Hydro-Mechanically Coupled Flow Through Heterogeneous Fractures

Abstract

Geothermal reservoir productivity strongly depends on fracture network and single fracture permeability. Fracture aperture and therefore reservoir permeability are subjected to changes in the lifetime of an enhanced geothermal system as a consequence of cold fluid injections in a hot reservoir. Associated effective normal stress changes, dilatant shearing and thermo-elastic effects affect both reservoir productivity and related seismic risk. Hydro-Mechanically (HM) coupled models are frequently used to estimate reservoir productivity and seismic risks. Due to the complexity of the above-mentioned processes and their couplings, extensive computational resources are required, and simplifications such as the assumption of planar fractures rather than heterogeneous fractures are often used. This study investigates HM-coupled processes in heterogeneous fractures. A HM-coupled model was developed in the GEOS framework that enables to explicitly account for complex fracture surface geometries and its influence of coupled fracture flow. This model is used to investigate flow velocity field evolution during different loading scenarios (i.e. normal and shear loading) on natural and artificial fractures in a granite. The model is tested for normal stresses up to 70 MPa, which results in fluid pressures above 10 MPa in individual cases. This enables detailed investigation of the normal stress dependent flow channeling, the increase of contact area and contact stress with increasing normal stresses and fracture opening at high fluid pressures. The flow rate and fluid pressure response to changing stress fields is verified against HM-coupled experiments performed on natural fractures. Furthermore, the contact area and contact stress increase with sample loading is tested against an experimental study on contact area and contact stress measurements on artificial fractures in granite. This contribution shows that the newly developed numerical framework for heterogeneous fracture surfaces allows reproducing experimental data at the laboratory scale, and may offer advanced understanding and prediction of the behavior of reservoirs that are subjected to high-pressure fluid injections.