Variations in Fracturing Mechanisms Observed by Broadband Microseismic Monitoring of Hydraulic Treatment

Abstract:

Hydraulic fracturing is the key stimulation technology to improve unconventional hydrocarbon recovery nowadays. Stimulation increases permeability of tight formations by causing fractures at depth. It involves pumping high-pressure fluid into reservoir rocks to force the opening of cracks, which could allow oil and gas to flow freely. The progress of a fracturing operation must be monitored carefully as fracturing could activate existing faults, leading the fluid mixed with chemicals to propagate beyond the targeted treatment zone. In order to study dynamic processes involved in hydraulic fracturing, we deployed a small-scale seismic array consisting of 22 broadband seismographs at the surface above a hydraulic fracturing area to monitor the whole fracturing progress. We made continuous recording for 20 days, and detected a total of 961 microseismic events with relatively high signal-to-noise ratio (SNR) recordings. We found that these events occurred either during the fracturing operation or after the fluid pumping. Some of the events also do not seem to be directly induced by the pumping, based on their locations and sizes. We determined the focal mechanisms of all events using the P-wave polarity data, and found that both the microseismicity and their focal mechanisms exhibit significant spatial and temporal variations. This variability can be associated with the hydraulic treatment, pre-existing faults, as well as the evolving stress field during the treatment. We computed the Coulomb stress changes of the observed seismicity to seek its contribution to the observed seismic variability.