The impact of rock heterogeneity on solute spreading and mixing

Abstract

In this thesis we have performed an experimental investigation on the impact of rock heterogeneity on solute spreading and mixing in porous rock using X-ray tomography. Furthermore, we have created a numerical model based on experimentally obtained statistical rock descriptions to investigate the impact of transport and chemical heterogeneity on reactive transport. We present a new core-flood test to characterize solute transport in 3-D natural-rock media. The test is carried out for three rocks with an increasing level of heterogeneity: Berea sandstone, Ketton carbonate and Indiana carbonate. The impact of heterogeneity on solute transport is analysed by: 1., quantifying spreading and mixing using metrics such as the transverse dispersion coefficient, the dilution index, and the scalar dissipation rate, and 2., visualizing and analysing flow structures such as meandering, flow-focusing and flow-splitting. The transverse dispersion coefficient, Dt, and the variation in Dt throughout the rock core, increases with Peclet number (Pe) and rock heterogeneity. The dilution index and scalar dissipation rate indicate that mixing is Fickian for the Berea sandstone and Ketton carbonate, but diverges for the Indiana carbonate. Heterogeneous rock features are observed to cause meandering, focusing or splitting of the plume depending on Pe. The impact of transport and chemical heterogeneity on reactive transport is investigated by modelling the injection of a HCl solution into the three rocks. The model shows that both transport and chemical heterogeneity are important and the dominating factor depends on the transport regime and reaction kinetics. The model is able to capture different dissolution regimes: compact dissolution is observed for low injection rates while the onset of wormholing and uniform dissolution is observed for the higher injection rates. The modelling results are a first indication that statistical descriptions of transport and chemical heterogeneity can improve continuum scale reactive transport modelling.