The Reservoir Characterization Project at Colorado School of Mines has been working in conjunction with Talisman Energy Inc. since 2009 to analyze two hydraulic stimulations in the Montney Shale play in Canada. Specifically, the project originates from the monitoring of two five stage horizontals from the Pouce Coupe area in North-Western Alberta. The completions were monitored using a variety of microseismic methods (surface, shallow water well and downhole). This paper will focus on analysis of the downhole arrays that were used to monitor the hydraulic stimulations.
Amplitude ratios of seismic waves (P, Sh and Sv) can be used to estimate focal mechanism solutions in areas where a full moment tensor inversion is not effective. In this case, due to the limited azimuths available from the two recording arrays, amplitude ratios are seen as a more robust tool to ascertain these mechanism solutions. A technique is implemented to model the radiation patterns from simple end member mechanisms (pure double-couple or tensile sources) to match the microseismic amplitudes recorded by the downhole arrays. Similar to previous studies in this area, this work finds that the superior solution is a vertical strike-slip mechanism striking at an angle close to that of maximum horizontal stress (N40°E) within the reservoir.
Previous amplitude ratio work has highlighted that microseismic events appear to be shear dominated in their amplitudes and has theorized that the events we record are an interaction between the hydraulic fracture and existing planes of weakness (natural fractures, bedding planes etc.). The focal mechanism solutions drawn from this work are compared with other natural fracture determination methods (time-lapse shear wave splitting and FMI logs) to see if there is any correlation. Results show that these solutions tie well with the natural fracture directions determined by the shear wave splitting study. The shear wave splitting map is consistent with the strike directions of the focal mechanism solutions in areas where microseismic events were detected. The FMI logs show near vertical fractures striking at angles close to that resolved by the amplitude analysis, but with variations from the dominant trend. This is interpreted to be an indication of the complication of fracture sets present in a naturally fractured reservoir. This study extends previous work in this area and also provides another example of the use of amplitude ratios in analyzing microseismic data without the need for high receiver coverage and costs.