Partially joint petrophysical inversion

Geophysical Journal International, vol. 244

Abstract

SUMMARY Joint petrophysical inversion is a powerful technique for using multiple geophysical modalities to estimate petrophysical or geotechnical parameters of the subsurface. A precise knowledge of the petrophysical laws for the full model domain is imperative to enable petrophysical coupling. In this work, we investigate the effect of partially invalid petrophysical laws on the inversion of a synthetic data set, using electrical resistivity tomography (ERT) and seismic traveltime data to image a $\text{CO}_2$ plume in a Carbon Capture and Storage (CCS) setup. We consider a model consisting of a reservoir and a caprock in which only the reservoir can be described by a petrophysical law. We first apply a conventional (joint) petrophysical inversion (JPI) and show that the use of wrong petrophysical laws leads to systemic artefacts within the parts of the model in which the petrophysical relations are invalid. We then present a new hybrid partially petrophysically coupled joint inversion (P-JPI) approach that combines petrophysical coupling for regions with valid petrophysical laws, and structural coupling, whenever no reliable petrophysical laws are available. The P-JPI approach outperforms tomography based on the individual ERT or seismic data set, as well as joint structural inversion (JSI) based on the cross-gradient functional. The partially petrophysically coupled joint inversion thus enables petrophysical coupling and provides a unique, quantitatively interpretable saturation model for the $\text{CO}_2$-plume. We further show that it is possible to detect zones with incorrect petrophysical relations by analysing the difference of the model updates based on the standalone data sets. Finally, we combine the detection of zones of incorrect petrophysical laws with the P-JPI to derive an inversion scheme that is independent of prior knowledge of the validity of petrophysical laws. Our novel methods facilitate direct estimation of the petrophysical subsurface parameters from multiple geophysical measurements if petrophysical relations are only available for parts of the model domain and provide means to quantify the spatial extent of regions where the petrophysical relations are valid.

Authors 3

1. Hagen Söding corresponding Aachen Geophysical Imaging and Monitoring

   RWTH Aachen University · ETH Zurich · Institute of Geophysics

   Affiliation as printed

   Geophysical Imaging and Monitoring, RWTH Aachen University , D-52062 Aachen ,

   Institute of Geophysics , ETH Zurich, CH-8092 Zurich ,

2. F M Wagner Aachen Geophysical Imaging and Monitoring

   RWTH Aachen University

   Affiliation as printed

   Geophysical Imaging and Monitoring, RWTH Aachen University , D-52062 Aachen ,

3. Hansruedi Maurer

   ETH Zurich · Institute of Geophysics

   Affiliation as printed

   Institute of Geophysics , ETH Zurich, CH-8092 Zurich ,