Three Phase Measurements of Nonwetting Phase Trapping in Unconsolidated Sand Packs

We perform a series of experiments in water-wet sand packs to measure the trapped saturations of oil and gas as a function of initial saturation. We start with brine-saturated columns and inject octane (oil) to reach irreducible water saturation followed by displacement by air (gas) from the top, allowing oil and air to drain under gravity for different amounts of time, then finally brine is injected from the bottom to trap both oil and gas. The columns are sliced and a sensitive and accurate measurement of saturation along the column is made using gas chromatography. The maximum residual gas saturation is over 20%, compared to 14% for two-phase flow (Al Mansoori et al. 2009). For lower initial gas saturation, the amount of trapping is similar to that reached in an equivalent two-phase experiment. We also find that the amount of oil trapped is insensitive to either the initial gas saturation or the amount of gas that is trapped. More oil is trapped than would be predicted from an equivalent two-phase system, although the trapped saturation is never larger than the maximum reached in two-phase flow (around 11%) (Pentland et al. 2008). These initially surprising results are explained in the context of oil layer stability and the competition between snap-off and piston-like advance. In unconsolidated two-phase water-wet systems, displacement is principally by cooperative piston-like advance with relatively little trapping, whereas in consolidated media snap-off is generally more significant. However, during three-phase waterflooding, oil layer collapse events rapidly trap the oil which acts as a barrier to direct water-gas displacement, except by snap-off, leading to enhanced gas trapping. Introduction The motivation of this research is to understand trapping of CO2 in carbon capture and storage projects, although the work also has application to enhanced oil recovery processes. Capillary trapping as been proposed as a rapid and effective way to store CO2 securely in simulation studies (Flett et al. 2004; Kumar et al. 2005; Hesse et al. 2006; Juanes et al. 2006; Obi et al. 2006; Hesse et al. 2008; Juanes et al. 2008; Saadatpoor et al. 2008; Qi et al. 2009). In this paper, we focus on CO2 capillary trapping in aquifers and oilfields through analogue laboratory experiments in water-wet systems. It is already well established that in drainage displacements, where gas displaces oil and water, very low residual oil saturations can be achieved in sand-packs similar to those we study here (see, for instance, Sahni et al. 1998; Dicarlo et al. 2000a;b). Here, however, we will study situations where the final displacement is a waterflood, trapping both oil and gas. Previous work (Holmgren et al. 1951; Kyte et al. 1956; MacAllister et al. 1993; Skauge 1996; Egermann et al. 2000) has shown that the residual oil saturation in three-phase flow is reduced from its two-phase value (where no trapped gas is present): p 3 gr p 2 or p 3 or S a S S − = (1) where p 3 gr S is the residual gas saturation in the presence of oil and water, p 3 or S is the residual oil saturation after waterflooding in the presence of gas, and p 2 or S is the residual oil saturation after two-phase waterflooding with no gas present. Water-wet data from Holmgren and Morse (1951) and Kyte et al. (1956) suggest that the coefficient a is 0.45. Egermann et al. (2000) reported similar results. Kyte et al. (1956) also reported data for Alundum rendered oil-wet by drifilm that indicates that a = 0. Skauge (1996) measured values of 0.5 to 1 for water-wet systems, 1 for weakly water-wet, and 0 for oil-wet systems. Data from MacAllister et al. (1993) for Baker dolomite indicate that a is 0.75, 0.25 and 0.04 for waterwet, mixed-wet and oil-wet conditions respectively. Kralik et al. (2000) found a = 0 for oil-wet media. Caubit et al. (2004)