Shear stimulation, so‐called hydroshearing, is believed to reactivate critical or near‐critical preexisting fractures by pressurized fluid injection, causing them to slip and dilate for economic production from geothermal and unconventional petroleum reservoirs. However, little or no experiments have been conducted to directly probe the coupled hydromechanical responses and permeability evolution during fracture shearing. In this paper, we present the results of novel injection‐induced shear tests on cylindrical granite samples each containing a single tensile or saw cut fracture. The shear test on rough fractures demonstrates that retainable permeability enhancement can be achieved through dilatant shear slip. The characteristic stick‐slip behavior of the fracture during shearing is also observed. Fracture aperture controlled by effective normal stress dominates the fluid flow in stick state, while in slip state, the major contributor to production increase is the irreversible normal dilation caused by fracture shear slip. The experimental data show that depending on the fracture roughness, the fracture slip process includes two quasi‐static slip intervals with a slip velocity of ~10⁻⁹ to ~10⁻⁶ m/s and a dynamic slip interval with a slip velocity of ~10⁻⁷ to ~10⁻⁵ m/s. The fracture slip correlates well with the associated stress relaxation: a faster fracture slip induces a quicker stress relaxation, and vice versa. The influence of surface roughness on the hydromechanical responses during fracture shearing and the shear‐induced asperity degradation are also studied. The various experimental observations on rough and smooth fractures clearly indicate that the key component to shear stimulation is fracture self‐propping by asperities.

Despite the wide application of hydraulic fracturing in enhanced geothermal system (EGS) development and unconventional hydrocarbon production, our understanding of the many factors affecting the propagation of hydraulic fractures has relied on circumstantial evidence. This is partly due to the scarcity of direct observations in the subsurface. The EGS Collab project attempts to address this issue in the context of EGS research by performing intermediate-scale (∼10 m) hydraulic stimulation experiments in a thoroughly characterized and heavily instrumented underground testbed. This paper analyzes the data collected from the first suite of hydraulic fracturing tests in this testbed, consisting of seven stimulation episodes. High-quality microseismic data delineate five planar features very clearly. Combining fracture-wellbore intersection observations from distributed temperature sensing (DTS) and visual observations from an open-hole well, as well as prior in situ stress measurements, we conclude with high certainty that the four larger planes were hydraulic fractures. The growth of the hydraulic fractures was temporarily halted by a prominent, open natural fracture in the testbed but they eventually crossed and slightly reoriented under continued stimulation. Mineral-filled (i.e. healed) natural fractures, though prevalent in the testbed, did not have perceivable effects on hydraulic fracture propagation. The high quality, mutually corroborating datasets allowed conclusions to be drawn with high confidence and attests to the advantage of intermediate-scale experiments in subsurface research.

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.