Measurements of Non-wetting Phase Trapping in Porous Media
Creators: name: Pentland, Christopher H.
Publisher: name: Imperial College London
Year: 2010

description: Saline aquifers have been identified as a preferred storage location for anthropogenic carbon dioxide emissions due to their large capacities and wide geographical spread. The storage of carbon dioxide in such formations must be carefully designed so that the carbon dioxide is trapped securely and will not escape to the surface. Of the various subsurface mechanisms, capillary trapping is a fast and secure means of rendering injected carbon dioxide immobile. The carbon dioxide is trapped on the pore scale as residual phase bubbles surrounded by formation brine. Local capillary forces prevent the movement of the carbon dioxide bubbles and retain them within individual pores or groups of pores. The trapping of a phase in this manner can be characterised by the relationship between initial and residual saturation, known as the capillary trapping curve. The maximum trapped saturation and the form of the capillary trapping curve are of key importance in characterising the underlying physics of this phenomenon, giving an important indication of system wettability. To date the form of the capillary trapping curve has not been measured for a carbon dioxide-brine-rock system and there is limited technical literature available on maximum trapped carbon dioxide saturations. The aim of this study is to measure the maximum saturation and the form of the capillary trapping curve in a carbon dioxide-brine-sandstone system at conditions representative of a storage location through coreflood experiments. Similar measurements are required on analogue systems of known wettability in order to infer the wettability of the carbon dioxide-brine-sandstone system. In addition measurements of capillary pressure are sought to better understand the migration of carbon dioxide after injection, and the influence of petrophysical properties on capillary trapping is investigated. We initially measure trapping in unconsolidated sand at ambient conditions with analogue fluids: oil-brine and gas-brine. The maximum trapped saturations are relatively low – 12.8% and 14.3% for oil and gas respectively. The form of the capillary trapping curve is shown to be slightly different in each case. We then measure the capillary trapping curve for Berea sandstone at temperature and pressure representative of a storage location (T = 343 K; P = 9.0 MPa). The maximum residual carbon dioxide saturation is measured as 35.3% and the form of the trapping curve is accurately predicted by the trapping correlation proposed by Spiteri et al., [1]. The primary drainage capillary pressure curve is also measured and it is shown that the irreducible brine saturation is 14.9% for an applied capillary pressure equivalent to a carbon dioxide column height of 37 m. For comparison oil-brine measurements are also made on the same sandstone. Trapping is shown to be different for each system. More oil than carbon dioxide is trapped in Berea sandstone – 48.3% versus 35.3%. Subtle changes in imbibition contact angle and system wettability are proposed as the cause – with the rock surface being slightly less hydrophilic in the presence of carbon dioxide. Brine remains the wetting phase however, explaining the substantial quantities of carbon dioxide trapped. A simple term to quantify trapping capacities is developed – the capillary trapping capacity – being the product of maximum trapped saturation and porosity. A capillary trapping capacity of up to 7.8% of the gross rock volume for a carbon dioxide-brine-Berea sandstone system is measured. We also investigate the influence of petrophysical properties on capillary trapping in oil-brine systems. Maximum trapped oil saturations are measured for five consolidated sandstone systems in addition to six unconsolidated sands. Strong relationships exist between trapped saturation and porosity, permeability and a characteristic geometric property (aspect ratio divided by coordination number). These relationships may prove valuable for future predictions of maximum trapped saturations when coreflood data is not available.