UNDERSTANDING MICROSEISMICITY ASSOCIATED WITH HYDRAULIC FRACTURE GROWTH USING LABORATORY EXPERIMENTS

There are uncertainties in the exact mechanisms that occur during hydraulic fracture growth and the interactions of hydraulic fractures with pre-existing fractures. In some cases fracture growth is seen to be purely tensile and in others a combination of tensile and shear mechanisms. In order to develop a seismic model to explain fracture propagation, there is a need for a complete classification of the microseismic events occurring during fracturing using waveform characteristics such as frequency, duration and magnitude. This classification would allow for more accurate prediction of the behavior of the hydraulic fracture from its initiation to when it intersects a natural fracture.

Due to the complicated nature of geological structures, it would be crucial to look at the microseismic events in the controlled lab environment to differentiate the real physics of the problem from environmental factors. Laboratory experiments, however should be scaled correctly to mimic the real field-scale problem. Two experiments are conducted using polymethyl methacrylate (PMMA) samples; one with a model fracture and the other without this feature. Microseismic data is collected during each experiment from 8 stations with 3 sensors each, in a Galperin arrangement, on the samples. Pressure data and camera data are also collected to examine the changes in pressure and the growth of the fracture throughout the experiment. By full moment tensor inversion, spectral and temporal analysis of the data, we present a catalog of microseismic event types and propose several mechanisms for their differences. It is expected that these microseismic events show elements of both tension and shearing related to the opening and closing of fractures, fracture propagation and interaction with pre-existing cracks.