Physical/Computational Framework for EGS In Situ Fracture Stimulation

Abstract

We employ a fully coupled finite element analysis of a thermal, hydraulic, and mechanical (THM) energy scheme to simulate stress/strain damage induced in an in situ poroperm medium stressed by wellbore fluid pressurization in the medium. Our poroperm medium is characterized by two empirical constraints: (i) a normally-populated fracture-density distribution that percolates fluid via long-range spatially correlated grain-scale fracture connectivity at all scale; and (ii) a (potentially) long-tailed (‘lognormal’) permeability distribution ê associated with percolation pathways related to normally distributed porosity distribution F expressed by ê = ê0exp(á(ö-ö0)) as attested by clastic reservoir well-core poroperm fluctuation systematics. The degree of fracture connectivity in such a medium is parameterized by á = ratio of standard deviations of logê and ö distributions. Small values of á describe low degrees of fracture connectivity and hence low bulk permeability, while large values of á describe high degrees of fracture connectivity giving high bulk permeability. Wellbore fluid pressurization creates shear strains in the fracture-heterogeneous poroperm medium, putatively generating grain-scale fracture damage additional to the pre-existing grain-scale fracture damage in the medium. Injecting grain-scale fracture damage can be seen as creating new fluid flow pathways and increased bulk permeability via newly created grain-scale fracture-connectivity. Pressure-induced fracture damage injection thus leads to greater fluid permeability equivalent to incrementing the value of the fracture-connectivity parameter á. Such wellbore pressurization could be conducted in interest of flow stimulation of an inter-wellbore EGS heat exchange volume.