Heterogeneous Fracture Slip and Aseismic-Seismic Transition in a Triaxial Injection Test

Fault/fracture slip and seismicity caused by fluid injection are of major interest to subsurface science and engineering. However, some fundamental aspects of the temporal and spatial evolutions of induced seismicity remain unresolved. In this paper, we present the results of a laboratory injection-induced shear test conducted on a rough granite fracture with concurrent acoustic emission (AE) monitoring. The results demonstrate a sequence of aseismic-seismic-aseismic fracture motion during fluid injection. It is observed that the temporal evolution of AE/microseismic activities was accompanied by the changes of slip velocity, stress drop, and friction coefficient. The seismic slip phase consists of three subphases, namely, a first quasi-static slip, a dynamic slip, and a second quasi-static slip. The dynamic slip occurred with an AE mainshock, simulating earthquake instability triggered by injection in deep crystalline rocks. In addition, slip heterogeneity controlled by the fracture surface roughness is directly evidenced by the spatially heterogeneous distribution of AE hypocenters and fracture surface topography. Plain Language Summary Inducing fracture slip, dilation, and propagation to create a highly conductive fracture network by injection has been viewed as an effective reservoir stimulation approach for subsurface energy extraction. Fault/fracture slip can cause seismic/microseismic events, which can be used to estimate the stimulated volume. There are also some concerns regarding seismic hazard related to fluid injection. In order to better understand the underlying physics of fault slip and induced seismicity, we performed laboratory experiments. Here, we report results from an injection-induced shear test conducted on a rough granite fracture, in which the fracture deformation, fluid flow, and acoustic emission were all concurrently measured. Our results demonstrate that faults/fractures may slip in different modes (from creep, slow quasi-static slip, to fast dynamic slip) during fluid injection and each slip mode is reflected by a different AE/microseismic response and frictional behavior (slip weakening or slip strengthening). Moreover, it is observed that the spatially heterogeneous distribution of induced AE events is directly linked with the fracture surface topography and the shear-induced asperity damage, indicating the controlling effect of surface topography on fault slip heterogeneity.