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Summary: Multicomponent multiphase fluid flows are commonly seen in the engineering practice of hydrocarbon production and transportation; thus, the phase‐wise heat and mass transfer mechanisms underneath the macroscopic flow and transport behaviors are essentially needed for better understanding of the physical phenomena and optimization of the industrial processes. Flash calculation, as the main approach computing phase equilibrium conditions, has arisen increasing interests to establish the thermodynamic foundations of multiphase flow simulation, as well as to determine whether two‐phase model is needed. In this paper, the general thermodynamically‐consistent flash calculation scheme will be developed, and the general adaptability to various special mechanisms will be analyzed. A unified framework of thermodynamics‐informed neural network will also be designed to accelerate conventional iterative flash calculation schemes that will be applied in various engineering scenarios to provide certain suggestions to the energy industry based on the predictions and analysis. Novelty Statement: A thermodynamically‐consistent flash calculation scheme incorporating various special mechanisms that are often met in energy industry.A unified thermodynamics‐informed neural network structure for various engineering demands in the energy industry.Suggestions to the energy industry to optimize the productions based on the phase transition predictions and analysis. [ABSTRACT FROM AUTHOR]

Over recent decades, studies in porous media have focused on many fields, typically in the development of oil and gas reservoirs. The imbibition phenomenon, a common mechanism affecting multi-phase flows in porous media, has shown more significant impacts on unconventional reservoir development, where the effect of the pore space increases with decreased pore sizes. In this paper, a comprehensive SPH method is applied, considering the binary interactions among the particles to study the imbibition phenomenon in porous media. The model is validated with physically meaningful results showing the effects of surface tension, contact angle, and pore structures. A heterogeneous porous medium is also constructed to study the effect of heterogeneity on the imbibition phenomenon; it can be referred from the results that the smaller pore throats and wetting surfaces are more preferred for the imbibition. The results show that the SPH method can be applied to solve the imbibition problems, but the unstable problem is still a sore point for the SPH method. [ABSTRACT FROM AUTHOR]

Droplet dynamics on a solid substrate is significantly influenced by surfactants. It remains a challenging task to model and simulate the moving contact line dynamics with soluble surfactants. In this work, we present a derivation of the phase-field moving contact line model with soluble surfactants through the first law of thermodynamics, associated thermodynamic relations and the Onsager variational principle. The derived thermodynamically consistent model consists of two Cahn–Hilliard type of equations governing the evolution of interface and surfactant concentration, the incompressible Navier–Stokes equations and the generalized Navier boundary condition for the moving contact line. With chemical potentials derived from the free energy functional, we analytically obtain certain equilibrium properties of surfactant adsorption, including equilibrium profiles for phase-field variables, the Langmuir isotherm and the equilibrium equation of state. A classical droplet spread case is used to numerically validate the moving contact line model and equilibrium properties of surfactant adsorption. The influence of surfactants on the contact line dynamics observed in our simulations is consistent with the results obtained using sharp interface models. Using the proposed model, we investigate the droplet dynamics with soluble surfactants on a chemically patterned surface. It is observed that droplets will form three typical flow states as a result of different surfactant bulk concentrations and defect strengths, specifically the coalescence mode, the non-coalescence mode and the detachment mode. In addition, a phase diagram for the three flow states is presented. Finally, we study the unbalanced Young stress acting on triple-phase contact points. The unbalanced Young stress could be a driving or resistance force, which is determined by the critical defect strength. [ABSTRACT FROM AUTHOR]

The modeling of multiphase fluid flow in porous medium is of interest in the field of reservoir simulation. The promising numerical methods in the literature are mostly based on the explicit or semi-implicit approach, which both have certain stability restrictions on the time step size. In this work, we introduce and study a scalable fully implicit solver for the simulation of two-phase flow in a porous medium with capillarity, gravity and compressibility, which is free from the limitations of the conventional methods. In the fully implicit framework, a mixed finite element method is applied to discretize the model equations for the spatial terms, and the implicit Backward Euler scheme with adaptive time stepping is used for the temporal integration. The resultant nonlinear system arising at each time step is solved in a monolithic way by using a Newton–Krylov type method. The corresponding linear system from the Newton iteration is large sparse, nonsymmetric and ill-conditioned, consequently posing a significant challenge to the fully implicit solver. To address this issue, the family of additive Schwarz preconditioners is taken into account to accelerate the convergence of the linear system, and thereby improves the robustness of the outer Newton method. Several test cases in one, two and three dimensions are used to validate the correctness of the scheme and examine the performance of the newly developed algorithm on parallel computers. [ABSTRACT FROM AUTHOR]

