The productivity and injectivity of hydraulically fractured geothermal wells in naturally fractured formations depend on the connectivity of fracture networks created by the interaction of hydraulic fractures with natural fractures. The primary objectives of this paper are (a) to define quantitatively the connectivity of the created fracture network, (b) to determine the factors that control the connectivity of fracture networks bounded by wells, and (c) to propose ways in which the flow capacity and fracture connectivity can be improved by changes to the hydraulic fracture design.
A fully 3D hydraulic fracturing simulator has been developed that considers the interaction of hydraulic fractures with natural fractures by solving for the stresses, fluid flow, heat transfer, fracture growth, and intersection. These propagated fractures, which include hydraulic fractures and reactivated natural fractures, are divided into backbone, dead-end, and isolated fractures. Different well patterns that aim to optimize the connectivity of the injector to the producer (optimize the area of the backbone fractures) are simulated. A sensitivity analysis is conducted to investigate the effect of various parameters on the connectivity of wells through fractures. An optimal well pattern is needed to maximize the connected fracture area that provides a conductive path for heat extraction from naturally fractured geothermal reservoirs. Our results show that the connectivity of fracture networks is dramatically impacted by the degree of deflection, crossing, and merging of hydraulic fractures with natural fractures. An example is used to investigate the effect of backbone and dead-end fractures on heat extraction from an enhanced geothermal system (EGS).
The detailed parametric study helps us better understand the factors that influence the geometry and connectivity of fracture networks and guide us in hydraulic fracture design and well spacing optimization in naturally fractured geothermal reservoirs.