Heterogeneous Pore Network Research Articles

Formation of Common Preferential Two‐Phase Displacement Pathways in Porous Media

AbstractIncluding specific interfacial area and saturation of the percolating phase into two‐phase porous media flow models, on the Darcy scale, enhances our ability to capture the physical properties of porous media flow more effectively. Using optical microscopy and microfluidic devices, we perform sequential drainage and imbibition experiments. The relevant processes, images, and boundary pressures are monitored, recorded, and logged at all times. For comparative purposes, two PDMS micromodels are used, one with an ortho‐canonical, homogeneous, and the other with a periodic heterogeneous pore network, with similar macro‐ but different pore‐scale properties. After processing the images, parameters like interfacial area belonging to percolating and non‐percolating phases and the corresponding phase saturations are determined. Our experimental results show that the relation between specific interfacial area and saturation of the percolating invading phase is a linear relationship with interesting properties. Additionally, after a number of fluid displacement processes (drainage and imbibition), and for both pore networks, unique flow paths for both phases are formed. We speculate that this happens due to the establishment of an effective porous medium, meaning a hydro‐dynamically active region within the pore space where the corresponding phase remains connected and flowing, where the capillary forces act as the guide for creating the “path of least resistance” in a highly viscous flow regime by keeping the non‐percolating phases in place. As the results can be specific to our experiments, more work needs to be done toward the potential generalization of these findings, especially in 3D flow domains.

Singular value decomposition for single-phase flow and cluster identification in heterogeneous pore networks

Singular value decomposition for single-phase flow and cluster identification in heterogeneous pore networks

Study of the Flow Field and Permeability Characteristics in the Goaf using the OpenPNM Package

The roof-caving step scale goaf behind the working face is sensitive to the region’s spontaneous combustion and gas concentration distribution, including many rock block cracks and holes. A severe deviation from the dynamics of fluids in porous media by representative element volume (REV), leading to the results of Computational Fluid Dynamics (CFD) simulation, has a significant error. A heterogeneous two-dimensional pore network model was established to simulate the goaf flow accurately. The network was first created using the simple cubic lattice in the OpenPNM package, and the spatial distribution of the “O-ring” bulking factor was mapped to the network. The bulking factor and Weibull distribution were combined to produce the size distribution of the pore and throat in the network. The constructed pore network model was performed with single-phase flow simulations. The study determined the pore structure parameters of the pore network through the goaf’s risked falling characteristics and described the flow field’s distribution characteristics in the goaf. The permeability coefficient increases as pore diameter, throat diameter, pore volume and throat volume increase and increases as throat length decreases. The correlation between throat volume and permeability coefficient is the highest, which indicates that the whole throat is the main control factor governing the air transport capacity in the goaf. These results may provide some guidelines for controlling thermodynamic disasters in the goaf.

Modeling Microscale Foam Propagation in a Heterogeneous Grain-Based Pore Network with the Pore-Filling Event Network Method

Foam flooding is an efficient and promising technology of enhanced oil recovery that significantly improves sweep efficiency of immiscible displacement processes by providing favorable mobility control on displacing fluids. Although the advantages in flexibility and efficiency are apparent, accurate prediction and effective control of foam flooding in field applications are still difficult to achieve due to the complexity in multiphase interactions. Also, conventional field-scale or mesoscale foam models are inadequate to simulate recent experimental findings in feasibility of foam injection in tight reservoirs. Microscale modeling of foam behavior has been applied to further connect those pore-scale interactions and mesoscale multiphase properties such as foam texture and the relative permeability of foam banks. Modification on a microscale foam model based on a pore-filling event network method is proposed to simulate its propagation in grain-based pore networks with varying degrees of heterogeneity. The impacts of foam injection strategy and oil-weakening phenomena are successfully incorporated. Corresponding microfluidic experiments are performed to validate the simulation results in dynamic displacement pattern as well as interfacial configuration. The proposed modeling method of foam propagation in grain-based networks successfully captures the effects of lamellae configurations corresponding to various foaming processes. The results of the simulation suggest that the wettability of rock has an impact on the relevance between reservoir heterogeneity and the formation of immobile foam banks, which supports the core idea of the recently proposed foam injection strategy in tight oil reservoirs with severe heterogeneity, that of focusing more on the IFT adjustment ability of foam, instead of arbitrarily pursuing high-quality strong foam restricted by permeability constraints.

