In situ stress and geology from the MH-2 borehole, Mountain Home, Idaho: Implications for geothermal exploration from fractures, rock properties, and geomechanics

The Snake River Plain in southern Idaho is a region of high crustal heat flow and has the potential for economic geothermal energy development. The MH-2 borehole was drilled into a high-temperature artesian reservoir in the western part of the Snake River Plain as part of the Snake River Scientific Drilling Project (Project Hotspot). This research characterizes the rock properties, fractures, and in situ stress state for Miocene basalts in the 1,821 m (5,974 ft) deep borehole near Mountain Home, Idaho. The methods and analyses demonstrate a workflow to conserve resources and utilize a variety of data sources in combination to fill data gaps when necessary.;Unconfined uniaxial compressive stress experiments on core samples provide direct measurement of rock properties that are used to calibrate petrophysical models that utilize wellbore wireline log data and laboratory shear and compressional sonic velocity data. These data show a large variation in elastic rock properties and rock strength which define three classes of rock that are used to describe the mechanical stratigraphy.;The distribution of rock properties controls the distribution of fractures that are found mostly in strong, elastic rocks that exhibit brittle failure. The fractures form connected pathways that are the primary porosity and permeability in the reservoir. Weak, plastic rocks act as a cap rock seal that prevents the loss of heat and fluids from the reservoir. The research demonstrates that dynamic petrophysical models of the rock properties can be used in this region in lieu of static measurements from core and in combination of borehole and laboratory data.;Analyses of borehole breakout fractures are used to interpret the orientation and magnitudes of the local stress field around the MH-2 borehole. The state of stress is used to describe the structure around the Mountain Home region. The borehole was possibly drilled near a northeast-oriented stepover fault on a ramp between the tips of two larger northwest-oriented oblique-slip normal faults that define the boundary of a gravity high that is interpreted to be an intrabasinal uplifted block. The stepover fault likely dips to the southeast and experiences both dip-slip and strike-slip motion during failure.