Characterization of Brittle Structures in Basalts of the Western Snake River Plain, Idaho: Implications for Fracture Connectivity in a Potential Geothermal Reservoir

James A. Kessler, James P. Evans, Doug R. Schmitt, and John Shervais
Utah State University

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). The DOE/ICDP Snake River Scientific Drilling Program drilled a borehole 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 1,608 m (5,276 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 tensile strength over a ~305 m (1,000 ft) interval of the borehole 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. The stratigraphic relationships indicate that a ~45 m (150 ft) section of weak, non-brittle, low-permeability, highly altered basalt may act as a caprock to the geothermal reservoir at depth. The induced fracture and breakout data will be used to conduct a stress analysis to identify the magnitude and direction of each of the three principal stress directions that describe the stress tensor. The results of fracture characterization, mechanical stratigraphy, lithological stratigraphy, and stress analysis will used as conditions of a discrete fracture network (DFN) model using the FracMan® code. FracMan® can build a deterministic model from fracture data measured in the borehole and stochastically model realizations of the fracture network on the reservoir scale. The model will allow us to make a first-order estimate of fracture permeability and fluid flow directions in the reservoir.

AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62^nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013