Probabilistic 3-D time-lapse inversion of magnetotelluric data: Application to an enhanced geothermal system

Geophysical Journal International
By: , and 

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Abstract

Surface-based monitoring of mass transfer caused by injections and extractions in deep boreholes is crucial to maximize oil, gas and geothermal production. Inductive electromagnetic methods, such as magnetotellurics, are appealing for these applications due to their large penetration depths and sensitivity to changes in fluid conductivity and fracture connectivity. In this work, we propose a 3-D Markov chain Monte Carlo inversion of time-lapse magnetotelluric data to image mass transfer following a saline fluid injection. The inversion estimates the posterior probability density function of the resulting plume, and thereby quantifies model uncertainty. To decrease computation times, we base the parametrization on a reduced Legendre moment decomposition of the plume. A synthetic test shows that our methodology is effective when the electrical resistivity structure prior to the injection is well known. The centre of mass and spread of the plume are well retrieved.We then apply our inversion strategy to an injection experiment in an enhanced geothermal system at Paralana, South Australia, and compare it to a 3-D deterministic time-lapse inversion. The latter retrieves resistivity changes that are more shallow than the actual injection interval, whereas the probabilistic inversion retrieves plumes that are located at the correct depths and oriented in a preferential north-south direction. To explain the time-lapse data, the inversion requires unrealistically large resistivity changes with respect to the base model. We suggest that this is partly explained by unaccounted subsurface heterogeneities in the base model from which time-lapse changes are inferred.

Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Probabilistic 3-D time-lapse inversion of magnetotelluric data: Application to an enhanced geothermal system
Series title Geophysical Journal International
DOI 10.1093/gji/ggv406
Volume 203
Issue 3
Year Published 2015
Language English
Publisher Blackwell Science
Publisher location Oxford
Contributing office(s) Geology, Minerals, Energy, and Geophysics Science Center
Description 15 p.
First page 1946
Last page 1960
Online Only (Y/N) N
Additional Online Files (Y/N) N