Abstract
Shear reactivation of pre-existing fractures can play a crucial role in hydraulic stimulation to enable production from unconventional reservoirs. Evidence of the phenomenon is found in microseismic/seismic events induced during stimulation by hydraulic fracturing. However, induced seismicity and permeability evolution in response to fracture shear reactivation by injection has not been extensively studied in laboratory tests under relevant conditions. In this work, a cylindrical Eagle Ford shale sample having a single fracture (tensile fracture) was used to perform laboratory injection test with concurrent acoustic emission (AE) monitoring. In the test, slip was induced on the fracture at near critical stress state by injecting pressurized brine water (7% KCL). Sample deformation (stress, displacement), fluid flow (injection pressure, flow rate) and AE signals (hits, events) were all recorded. The data was then used to characterize the fractures’ fully coupled seismo-hydro-mechanical response during shearing. Results shown that the induced AE/microseismic events correlate well with the fracture slip and the permeability evolution. Most of the recorded AE hits and events were detected during the seismic slip interval corresponding a rapid fracture slip and a quick stress relaxation. As a result of dilatant shear slip, a remarkable enhancement of fracture permeability was achieved. Prior to this seismic interval, an aseismic slip interval was evident during the tests, where the fracture slip, the associated stress relaxation, and the permeability increase were limited. The test results and analyses clearly demonstrate the role of shear slip in permeability enhancement by hydraulic stimulation for unconventional shale reservoirs. Finally, it is further revealed that the transition from aseismic slip to seismic slip is highly dependent on the fracture slip velocity.
Introduction
Shear slip of pre-existing fractures has been recognized as a major permeability creation and micro-seismicity mechanism of reservoir stimulation for a long time (e.g. Pine and Batchelor 1984, Mayerhofer et al. 1997, Rutledge et al. 2004, Zoback et al. 2012). Most reservoir rocks contain abundant pre-existing fractures, some of which may be sealed with calcite or other infill minerals. Usually, these fractures are inactive and without sufficient permeability before stimulation. A number of modeling and field studies have shown that when the pre-existing natural fractures are favorably oriented with respect to the in-situ stresses, an injection or the leak-off from a hydraulic fracture (even with low fluid pressure) can cause the fractures sliding and propping due to asperities. Also, it is widely observed that the induced microseismic or seismic events during hydraulic injection are mostly related to the shear failure of pre-existing fractures. However, fracture slip-permeability evolution and the associated microseismic signature during injection are still poorly understood. The fundamental investigations through laboratory experiments and field injection tests are insufficient.
