Understanding the interplay of capillary and viscous forces in CO2 core flooding experiments

Interaction between capillary and viscous forces significantly affects the flow instability in immiscible displacement, which is usually investigated by visualization of flow patterns in 2d porous micromodels or in 3d system equipped with X-ray CT. However, in most practical applications, visualization of flow in porous media is not possible and the pressure signal is often as one of the important sources of information. Core flooding experiments were implemented in this study to investigate the interplay of capillary and viscous effects by analysis of differential pressure. Water and crude oil were employed as defending fluid, and different states of CO2 were injected as invading fluid. The inlet was set as the constant injection flow rate while the outlet as the constant pressure. In viscous-dominated displacement, differential pressure evidently depends on the injection rate and the pressure decline curve is fitted by a power function. The exponent of the function is found to be significantly larger at the crossover between capillary-dominated and viscous-dominated regions. In capillary-dominated displacement, the pressure profile is characterized by a pressure jump at the beginning and intermittent fluctuations during the displacement. Further analysis by wavelet decomposition indicates a transition point existing in standard deviation of pressure fluctuations when the displacement is transformed from capillary-dominated to viscous-dominated. The experimental results are finally verified by a macroscopic capillary number, which characterizes the interaction between capillary and viscous forces at a critical value of Ncamacro∼1, agreeing well with the Log Nca-Log M phase diagram.