Coreflood Measurements of CO2 Trapping
- Rehab El-maghraby (Imperial College) | Christopher Holst Pentland (Shell Netherlands Natural Gas) | Martin Julian Blunt (Imperial College)
- Document ID
- Society of Petroleum Engineers
- SPE Annual Technical Conference and Exhibition, 30 October-2 November, Denver, Colorado, USA
- Publication Date
- Document Type
- Conference Paper
- 2011. Society of Petroleum Engineers
- 5.4.2 Gas Injection Methods, 4.3.4 Scale, 5.4.1 Waterflooding, 4.1.2 Separation and Treating, 5.1.1 Exploration, Development, Structural Geology, 5.5.2 Core Analysis, 5.4 Enhanced Recovery, 5.2 Reservoir Fluid Dynamics, 6.5.3 Waste Management, 6.5.7 Climate Change, 4.2.3 Materials and Corrosion, 4.1.5 Processing Equipment, 5.5 Reservoir Simulation
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We present a coreflood experiment that measures three properties that control the long-term fate of CO2 injected into the subsurface during carbon capture and storage (CCS): the residual saturation of CO2 after brine flooding as a function of initial saturation; the amount of CO2 that can dissolve in the brine; and the primary drainage capillary pressure. We employ a porous plate method to establish initial saturation, a stirred reactor ensures that the injected brine is pre-equilibrated with CO2 and we use isothermal de-pressurization to determine CO2 saturation. The fluid pressure was 9 MPa and the temperature varied between 33 and 70oC - the CO2 is in a super-critical (sc) phase at a temperature and pressure typical of likely storage aquifers with a density between 211 and 705 kgm-3. We find that significant quantities of the CO2 can be trapped, with residual saturations up to 35%; the variation of trapped saturation with initial saturation is accurately matched using the Spiteri et al. model (a quadratic function). We compare the results with experiments performed at similar conditions using decane as the non-wetting phase. More decane is trapped than CO2, suggesting that the CO2-brine systems are not completely water-wet.
We show that temperature (density) variation has no effect on the saturation of scCO2 that is residually trapped. The measured dissolution constant lies between 0.84 and 0.97 moles CO2/kg brine. The primary drainage capillary pressure is consistent with a strongly water-wet system and the same - to within experimental error - as that measured on an analogue decane-brine system.
Deep saline aquifers are promising sites for long term storage of CO2 [1-4] with a huge storage capacity . CO2 will likely be stored as a dense, supercritical phase [3, 6-7], as the injection site is likely to be deeper than 800 m; this will maximize the stored mass of CO2.
There are four mechanisms by which CO2 could be stored safely underground: structural or stratigraphic trapping; dissolution trapping; residual trapping; and mineral trapping . Structural and stratigraphic trapping prevents the buoyant scCO2 rising to the surface . In addition, some of the injected CO2 will dissolve in the brine over hundreds to thousands of years [3, 10-11]. Moreover, as the scCO2 plume travels away from the injector it leaves behind scCO2 bubbles trapped by capillary forces in the pore network of the rock [11-14]. Last, reaction with the host rock can transfer the CO2 to a more stable solid form as carbonates (mineral trapping) over thousands to millions of years [15,16].
In the literature there are few measurements of the amount of CO2 that could be residually trapped. Some papers [17-22] determined the end point of the scCO2/brine trapping curve, see . In previous work we measured the full trapping curve in Berea sandstone for one set of conditions (70oC and 9 MPa) . While significant trapping was observed, this could have been a result of the rather low CO2 density, 211 kgm-3, which is less than that at most putative storage sites: it has been suggested that CO2 interacts with the solid surface and alters the wettability of the rock away from strongly water-wet conditions resulting in less trapping  and it is not unreasonable to suggest that this effect could be a function of CO2 density. In this work we study trapping for a range of temperatures that allows a wide range of CO2 density to be studied - see Table 1.
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