Hydraulic Fracture Geomechanics and Microseismic Source Mechanisms

Abstract

Abstract Interpretation of microseismic results and attempts to link microseismic source mechanisms to fracture behavior require an understanding of the geomechanics of the fracturing process. Stress calculations around fractures show that the area normal to the fracture surface is stabilized by a pressurized fracture as a result of increased total stress and decreased shear stress. In this area, microseisms can only occur if leakoff pressurizes natural fractures, bedding planes, or other weakness features, and source mechanisms are thus likely to show a volumetric component. Conversely, the tip tensile region is destabilized by a reduction in total stress and an increase in shear stress, with the likelihood that microseisms would be generated in this region because of these changes. Such microseisms would not yet be invaded by the fracturing fluid, and events that are mostly shear would be expected. Systems with multiple fractures, such as those that are potentially created in multiperforationcluster stages, are much more complex, but similar elements can be outlined for those as well. Source mechanisms can help delineate these different types of microseismic behaviors, but evaluation of such mechanisms reveals that they provide no significant information about the hydraulic fracture. While it would be valuable if source mechanisms could provide information about the mechanics of the hydraulic fracture (opening, closing, proppant, etc.), calculations show that both the energy and volume associated with microseismicity are an insignificant fraction of the total energy and volume input into the stimulation. Thus, hydraulic fractures are almost entirely aseismic. Analysis of source mechanisms should be concentrated on what that data reveals about the reservoir (e.g., natural fractures, faults, etc.). Integrated diagnostic studies provide more value in understanding both the microseismicity and interpretation of the microseismic results.