Rock heterogeneity can be observed at a range of scales and can result in the spatial variation of capillary pressure, termed ‘capillary heterogeneity’. Over centimetre to metre length scales, studies have shown that capillary heterogeneity is one of the dominant fluid distribution mech- anisms. On larger scales, recent literature indicates that capillary heterogeneity can lead to anisotropy and flow rate dependency in the observed relative permeability, which is one of the key parameters controlling fluid behaviour. Albeit these findings, there remain significant gaps in our understanding, characterisation and modelling of capillary heterogeneity and its impact on multiphase flow. This poses challenges to the predictive modelling of subsurface CO2 flow and other scientific and engineering processes where multiphase fluid flow in porous media is central.
We examined the importance of, and the challenges associated with, characterising core-scale rock heterogeneities. We applied a capillary heterogeneity characterisation workflow to five rock cores (two sandstones and three carbonates) with varying degrees of heterogeneity to systematically evaluate the inversion approach against a systematic variation in rock structures. For the carbonates, we found the success of the workflow to correlate with the size of the controlling heterogeneous features resolved by the scanner. Strongly-heterogeneous rocks such as carbonates introduce complexities that warrant further advancements in core characterisation approaches. Utilising the successful digital core model of a carbonate, we also discovered that isotropic heterogeneity results in non-monotonic rate dependency behaviour of the relative permeability.
Following these discoveries, we focused our work on the proposed Endurance CCS site in the UK to evaluate the potential impacts of capillary heterogeneity on CO2 plume migration at this site. We conducted an experimental and characterisation study on reservoir samples from the site. We successfully obtained capillary pressure and relative permeability characteristics, which were subsequently used as input into modelling studies. We discovered that the Endurance site exhibits strong capillary heterogeneity on a range of 10s metres along the proposed injection interval.This motivated the investigation into the impact of rock heterogeneity on CO2 plume migration at the metre scale varying flow rate, heterogeneity structure and tilt, using fine-scale simulations incorporating these experimental observations. Capillary heterogeneity can lead to a noticeable increase or decrease in the CO2 migration speed depending on the orientation of heterogeneity relative to the flow axis. In a tilted domain the impact of unfavourable heterogeneities aligned perpendicular to the flow axis is enhanced. We also discovered that the e↵ects of capillary heterogeneity decreased in a 2D model, which suggests that reduced-dimensionality models under-predict the importance of capillary heterogeneity.
Based on these findings, we explored the upscaled impact of capillary heterogeneity on the multiphase flow properties, capillary pressure and relative permeability, by developing a novel 3D, capillary-limit, steady-state upscaling scheme. Using the algorithm, we built a 3D reser- voir model of the Endurance site and simulated CO2 injection. Our work demonstrated that capillary heterogeneity has a significant impact on 3D CO2 plume migration. Near the injector, it enhances the formation of CO2 channels, which significantly improves sweep e ciency and may result in an increase in capillary trapping and dissolution. At distance from the injector, the gravity tongue is thinned and the plume travels faster as a result of capillary heterogeneity. These are novel insights and underpin the importance of characterising capillary heterogeneity in reservoir models of geological carbon storage sites.