Residual saturation: An experimental study of effect of gravity and capillarity during vertical and horizontal flow

Minimum residual oil or gas saturation is desired in any oil and gas field operation while maximum residual gas saturation is desired in gas (CO2) sequestration projects. In the literature, many studies have reported various factors that affect residual saturations such as wettability, interfacial tension, viscosity ratio, and injection scheme. There have also been reports of the directional dependence of residual saturation due to anisotropy. The anisotropy is believed to be caused by variation in rock minerals and/or properties in different directions. However, the effect of the interplay between gravity and capillarity in a low velocity flow away from the injection wells during gas sequestration in saline aquifer has often been neglected in many studies. Gas flows vertically upward and against gravity at a very low rate after they have been injected into the underground aquifer. During this flow process, some key parameters that are often ignored in the laboratory estimation of the relative permeability curves that are used to model and forecast the multiphase behavior, are present. They are gravity and capillary effects. In addition, vertical upward flow core flooding experiments are rarely performed when generating relative permeability curves in the lab for CO2 sequestration modeling. In this study, the influence of the interplay between gravity and capillary forces on residual saturation during flow in horizontal and vertical direction was investigated and compared. Series of core flooding experiments at reservoir flow conditions was performed in both horizontal and vertical flow direction on different rock samples of varying mineralogy and permeability. Results obtained so far indicate directional dependence of residual saturation even for homogeneous and isotropic rocks. Residual fluid saturation is higher when flow is in vertical direction as compared to horizontal flow direction. It was concluded that directional dependence of end saturation is not due only to heterogeneity but also due to the flow direction itself as observed in homogeneous and isotropic rocks tested.