Experimental and Numerical analysis for the effect of capillary number on two-phase flow in porous sandstone

Abstract:

The capillary number is essential clue to understand two-phase fluid-flow mechanism in porous material. In Carbon Capture and Storage (CCS) project, Capillary number becomes key-issue, because it is strongly related the accessibility of total pore space of the reservoir. In this study, we conducted two different types of investigations to discuss the effect of capillary number for high-permeable Mt. Simon sandstone (Φ:26.4%). First, we performed a CO₂ injection experiment and measure P-wave velocity and strain during injection. The Mt. Simon sandstone has high absolute-permeability (105 mD) and single modal pore-size distribution (peak size: 23 mm). The sample was cut into column shape (d:50mm, L50mm). In this study, we set three V_p-measurement lines by each 10 mm and two strain gages (vertical and horizontal) at same height (center of core). During CO₂ injection (drainage), we kept a constant flow-rate at 0.5ml/min and differential pressure under 120 kPa and injected totally 7.7 PV (200ml ) of CO₂. All V_p-measurement lines indicated small reduction (< 2%), simultaneously after 7.7PV CO₂ injection. It is obviously lower value. Some previous studies reported over 10 % V_p-reduction in drainage. On the other hand, both of strains data indicated the expansions over 2000με and 1400με, respectively. In generally, V_p-reduction is directly related the increasing of CO₂-saturation and it consider that there are no large-change of CO₂ saturations in the samples. However, strains indicate the existence of injected CO₂ in pore space. Thus, we assumed that the CO₂ makes a channel out of the Fresnel zones of all V_p-measurements lines. To elucidate this assumption, we discussed the CO₂ flow pattern by using TOUGH-2 simulation and confirmed the strong effect of flow–rate on the CO₂ flow pattern channels.