PLAY FAIRWAY ANALYSIS FOR GEOTHERMAL EXPLORATION IN THE WASHINGTON CASCADES

Abstract

A statewide geothermal resource assessment by the Washington Division of Geology and Earth Resources (WDGER) revealed areas with elevated heat and permeability, within close proximity to transmission lines, at accessible areas with elevations reasonable for development, and geothermal lease lands, defining three promising plays along the central axis of the Cascade magmatic arc in Washington: Mt. St. Helens, Wind River valley, and Mt. Baker. The three plays were selected by the United States Department of Energy (DOE) Geothermal Technologies Office for geothermal Play Fairway Analysis (PFA) by WDGER and with AltaRock Energy Inc. The PFA adapts fast-track exploration and development methods used in oil and gas to geothermal resources. The geothermal fairway is constituted by the collocation of heat, permeability, and saturated porosity in sufficient volume that provide adequate heat withdrawal at depths accessible by modern drilling technologies. The three sites showed regional scale characteristics consistent with these attributes and sufficient data exists to support more detailed analysis. Phase 1 of the PFA included a desktop study of publicly available data to assess where commercial geothermal potential is highest. Phase 1 revealed where new data was needed to increase resolution and reduce uncertainty, which was gathered from field surveys during Phase 2. Permeability potential was assessed at a local level (along fault planes) and incorporating regional strain. The heat potential at the surface is based on indicators such as hot springs, steam and volcanic gas vents, and hot shallow wells, and employs a radius of influence as a spatial indicator of favorability. The favorability criteria in Phase 2 is being refined to include numerous geophysical datasets that were gathered in the field (by WDGER, AltraRock Inc., and USGS). The revised favorability modeling aims to identify areas where there is reservoir permeability, heat sources at depth, and the presence of reservoir fluids that are accessible by drilling and producing wells.