Probabilistic electrical resistivity tomography of a CO2 sequestration analog

Abstract

article Electrical resistivity tomography (ERT) is a well-established method for geophysical characterization and has shown potential for monitoring geologic CO2 sequestration, due to its sensitivity to electrical resistivity contrasts generated by liquid/gas saturation variability. In contrast to deterministic inversion approaches, probabilistic in- version provides the full posterior probability density function of the saturation field and accounts for the uncer- tainties inherent in the petrophysical parameters relating the resistivity to saturation. In this study, the data are from benchtop ERT experiments conducted during gas injection into a quasi-2D brine-saturated sand chamber with a packingthatmimicsa simpleanticlinal geologicalreservoir. The saturation fields are estimated by Markov chain Monte Carlo inversion of the measured data and compared to independent saturation measurements from light transmission through the chamber. Different model parameterizations are evaluated in terms of the recov- ered saturation and petrophysical parameter values. The saturation field is parameterized (1) in Cartesian coor- dinates, (2) by means of its discrete cosine transform coefficients, and (3) by fixed saturation values in structural elementswhoseshapeandlocationisassumedknownorrepresentedbyanarbitrary GaussianBellstructure.Re- sultsshowthattheestimatedsaturation fieldsare inoverallagreementwith saturationsmeasuredbylighttrans- mission, but differ strongly in terms of parameter estimates, parameter uncertainties and computational intensity. Discretization inthe frequency domain(as inthe discrete cosinetransform parameterization) provides more accurate models at a lower computational cost compared to spatially discretized (Cartesian) models. A priori knowledge about the expected geologic structures allows for non-discretized model descriptions with markedly reduced degrees of freedom. Constraining the solutions to the known injected gas volume improved estimates of saturation and parameter values of the petrophysical relationship.