Hydraulic Fracturing and Induced Seismicity: A Semi-Analytical Approach

Abstract

The injection of fluids into the ground is undertaken in a number of engineering practices including hydraulic fracturing, liquid waste disposal, CO2 sequestration and extraction of geothermal heat (EGS). Study of mechanisms for slip on faults/natural fractures due to hydraulic fracturing is of great interest for the energy resource industry in view of efficiency of hydraulic fracturing, seismic monitoring and seismicity related safety hazards. In this thesis, semi-analytical approaches are applied to investigate physical processes associated with hydraulic fracturing induced seismicity and to develop a deeper understanding of the problem. The primary focus is on the modeling of initiation and growth of slip on a pre-existing fault/fracture due to interaction with a single propagating hydraulic (tensile mode) fracture. The first part of the thesis is concerned with the relationship between hydraulic fracturing injection into a fault and the possibility of a seismic slip. The results show that the nucleation of dynamic slip on a fault with slip-weakening friction is only weakly dependent on the magnitude of the stress perturbation ahead of the propagating hydraulic fracture (HF), or the HF propagation regime, and is mainly controlled by the hydraulic fracture length (i.e., the size of the fully unloaded fault segment at a given time). The growth of the fault slipping patch remains stable when the background shear stress is smaller than the residual fault strength under ambient conditions. Otherwise, nucleation of dynamic slip takes place when the hydraulic fracture grows to the critical size, which is vanishingly small for critically-stressed faults (i.e., when the background stress approaches the fault peak strength) and is diverging when the stability boundary is approached. In addition, no dynamic slip transients are predicted when background shear stress is less than the residual fault strength. In the second part of the thesis we examine the possibility of a microseismic slip on a natural fracture as a result of poromechanical interactions with an advancing hydraulic fracture. Nucleation of slip on the frictional fracture approached by the hydraulic fracture depends on the state of in-situ stresses, hydraulic fracture pressure, angle of approach and friction of the fracture. Slip instability, consistent with field observations, occurs on the critically stressed and favorably oriented strike-slip fracture. Nucleation takes place as the slipping patch reaches the critical length. Finally, we have studied nucleation of slip on a natural fracture crossed by a propagating hydraulic fracture. Slip initiates along the edge crack as the frictional strength drops due to increase in pore pressure by diffusion of fluid from the hydraulic fracture.