Strength and ductility of four dry igneous rocks at low pressures and temperatures to partial melting

Abstract

Energy extraction from magma requires stable boreholes at relatively shallow depths (< 10 km) in rocks at temperatures of the order of 1000/sup 0/C. Accordingly, the failure strengths, strains at failure, and associated deformation mechanisms of room-dry andesite, besalt, granodiorite, and obsidian are determined at temperatures to partial melting (> 1050/sup 0/C), at confining pressures of 0 and 50 MPa, and strain rate of 10/sup -4//s. The strength reductions of the crystalline rocks are more or less linear until they steepen suddenly with approach to melting. When that occurs, strengths vanish and deformations become quasiviscous. The obsidian is stronger than the crystalline rocks to 600/sup 0/C where glass softening begins and strength goes to zero at 800/sup 0/C. All rocks are brittle throughout the entire temperature range until melting or softening occurs. Shortenings at failure are 3 percent or less. The crystalline rocks tend to deform primarily by precursive microscopic extension fracturing and its coalescence into macroscopic faults. The abundance of load-induced fractures remains about constant, but thermal cracking increases with increasing temperature. Results from tests at 25/sup 0/C on specimens that previously had been heated to 900-1000/sup 0/C clearly show that the weakening of unconfined specimens is due to the thermal cracking. Weakening of confined specimens, however, probably is due to an inherent temperature effect on the load-induced fracturing process. Comparisons of instantaneous failure-strengths with stresses likely ot occur at the walls of boreholes show that a hole as deep as 10 km in impermeable crystalline rock is not likely to fail under short-time loading even at 1000/sup 0/C, unless the maximum in-situ horizontal stress is greater than or equal to vetical stress and the hole is open (i.e., borehole pressure is zero). 27 refs., 10 figs., 7 tabs.