Similarity of trapping efficiencies for N₂, CO₂, H_2, and air in indiana limestone based on capillary gas displacement and spontaneous water imbibition experiments

The trapping coefficient is a parameter that describes the relationship between the displacement efficiency and capillary trapping of a gas in a porous medium. Accurate prediction of gas residual trapping is essential in evaluating underground carbon storage projects. The determination of the gas trapping coefficient could be complicated when a high-pressure and high-temperature experiment is needed to mimic subsurface conditions and when a complex gas is involved. The objective of this study is to investigate the role of gas type and operating conditions on trapping coefficient using Indiana limestone core samples with different permeability ranges and different gases such as CO₂, N₂, H₂, and air. A variety of displacement methods was employed such as a simple benchtop porous plate drainage chamber, an electrical resistivity-based saturation monitoring core flooding equipment, and a vapor desorption chamber to drain water-saturated rock samples to different levels of water saturation. The different methods displaced brine at different levels of capillary pressures. A benchtop spontaneous imbibition chamber was then used to spontaneously imbibe brine again under a capillary-dominated process until a residual gas saturation was attained in each case. A nuclear magnetic resonance (NMR) relaxation technique was used to monitor fluid distribution in the pores of the saturated samples, gas saturations after drainage, and the trapped gas saturations. An initial-residual saturation curve based on Land's (1968) trapping model was then used to compare the trapping coefficient of the rock samples to the different gases. The trapping coefficients of the rock samples were similar for the tested gases, although hydrogen exhibited a relatively lower residual trapping efficiency. This observation further elucidates previous results and confirms that the trapping coefficient is mainly a function of pore structure. However, this observation is not conclusive until further tests are completed on rocks with a wider range of petrophysical properties and under the same flow conditions.