Injection-Induced Propagation and Coalescence of Preexisting Fractures in Granite Under Triaxial Stress

Abstract

The dilatant shear slip of preexisting fractures/faults by injection at pressures below the minimum in situ stress has been recognized as an important permeability creation mechanism during reservoir stimulation. However, interconnected fracture network generation through the propagation and coalescence of preexisting fractures during shear stimulation has been rarely studied through laboratory experiments. To examine permeability enhancement through the propagation and coalescence of preexisting fractures, we conducted novel triaxial-injection experiments under representative crustal stress conditions on cylindrical granite samples containing single or double preexisting fracture(s)/flaw(s). In the sample with a single fracture, SW-1, new cracks propagated from the preexisting fracture due to injection. In the sample with two fractures, SW-2, a highly conductive fracture network was created by the coalescence of newly formed cracks with preexisting fractures. As a result, the equivalent permeability of the sample was enhanced by 17 to 35 times. Both tensile wing cracks and shear and/or mixed-mode secondary cracks were detected from the scanning electron microscope images of the tested samples. Our results suggest that in addition to dilatant shear slip, propagation and coalescence of preexisting fractures significantly contribute to reservoir permeability creation during low injection pressure stimulation in fractured rocks. Although the experiments focused on reservoir stimulation, the results have relevance to crustal permeability evolution and induced seismicity.