In this work, two models to simulate the single-phase multicomponent flow in reservoirs are introduced: single-phase multicomponent flow model and two-phase compositional flow model. Because the single-phase multicomponent flow is a special case of the two-phase compositional flow, the two-phase compositional flow model can also simulate the case. We compare and analyze the two models when simulating the single-phase multicomponent flow, and then demonstrate the equivalence of the two models mathematically. An experiment is also carried out to verify the equivalence of the two models. [ABSTRACT FROM AUTHOR]

In this work, we apply the experimenting pressure field technique to the problem of the flow of two or more immiscible phases in porous media. In this technique, a set of predefined pressure fields are introduced to the governing partial differential equations. This implies that the velocity vector field and the divergence at each cell of the solution mesh can be determined. However, since none of these fields is the true pressure field entailed by the boundary conditions and/or the source terms, the divergence at each cell will not be the correct one. Rather the residue which is the difference between the true divergence and the calculated one is obtained. These fields are designed such that these residuals are used to construct the matrix of coefficients of the pressure equation and the right-hand side. The experimenting pressure fields are generated in the solver routine and are fed to the different routines, which may be called physics routines, which return to the solver the elements of the matrix of coefficients. Therefore, this methodology separates the solver routines from the physics routines and therefore results in simpler, easy to construct, maintain, and update algorithms. [ABSTRACT FROM AUTHOR]

We analyze a combined method consisting of the mixed finite element method for pressure equation and the discontinuous Galerkin method for saturation equation for the coupled system of incompressible two-phase flow in porous media. The existence and uniqueness of numerical solutions are established under proper conditions by using a constructive approach. Optimal error estimates in L2( H1) for saturation and in L ∞ ( H(div)) for velocity are derived. Copyright © 2013 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Abstract: A new technique for the numerical solution of the partial differential equations governing transport phenomena in porous media is introduced. In this technique, the governing equations as depicted from the physics of the problem are used without extra manipulations. In other words, there is no need to reduce the number of governing equations by some sort of mathematical manipulations. This technique enables the separation of the physics part of the problem and the solver part, which makes coding more robust and could be used in several other applications with little or no modifications (e.g., multi-phase flow in porous media). In this method, one abandons the need to construct the coefficient matrix for the pressure equation. Alternatively, the coefficients are automatically generated within the solver routine. We show examples of using this technique to solving several flow problems in porous media. [Copyright &y& Elsevier]

Fundamental study on the pore scale dissociation and transportation mechanism of methane hydrate in porous sediments contributes to understanding the heat and mass transfer in the multiple physicochemical and thermal processes. This paper proposes a novel enthalpy-porosity technique coupling with volume of fraction (VOF) method for modeling the phase-change process of the hydrate dissociation and the multi-phase flow. The mathematical models are programed by C language and used as the subroutine for the commercial FLUENT software. The proposed theoretical model is validated by comparison with the experiment and numerical modeling in literature. The distribution of fluid saturation, velocity and temperature in the MH dissociation are presented, analyzed and discussed comparatively. As the first effort in literature, the proposed model can properly simulate the effects of the phase change on the pore structure evolution, multiphase flow, heat and mass transfer and kinetic reaction process in porous media in real-time. Both the normalized permeability of water (K Nw) and the normalized absolute permeability (K N) of the porous media with different hydrate saturation are acquired and analyzed comparatively with the results in the previous studies. This study provides a new insight into pore scale modeling on the multiphase flow with phase change. • The coupling enthalpy-porosity with VOF is proposed to model hydrate dissociation. • Pore structure evolution, heat & mass transfer in MH dissociation are studied. • K Nw and K N of the porous media with different hydrate saturation are acquired. • New insight into pore scale multiphase flow with phase change is achieved. [ABSTRACT FROM AUTHOR]

