Phase-Field Modeling of Pore-Scale Oil Replacement by Spontaneous Imbibition in Fractured Porous Media

Abstract

Horizontal wells with complicated fracture networks have become a key technical measure to improve the oil recovery of low-permeability and tight reservoirs in China. Spontaneous imbibition is regarded as the major oil recovery mechanism after fracturing. Due to the limitation of the observation scale, the traditional experimental methods cannot accurately describe multiphase fluid flow in the micro-/nanopore space of low-permeability and tight reservoirs, and the pore-scale oil recovery mechanism during spontaneous imbibition was not clearly understood. In this study, a novel mathematical model of oil replacement by spontaneous imbibition in fractured porous media is developed and then numerically solved using the phase-field method. By comparing the numerical results with the analytical solution of single-tube capillary-driven flow, which is widely described by the classical Lucas–Washburn equation, the accuracy of the proposed method is validated. The effects of rock wettability, oil–water viscosity ratio, interfacial tension, and fracture network on oil imbibition recovery are further explored. The results demonstrate that the pore-scale dynamic events of oil droplets including snap-off and coalescence can be well observed. The stronger the degree of water-wet and the lower the oil–water viscosity ratio, the higher the oil imbibition recovery. The oil–water interfacial tension exerts little impact on the oil imbibition recovery, while it can significantly affect the imbibition time. As the oil–water interfacial tension decreases, the imbibition time will become longer. The existence of a fracture network can enlarge the contact area of oil–water exchange, thus greatly improving the oil imbibition recovery during spontaneous imbibition. It is concluded that the pressure difference between fracture and matrix is of particular importance to achieve a high oil imbibition recovery in fractured porous media. The above understandings can provide a theoretical basis for the efficient development of similar reservoirs.

Review of the Development Status and Technology of Tight Oil: Advances and Outlook

“…According to Schlumberger Business Consulting, the technically recoverable reserves tight oil worldwide are approximately 28–63 billion tons (as shown in Table ). − Time magazine believes that the world’s highest tight oil reserves are 42 billion tons . BP pointed out that North America has approximately 10 billion tons of tight oil resources, making it the largest region in the world with tight oil resources .…”

Pore-Scale Modeling of Pressure-Driven Flow and Spontaneous Imbibition in Fracturing-Shut-In-Flowback Process of Tight Oil Reservoirs

“…Previous studies [4][5][6] indicated that spontaneous imbibition is one of the primary underlying mechanisms for crude oil extraction in the tight oil reservoir. In recent years, a large number of researchers have adopted the development method of "horizontal well + volume fracturing + shut-in well" to develop tight oil, which involves immediately shutting down the well after volume fracturing and shutting it for a period of time before opening the well for production.…”

Spontaneous Imbibition in Nanomatrix–Fracture of Low Permeability Using Multiscale Nanofluidic Chips

“…However, numerical simulations of multiphase flow have some limitations. For instance, the modeling process is restricted by severe computational space . It often discretizes complex porous structures into connected pore elements, which are affected by the element’s density and size.…”

“…In previous studies, extensive explorations of the imbibition mechanisms were conducted through numerical simulation, core experiments, and microfluidic models. In numerical simulations, various modeling methods have been employed to investigate the imbibition process, including lattice Boltzmann, , molecular dynamics, pore network model, , phase-field modeling, and so on. The numerical simulation makes a remarkable contribution to providing a more precise depiction of fluid dynamics on the pore scale.…”

“…For instance, the modeling process is restricted by severe computational space. 14 It often discretizes complex porous structures into connected pore elements, which are affected by the element's density and size. Consequently, experiments are also essential components in studying the mechanisms of imbibition.…”