Natural fracture networks enhancing geothermal producibility, mapping or predicting!

Abstract

Subsurface natural fracture networks that act as conduits for thermal fluids play an important role in geothermal reservoirs. The success of enhanced geothermal systems is mainly reliant on modelling the orientation of the pre-existing natural fracture networks within the reservoir. This will help modelling the direction of movement of the hydrothermal fluids, and in locating sweet spots and/or possible orientation of susceptible fractures for further stimulation programs. We applied both mapping and predicting techniques on sand and shale intervals in the Cooper Basin/South Australia and calibrated these techniques using image logs, well data, and quality seismic in order to validate their ability to model subsurface fracture networks. We used most positive curvature attribute to generate a workflow for modelling subsurface fractures with high confidence. As the curvature attributes are known to be sensitive to the acquisition direction, we reduced all acquisition artefacts using structural smoothening and generated a seismic cube that is free of data acquisition artifacts. A final curvature volume was produced after eliminating low values that don’t reflect any structural features. A validation procedure was applied using image logs, well data, and seismic sections and a high correlation was found between the curvature mapped fractures and the image logs fractures. Another technique used in this study was to integrate geological and geophysical data extracted from fault and horizon seismic interpretation with geomechanical analyses of stress, strain, and displacements associated the structural development of the basin. Finite element method (FEM) and boundary element method (BEM) are two ways used to predict fractures generated during the tectonic events of basins. FEM provides a physically-based soluiton for subsurface issues related to fractures and basin evolution taking into consideration horizon geometry, heterogeneous rock properties and stresses generated from structural features. The BEM method considers the effect of fault displacement on generating stress and near the fault. One of the disadvantages of BEM is that it doesn't consider rock heterogeneity or the effect of intra-seismic relaxation on fracture generation. Also, BEM ignores far field stress data, and thus does not predict fracture generation away from the major faults. The validation procedure was applied on fractures predicted from FEM and BEM, and a good correlation was found between the predicted fracture network and the image logs fractures next to the major faults in both methods as fractures in these areas were mostly generated due to strain exerted during fault displacement. FEM succeeded in predicting fractures close to and away from major faults with higher accuracy while BEM didn’t map fractures away from faults. Thus, both FEM and enhanced most positive curvature attributes can be used successfully to model subsurface fracture networks that will locate productive spots with good permeability.