Over the past few decades, pore-network models (PNMs) have emerged as a pivotal tool in the investigation of fluid flow within porous media. The crux of PNM lies in the extraction of the topological structure of porous media, as abstracted from geological scans, commonly referred to as the pore network. Conventional methods for pore-network extraction rely on pixel-based techniques and necessitate high-quality images to accurately capture pore information. In recent times, the flashlight search medial axis (FSMA) algorithm has been introduced, offering a novel approach to extract pore networks within continuous spatial domains. This innovation enables the algorithm to operate independently of specific pixels, thereby significantly reducing computational complexity. Building upon the foundational principles of the FSMA algorithm, this paper presents an efficient search algorithm in conjunction with string methods. This algorithm facilitates the precise determination of pore and throat center locations within porous media using a minimal number of computational points and can accurately compute the positions of pore medians. Furthermore, this algorithm can effectively circumvent the issue of dead-end pores encountered in the FSMA algorithm, a feature of paramount importance in the study of multiphase flow within porous media. [ABSTRACT FROM AUTHOR]

Natural gas hydrate (NGH) has been regarded as a fossil fuel reserve for the future on account of its tremendous potential. The numerical modeling on NGH formation/dissociation mechanism contributes to better understanding its accumulation and distribution feature, and optimizing the development program. This paper aims to develop a new simulator for the NGH formation in the core-scale sandy sediments based on the computational fluids dynamic (CFD) methods. The mathematical model is established based on the kinetic reaction model of hydrate formation, the permeability reduction model by the NGH, model of heat and mass transfer in porous media. The hydrate formation model is programmed by C language, and used as a subroutine for Fluent software which is adopted to solve the governing equations of the multiphase flow. The simulator scheme is verified by comparison with the experiment and numerical simulation in literature. What's more, this study reproduces the same fluctuant tendency of temperature as the experiment during the 1.0 h–2.0 h for the first time. Different reaction surface models of NGH formation/dissociation are evaluated by the developed codes. The effects of the reaction surface of hydrate (RSH) model and the initial fluids distribution on the hydrate formation process are simulated and analyzed. The variation of the RSH in NGH formation/dissociation should be taken into consideration when modeling the hydrate re-formation in the exploitation of NGH. The initial distribution of water and gas has a great impact on the hydrate formation in the sealed reactor. The hydrate distribution is ununiform, even when assuming the water and methane are mixed uniformly in a homogeneous porous media. This study provides new insight for the parametric estimation of the RSH model in the hydrate formation and dissociation modeling. • A subroutine for Fluent is developed to model hydrate formation in porous media. • Different reaction surface models of NGH formation/dissociation are evaluated. • Effects of RSH model and initial fluids distribution on NGH formation are analyzed. • The heterogeneity of hydrate distribution synthesized in lab is studied. [ABSTRACT FROM AUTHOR]

With the ongoing exploration and development of oil and gas resources all around the world, applications of petrophysical methods in natural porous media have attracted great attention. This special issue collects a series of recent studies focused on the application of different petrophysical methods in reservoir characterization, especially for unconventional resources. Wide-ranging topics covered in the introduction include experimental studies, numerical modeling (fractal approach), and multiphase flow modeling/simulations. [ABSTRACT FROM AUTHOR]

The development of natural hydrogen is an emerging topic in the current energy transition trend. The production process involves compositional multiphase flow via subsurface porous media. This makes studying the compositional phase equilibrium behavior essential for reliable reservoir simulation and prediction. Herein, we develop an iterative flash calculation scheme and a deep learning algorithm using a thermodynamics-informed neural network (TINN) to perform accurate, robust, and fast phase equilibrium calculations for realistic fluid mixtures of natural hydrogen. The application of TINN architecture can accelerate the calculations for nearly 20 times. The effect of capillarity on phase equilibrium states is demonstrated. Based on simulation results, suggestions for the natural hydrogen industry chain are provided to control the possible phase transitions under certain environmental conditions that may be observed in the natural hydrogen reservoirs, storage and transportation facilities. The extremely low critical temperature of hydrogen challenges the robustness of flash calculations but facilitates the separation of impurities by liquefying certain undesired species. Moreover, phase transitions under control can be an effective approach for carbon dioxide capture and sequestration with optimized operating conditions over the phase equilibrium analysis. [ABSTRACT FROM AUTHOR]

