Sheared Fracture Conductivity

Abstract

Experimental and numerical evaluations have identified features related to conductivity in shear-induced self-propped conduits that could be found in ideal EGS reservoirs. Does reopening or reactivating the natural fractures help to maintain or augment fracture conductivity? Does shear-induced asperity override provides adequate conductivity for geothermal applications? Shearing contributions are difficult to legitimately quantify. Long-term integrity of these fractures needs thorough assessment in order to investigate the potential to alter fluid flow within fracture pathways. The characteristics of hydraulic conductivity change in shear-induced conductive fractures were studied and compared; experimentally and numerically. The intent has been to investigate changes in effective fracture width in-situ which may manifest complex interlocking of influential parameters like lithology, fluid gradients, creep, etc. Baseline laboratory conductivity measurements were completed for an artificially-sheared, self-propped, granite sample subsequently subjected to confining pressure. Internal surfaces of an axially-split core sample were relatively translated to ensure interfering roughness that may cause self-propping. Mechanical stressing and degradation of conductivity associated with changes in fracture width were simulated using a commercially available simulator FLAC3D (developed by Itasca). The contours of shear-induced fractured surface were re-created from X-ray microtomographic images. Numerical runs have been performed to simulate fluid flow within these fractured surfaces subjected to increasing closure stresses. Fracture conductivity measurements for a shear-induced fracture in granitic sample, at room temperature and a nominal closure stress of 1000 psi were recorded. The temperature was varied from ambient temperature to 200°C. This study introduced a new avenue for in-situ measurements of fracture conductivity in laboratory setup. The insights gained from this study are invaluable in understanding the potential of shear-induced fracture pathways to alter hydraulic conductivity.