Abstract

We have used a hydraulic fracture model to quantify fault rupturing promoted by injection and migration of fluid into a fault, which contains in-plane high-conductivity segments and out-of-plane jogs or branches. The fluid under elevated pressure can promote extension-shear fracturing. The model provides numerical results on the opening and pressure variations with position and time. High-conductivity segments aid penetration of the pressurized fluids, but limit fluid pressure increases, so as to sustain a stable rupture growth mode. In the presence of varying normal stresses, stable growth cannot be maintained by pressure variations leading to the unstable growth in shear, and rapid fault movement events can be triggered as the fault re-establishes stable growth, radiating seismic energy and accompanied by local backward slip. These coupled seismic and aseismic faulting processes are applicable to faults with jogs and branches, which interact with the main fault to produce changes in the local stress states. The opening and slip along jogs and branches, either pre-existing or induced by fluid flow, will not only contribute to fluid storage due to their suction pumping action, but also produce changes in the downstream flow rates. The slip along them can produce associated opening along the main fault, but their opening can increase the compressive stress across the main fault which restricts its opening. The deformation transfer at junctions can complicate the fracture and flow responses.

1. INTRODUCTION

Injection of liquid waste has been found to result in generation of seismic events and some of these may be large enough to be felt at the surface [1, 2]. The total volume injected and the maximum size of the seismic events generated have been found to be correlated [3]. Microseismic monitoring of low-level induced seismicity generated by hydraulic fracturing is a technology applied in unconventional gas reservoirs [4- 6] for the purpose of mapping the extent of fracturing.

Considerable effort has been devoted to detecting and locating seismic events associated with fluid injection. However, little attention has been paid to understanding the source mechanisms of seismic events for a pressurized fault [7, 8]. Hydraulic fracturing in lowpermeability reservoirs is strongly affected by natural fractures within the targeted rock layers. Stimulation improves the connection of these fractures to one another and to the hydraulic fracture. Slip on these natural fractures can be activated by stress changes to generate low-magnitude seismic events. Meanwhile, shearing and shear-induced dilation on the natural fractures enhances conductivity and increases the stimulated volume. The aperture distribution along a fracture, including the initial and subsequent propagation paths, controls the conductivity and pressure distributions resulting from the injection. An integrated coupled hydraulic fracture model is applied in this paper to this problem to predict the relationship among stress, pressure, deformation and rupture growth.

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