Uncertainty analysis for heat extraction performance from a stimulated geothermal reservoir with the diminishing feature of permeability enhancement

An enhanced geothermal system (EGS) has two critical features in the stimulated reservoir volume (SRV): random distribution of permeability and definite diminishing enhancement of permeability. However, the effects of these two critical features on heat extraction simulations of hot dry rock (HDR) reservoirs have been rarely studied. This paper will propose a theoretical framework to incorporate these two features into an uncertainty study of heat extraction performance. First, five sets of random fluctuations are generated with turning bands method (TBM) to characterize the uncertainty feature of permeability. Five descend trends in an elliptic SRV are constructed to characterize the definite diminishing feature of permeability. Then, nine hundred stimulated virtual reservoirs are constructed in two groups by assigning the superposition of random fluctuations and descend trends to the permeability. Third, a thermal-hydraulic-mechanical coupling model is established to simulate the heat extraction from these virtual reservoirs. Finally, several indices are defined for the elevation of EGS performance. The statistical analysis on production flux and thermal breakthrough time of these simulations identified three modes of EGS performances. Larger correlation length and variance result in higher uncertainty of EGS performance. Larger permeability enhancement in the SRV zone consistently increases the production flux of a stimulated reservoir. Small flux induced by large impedance is the first barrier to a satisfactory EGS performance of low permeability HDR reservoirs with current reservoir stimulation techniques.