Nucleation of Dynamic Slip on a Hydraulically Fractured Fault

Abstract:

This work is concerned with the relationship between hydraulic fracturing injection and induced seismicity on a fault. This is applicable to safety hazard assessment of the nucleation of dynamic slip along the fault as a result of hydraulic fracturing injection into or near the fault. The hydraulic fracture (HF) injection into the fault can be purposeful, for instance to trigger an earthquake in more controllable conditions than would happen otherwise; or in order to use the open portion of the fault as a reservoir heat exchanger for the extraction of geothermal heat. It can be unintentional due to lack of accurate subsurface characterization prior to the injection.

Injection with constant flow rate into an impermeable fault will initiate slip along the fault. The slip occurs due to the lack of frictional strength along the open part of the fault (the hydraulic fracture) as well as from the reduction of the normal stress ahead of the hydraulic fracture front. Slip in front of the hydraulic fracture is assumed to degrade the tensile strength of the fault to zero, so that the hydraulic fracture propagation takes place in the viscous-dominated regime (i.e. dominated by viscous losses in the fluid flow inside the fracture). In our model, the nucleation of dynamic slip is related to the slip-weakening nature of the friction, and depends on in-situ stresses, pressure distribution inside the hydraulic fracture, and the evolving length of the hydraulic fracture. The results of this study show that the growth of the fault slipping patch remains stable, with no episode of dynamic rupture, when the background shear stress τ_b is smaller than the residual shear strength τ_r of the fault under ambient conditions. Otherwise (τ_b > τ_r), nucleation takes place when the extent of the hydraulic fracture reaches the critical length ~ (μ/τ_p) δ_c, where μ is the elastic shear modulus of the rock, τ_p is the peak shear strength of the fault, and δ_c is the characteristic slip weakening distance. The size of the slipping patch ahead of the HF tip obeys roughly similar scaling. This scaling changes when τ_b is just slightly larger than τ_r in which case both critical hydraulic fracture and slipping patch lengths increase strongly with diminishing τ_b, and become unbounded when stress stability boundary (τ_b= τ_r) is approached.