Hydro-Shearing and Hydraulic Fracturing for Enhanced Geothermal Systems in Archetypical Normal, Strike-Slip, and Thrust Faulting Terrains

摘要

This paper advances understanding of the feasible space of Enhanced Geothermal Systems (EGS) by evaluating the effects of in situ stress, natural fracture patterns, and hydraulic stimulation on processes of geothermal conduction and convection. EGS require stimulation techniques to improve fracture network transmissivity in sparsely fractured rock masses in a way that balances conductive and convective heat transport. Three archetypical terrains with in situ stress typical of normal, strike-slip and thrust faulting regions of North America are hydraulically fractured and hydro-sheared in Discrete Fracture Network (DFN) models so that the total available fracture area and fracture spacing at depth become sufficient for geothermal energy production. The adequacy of fracture spacing and area was determined using analytic equations for parallel fractures of uniform aperture which are considered a best case scenario. Connected fracture networks are developed using simulation of both hydro-shearing of existing natural fractures and induced tensile fracturing. The average initial temperature of the reservoir was required to be 200°C and this constraint along with the regional thermal gradient was used to define the depth of the geothermal reservoir. Pumping pressures and durations required to develop and later utilize these systems are discussed. Low geothermal gradients and high minimum stress values limit the practicality of creating EGS resources in major thrust faulting regions of the United States. Development of EGS in normal and strike-slip faulting regions seems to be more tractable, but potentially requires horizontal drilling for both the stimulation well and any injection or production wells. Differences in rock lithology affect constraints on fracture spacing. Designing an appropriate stimulation program that balances enhancing the reservoir permeability while maintaining adequate fracture spacing is challenging.