In this paper we consider efficient and fully mass-conservative numerical methods for the multicomponent compressible single-phase Darcy flow in porous media. Compared with the classical IMplicit Pressure Explicit Concentration (IMPEC) scheme by which one of the components may be not mass-conservative, the new scheme enjoys an appealing feature that the conservation of mass is retained for each of the components. The pressure–velocity system is obtained by the summation of the discrete conservation equation for each component multiplying an unknown parameter which is nonlinearly dependent of the molar concentrations. This approach is quite different from the conventional method which is used in the classical IMPEC scheme. We utilize a fully mass-conservative iterative IMPEC method to solve the nonlinear system for molar concentration, pressure and velocity fields. The upwind mixed finite element methods are used to solve the pressure–velocity system. Although the Peng–Robinson equation of state (EOS) is utilized to describe the pressure as a function of the molar concentrations, our method is suitable for any type of EOS. Under some reasonable conditions, the iterative scheme can be proved to be convergent, and the molar concentration of each component is positivity-preserving. Several interesting examples of multicomponent compressible flow in porous media are presented to demonstrate the robustness of the new algorithm. [ABSTRACT FROM AUTHOR]

• The flow behavior of multicomponent shale oil is studied in the realistic kerogen nanochannel. • The fictitious slip boundary of shale oil flow is defined and verified. • The effects of pressure gradient, temperature and pore size are analyzed. • The velocity of shale oil tends to be stable within larger pore. • The branch chain of kerogen on the wall cause the peristaltic flow behavior. In the last decades, shale oil, mainly distributed in the nanopores of shale, has been considered as the representative of unconventional energy to alleviate the energy crisis. But the ideal pore models greatly overestimate the flowing capability of shale oil. To get more accurate and reasonable flow behavior, the multicomponent shale oil in the realistic kerogen channel is studied by using molecular dynamic simulation. Both density and velocity distributions that along and perpendicular to the flow direction are studied in kerogen channel, where the influence of branch chain of kerogen is also took into consideration. The heavy component tends to form the adsorbed layers on the kerogen wall, as a result of the extremely strong affinity between kerogen and hydrocarbons, and some asphaltene molecules in bulk phase form the cluster in middle of slit. On the flow direction, the velocity profile preforms the peristaltic behavior due to the effect of branch chain of kerogen, and the toluene and asphaltene components contribute it mostly. According to the heterogeneous characteristics of shale oil flow, we define the fictitious slip boundary, which corresponds to the boundary between bulk phase and adsorbed phase, to describe shale oil flow precisely. We also examine the effects of driving pressure gradient, temperature and pore size on the flow behaviors. The enhancements of driving force and temperature both facilitate shale oil flow, and the maximum velocity reaches the stable value within larger kerogen pores. The potential energy distribution and the interaction force contour verified the peristaltic flow behavior and the validity of fictitious slip boundary. [ABSTRACT FROM AUTHOR]

