permeability. In order to interrogate a complex heterogeneous system such as Coso we employed a number of complementary methods to determine the mineralogy and the mineral-pore associations, including polarized light microscopy (PLM), powder X-ray Diffraction (XRD), scanning electron microscopy (SEM) including energy dispersive X-ray spectroscopy (EDXS) and Backscattered Electron Imaging (BSE), Quantitative Evaluation of Minerals by Scanning Electron Microscopy (QEMSCAN^®), gas sorption porosimetry, and neutron scattering. Additionally preliminary assessment has been made of key samples using micro-X-ray Computed Tomography (µ-XCT) which revealed the presence of numerous very small unconnected pores. Collectively, our results suggest that the Coso geothermal field is a rock buffered system, where connected porosity is generally a very small fraction of the total porosity. Among the minerals, plagioclase has the largest area percent of pores within the grains or adjacent to them. The alteration mineralogy evolved over a temperature range from approximately 300-325^oC to 200^oC. The vertical alteration zoning pattern so common to many geothermal systems is not as well developed at Coso, but there is a reasonably definitive paragenetic sequence of mineralization that varies depending on the location of the producing part of the system.