| Abstract |
Despite active volcanism, geothermal energy remains an under-utilized resource in the Cascades region of the Pacific Northwest. The lack of development partly reflects the difficulty in locating potential reservoirs. The geothermal Play-Fairway Analysis (PFA) for Washington state developed and implemented an exploration method (Swyer, et al., 2018a,b, Forson et al., 2017) to improve the search for blind geothermal systems by assessing the likelihood of key elements of a complete geothermal system, including: (a) a heat source, (b) a permeable conduit connecting the heat source to a (c) shallow porous and permeable reservoir that is (d) saturated. One of the sites identified with a high potential for these characteristics, as well as necessary infrastructure and development potential, is approximately 15 km north of Mount St. Helens within the NNW-SSE trending St. Helens seismic zone. In the summer of 2018, we began two studies to test this prediction. First, the potential for fault-related porosity—to support a reservoir—is assessed through combined analysis of image and petrophysical logs in the Plate Boundary Observatory borehole B201 and outcrop and petrographic mapping of faults and fracture systems. These studies characterize the geometry, kinematics, and associated healing or alteration of the fracture and fault population. The goal is to determine if faults and fractures in basalt flows or indurated volcaniclastic deposits at reservoir depths are likely to provide connected porosity sufficient to host commercially viable geothermal reservoirs. In addition, borehole wall failure in image logs of B201 and slickenlines on the mapped faults provide new constraints on the regional geomechanical model. Second, the presence of heat is being assessed with a temperature-gradient hole drilled in the summer of 2018. In this paper, we: 1) review fault-associated porosity in the Mount St. Helens study area, and 2) present preliminary drilling results. Forson, C., A.N. Steely, T. Cladouhos, M. Swyer, N.C. Davatzes, M. Anderson, B. Ritzinger, J. Glen, J. Peacock, W. Schermerhorn, E. Burns, P. Stelling (2018) GEOTHERMAL PLAY-FAIRWAY ANALYSIS OF WASHINGTON STATE PROSPECTS: PHASE 2. Geothermal Resources Council Transactions, 41. October, 1-4, 2017, Salt Lake, City, UT, p. 2265-2311. Sywer, M.W., M.E. Uddenberrg, T.T. Cladouhos, A.N. Steely, C. Forson, N.C. Davatzes (2018a), Simulating local sources of crustal deformation for Washington State Geothermal Prospects using Geomechanical Models. Proceedings of the American Rock Mechanics Association, 52nd Annual Rock Mechanics / Geomechanics Symposium, Seattle, WA, USA 17-20 June 2018. ARMA 18-464, 8 p. Sywer, M.W., M.E. Uddenberrg, T.T. Cladouhos, A.N. Steely, C. Forson, N.C. Davatzes (2018b), Preliminary Geothermal Resource Assessment of the St. Helens Seismic Zone Using the Results from the Geothermal Play-Fairway Analysis of Washington State Prospects. PROCEEDINGS, 43rd Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, February 12-14, 2018 SGP-TR-213, 9 p. |