Capillary forces can hold fluid within small or irregular pores after displacement begins. Interfacial tension at the boundary between fluids and the geometry of connected pore spaces promote the formation of disconnected regions. Once these regions lose continuity, an applied pressure gradient may no longer move them, increasing the amount of immobilized fluid.
Wettability influences how each fluid interacts with the surfaces of the porous medium and therefore affects where fluids remain during drainage or imbibition. Because surface preference changes the distribution of fluids within pore spaces, it also changes how readily regions become disconnected. Residual saturation consequently depends on the fluid, solid, and displacement conditions together.
Drainage and imbibition displace fluids in opposite directions, so they reorganize fluid regions differently within the pore network. Changes in capillary forces, interfacial tension, pore geometry, and wettability can alter which regions remain connected or become trapped. This means the retained-fluid outcome depends not only on the medium, but also on the displacement process.
Immobilization depends on whether capillary forces and interfacial tension can maintain disconnected fluid regions against the applied pressure gradient. Pore geometry and wettability also control the stability and distribution of those regions. When these effects prevent reconnection or movement, additional pressure may not produce further displacement, leaving residual saturation as an important flow-model parameter.
Engineers use residual saturation as a parameter describing the portion of pore volume that remains unavailable for further movement by the displaced fluid. Including it helps models represent the effects of trapped, disconnected regions rather than assuming complete displacement. More accurate values improve predictions of multiphase flow and support engineering decisions involving porous subsurface materials.
Estimation requires attention to the porous medium, the fluids involved, and the displacement history, including whether drainage or imbibition occurs. Engineers account for capillary forces, interfacial tension, pore geometry, and wettability because these controls determine fluid trapping. The resulting estimate can then be used to improve multiphase-flow models for the relevant system.
Residual saturation indicates how much displaced fluid may remain trapped in reservoir-rock pore spaces rather than continuing toward production pathways. This information helps engineers evaluate the limits of displacement and represent trapped fluids in recovery predictions. Better estimates support resource-extraction decisions by connecting pore-scale immobilization with expected multiphase-flow behavior.
In groundwater and soils, retained fluid affects predictions of contaminant transport through porous materials. In subsurface carbon dioxide storage, it helps engineers evaluate how much fluid may remain immobilized and how storage performance should be represented. Across these applications, improved estimates strengthen decisions about remediation and subsurface storage by refining multiphase-flow predictions.