Numerical simulation of counter-current spontaneous imbibition in water-wet fractured porous media: Influences of water injection velocity, fracture aperture, and grains geometry

Abstract

Counter-current spontaneous imbibition (SI), in which water and oil flow through the same face in opposite directions, is known as one of the most significant oil recovery mechanisms in naturally fractured reservoirs; however, this mechanism has not received much attention. Understanding the dynamic of water-oil displacement during counter-current SI is very challenging because of simultaneous impacts of multiple factors including geometry complexity and heterogeneity of naturally fractured reservoir materials, e.g., high permeability contrast between the rock matrix and fracture, wettability, and porosity. This study investigates the effects of water injection velocity, fracture aperture, and grain shape during counter-current SI at pore-scale. A robust finite element solver is used to solve the governing equations of multiphase flow, which are the coupled Navier–Stokes and Cahn–Hilliard phase-field equations. The results showed that the case with the highest injection velocity (uinj = 5 mm/s) recovered more than 15% of the matrix oil at the early times and then reached its ultimate recovery factor. However, in the case of the lowest injection velocity, i.e., uinj = 0.05 mm/s, the lowest imbibition rate was observed at the early times, but ultimately 23% of the matrix oil was recovered. The model with uinj = 5 mm/s was able to capture some pore-level mechanisms such as snap-off, oil film thinning, interface coalescence, and water film bridging. The obtained results revealed that changing the fracture aperture has a slight effect on the imbibition rate at the earlier times and ultimate recoveries would be almost equal. To assess the influences of grain shape on the imbibition process, the simulated domain was reconstructed with cubic grains. It was noticed that because of higher permeability and porosity, relatively larger oil drops were formed and resulted in higher oil recovery compared with the model with spherical grains. The developed model can be used as a basis for phase-field counter-current simulations and would be useful to study the qualitative and quantitative nature of this phenomenon.

Pore-scale study of counter-current imbibition in strongly water-wet fractured porous media using lattice Boltzmann method

“…At the solid surfaces, no-slip boundary condition is imposed by using the half-way bounceback scheme 53 and the desired contact angle is achieved by using the wetting boundary condition proposed by Xu et al 24 , which modifies the direction of the colour gradient ∇ρ N at the boundary to match the specified contact angle θ . These boundary conditions are to maintain a counter-current imbibition environment in which the oil in the matrix can only flow into the fracture that supplies water, which have been widely adopted in the previous numerical and experimental studies of counter-current imbibition 4,9,23,54 . Among various factors that influence the capillary imbibition, surface wettability is of vital importance for both imbibition rate and ultimate oil recovery, and its effect has been thoroughly investigated by Rokhforouz and Amiri 4 .…”

“…The counter-current imbibition occurs more often although it is less effective in terms of oil recovery. The spontaneous imbibition is known to be affected by many factors including the wettability 4 , the porosity, permeability and heterogeneity of the matrix [5][6][7][8][9][10][11] , the viscosity ratio of the wetting to non-wetting fluids 12,13 and their interfacial tension 14,15 , as well as initial saturation and boundary conditions 10,14,16 , etc. The spontaneous imbibition in conventional porous media has been extensively studied [17][18][19][20] due to its wide existence in various disciplines such as oil recovery, polymer composite manufacturing, soil science and subsurface hydrology.…”

“…Jafari et al 9 also used the phase-field method to investigate the effects of water injection velocity, fracture aperture, and grain shape during counter-current spontaneous imbibition. In these pore-scale simulations, the solid grains in the matrix are generally idealized as simple circular 4,9 or square cylinders 23 . However, the pore structures in natural rock matrix are extremely complex and irregular.…”

Review of studies on pore-network modeling of wettability effects on waterflood oil recovery

“…Fluid flow and storage in naturally fractured reservoirs are largely influenced by the geometry, complexity and heterogeneity of the reservoir, e.g., large permeability difference between the rock matrix and fracture, wettability, and porosity, fracture aperture, and grain shape, permeability pathways and/or impermeable barriers formed by the fracture network and their interaction with the host rock matrix (Rezaveisi et al, 2012;Leu et al, 2016;Jafari et al, 2017). The complex multiphase fluid flow behaviour of fractured reservoirs has traditionally proved difficult to predict, resulting in high degree of uncertainty in their characterization and economic development.…”

Investigation of Spontaneous Imbibition Behavior in a 3D Pore Space Under Reservoir Condition by Lattice Boltzmann Method

“…Several simulation methods can be used to investigate the multiphase flow dynamics in complex porous structures, such as computational fluid dynamics modeling (Jafari et al, 2017), pore network modeling (Qin et al, 2021;Valvatne & Blunt, 2004;Xie et al, 2017), and lattice Boltzmann method (LBM) (Ju et al, 2020;Li et al, 2016;H. Wang et al, 2019;Xie, Lv, & Wang, 2018).…”