Correlation Between Spatial and Magnitude Distributions of Microearthquakes during Hydraulic Fracture Stimulation

Seismic and microseismic event populations can be characterized by their spatial and magnitude distributions. The magnitude distribution is described by the Gutenberg-Richter b value. The spatial distribution can be described by the two-point correlation coefficient, Dc. We observe spatio-temporal variations in both b and Dc during hydraulic fracture stimulation in the Cotton Valley tight gas reservoir. We find that during the initial stages of the fracture stimulations, b is high while Dc is low, implying that deformation is dominated by smaller events clustered at points near the injection well. As injection progresses, b decreases and Dc increases. We investigate correlation between b values and Dc, which can be indicative of the style of fracturing. Initially, b and Dc are negatively correlated, consistent with mechanical weakening during the early stages of fracturing. As the stimulation progresses, b and Dc become positively correlated, consistent with later stabilization of the fractures and mechanical hardening when the induced fracture intersects a pre-existing fault. The mechanism of hardening may be crack blunting while propagating into a pre-existing fractured or damaged zone.