What Does Microseismicity Tell Us About Hydraulic Fracturing?

Abstract

Abstract Interpreting hydraulic fracture geometry isn't the only proven value application of microseismic imaging. With careful geophysical analysis, the microseismic deformation can also be determined in terms of the fracture strain that results in the source of the observed microseismic signals. The microseismic strain or deformation is potentially of interest, since the deformation associated with the hydraulic fracture controls the fracture dilation and complexity and ultimately the effectiveness of the stimulation. However, evidence is presented here which points to the microseismic being primarily a fast, shearing deformation in contrast to the relatively slow and tensile hydraulic fracture growth. Comparing the deformation energy balance between the recorded microseismic and the hydraulic fracture indicates that the microseismic is a very small component of total deformation. The majority of the fracture deformation is aseismic, beyond what is captured with conventional microseismic monitoring. The microseismic and hydraulic fracture deformations can be rationalized through a geomechanical model that honors the fracture mechanics and mass balance to simulate fracture growth. A workflow is presented that describes calibrating a geomechanical model using the location, timing and strains of the recorded microseismic to match the modeled interaction between the hydraulic fracture and pre-existing fractures. A rock physics model is also required to define synthetic microseismic data from the geomechanical model, for comparison with the observed microseismic data. The simplest form of calibration is a qualitative match between the relative microseismic density of either number of events or source strength. However, more advanced quantitative matches can also be made between the simulated and monitored microseismicity, by matching the microseismic displacements and mechanisms with the additional constraint of matching the energy balance. Through such a calibrated geomechanical model, the interrelation between stress, fractures, and injected fluid can be examined and used to characterize the hydraulic fracture growth. The validated hydraulic fracture model can then be incorporated into a reservoir simulator to ultimately predict the associated production characteristics. Therefore, beyond hydraulic fracture geometry, microseismic source deformation can be used to constrain a geomechanical model of the hydraulic fracture growth and thereby help describe the overall effectiveness of the stimulation.