Facies Analysis and b-Value for Operationally Induced Microseismicity

Abstract:

Operationally induced microseismicity reveals brittle failure processes that occur during hydraulic fracturing stimulation of a rockmass. The b-value of the magnitude-frequency distribution is commonly used to characterize the relative abundance of high- to low-magnitude events. In contrast to earthquake fault systems with b ~ 1, microseismicity that is directly associated with hydraulic fracturing is typically characterized by b > 1.5. Recent studies show that such unusually high b-values may be primarily controlled by scaling properties of mechanical bed thickness within the treatment zone, since the occurrence of fracture arrest at bedding boundaries gives rise to stratabound fracture networks. Stress variations also influence the b-value. These concepts provide a framework for microseismic facies analysis, a novel approach for interpretation of microseismicity that facilitates delineation of lithofacies units and stress compartments for characterization of unconventional reservoirs. Microseismic facies units are recognized on the basis of clustering analysis and recognition of distinct sets of microseismic attributes, which include b-value (and other magnitude statistics), duration, transience, seismic moment density and seismic moment release rate in addition to commonly reported characteristics of azimuth and dimensions. A case study is presented, in which microseismic facies analysis is applied to characterize an unconventional reservoir.