With the increasingly accurate modeling and simulation demands and techniques, obtaining the knowledge of the multi-component phase equilibrium states holds the bottleneck challenge in the study of multi-phase fluid flow. In order to resolve the shortages in computational efficiency and stability in the conventional iterative flash calculation, a new phase equilibrium prediction method is proposed by solving the saddle point of the multi-phase system. In this paper, the HiSD (High-index saddle point dynamics) algorithm is used for the first time to calculate the saddle points on the energy landscape of the two-phase two-component NVT flash model based on the Peng-Robinson equation of state, and the up-down search algorithm of HiSD is applied to generate the solution landscape of the flash system. The Rosenbrock-Euler ETD (exponential time differencing) format is involved to reduce the interference of system rigidity to the calculation. It can be referred from the numerical analysis that there are at most the 1 s t -order saddle points in the energy landscape of the two-component two-phase NVT flash system, and all these saddle points are located on one straight line of the hyperplane, where the energy is equal everywhere. All these 1 s t -order saddle points can converge to the same or equivalent local minima, which indicates that the two-component two-phase flash system is a system with only one single solution with physical meanings. In addition, the saddle points also obey a linear relationship and the energy remains the same at different temperatures. Therefore, using the method proposed in this paper, the conventional two-step efforts of phase stability test and phase separation calculation can be simplified. The 1 s t -order saddle points of the system can be directly calculated, reducing the need for an initial guess. The local minima can be directly searched through the downward direction of the saddle point, which greatly reduces the calculation amount of phase equilibrium calculations. Furthermore, the minimum states at different temperatures can be calculated in batch by using one certain initial value, which significantly improves the adaptability and reliability for complex engineering problems with drastic temperature changes. • The HISD algorithm used for the first time in saddle point calculation in NVT flash. • The Rosenbrock-Euler ETD format used to reduce the interference of system rigidity. • At most the 1st-order saddle points in the energy landscape of the 2-component system. • There is only one solution in the 2-component 2-phase NVT flash problem. • All the 1st-order saddle points can converge to the equivalent local minima. [ABSTRACT FROM AUTHOR]

As the first effort in literature, this paper conducts pore scale modeling on the methane hydrate dissociation and transportation in the reconstructed three-dimensional models of the MH-bearing sediment. The porous MH sample is synthesized using excess-gas method and imaged by micro-CT, which is used as input for the reconstructed mesh models. The real-time distribution of MH & water & methane, velocity and temperature is investigated. The effects of the temperature, pressure and flow rate of the injected water on MH dissociation and transportation are simulated and discussed. The results indicate that: 1) The hydrate generated by the excess - gas method is mainly cementing and mineral-coating on the sands surface, and occupies the small pores firstly. 2) The heterogeneity of the porous MH sediments is one of the key factors which influences the dissociation and the transportation process of the MH. 3) A lack of heat supply will restrict the dissociating rate of the MH reaching the maximum under the given PT conditions. 4) The gathering of the gas will decrease the flowing capacity of both water and methane. This study provides a new method to predict the multiple physicochemical and thermodynamical properties of the porous MH bearing sediments. • Pore scale MH dissociation and transportation in the real-shape porous media is modeled. • Real-time phase-change, multiphase flow, heat & mass transfer in porous media are studied. • Absolute and dynamic permeability of water & methane in the MH samples are predicted. • New method to predict petrophysical-thermal properties of MH sediments is provided. [ABSTRACT FROM AUTHOR]

Accurate characterization of surface roughness and understanding its influence on multiphase flow behavior are important for industrial and environmental applications such as enhanced oil recovery, CO 2 geological sequestration, and remediation of contaminated aquifers. Although some experimental and simulation studies have been conducted for investigating surface roughness in regular geometry structures, a more realistic description of roughness and its quantitative influence on multiphase flow need to be further explored. In this study, an optimized color-gradient lattice Boltzmann model is applied to simulate the steady-state two-phase flow in two-dimensional porous media modeled by a fourth-order Sierpinski carpet. The model is validated by comparing with the analytical solution and literature results, indicating reliability of our method. Then, rough surfaces with different roughness height and surface fractal dimension are characterized by a modified Weierstrass-Mandelbrot function and these effects on two-phase flow are investigated systematically by our model. The results show that the surface roughness has a negative effect on single-phase and two-phase fluid flow, which implies that the absolute and relative permeabilities for both wetting phase and nonwetting phase decreases with the increase of roughness height or surface fractal dimension. In addition, the surface roughness has influence on the two-phase distribution, velocity distribution and fluid-fluid/fluid-solid interface area, especially under the neutral wetting condition. Our study provides a pore-scale insight into the effect of surface roughness on two-phase flow, which is important for a fundamental understanding on macroscopic multiphase flow behaviors. [ABSTRACT FROM AUTHOR]