Rate dependency of capillary heterogeneity trapping for CO2 storage

TL;DR

Addressing the rate dependence of capillary heterogeneity trapping in CO$_2$ storage, the authors combine steady-state drainage/imbibition core floods in a layered Bentheimer rock with in situ X-ray CT and a 1D continuum model using Brooks-Corey capillary pressure and Corey relative permeabilities to predict trapped saturation distributions. The model introduces a dimensionless trapping length $\overline{x_T}$ and dimensionless groups $N_{v/c}$ and $N_{g/c}$ to quantify how flow rate, gravity, and heterogeneity control trapping, with experimental data validating the predictions. Results show that trapping upstream of capillary barriers increases at lower imbibition rates, informing how rate conditions influence initial-residual trapping relationships and enabling rapid field-scale estimates of capillary heterogeneity trapping. These findings guide upscaling of laboratory measurements to reservoir simulations and have practical implications for screening and designing field-scale CO$_2$ storage projects.

Abstract

In this paper, we experimentally quantify and analytically model rate dependent capillary heterogeneity trapping. Capillary heterogeneity trapping enhances non-wetting fluid trapping beyond pore-scale residual trapping through the isolation of non-wetting phase upstream of heterogeneities in the continuum capillary pressure characteristics. Whilst residual trapping is largely insensitive to the range of flow regimes prevalent in engineered reservoir settings, continuum theory anticipates that capillary heterogeneity trapping will be more sensitive to the balance of viscous and capillary forces that occur. We perform steady-state drainage and imbibition multiphase flow experiments at varying flow rate on a layered Bentheimer sample with in-situ medical X-ray CT scanning to quantify saturation. Saturation discontinuities are observed upstream of capillary pressure barriers as a result of capillary pressure discontinuities, trapping the non-wetting phase at a saturation greater than pore-scale residual trapping alone. We confirm the flow rate dependence predicted by theory whereby the relationship between the initial and residual saturations approach a 1:1 dependence as flow rate is decreased. We develop a one-dimensional analytical model to quantify the proportion of capillary heterogeneity trapping in the system and the dimensionless trapping length scale, which agrees with the experimental data and allows for rapid estimates of trapping up to the field-scale.

Figures (6)

• Figure 1: (a) Diagram of the model system with length L. A region of high capillary entry pressure (layer 2) lies above a region of low capillary entry pressure (layer 1), with heterogeneity length h. (b) The Brooks-Corey capillary pressure - saturation functions for the two layers of different entry pressure are shown. During the steady state imbibition of water, the critical CO2 saturation (Scritical) forms in the low capillary entry pressure layer, upstream of a capillary pressure barrier, as a consequence of capillary pressure continuity.
• Figure 2: Grey scale visualisation of CT number for a central YZ slice (top) and slice average porosity along the normalised core length (bottom), calculated from medical-CT scans of the Bentheimer core.
• Figure 3: Voxel-scale Land trapping coefficient within the Bentheimer sample for three flow rate experiments, displayed in 3D. Flow is from left to right in all images.
• Figure 4: (a) Slice average initial-residual N2 saturation for three flow rate experiments in the Bentheimer sample (blue, orange and yellow markers). The maximum Land trapping relationship, calculated as the slice average relationship from the high flow rate (0.5 cm$^3$min$^{-1}$) experiment, is plotted as a solid line ($C = 1.24 \pm 0.13$). In addition literature comparisons from Jackson et al. (2020) for a homogeneous and heterogeneous Bentheimer Jackson2020REV are shown (grey and black markers). The average trapping relationships for the homogeneous $C=1.34$ and heterogeneous $C=0.78$ literature samples are also shown (dotted lines). (b) Slice average initial-residual N2 saturation for the imbibition rate 0.05 cm$^3$min$^{-1}$ experiment. The colour gradient shows where the saturations occur along the core, from inlet ($x$=0) to outlet ($x$=$L$). The Land trapping relationship upstream of the heterogeneity $C=0.55$ (dashed line), as well as the maximum $C=1.24$ (solid line) and average $C=0.86$ (dotted line) Land trapping relationships are displayed
• Figure 5: (a) Proportion of capillary heterogeneity trapping ($H$%) over different flow rates, estimated for the Bentheimer experiments from the analytical model for a range of capillary entry pressures. In addition, the experimental results (The proportion of capillary heterogeneity trapping in the system as a function of water imbibition rate, with $\pm$ 1 standard deviation of uncertainty in $C$ shown.) and associated uncertainties are shown. The capillary pressure parameters in the analytical model are optimised to provide a good fit to the experimental data. (b) Analytical solution of trapped saturation compared to experimental results, for flow rate 0.05 cm$^3$min$^{-1}$. The capillary pressure parameters in the analytical model are optimised to provide a good fit to the proportion of capillary heterogeneity trapping from the experimental data.