Hydraulic Fracturing and Hydrothermal Brecciation in Active Geothermal Systems

Abstract

Hydrothermal breccias in active geothermal systems form when fluid pressures exceed the least principal effective stress (sigmax) by an amount equal to the tensile strength (T) of the affected rock. In high temperature systems in extensional tectonic setting, sigmax is horizontal and approximately equal to one third the lithostatic load (L) less hydrostatic fluid pressure (P) at the boiling point. The critical pressure required to hydrofractrure (Pfr) is provide by the equation: Pfr=(L-P)/3+T. Boiling point vs. depth curves based on Pfr values graphically decline the phsycical conditions of hydraulic rock rupture. Increasing vapor pressure beyond Pfr at a given depth, through seismic activity or increase heating, will induce rock failure by hydrofracturing.