The microseismic method is the dominant geophysical tool for monitoring hydraulic-fracturing. High-pressure injection of fluids creates fractures in the sub-surface, recorded as small seismic events. Inverting such events yields the temporal and spatial extent of deformation. However, there is not a straightforward relationship between location of micro-deformations and migration of injected fluid. In this paper we present an alternative approach to deep-fluid monitoring using the magnetotelluric (MT) method. As MT is primarily sensitive to the presence of fluids (via changes in the bulk electrical resistivity), and the orientation of fluid connection, the technique can yield important constraints on how fluids propagate from the well with time. This technique has been successfully used to monitor hydraulically-induced fractures in two geothermal settings at depths of more than 3 km. A ten-stage horizontal-well hydraulic-fracture of a shale-gas reservoir at 3 km depth in the Cooper Basin was monitored using MT. From the central injecting well, four lines of ten electric-field loggers were deployed in a cross-pattern from the well for 65 days of continuous monitoring at 650 Hz sampling. As changes in magnetic-field are spatially uniform over the extent of the survey area, only two magnetic-field loggers were placed within the array, and a third logger 10 km from the well as a remote reference. Measurements were taken from mid-May to mid-July 2014, while the injection and drawback occurred over approximately one month in June. During the hydraulic fracturing, we observe a small change of about 1–2% in the measured MT response at sites close to the injection points. Such changes were most evident in the longer period apparent resistivity (outside the dead-band) and in the phase tensor residuals which indicate orientation and magnitude of change.
AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015