Magnetotelluric monitoring of coal seam gas depressurization

The depressurization of coal seam gas formations causes in situ fluids to migrate through pores and fractures in the earth. The removal or discharge of large volumes of water from coal measures reduces in situ fluid pressure allowing natural gas to be released from the coal matrix. This process results in a time-dependent resistivity variation in the subsurface. Increasing the connectivity of in situ fluids may lead to a reduction in resistivity of the targeted lithologies. A correct assessment of such resistivity variations is of significant interest not only for the industry to optimize production and extraction well locations but also for the regulatory bodies, in which a desire for a reliable method for monitoring changes in subsurface fluid distribution allows sound risk assessment of potential environmental hazards. From an industrial field study conducted in Queensland, Australia, we have found that the magnetotelluric (MT) method in the bandwidth of 100 Hz to 100 s could be used to monitor changes in the bulk resistivity of depressurized lithologies. Results from our study indicate the orientation of fluid flow resulting from depressurization, which can be mapped and directly attributed to spatial and temporal variations in permeability. The MT method is introduced as a low-cost, low-impact technology that can be used for short-and long-term environmental monitoring.