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3-D Seismic Workflows Developed to Evaluate Out-of-Zone and Stealth Zone Microseismic Behaviors: Marcellus Shale, Central Appalachians, USA

Abstract

Out-of-zone and cross-stage microseismic activity along with stealth- or aseismic-zone phenomena are often observed during hydraulic fracture treatment. In this study we examine interrelationships between 3-D seismic response and anomalous microseismic distribution observed during hydraulic fracture treatments from a cluster of Marcellus Shale wells. We preprocess seismic data for enhanced 3-D visualization and seismic discontinuity extraction in the vicinity of the treatment area using time-variant trace amplitude slicing and differential attenuation (t*attenuation) computations. Amplitude slicing uses absolute values of trace amplitude followed by bandpass filtering in a series of two to three steps to increase apparent frequency content. The output retains direct relationship to variations in signal phase and frequency content through time. The process improves visualization and interpretation of subtle amplitude and phase variations related to local structural and stratigraphic features. High frequency attenuation is commonly observed in wave propagation through fracture zones and faults. We use t*attenuation to identify areas of high frequency loss across windowed regions throughout the 3-D seismic volume. The 3-D attenuation volume is then used as an input to a seismic discontinuity detection process (in this case, Ant Tracking). Enhanced 3-D displays reveal stealth-zones in areas of more coherent signal in the overlying Hamilton to Sonyea section. Out-of-zone events are observed in the overlying Sonyea to sub-Elk strata and are among the earliest events to occur. Out-of zone events occur in areas disrupted by small faults/fracture zones and areas of interpreted local shale flowage. They are separated from contemporary microseismicity within the Marcellus by a thick stealth-zone. In a process similar to cross-stage fracturing, out-of-zone microseismic regions are repeatedly activated during hydraulic fracturing of nearby wells. Novel post-stack processing workflows developed in this study form the basis for 3-D seismic interpretation of out-of-zone and stealth-zone microseismic behavior. Out-of-zone events appear to occur in locally deformed strata while stealth-zones separating the reservoir and out-of zone events appear in less disrupted strata. The interrelationships are approximate and might be improved through use of a revised velocity function for microseismic event location.