Monitoring of Unconventional Resources Using Magnetotellurics

Abstract

The success of unconventional gas extraction is dependent on establishing sufficient permeability in otherwise low-porosity and low-permeability formations. In the case of shale gas, permeability can be established through hydraulic stimulation of deep formations, either through existing fracture networks or by creating new pathways for fluids to flow. Coal seam gas (CSG) permeability can be established through de-pressurisation of coal beds by extracting existing sub-surface fluids.

The primary geophysical technique for the monitoring of hydraulic stimulation and de-pressurisation has been microseismic, which measures small seismic events associated with rock fractures. The magnetotelluric method (MT) presents itself as an alternative geophysical approach for monitoring unconventional resource development. MT is directly sensitive to electrical resistivity with depth and orientation and could be used to infer fracture orientation, fluid migration and hydraulic conductivity.

We report on the first industrial MT field surveys for the spatial and temporal monitoring of fluid movement resulting from both hydraulic fracturing of a shale gas reservoir and de- pressurisation of a CSG formation. We show that increasing permeability enables conductive fluids to connect resulting in small drops in bulk resistivity. Such changes in resistivity can be mapped through modelling and inversion allowing a determination of areas with greater permeability and hence production capacity.