Use of hydraulic tests to identify the residual CO2 saturation at a geological storage site

Abstract

Estimating long term capillary trapping of CO2 in aquifers remains a key challenge for CO2 storage. Zhang etal, (2011) proposed a combination of thermal, tracer, and hydraulic experiments to estimate the amount of CO2 trapped in the formation after a CO2 push and pull test. Of these three types of experiments, hydraulic tests are the simplest to perform and possibly the most informative. However, their potential has not yet been fully exploited. Here, we present a methodology to interpret these tests and analyze which parameters can be estimated. We use numerical and analytical solutions to a continuous injection in a medium where residual CO2 has caused a reduction in hydraulic conductivity and an increase in storage coefficient over a finite thickness (a few meters) skin around the injection well. We interpret the model results using conventional pressure buildup and diagnostic plots as well as the Agatwal (1980) recovery plots. We find that both skin hydraulic conductivity and storage coefficient (and thus residual CO2 saturation) can be obtained from the water injection test provided that water flow rate is carefully controlled and head buildup is measured with high frequency at the beginning of the test. (C) 2013 Elsevier Ltd. All rights reserved.