CHARACTERIZATION OF BRITTLE STRUCTURES IN BASALTS OF THE WESTERN SNAKE RIVER PLAIN, IDAHO: IMPLICATIONS FOR FRACTURE CONNECTIVITY IN A POTENTIAL GEOTHERMAL RESERVOIR

The western Snake River Plain is a region of high crustal heat flow and has the potential for commercial geothermal energy development. High-temperature crystalline reservoirs commonly have connected fracture networks and other discontinuities that provide the primary fluid storage and permeability (Type I fractures). A borehole was drilled through the DOE/ICDP Snake River Scientific Drilling Program near Mountain Home, Idaho to a depth of ~1,800 m (6,000 ft) with 85 - 90% slimhole core recovery to assess the potential for geothermal energy development. An artesian flow zone was encountered in basalt at a depth of 1,745 m (5,726 ft) in the MH-2 borehole with fluid temperatures above 140°C. Analysis of geomechanical behavior of rocks requires an understanding of basic physical and elastic properties under dynamic in-situ stress conditions. We conduct unconfined uniaxial stress experiments on core samples to measure static elastic properties and compressive strength over a ~305 m (1,000 ft) interval of the borehole at 55 sample locations above and including the geothermal reservoir. We compare the static elastic properties to the dynamic elastic properties calculated from full wave train downhole sonic data. The comparison demonstrates that the method to calculate dynamic elastic properties is effective in the case that core is not available for analysis. Natural fractures, induced fractures, and breakouts are mapped in acoustic televiewer data. Fracture density is calculated and compared to lithological and mechanical stratigraphy, defined by the physical properties, elastic properties, and strength measurements. 7 - 10 mechanical stratigraphic units are mapped have been identified from static and dynamic elastic properties and fracture density data. The stratigraphic relationships indicate that a ~15 m (50 ft) section of weak, non-brittle, low-permeability, highly altered basalt may act as a caprock to the geothermal reservoir at depth.