Capillary Trapping In Carbonate Rocks
- Celine Marie Marguerit Lamy (Institut Francais du Petrole IFP) | Stefan Iglauer (Imperial College) | Christopher Holst Pentland (Imperial College) | Martin Julian Blunt (Imperial College) | Geoffrey Colin Maitland (Imperial College)
- Document ID
- Society of Petroleum Engineers
- SPE EUROPEC/EAGE Annual Conference and Exhibition, 14-17 June, Barcelona, Spain
- Publication Date
- Document Type
- Conference Paper
- 2010. Society of Petroleum Engineers
- 5.1 Reservoir Characterisation, 5.2.1 Phase Behavior and PVT Measurements, 6.5.7 Climate Change, 5.8.7 Carbonate Reservoir, 5.4.1 Waterflooding, 4.3.4 Scale, 4.1.2 Separation and Treating, 5.5 Reservoir Simulation, 1.6.9 Coring, Fishing, 5.3.4 Reduction of Residual Oil Saturation, 6.5.3 Waste Management, 1.2.3 Rock properties, 6.5.1 Air Emissions, 1.8.5 Phase Trapping
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Carbonate reservoirs represent a possible geological storage option for carbon dioxide from anthropogenic sources. We conducted capillary trapping experiments on different carbonate rocks to assess their suitability for storage. We measured the trapped non-wetting phase saturation as a function of the initial non-wetting phase saturation and porosity. We used refined oil - with a density similar to that of supercritical CO2 - as the non-wetting phase and brine as the wetting phase. The experiments were performed at ambient temperature and slightly elevated pressures. Saturations were determined by mass and volume balance. We found that the trapped non-wetting phase saturation rises approximately linearly with initial saturation. The porosity was shown to have a significant effect on both initial saturation and residual saturation.The influence of effective stress was also investigated. It was shown that carbonates have significantly different stress behavior compared to sandstones. As the pressure of the non-wetting phase increases during primary drainage, the initial oil saturation increases to a maximum value and then decreases, as the fluid pressure affects the pore structure of the rock.
Climate change caused principally by anthropogenic emissions of carbon dioxide (CO2) is a major societal concern (IPCC, 2005). One way to reduce CO2 emissions to the atmosphere is to capture and store the CO2 emitted (carbon capture and storage or CCS) in geological formations (IPCC, 2005; Orr, 2004; Bachu et al., 1994). In this context, simulation studies have proposed that capillary trapping - where CO2 as the non-wetting phase is isolated in pore-space bubbles surrounded by water - is a rapid and effective way to store the CO2 safely (Qi et al., 2007; Qi et al., 2008; Qi et al., 2009; Kumar et al., 2005; Obi and Blunt, 2006; Hesse et al., 2008; Juanes et al., 2006). The same process occurs during oil and gas production by water flooding - here it is the hydrocarbon that is trapped. More than 60% of the world's oil reserves and 40% of the gas reserves are held in carbonate reservoirs of which the majority is in the Middle East (Scholle and Ulmer-Scholle, 2003). These structures are also possible storage sites - either depleted hydrocarbon fields or geologically similar deep saline aquifers.
While capillary trapping in siliclastic material has been extensively studied (a review of the literature is available - Pentland et al., 2008), fewer data are available for carbonate systems - Figure 1. The literature includes data from gas-oil experiments where drainage was achieved by evaporation (Keelan and Pugh, 1975), oil-water coreflood experiments (Kamath et al., 2001), and gas-water coreflood experiments (Maloney and Zornes, 2003). The broad distribution of residual non-wetting phase saturations, S(nw)r, for a given initial saturation,S(nw)i, can be explained in terms of different fluid systems, different displacement mechanisms, and the variation in pore structures investigated. No clear trend emerges from the combined data - although some authors (Maloney and Zornes, 2003) proposed a relationship between S(nw)i and S(nw)r based upon the Land correlation (Land, 1968).
To investigate capillary trapping in carbonates further and to determine the functional trapping relationship, we measured the trapped saturation as a function of porosity, initial saturation and stress state for four different carbonate rocks and a carbonate sandpack.We establish initial and residual saturations in the core by displacement using the porous plate method which is deemed more representative of reservoir flow than evaporation of the non-wetting phase.
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