| Abstract |
The mechanism of stimulation in Enhanced Geothermal Systems (EGS) is typically assumed to be induced slip on preexisting fractures (Pure Shear Stimulation, PSS). In oil and gas fracturing, the mechanism of stimulation is typically assumed to be opening and propagation of new fractures (Pure Opening Mode, POM). In this paper, we explore the possibility that stimulation in EGS sometimes occurs through a combination of propagation of new fractures and induced slip on preexisting fractures (Mixed- Mechanism Stimulation, MMS). Using a discrete fracture network model that couples fluid flow and fracture deformation in large, complex fracture networks, we show that there are many geological conditions that must be satisfied in order for PSS to be the mechanism of stimulation in the low matrix permeability settings typical for EGS. These conditions are (1) adequate storativity in closed natural fractures, (2) adequate initial transmissivity of natural fractures, (3) percolation of the natural fracture network, (4) natural fractures well oriented to slip in the local stress state, (5) natural fractures that experience enhanced transmissivity with slip, and (6) adequate stimulated transmissivity. These conditions have likely been met in some, but not all, historical EGS projects. We argue that in cases where these conditions are not met, the MMS mechanism is more likely than the PSS mechanism. We summarize some of the arguments that have been used to justify the PSS interpretation, and discuss how these arguments may not hold if the intact rock tensile strength is not negligible. We discuss techniques that might be used to diagnose stimulation mechanism from field data. Stimulation mechanism is discussed in the context of the EGS project at Fenton Hill, USA, and it is shown how evidence is consistent with the idea that substantial fracture opening and propagation occurred during stimulation at that project. We conclude by discussing implications of stimulation mechanism for EGS modeling and design. |