Summary With the increasing preference for long lateral wellbores for recovery of unconventional gas, and the lack of suitable vertical observation wells to monitor microseismic activity related to the treatment of these wells, more and more hydraulic fracture stimulations are being monitored with geophone arrays deployed in horizontal wellbores. However, since fracture height growth is often the most important concern in microseismic monitoring, single or multiple horizontal arrays do not provide adequate depth resolution to satisfy the needs of the petroleum industry. The study presented here explores the effectiveness of using horizontal arrays to provide the necessary depth constraint to provide realistic estimates of height growth in hydraulic fracture stimulations.
Microseismic events from six stages of a hydraulic fracture treatment were recorded using two horizontal sensor arrays in adjacent wellbores. The events were located with a depth constraint, which confined the event locations to the depth of the treatment wellbore, and without a depth constraint. Comparison of the two sets of results reveals that the depths obtained without confinement are highly uncertain, with the scatter of the locations in depth increasing with greater distance from the sensor arrays. The event distributions in all stages tend to follow isochronal misfit surfaces upwards and away from the treatment well.
Based on the results presented here, we conclude that horizontal array geometries are not able to effectively resolve depth for events associated with hydraulic fracture treatments excepting when events occur between arrays. Confining the event locations to a horizontal plane at reservoir depth is a reasonable assumption to obtain meaningful event distributions in plan view. The interpretation of the results in plan does not change significantly with and without the depth constraint, and the shapes of the misfit fields indicate that there is not a marked improvement in the fit of the waveforms when events drift vertically away from the treatment well.