Trapping of the non-wetting phase fluid, known as residual trapping, is one of the key mechanisms in geological storage of CO2. When modelling this process, the hysteretic behaviour in the relative permeability functions needs to be accounted for. Furthermore, an important parameter that needs to be considered, yet is commonly not taken into account, is the parameter of critical gas saturation (Sgc) (Egermann, Schaaf, and Bréfort 2010). Sgc determines at what saturation the trapped gas can be remobilized during pressure depletion (Bennion et al. 1998). Knowledge of this in turn affects the interpretation of any CO2 residual trapping experiments. Although a number of studies have addressed the effect of Sgc on gas recovery at the laboratory scale and a few even at the field scale, none of them has considered it in the context of determining CO2 residual trapping in situ. Small-scale push-pull injection experiments have been carried out at Heletz, Israel, to determine CO2 residual trapping in-situ in a 1.6 km deep reservoir layer. In the experiment carried out in 2017, partitioning tracer Krypton was used to determine the CO2 residual trapping (Niemi et al., 2020). This paper presents the interpretation of the experiment with a model accounting for the effect of Sgc. The simulations are performed with iTOUGH2 with the equation of state module EOS7C (Oldenburg et al. 2004), to which we have incorporated modifications to account for Sgc in the relative permeability functions. Effects of critical gas saturation on tracer breakthrough and other key parameters are analysed. The results provide a better understanding of how gas remobilization can influence CO2 residual trapping and partitioning tracer arrival in situ.
Keywords: residual trapping, geological CO2 storage, relative permeability, critical gas saturation, partitioning tracer
References:
Bennion, D Brant, F Brent Thomas, A K M Jamaluddin, and T Ma. 1998. “The Effect of Trapped Critical Fluid Saturations on Reservoir Permeability and Conformance.” In Annual Technical Meeting of the Petroleum Society of CIM, Calgary, Alberta, Canada, 8–10.
Egermann, P, T Schaaf, and B Bréfort. 2010. “A Modified Hysteresis Relative Permeability Including a Gas Remobilization Threshold for Better Production Forecasts of Gas Storages.” Petrophysics 51 (05).
Oldenburg, Curtis M, George J Moridis, Nicholas Spycher, and Karsten Pruess. 2004. “EOS7C Version 1.0: TOUGH2 Module for Carbon Dioxide or Nitrogen in Natural Gas (Methane) Reservoirs.”
Niemi, Auli, Jacob Bensabat, Saba Joodaki, Farzad Basirat, et al. 2020. “Characterizing CO2 Residual Trapping in-situ by means of Single-Well Push-Pull Experiments at Heletz, Israel, Pilot Injection Site –Experimental Procedures and Results of the Experiments” accepted with minor revision to International Journal of Greenhouse Gas Control.
