The impact of capillary heterogeneity on CO2 plume migration at the Endurance CCS target site in the UK

Characterisation of multiphase flow properties is crucial in predicting large-scale fluid behaviour in the subsurface, for example carbon dixoide (CO₂) plume migration at Carbon Capture and Storage (CCS) storage sites. Many of the CO₂ storage sites worldwide have displayed unexpected fluid flow behaviour. The CO₂ injected underground has migrated in reservoirs away from injection points at much faster rates than had previously been predicted with reservoir simulations [1]. It has emerged that conventional flow simulations are not representing the impact of small-scale heterogeneities in multiphase flow properties, which is a key driver behind these unexpected CO₂ migration observations [2]. Heterogeneity in the underlying rock structure can cause large variations in porosity and permeability, which manifest as capillary pressure heterogeneity [3-4]. At the low flow potentials typically encountered during CO₂ injection, these heterogeneities can significantly impact fluid flow behaviour, typically observed as large saturation variations within the rock [5-6]. In this work, we have combined experimental and numerical methods to characterise the impact of capillary heterogeneities on plume migration at the Endurance proposed storage site to support the Northern Endurance Partnership (NEP) serving the Zero Carbon Humber and Net Zero Teesside projects in the UK. We built on an approach to characterising capillary heterogeneity at the core scale originating in the work of Krause et al. (2011). The workflow combines core flood experimental data with numerical simulations in a history match, with the experimental 3D saturation distribution as a matching target and the capillary pressure characteristics as a fitting parameter [6]. Through this a 3D digital model of the rock core is built, which incorporates spatial variations in permeability, porosity and capillary heterogeneity. We applied this characterisation effort to reservoir samples from a range of depths within the target interval. Subsequently, these digital core models were used in an upscaling procedure to characterise the impact of small-scale heterogeneities on field scale simulations. The workflow has enabled us to make informed predictions on the observed fluid behaviour at the Endurance storage site. The results emphasize the prevalent impact of small-scale capillary heterogeneities on CO₂ plume migration, thus underscore the importance of characterising and incorporating them in reservoir models.

1. Global CCS Institute (2019), Global Status of CCS: 2019.
2. Jackson, S. J. and Krevor, S. (2020), ‘Small-Scale Capillary Heterogeneity Linked to Rapid Plume Migration During CO2 Storage’, Geophysical Research Letters 47(18).
3. Pini, R., Krevor, S.C. and Benson, S.M., 2012. Capillary pressure and heterogeneity for the CO2/water system in sandstone rocks at reservoir conditions. Advances in Water Resources, 38, pp.48-59.
4. Reynolds, C.A., Blunt, M.J. and Krevor, S., 2018. Multiphase flow characteristics of heterogeneous rocks from CO 2 storage reservoirs in the United Kingdom. Water Resources Research, 54(2), pp.729-745.
5. Krause, M.H., Perrin, J.C. and Benson, S.M., 2011. Modeling permeability distributions in a sandstone core for history matching coreflood experiments. SPE Journal, 16(04), pp.768-777.
6. Jackson, S. J., Agada, S., Reynolds, C. A. and Krevor, S. (2018), ‘Characterizing Drainage Multiphase Flow in Heterogeneous Sandstones’, Water Resources Research 54(4), 3139–3161.