Investigation of changes in permeability due to poro-thermo-elastic effects in enhanced geothermal reservoirs

An innovative technique for estimating changes in stresses due to poro-thermo-elastic effects during stimulation and circulation of cold water over a longer term in geothermal reservoirs is presented. A hybrid of tectonic simulation and stochastic analysis of field data is utilized to generate the subsurface fracture map of three formations. For this purpose, a geological structure is reconstructed according to its past tectonic history. Then fractures are stochastically simulated using Gaussian simulation. After generating the subsurface fracture map, the fluid flow is simulated using finite element method in a poro-thermo-elastic framework. Time-dependent heat transfer is modelled based on conductive heat transfer within the reservoir rock and convective (including conduction) heat transfer in discrete fractures. Changes of stress due to injection and circulation of cold fluid is studied by using roughness induced shear displacement principle in a poro-thermo-elastic environment. An analytical model for dilation of fracture surfaces based on the distributed dislocation technique is used to estimate changes in fracture aperture. The roughness of fracture surfaces is used in the calculation of residual fracture aperture.The proposed methodology is applied to a section of Soultz geothermal reservoir at a depth of 3650m and a number of numerical experiments were conducted to evaluate potential for permeability enhancement. Results of this study agree well with the observed field data. The results show that the average residual (retained) aperture is much lower and the time required to reach maximum shear dilation events were greater than those predicted by earlier studies including authors’ own study. In this study it was also observed that the effective tensile normal stresses due to circulation of cold fluid tend to increase fracture apertures within the zone of cooling. This increase in aperture and hence the permeability is evident predominantly near wellbore region in the early circulation period. Over longer period, a significant part of the reservoir, through which circulation is well established, is subjected to larger thermal stresses. These thermal stresses consequently increased permeability of the major interconnected fractures which led to significant changes in pressure distribution (decrease in impedance) and hence, increase in the flow rate