Geothermal Well Targeting Method Using Structural Irregularities

Abstract

Studies of the association of hot springs and fumaroles with fault structures indicate a statistically valid preference for enhanced permeability at structural irregularities such as fault step-overs, intersections, and bends near the surface (e.g. Curewitz and Karson, 1995 and Faulds et al, 2010). Other sites of local deformation that can result in high geothermal permeability include volcanic or magmatic brecciation (Lawless and White, 1990). Meanwhile statistical rock fracture studies demonstrate the importance of varying rock strength for the creation of abundant fractures. Structural locations of thermal features in geothermal areas, drilling results, and mechanical considerations show that tensional zones in areas of variable rock strength are particularly permeable. Meanwhile study of an exhumed fault zone originally formed at over 10 km depth shows remnant high porosity and permeability in tensional fault breccias (Melosh et al, 2014) even within the brittle-ductile transition zone. Comparison of these surficial and very deep observations to geothermal drilling history shows significant success at intervening depths in encountering high “sweet spot” commercial deliverability wells in similar patterns. A direct comparison of structural setting to well success at Blue Mountain, Nevada demonstrates the co-location of sweet spot well success with a tensional zone in a relatively simple structural irregularity. These consistent patterns from outcrop and well results along with practical exploration drilling considerations are applied in a step-by-step conceptual well targeting method that applies the stress orientation, fault and volcanic patterns, identification of tensional zones, interpreted depth extent of structures, rock strength patterns, and practical well geometry.