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.