Pattern Formation And Pore-scale Mechanisms Of Two-phase Flow In Porous And Fractured Media

Multiphase flow in porous and fractured media is a critical issue in many natural and industrial processes,including enhanced oil recovery,geologic CO₂sequestration,and groundwater contaminant migration and remediation.Heterogeneity of porous and fractured media and difference in the viscosity of the multiple fluids leads to the instability of fluid-fluid interface,known as fingering.The fingering would directly affect the flow structure and displacement efficiency,and thus has a critical impact on the geological CO₂storage efficiency and enhanced oil/gas recovery.In this thesis,we combined experiments and theoretical analysis to investigate the pattern formation and pore-scale mechanisms of two-phase flow in porous and fractured media.The major achievements of this study are summarized below:(1)We proposed a full phase diagram for immiscible displacement pat-terns in a rough fracture proposed and revealed the physical mechanism response for the onset of interface instability under favorable condition.Controlled by capillary and viscous forces,displacement patterns of one fluid dis-placing another more viscous one exhibit fingering flow pattern.Using a transpar-ent fracture-visualization system,we studied the immiscible displacement pattern in a rough fracture.We found that the displacement efficiency is a nonmontonic function of capillary number under the unfavourable condition(M<1,M is the viscosity ratio).Experimental results also showed that the interface instability lead to an earlier break-through at the compact-dominated regime under the favourable condition.Based on the force balance analysis of viscous force and capillary force,a full phase diagram for immiscible displacement pattern in a rough fracture was proposed and the mechanism of interface instability under favorable condition was revealed.(2)We investigated energy conversion in forced imbibition of two phase flow in a rough fracture,and found the mechanism of energy dissipation in micro-scale.By performing the forced imbibition experiment in a fracture-visualization system,we found that the pressure difference between inlet and outlet exhibits the characteristic of fluctuation,known as Haines jump.We observed that in this Haines jump the invading phase flows from the zone with small aperture to the large one.Furthermore,we studied the energy conversion amony surface energy,external work and dissipated energy,indicating that this fluctuation will significantly increase the local flow velocity in capillary-dominated regime,which dissipates 51?58%of surface energy.(3)We bridge the gap between energy conversion/dissipation and imbi-bition regime transition and consequently proposed a criterion for identify-ing imbibition regime for the respect of energy conversion.By analyzing energy balance in multiphase flow system,we find a fundamental link between regime transition and energy conversion.In capillary regime,surface energy is partially transformed into external work and dissipated energies;while in capillary-viscous regime,surface energy together with external work is transformed into kinetic and dissipated energies.This transition,corresponding to critical capillary number,is evidenced by quantitative analysis of invasion morphologies.Our work bridges the gap between energy conversion/dissipation and multiphase flow and a criterion for two-phase flow regime based on energy conversion was proposed.(4)We proposed a phase diagram for displacement patterns with con-sideration of pore-scale disorder and capillary number in porous media and revealed the pore-scale mechanism of interface instability controlling the patterns.Pore structure directly determines the magnitude of the capillary force,which in turn has a decisive influence on the displacement patterns.Using the microfluidic experiment,we studied the interface characteristic length and fractal dimension in the drainage process(??120°)and we showed that the displacement patterns shift from compact displacement to capillary fingering to viscous fingering to ordered dendritic as pore disorder and flow rate both increase.By performing strong imbibition experiments in microfluidics(??7°),we found that imbibition pattern shifts from compact capillary domain to liquid thin film to capillary/thin film to incomplete displacement as capillary number increases.Based on the analysis of the evolution of meniscus shapes at the pore-scale,a critical capillary number for the transition of the capillary fingering into the liquid thin film was determined.