We quantify the influence of the initial nonwetting-phase saturation and porosity on the residual nonwetting-phase saturation using data in the literature and our own experimental results on sandpacks and consolidated sandstones. These experiments were conducted at ambient or elevated pressure and temperature (ETP) conditions. The principal application of this work is for carbon capture and storage (CCS) where capillary trapping is a rapid and effective way to render the injected CO 2 immobile, guaranteeing safe storage.We introduce the concept of capillary-trapping capacity (C trap ) which is the product of residual saturation and porosity that represents the fraction of the rock volume that can be occupied by a trapped nonwetting phase. We show that the measured trapping capacity reaches a maximum of approximately 11% for porosities of 22%, which suggests an optimal porosity for CO 2 storage.

We measure the trapped nonwetting-phase saturation as a function of initial saturation in sandpacks. The application of the work is for carbon dioxide (CO 2 ) storage in aquifers, where capillary trapping is a rapid and effective mechanism to render the injected fluid immobile: The CO 2 is injected into the formation followed by chase-brine injection or natural groundwater flow that displaces and traps it. Current models to predict the amount of trapping are based on experiments in consolidated media; while CO 2 is likely to be injected at depths greater than approximately 800 m to render it supercritical, it may be injected into formations that tend to have a higher porosity and permeability than deep oilfield rocks. We use analog fluids-water and refined oil-at ambient conditions. The initial conditions are established by injecting oil into vertical or horizontal sandpacks 0.6 m long at different flow rates and then allowing the oil to migrate under gravity. The packs are then flooded with water. The columns are sliced, and the residual saturation is measured with great accuracy and sensitivity by gas chromatography (GC). This method allows low saturations to be measured reliably. The trapped saturation initially rises linearly with initial saturation to a value of approximately 0.13, followed by a constant residual as the initial saturation increases further. This behavior is not predicted by the traditional Land (1968) model but is physically consistent with poorly consolidated media where most of the larger pores can be invaded easily at relatively low saturation and there is, overall, relatively little trapping. The best match to our experimental data is achieved with the Aissaoui (1983) and the Spiteri et al. (2008) trapping models.

We measure the trapped non-wetting phase saturation as a function of initial saturation in sand packs. The application of the work is for carbon dioxide (CO 2) storage in aquifers, where capillary trapping is a rapid and effective mechanism to render the injected fluid immobile: the CO 2 is injected into the formation followed by chase brine injection or natural groundwater flow that displaces and traps it. Current models to predict the amount of trapping are based on experiments in consolidated media; while CO 2 is likely to be injected at depths greater than around 800 m to render it super-critical, it may be injected into formations that tend to have a higher porosity and permeability than deep oilfield rocks. We use analogue fluids-water and refined oil-at ambient conditions. The initial conditions are established by injecting oil into vertical or horizontal sand packs 1 m long at different flow rates and then allowing the oil to migrate due to buoyancy forces. The packs are then flooded with water. The columns are sliced and the residual saturation measured with great accuracy and sensitivity by gas chromatography. This method allows low saturations to be measured reliably. The trapped saturation initially rises linearly with initial saturation to a value of around 0.11, followed by a constant residual as the initial saturation increases further. This behavior is not predicted by the traditional Land (1968) model, but is physically consistent with poorly consolidated media where most of the larger pores can easily be invaded at relatively low saturation and there is, overall, relatively little trapping. The best match to our experimental data was achieved with the Aissaoui (1983) trapping model.

We measure residual nonwetting phase saturation in six unconsolidated sands of different average grain sizes. We also analyze the pore structure using three-dimensional images from which topologically equivalent pore networks are extracted. The residual saturations range from 10.8% to 13.1%, which is lower than for most consolidated media. Higher porosity is associated with lower residual saturations, while there is little correlation between grain and pore shape and the degree of trapping. We also study layered packs: the residual saturations are reduced compared to comparable homogeneous systems. We discuss the results in the context of capillary trapping during carbon storage in aquifers.

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