The productivity and injectivity of hydraulically fractured geothermal wells in naturally fractured formations depends 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 determine the factors that control the connectivity of fracture networks bounded by wells, (b) propose ways in which the flow capacity and fracture connectivity can be improved by changes to the hydraulic fracture design.
A fully 3-D hydraulic fracturing simulator has been developed that considers the interaction of hydraulic fractures with natural fractures by solving for the stresses, fluid flow, fracture growth and intersection. An efficient and practical algorithm is developed to automatically detect and quantify the connectivity of the propagated fracture networks. 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 maximize the connectivity of the injector to the producer (maximize 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 suggested 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.
The detailed parametric study based on the fracture-type-detection algorithm helps us better understand the factors that influence the geometry of fracture networks and guides us in hydraulic fracture design and well spacing optimization in naturally fractured geothermal reservoirs.