Residual CO2 imaged with X‐ray micro‐tomography

Carbon capture and storage (CCS), where CO2 is injected into geological formations, has been identified as an important way to reduce CO2 emissions to the atmosphere. While there are several aquifers worldwide into which CO2 has been injected, there is still uncertainty in terms of the long‐term fate of the CO2. Simulation studies have proposed capillary trapping – where the CO2 is stranded as pore‐space droplets surrounded by water – as a rapid way to secure safe storage. However, there has been no direct evidence of pore‐scale trapping. We imaged trapped super‐critical CO2 clusters in a sandstone at elevated temperatures and pressures, representative of storage conditions using computed micro‐tomography (μ‐CT) and measured the distribution of trapped cluster size. The clusters occupy 25% of the pore space. This work suggests that locally capillary trapping is an effective, safe storage mechanism in quartz‐rich sandstones

Pore-scale imaging and modelling

Pore-scale imaging and modelling – digital core analysis – is becoming a routine service in the oil and gas industry, and has potential applications in contaminant transport and carbon dioxide storage. This paper briefly describes the underlying technology, namely imaging of the pore space of rocks from the nanometre scale upwards, coupled with a suite of different numerical techniques for simulating single and multiphase flow and transport through these images. Three example applications are then described, illustrating the range of scientific problems that can be tackled: dispersion in different rock samples that predicts the anomalous transport behaviour characteristic of highly heterogeneous carbonates; imaging of super-critical carbon dioxide in sandstone to demonstrate the possibility of capillary trapping in geological carbon storage; and the computation of relative permeability for mixed-wet carbonates and implications for oilfield waterflood recovery. The paper concludes by discussing limitations and challenges, including finding representative samples, imaging and simulating flow and transport in pore spaces over many orders of magnitude in size, the determination of wettability, and upscaling to the field scale. We conclude that pore-scale modelling is likely to become more widely applied in the oil industry including assessment of unconventional oil and gas resources. It has the potential to transform our understanding of multiphase flow processes, facilitating more efficient oil and gas recovery, effective contaminant removal and safe carbon dioxide storage