Laboratory simulation of binary and triple well EGS in large granite blocks using AE events for drilling guidance

•Multi-well laboratory experiments show challenges for EGS in granite.•We observe the influence of natural discontinuities on AE and fracture geometry.•Fracture location estimation with AE improved by correlation with pressure drops.•We measure hydro-thermal fluid flow through hydraulic fracture networks.•Injectivity was more stable with constant pressure control than constant flow rate. Multiple-well Enhanced Geothermal Systems (EGS) can enable economic recovery of energy from underutilized hot dry rock (HDR) reservoirs. Hydraulic fracturing is a promising stimulation method for improving fluid flow and heat extraction in EGS. Laboratory simulations of EGS with hydraulic fracture stimulation have recently been completed in two large 300×300×300mm3 granite block specimens to better understand this complex process of geothermal energy recovery. The first experiment implemented a binary well layout with an injector and producer. The second experiment used a triplet well layout with one injector and two producers. Selection of production well trajectory so as to intersect the hydraulic fractures was guided by acoustic emission (AE) events collected during stimulation. Both model reservoirs were subjected to heating and true-triaxial stress confinement throughout a series of drilling, stimulation, and flow and heat circulation tests. Stimulated thermal reservoir flow was characterized by a series of constant pressure, constant flow rate, stepped constant pressure and stepped constant flow rate injection tests. Tested blocks were cross-sectioned to characterize final locations and 3D geometries of the induced fractures. Insights and lessons learned from these experiments are presented with focus on application to field-scale EGS.