CHARACTERIZING FELT SEISMICITY DURING HYDRAULIC FRACTURE STIMULATIONS

Hydraulic fracturing in naturally fractured reservoirs is known to generate seismicity due to the interaction of injected fluids with the pre-existing fracture network. Typically, the observed moment magnitudes for such operations are small, usually with Mw < 0. To map the seismicity during these injections, geophones (utilizing 15 Hz) are deployed in arrays in nearby wells. From such configurations information on the relative stimulation volumes and overall fracture dimensions can be obtained. However, the ubiquity of these high-frequency instruments has profound implications for the reliability of magnitude estimates for the largest events associated with these treatments. To address this concern, accelerometers and lower-frequency geophones along can be installed close to surface (within 50m) to characterize events over a wider magnitude band. Furthermore, these sensors can be combined with the high-frequency downhole geophones to monitor (hybrid sensor network) the full bandwidth of activity that can occur during fracture stimulation programs.

Recently, the British Columbia Oil and Gas Commission reported the occurrence of several M3+ events in the Horn River Basin associated with hydraulic fracturing of several well pads. We detail the results of monitoring one such well pad over their summer completion program with a modest network of near-surface 4.5 Hz geophones and force balance accelerometers. Over the approximately six weeks it took to complete the fracturing, a sequence of 1000+ were detected and located. These events have moment magnitudes generally greater than Mw1, reaching as high as Mw2.9. These largest events, felt on surface, align along a cluster of seismicity oriented within 30 degrees of the principle horizontal stress direction below the reservoir. This alignment suggests that the stress redistribution from hydraulic injection is sufficient to cause larger-scale, optimally oriented features to slip in surrounding formations.