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Abstract

Capillary trapping has been identified as a fast and effective method to render injected carbon dioxide (CO₂) immobile as disconnected pore-scale droplets surrounded by brine. We measure trapped CO₂ saturations in sandstones at conditions representative of storage locations. We compare the unsteady state and porous plate methods of achieving initial CO₂ saturations before waterflooding to reach residual saturation. Brine and CO₂ are equilibrated prior to injection to ensure immiscible displacements occur on the pore scale. The problems faced with un-equilibrated phases are discussed.

The unsteady state and porous plate methods are shown to give different results in terms of maximum initial and residual saturations for Berea sandstone samples. With the unsteady state method maximum residual CO₂ saturations of 25-28% are measured for corresponding maximum initial saturations of 35-40%. With the porous plate method a maximum residual saturation of 37% is measured for a maximum initial saturation of 90%. The implications for coreflood method selection during data gathering are discussed.

The porous plate results are compared with oil-brine porous plate saturations measured on the same samples. CO₂-brine residual saturations are shown to be slightly lower than the corresponding oil-brine measurements. We suggest that considerable carbon dioxide capillary trapping is possible in clean sandstones and discuss the implications for carbon storage in aquifers.

Introduction

Saline aquifers have been identified as a suitable long term storage location for anthropogenic CO₂ emissions due to their large capacities and wide geographical spread (Lackner 2003; Orr 2004; Hawkes et al. 2005; IPCC 2005). The storage of CO₂ in saline aquifers would likely occur at depths greater than 800 m where the formation temperature and pressure would render the CO₂ in a dense supercritical phase – increasing storage capacities. Upon injection into a saline aquifer there are a number of trapping mechanisms which prevent the CO₂ escaping from the formation. The CO₂ will dissolve in the resident host brine; the resultant denser – CO₂-rich – brine will convectively mix and move deeper within the formation (Lindeberg & Wessel-Berg 1997; Ennis-King & Lincoln 2002). The CO₂ that does rise in a plume upwards through the formation may eventually reach the cap rock which will constrain its further upwards movement due to its capillary entry constraints. This process is known as structural and stratigraphic trapping (Bachu et al. 1994). Mineral trapping occurs over longer time scales than other trapping methods. As CO₂ dissolves in formation brine, carbonic acid (H₂CO₃) is formed and subsequently dissociates and reacts with the host rock or brine to generate solid minerals (Gunter et al. 1993; Gunter et al. 1997; Egermann et al. 2005; Lin et al. 2007). The final trapping mechanism is capillary trapping where the re-imbibition of water into pores occupied with CO₂ displaces and traps the CO₂ as discontinuous residual droplets surrounded by brine. The residual CO₂ droplets are held immobile by local capillary forces. Capillary trapping is a rapid and effective mechanism that reduces the requirement to ensure caprock integrity. The re-imbibition of brine will occur at the trailing edge of the rising CO₂ plume or it can be engineered to maximize CO₂ storage through the co-mingled injection of CO₂ and brine, followed by a period of chase brine injection (Kumar et al. 2005; Hesse et al. 2008; Obi & Blunt 2006; Juanes et al. 2006; Ide et al. 2007; Qi et al. 2009; Saadatpoor et al. 2009).

Brine and CO₂ are mutually soluble. Mass transfer will occur between the phases when CO₂ is injected into an aquifer. However where the resident brine has become saturated with CO₂ – and the CO₂ is saturated with brine – there will be a region within the aquifer where immiscible displacement occurs. This will be in-between the near wellbore region and the leading edge of the CO₂ plume (Figure 1). This important immiscible displacement region is likely to be substantially larger in terms of aquifer volume than the wellbore or leading edge regions where there is significant phase transfer. It is therefore the critical region for assessing CO₂ storage potential.