A reduced degree of freedom model for thermal permeability enhancement in blocky rock

Abstract

An upwind-weighted finite-element model is presented for the analysis of non-boiling hot dry rock geothermal systems. The model accommodates the essential mechanisms of permeability enhancement or degradation resulting from injection of fluid at a temperature different from ambient. The effects of induced thermal strains and fluid pressures in conditioning both normal and shear displacements in an ubiquitously jointed continuum are accommodated. The mass is idealized as a blocky assemblage where diffusive-advective energy transport in the fracture system is augmented by transient heat supply from intact rock blocks. The true transient nature of both energy supply from the blocks to the percolating fluid and the development of thermal strains within the medium are determined analytically. The local assumption of full lateral restraint coupled with analytical representation of thermal strains renders the nonlinear initial-value problem fully defined in terms of the two dependent variables of fluid pressure and fluid temperature only. Subject to these assumptions, complete fluid-pressure and fluid-temperature histories of large, thermally stimulated reservoirs may be determined effectively and efficiently. Results are presented for both single point injection and dual point injection-withdrawal scenarios to illustrate the possible scope of the method.