Magnetotelluric monitoring of coal-seam gas and shale-gas resource development in Australia

Abstract

Extraction of unconventional energy has become a major global industry in the last decade and is driven by changes in technology and increasing demand. One of the key factors for the success of gas extraction is establishing sufficient permeability in otherwise low-porosity and low-permeability formations. Permeability can be established through hydraulic stimulation of deep formations, either through existing fracture networks or by creating new pathways for fluids to flow, and through depressurization of coalbeds by extracting existing subsurface fluids. Geophysical monitoring of hydraulic stimulation and depressurization can be used to determine lateral and vertical constraints on fluid movements in the target lithologies. Such constraints help to optimize production and well placement. In addition, independent verification is critical for social and environmental regulation, to ensure that hydraulic stimulations and depressurization do not interact with overlying aquifers. To date, the primary and most successful geophysical technique has been microseismic, which measures small seismic events associated with rock fractures from arrays of surface and downhole geophones. The microseismic approach has been used widely for many types of unconventional energy-resource development. The magnetotelluric (MT) method is an alternative approach to monitoring hydraulic stimulations and depressurization. In contrast to microseismic, which delineates the locations of rock fractures, MT is sensitive directly to the presence of fluid as measured by the earth's bulk electrical resistivity, which is dependent on permeability. MT is sensitive to the direction of fluid connection, so it might yield important information on how fluids migrate with time. Because subsurface fluids conduct electrical current dependent on the porosity, connectivity, and ionic saturation of the fluid, it follows that the introduction or removal of fluids will change the electrical resistivity of the formation. The physics of the approach is outlined, and the feasibility of the MT method for monitoring unconventional energy-resource development is demonstrated. Two case studies are conducted, one for a shallow (CSG) depressurization and the second for a deep hydraulic stimulation of a shale-gas reservoir.

Time-lapse inversion of one-dimensional magnetotelluric data

“…We use time-lapse MT data from a 2013/2014 survey of a coal-seam gas field in the Surat Basin, Queensland, Australia (Rees et al 2016). Data for this survey were collected along two lines near several production wells.…”

“…Firstly, we present the results from synthetic inversions of 1D magnetotelluric data to test the validity of the algorithm. We then present inversions from a case study of data from a coal-seam gas production survey (Rees et al 2016).…”

“…Another approach has been to use 1D layer-stripping, where the effect of overlying structures is removed to model the time-varying magnetotelluric responses at depth (Ogaya et al 2016). Standard MT modelling tools such as inversion can also be adapted, for example Rees et al (2016) use cascading two-dimensional (2D) inversions to model a coal-seam gas depressurisation, where the results from the preinjection inversion are used as a prior-model for the postinjection. An improvement on cascaded inversion has been implemented by Rosas-Carbajal et al (2012), where differenced 2D MT inversions have achieved high accuracy by subtracting the prior-model response from all data and reducing the data error.…”

“…High-accuracy simultaneous time-lapse 1D MT inversions have also been approximated using 2D MT codes (e.g. Rees et al 2016;Didana et al 2017); however, this approximation leads to small errors in the calculation of the forward-model and hence inversion result.…”

“…In recent years, the magnetotelluric method (MT) has been increasingly used as a cost-effective technique for subsurface resistivity monitoring (see Rees et al 2016, for a general introduction). As this is a relatively new application for MT, there are few modelling tools available.…”