Despite the volatility in the energy market, various predications have shown that the demand for oil and gas will continue to rise in the next a few decades. Hence, significant attention has been paid to increase hydrocarbon production using novel methods of enhanced oil recovery (EOR). Although numerous research work reported in the literature on the fundamental mechanisms of EOR, our understanding of multiphase flow behaviors is still very limited at the pore scale, especially for micro/nano EOR related processes.
Firstly, the effects of capillary number, Ca, wettability, viscosity and interfacial tension (IFT) on the oil/water flow characteristics in a capillary during the flooding process were numerically investigated using the computational fluid dynamics (CFD) method at the pore scale. Volume of fluid (VOF) model was used to capture the interface between oil and water in a pore-throat connecting structure. The results showed that during a water displacement process, an initial continuous oil phase could be snapped off in the water-wet pore due to the capillary effect. By altering the viscosity of the displacing fluid and the IFT between the wetting and non-wetting phases, the snapped-off phenomenon could be eliminated or reduced during the displacement. A flow chart was further developed relating the oil recovery factor with various flow states.
With a focus on the wettability, the second phase of this work looks at the atomistic scale using classical molecular dynamics (MD) simulation method to investigate the low salinity effect, which includes i) water salinity effect on the nano-scale water/oil interfacial equilibrium, ii) surface charge and water salinity effects on the wettability variation of a mineral surface, and iii) the wettability effect on oil displacement in a nano-pore via non-equilibrium molecular dynamics (NEMD) simulation. Increasing water salinity showed little effect on the wettability modification of a nano-pore comprised of neutral calcite surfaces, however it affected dipole-ion interactions significantly for charged surfaces, resulting in a more hydrophilic effect due to increased hydration effect of ions at higher salt concentrations. While a partially oil-wet neutral nonpolar calcite surface inhibited the movement of an oil droplet in the pore, greater oil mobility was achieved for dipolar nano-pores, especially at elevated salt concentrations.
Finally, to achieve both simulation efficiency and accuracy, a multi-scale hybrid CFD-MD coupling scheme for EOR applications was developed to resolve multi-scale features of multiphase fluid flow dynamics. An open source code, OpenFOAM, was employed for the continuum part of the simulation, and LAMMPS was adopted for the MD simulations. The coupling schemes and data interfaces were implemented through developing C++ codes based on the OpenFOAM interface. The accuracy of this multi-scale hybrid CFD-MD coupling model was firstly demonstrated through the simulation of Couette flow with LJ argon liquid. An example study was then conducted to probe the effect of water salinity on the multi-scale single phase and multiphase flow dynamics in a calcite nano-pore. The hybrid model showed its good capability to achieve both continuum and atomistic scale phenomenon with great speedup performance as compared to a fully-atomistic simulation.
Consequently, with the identification of the governing parameters and their effects, this work advances our understanding of EOR related multiphase flow processes at both microscale and nanoscale, applicable to both conventional (i.e. sandstone, carbonates) and non-conventional reservoirs (i.e., shale gas, shale oil). The established hybrid scheme allows the simulation extending to a large domain, achieving both atomistic insight and good computational efficiency for multiscale multiphase flow processes.
