Micromechanics-based continuum model for hydraulic fracturing of jointed rock masses during HDR stimulation

Abstract

In this paper, the mechanical response of rock masses to high-pressure hydraulic injections applied during hot dry rock (HDR) stimulation is studied. The sliding of natural joints originated by the action of water pressure is assumed for the governing mechanism of hydrofracturing. The constitutive relationship for the mechanical behavior of jointed rock masses under hydraulic fracturing is formulated by extending the MBC (micromechanics based continuum) model which is developed for jointed rock masses under excavation. After implementing the derived relationship into a finite element analysis code, HDR stimulation problems are analyzed and a successive formation of the fractured zone and the corresponding water pressures are studied, neglecting complex time-dependent processes. Variation of the mechanical response for different geological conditions is shown by two- and three-dimensional FEM analysis. In order to verify the assumed fracturing mechanism, input parameters from two HDR test project sites are employed for analysis. Comparison of the analysis results with the data recorded during the experiments supports the assumption that sliding and associated opening of natural joints assisted by the action of water pressure is the governing mechanism of hydrofracturing during HDR stimulation.