An innovative stimulation technology for permeability enhancement in enhanced geothermal system - Fully coupled thermo-poroelastic numerical approach

This paper presents a new technique for stimulation of enhanced geothermal systems utilising roughness induced opening of fractures in the presence of compressive and shear stresses as well as fluid pressure inside the fracture. The thermal model is based on conductive heat transfer within the reservoir rock, convective (including conduction) heat transfer in arbitarily oriented discrete frectures and time dependent thermal equilibrium between the rock and fluid. The roughness induced shear displacement model in a thermo-poroelastic environment combined with an advanced computational technique has allowed us to simulate the opening of fractures and changes in the permeability in a geothermal reservoir. The technique was applied to a section of Soultz geothermal reservoir at a depth of 3650m and a number of numerical experiments were conducted to evaluate its geothermal potential. Results of this study show that the average residual (retained) aperture is much lower and the time required for maximum number of observed shear dilation events due to stimulation was greater than those predicted by earlier studies including authors own study. This is due to the fact that in all these studies the surface roughness was ignored and best guess approach is used in the calculation of shear displacement. These results confirmed that the reservoir volume (contactable rock matrix by the circulating fluid) created by stimulation is much lower than the initial estimation at Soultz and Cooper Basin geothermal test sites. Results of this study also demonstrate that the effective tensile normal stresses from the injected cold fluid tend to increase fracture apertures, and hence, increase the fracture permeability within the zone of cooling. This increase in permeability is evident predominantly near wellbore region in the early circulation period. Over longer term, a significant part of the reservoir, through wich circulation is well estalished, is subjected to larger thermal stresses. These thermal stresses consequently increase permeability of the fractured network, leading to significant changes in the pressure distribution (decrease in impedance) and hence, an increase in the flow rates.Gholizadeh Doonechaly, N., S. S. Rahman, and A. Kotousovhttps://www.geothermal-library.org/index.php?mode=pubs&action=view&record=103026