The deformation behavior and transport properties of subsurface reservoir rock masses are highly influenced by the existence of natural fractures. Shear reactivation of pre-existing fractures by pressurized fluid injection has been recognized as an important mechanism of reservoir stimulation and induced seismicity for a long time. In order to fundamentally reveal the roles of pre-existing fractures in reservoir stimulation and induced seismicity, novel laboratory injection experiments were conducted for this dissertation.
First, the dissertation presents the injection-induced shear tests on both granite and shale fractures (either rough or smooth). The results show that significant shear-induced permeability enhancement can be achieved on rough rock fractures by injection. Also, the enhanced permeability can be retained in the sheared fractures after the fluid pressure is deceased, indicating that the permeability increase by fracture shearing may be permanent. Moreover, the various observations for rough and smooth fractures demonstrate that normal dilation and self-propping of a rough fracture during shearing is key to the success of shear stimulation. Additionally, the propagation and coalescence of pre-existing fracture(s) by injection is explored for the first time using triaxial-injection experiments. It is observed that the pre-existing fracture(s) can be propagated as tensile, shear or mixed-mode at injection pressures below the minimum principal stress. As a result, interconnected fracture networks can be created through the coalescence of newly propagated cracks with the pre-existing fractures, resulting in a remarkable permeability enhancement. This indicates that in addition to commonly accepted dilatant shear slip, tensile or shear mode propagation, coalescence of pre-existing fractures is also significant in reservoir stimulation and could be viewed as integrated mechanisms of permeability enhancement in low injection pressure shear stimulation. At last, the fully coupled seismo-hydro-mechanical response of rock fractures (granite or shale) is probed through laboratory injection-induced fracture slip tests with concurrent acoustic emission monitoring. The tests reveal a transitional seismic response during fracture shear slip namely, aseismic (or creeping) slip to seismic slip during fluid injection. In the aseismic creep interval, the fracture sheared at a relatively low rate, and very few or no acoustic emission events were triggered concomitant with a small decline of the shear stress and a slight enhancement of flow rate into the fracture. A significant number of acoustic emission events were detected in the seismic slip interval with a relatively high slip rate. The shear failure of the fracture occurred in the seismic slip stage, and was accompanied by a large stress drop and a dramatic fluid flow rate increase. In granite fracture test, a large number of AE events were detected during the dynamic slip interval corresponding with a frictional weakening behavior. The spatio-temporal evolution of acoustic emission hypocenters demonstrates the existence of slip heterogeneity on the fracture plane having a heterogeneous roughness distribution. In the shale fracture test, the fracture reactivation occurs with a limited number of AE events, which is likely related to the friction strengthening response during shearing. The results also show that microseismicity, permeability, fracture slip correlate well with each other: more microseismic events possibly indicate a better production performance. This laboratory observation is consistent with some field measurements. The research findings in this dissertation demonstrate the important role of pre-existing fractures in reservoir stimulation and induced seismicity. Observations and results enhance the physical understanding of the mechanical deformation and fluid flow processes of crustal rocks (having natural fractures/discontinuities), and help engineer solutions for subsurface energy developments and management of induced seismicity, and potentially provide insights into crustal permeability dynamics.