Tip-Effect Microseismicity - Numerically Evaluating the Geomechanical Causes for Focal Mechanisms and Microseismicity Magnitude at the Tip of a Propagating Hydraulic Fracture

摘要

There have been extensive industry efforts to understand the geophysical implications – and limitations – of microseismic analyses; however, a critical issue that is often overlooked is the geomechanics of the rock failure that is represented by microseismicity. Recall that microseismicity is the acoustic representation of rock failure, whether tensile failure or shear failure, which is driven by the coupled hydro-thermo-mechanical effects of injecting cool fluids at high rates into naturally fractured formations. Often overlooked in the analysis of microseismic data is the stress and deformational effects at the tip of a propagating fracture that cause a significant percentage of the total microseismic record. Previous publications, for example, have noted that at the horizontal leading edge of a propagating fracture, the dominant shear is in a horizontal plane. Conversely, at the upper and lower vertical leading edge of the propagating fracture, vertical shear has been reported to dominate. These would be expected to not only cause a different microseismic response, but also, likely, a different stimulation response. In this paper, we present a detailed numerical evaluation of the stresses generated at the tip of a propagating hydraulic fracture under varying field conditions. The simulations were performed with a finite difference continuum code. The results of the simulations show that field conditions and position along the perimeter of the propagating hydraulic fracture can significantly impact local stresses, which will have then have an impact on the generated microseismicity. The results of the work will allow for a better interpretation of field microseismicity for completion optimization.