We present the results from a novel experimental apparatus that can image – at the micron scale – fluid displacements at elevated temperatures and pressures. The images are acquired using a micro-CT scanner (Xradia Versa 500) that has been adapted to allow core flood experiments to be conducted in situ, allowing continuous imaging at resolutions down to around 1 µm. A small cylindrical core – approximately 6 mm in diameter – is placed in a carbon fibre core holder that allows high pressures and temperatures to be imposed, while remaining largely transparent to x-rays. Fluids are injected into this mini-core holder, with flexible tubing to allow the core to rotate during scanning.
We use this apparatus to study the displacement of supercritical carbon dioxide by brine, with application to carbon storage in aquifers. We study displacement in carbonate and sandstone rocks. Experiments in carbonates introduce additional challenges, since the carbon dioxide, brine and rock need to pre-equilibrated to prevent dissolution of carbon dioxide and chemical reaction (dissolution) with the rock during the experiments: this then reproduces conditions in the centre of a carbon dioxide plume where local thermodynamic equilibrium has been reached.
We study displacement in Ketton limestone and Bentheimer sandstone. Both rocks have large inter-granular pores. In Ketton there is also significant intra-granular micro-porosity that remains brine-saturated during the experiments. We study primary drainage (injection of carbon dioxide) followed by secondary imbibition (injection of brine). We image the distribution of the phases during and at the end of the experiment.
We show that significant quantities of carbon dioxide can be trapped as a residual phase in the pore space of both rock types, with a saturation matching that measured in core-scale (cm scale) experiments (0.202±0.012 in Ketton and 0.320±0.009 in Bentheimer). Trapped ganglia of all sizes are observed, with an approximately power-law distribution of size. The exponents for the power laws are larger than expected through percolation theory (2.189 as against measured values of 2.39±0.12 in Ketton and 3.0±0.3 in Bentheimer). We can observe how the supercritical carbon dioxide is displaced at the pore scale during waterflooding, providing a benchmark dataset for pore-scale modelling and further analysis.
Overall, this work presents a novel methodology for in situ reservoir-condition pore-scale multi-phase flow analysis, accounting for phase exchange and chemical reaction. In application to carbon storage, we show that significant amounts of carbon dioxide can be trapped as a residual phase in both quartz and calcite rich rocks.