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Summary
Understanding out of zone frac growth can lead to designing stimulation programs that can effectively enhance production from adjacent horizons. The effectiveness of the stimulation program can be assessed by incorporating monitoring programs that include instrumentation to detect seismic events over a broad range of magnitudes from the smallest detectable events with magnitudes below zero (microseismic) to larger events with magnitudes above zero (induced seismic), typically related to larger pre-existing fractures or faults. Stimulating these larger structures could lead to loss of fluid from the reservoir and affect estimates of stimulated volume. In this study, we examine data recorded using a typical downhole microseismic wireline supplemented with a near surface array designed to record induced seismic events. In this study, two horizons were stimulated. The intent of the program was to stimulate both zones by stimulating wells in the lower horizon by increasing pressure rates both early and late into the injection program in the upper horizon. About 4500 microseismic events and 28 induced seismic events were observed. These larger events represent approximately 83% of the total seismic energy released during the stimulation, which, if only using standard recording, would have been misinterpreted as microseismic events and thereby would not have contributed to the overall energy dissipation levels. The larger events were associated with fractures with lengths varying from about 40 m to over 110 m, whereas the microseismic fracture lengths varied from ~5 m up to ~ 35m. The microseismic and induced seismic events could be used to identify growth from the upper to lower horizon at different pressure rates. The occurrences of larger magnitude events appeared to precede pressure increases in the program, suggesting that larger structures were activated as a result of the injection program even before pressures were increased. This observed process sets the foundation to better control stimulation programs.
Introduction
Hydraulic fracturing in naturally fractured reservoirs is known to generate seismicity due to the interaction of injected fluids with the pre-existing fracture network. Typically, the observed moment magnitudes (Mw) for such operations are small, usually with Mw < 0. To map the seismicity during these injections, geophones (utilizing 15 Hz) are typically deployed in arrays in nearby wells. From such configurations, information on the relative stimulation volumes and overall fracture dimensions can be obtained. However, the ubiquity of these high-frequency instruments has profound implications for the reliability of magnitude estimates for the largest events associated with these treatments. To address this concern, accelerometers and lower-frequency geophones can be installed close to surface to characterize events over a wider magnitude range. Furthermore, these sensors can be combined with the high-frequency downhole geophones to monitor (hybrid sensor network) the full bandwidth of activity that can occur during fracture stimulation programs thereby providing a more complete picture of fracturing for stimulation design purposes.
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