Monitoring Coal Seam Gas Depressurisation Using Magnetotellurics

Abstract

The depressurisation of coal seam gas (CSG) formations causes in situ fluids to migrate through pores and fractures in the Earth. The removal of large volumes of water from coal seams has the potential to affect water table levels and groundwater flow. Magnetotellurics (MT) is a passive electromagnetic technique that utilises the natural fluctuations of electric and magnetic fields at the Earth's surface to determine the conductivity structure of Earth. The bulk movement of fluids during CSG depressurisation causes a conductivity change in the subsurface and this change can be continuously monitored by deploying an array of magnetotelluric instruments. Various techniques will be presented to analyse a magnetotelluric CSG monitoring dataset. Firstly, we examine electric phase tensors and quasi-electric phase tensors and compare these with standard MT responses. These tensors relate the electric fields at survey sites with the electric or magnetic fields at base sites and are almost or entirely free from distortion effects. Secondly, we apply eigenanalysis and singular value decomposition (SVD) methods to the distortion tensor. Both techniques can be used to determine the geologic strike direction for the two-dimensional (2D) case as well as determining if a situation is far from two-dimensional such that 2D modelling is not justified. The results of eigenanalysis and SVD can be displayed on a Mohr diagram, which is a useful way to display a wide range of properties of the distortion matrix. Finally, we link the above analysis to standard 1D and 2D inversions of our dataset. 2D models of resistivity show the spatial pattern of change preand postCSG production. 1D time-lapse inversions show the temporal variations in sub-surface resistivity as a function of time. Monitoring Coal Seam Gas Depressurisation Using Magnetotellurics Nigel Rees, Graham Heinson, Lars Krieger Project Objectives Monitor the movement of fluids and changes in the Earth resulting from CSG production Understand what direction fluids and gases move and how far they migrate Determine the short and long-term consequences of CSG production Records time variations of Earth’s magnetic and electric fields over a wide frequency range to image electrical resistivity (conductivity) structure with depth How does MT work? Source fields Low frequencies ( 1 Hz): World-wide thunderstorm activity, usually near the equator. Dead Band: 100 to 10-1 Hz (1 to 10s): Natural EM fluctuations have a low intensity. Skin depths 1.5 to 15 km, upper middle crust. Depth of investigation – skin depth High frequencies image the near-surface Low frequencies penetrate to greater depths T is the period ρ is the apparent resistivity Apparent resistivity and phase Forward modelling Before After