Thermodynamic-force mixing activity and Darcy-scale flow-dissipation diagnostics in three-phase compositional flow through porous media

Miscible and near-miscible porous-media displacement is commonly described by fronts, phase-behavior thresholds, and phase-occupancy maps. These descriptors locate compositional advance but not thermodynamic-force mixing activity or its relation to Darcy-scale pressure-work dissipation. We combine conservative three-phase compositional transport, equation-of-state fugacity, and Darcy fluxes to diagnose the compositional transition envelope ΩZ, high mixing-activity region ΩS, and high pressure-work core ΩF. The framework is evaluated in one-dimensional displacement and two-dimensional continuous-gas injection, water-alternating-gas (WAG), and concentration-controlled gas-mobility-reduction calculations. The one-dimensional case shows that the relative-fugacity-gradient indicator Imμ follows the component transition more closely than the gas-occupancy front. Within the tested covariance family, all five realizations at each nonzero permeability contrast have a broader pore-volume-averaged transition envelope than the homogeneous reference. The largest resolved increase occurs between the homogeneous and lowest-contrast cases; subsequent increments remain within realization variability, and the activity-weighted position has no monotonic trend. Chance-corrected, phase-resolved analyses show that WAG cycles reorganize contact between pressure-work pathways and hydrocarbon bands retaining relative-fugacity gradients. Water slugs redirect mechanical dissipation to water-connected pathways, while high mixing activity persists in incompletely homogenized oil-rich bands. Gas-mobility reduction requires a larger phase-potential gradient to sustain the imposed flux and restricts gas-rich compositional advance, increasing pressure work while reducing envelope area and integrated mixing activity. Carrier-phase switching, compositional relaxation, and permeability connectivity therefore jointly govern flow–mixing coupling.