Stimulated Shale Volume Characterization: Multiwell Case Study from the Horn River Shale: I. Geomechanics and Microseismicity

摘要

We carried out an integrated geomechanical and microseismic analysis of a gas shale production pad in the Horn River Basin containing 16 horizontal wells, over 270 individual hydraulic fracture stages, and more than 15,000 located microseismic events to better characterize the reservoir response to hydraulic fracture stimulations. Geomechanical constraints indicate that the pad is generally characterized by a strike-slip stress regime (SHmax > SV > Shmin) with the maximum horizontal stress (SHmax) oriented NE-SW. A pre-existing NNE-SSW-trending fault passes through the pad and appears to be well-oriented for slip in the current stress field. Measurements of Shmin, the minimum principal stress, are moderately elevated in some stages in the vicinity of this fault, and also appear to gradually increase from toe to heel in most wells. Microseismic b-values, which quantify magnitude scaling relationships for a population of events, vary from stage to stage but typically range between 1.0 and 2.5, indicating a relative abundance of small magnitude to large magnitude events compared to naturally- occurring earthquake populations (b ≈ 1.0). This observation has also been reported in other shale gas stimulation projects. We also utilize the double-difference relocation technique in an attempt to improve the accuracy of microseismic event hypocenters recorded during individual hydraulic fracture stages. The technique produces clear hypocenter lineations, which, unfortunately, appear to be artifacts arising from the limited monitoring array configuration and not actual structures within the reservoir. Finally, we analyze a population of microseismic events recorded during a single hydraulic fracture stage and find that the events can be broken into small sub-groups of events based on high waveform similarity, which suggests repeated slip on small-scale reservoir structures. We conclude that reservoir response to hydraulic fracturing is generally characterized by seismic deformation occurring on small fractures created or reactivated by the hydraulic fracturing stimulations, although there is also some evidence for seismic deformation occurring on larger-scale localized structures within the reservoir.