The Behavior of N2 and CO2 Foams under Varying Pressure Conditions

Abstract

Abstract Knowledge of how CO2 foam behaves when flowing through rock is important for many CO2 Capture, Utilization and Storage (CCUS) applications. Typically, laboratory foam flow tests are carried out at relatively low pressures as compared to reservoir pressures. However, the possibility of foam behavior changing with pressure introduces uncertainty in results that are extrapolated up to reservoir pressure. The current work consists of a series of coreflood experiments to investigate how system pressure affects the behavior of both nitrogen and CO2 foams flowing through Bentheimer sandstone. For each gas, the system pressures were varied in steps from 5 bar up to 200 bar, with values of 5, 50, 100, 150, and 200 bar for the nitrogen tests, and 5, 25, 50, 70, 80, 120, 160, and 200 bar for the CO2 tests. Foam-quality scans were carried out at each pressure step to provide full data on the foam behavior at each individual pressure. Implicit-texture foam model parameters were then fitted for each pressure step. Two corefloods were also carried out in the CT scanner to determine the water saturations within the core and consider the variation in water saturation Sw with injected foam quality and injected gas type─nitrogen or CO2. It was found that for nitrogen, the apparent foam viscosity was only weakly affected by pressure, with an increasing pressure giving only a small increase in the maximum apparent viscosity. In contrast, the foam flow behavior of CO2 changed significantly with pressure, and it is suggested that this variation is linked to four factors that vary strongly with pressure: 1) CO2 density, 2) the interfacial tension between the CO2 and the surfactant solution, 3) the effect of CO2 chemistry on surfactant behavior, and 4) the contact angle of the CO2 on the rock. These four factors are all pressure-dependent and have the potential to affect foam generation, trapped-gas fraction, and foam stability.