An experimental study on the imbibition characteristics in heterogeneous porous medium

Capillary imbibition plays an essential role in the flow behavior of unconventional reservoirs. The severe heterogeneity of pore structures in unconventional formations can lead to different imbibition processes and flow dynamics compared to conventional reservoirs. This study investigates the imbibition process in heterogeneous pore networks by first examining the imbibition process between different pores using an ideal capillary model with interacting microchannel micromodels. The results reveal that water preferentially imbibes into small microchannels rather than large ones, and the imbibition velocity decreases with the microchannel width due to crossflow between different microchannels. Furthermore, heterogeneous matrix–fracture micromodels are used to examine the influence of boundary conditions, pressure conditions, and pore structure distribution on the imbibition process. The results show that the imbibition pattern is primarily governed by the boundary condition and is unaffected by the driving pressure condition. The conventional dimensionless time model fails to capture the spontaneous imbibition characteristics due to the interaction of different pores and the change in the imbibition pattern. Both increasing the injection pressure and increasing boundary openness can lead to higher oil recovery enhancement, and the distribution of the pore structure also influences the final oil recovery. Finally, the imbibition characteristics in the core scale are monitored using Nuclear Magnetic Resonance, demonstrating the similar phenomenon that water can imbibe into small pores and displace oil into larger pores. These findings enhance our understanding of the imbibition mechanism in heterogeneous porous media.

Activate the filtration capability of 3.5 nm ultra-large pores and eliminate the requirement of pore homogeneity for carbon nanotube array membranes

Activate the filtration capability of 3.5 nm ultra-large pores and eliminate the requirement of pore homogeneity for carbon nanotube array membranes

Study of void space structure and its influence on carbonate reservoir properties: X-ray microtomography, electron microscopy, and well testing

Study of void space structure and its influence on carbonate reservoir properties: X-ray microtomography, electron microscopy, and well testing

Evaluation of Gas Adsorption in Nanoporous Shale by Simplified Local Density Model Integrated with Pore Structure and Pore Size Distribution.

Simplified local density (SLD) model has been widely used to describe the gas adsorption behaviors in porous media. However, the slit pore geometry and constant pore width associated with the SLD model may fail to represent the heterogeneous pore network structure in shale. In this study, a new method to integrate the SLD model with the slit and cylindrical pore structures as well as the pore size distribution (PSD) is proposed and validated by the grand canonical Monte Carlo (GCMC) simulations and the experimentally measured adsorption of methane on shale with complex pore network. Comparison results show that reasonably good agreement is achieved between the SLD model and GCMC simulations for both the gas adsorption isotherms and discrete-density profiles in multiwalled carbon nanoslit and nanotube. The corresponding average absolute percentage deviations (% AADs) are below 0.3 and 9.3 for gas adsorption isotherm and discrete-density profile, respectively. In addition, the SLD model coupled with the PSD of slit and cylindrical pores ranging from micro- to macropores properly characterizes the measured excess adsorption of methane on Wolfcamp shale core sample with % AADs between 1.7 and 3.6. It is found that when the pore volume is fixed, the gas adsorption isotherm and gas density profile are heavily dependent on the pore geometry and pore size. Furthermore, integrating the PSD into the SLD model can guarantee the valid identification of the adsorbed- and free-gas regions in flow channels with different sizes based on the gas density profiles. The findings of this study shed light on the effects of pore structure on gas adsorption in nanopores and enable us to precisely evaluate and predict the gas adsorption behaviors in slit and cylindrical pores over a wide range of pore sizes.

A Scaling Approach for Retention Properties of Crystalline Rock: Case Study of the In‐Situ Long‐Term Sorption and Diffusion Experiment (LTDE‐SD) at the Äspö Hard Rock Laboratory in Sweden

AbstractThe in‐situ long‐term sorption and diffusion (LTDE‐SD) experiment performed at the Äspö Hard Rock Laboratory in Sweden aims at increasing knowledge of sorption and diffusion processes of radionuclides in natural fractures and granodiorite matrix under in‐situ conditions. This paper presents a comprehensive modeling exercise focusing on the scaling approach for sorption and diffusion parameters from laboratory to in‐situ conditions using the LTDE‐SD data set for six representative radionuclides. The near‐surface heterogeneities at both the fracture surface and rock matrix were evaluated by the conceptual model with high porosity, diffusivity, and sorption capacity, as well as their gradual changes at the near‐surface zones. The modeling results for non‐sorbing Cl‐36 and weak‐sorbing Na‐22 validated the model concept and parameter estimation of porosity and diffusivity by considering a 5‐mm‐thick disturbed zone with gradual parameter changes and an undisturbed matrix characterized by constant parameters. The effective diffusivities (De) of these cationic and anionic tracers showed typical cation excess and anion exclusion effects; however, the cation excess diffusion was more pronounced due to the heterogeneous pore networks in this granodiorite. The modeling results for high‐sorbing tracers (Cs‐137, Ra‐226, Ni‐63, and Np‐237) with different sorption mechanisms validated the scaling approaches of distribution coefficients (Kd) as a function of particle size and their relation to near‐surface disturbances. Sorption dependence on particle size could be understood in relation to different sorption mechanisms and mineralogical features. This modeling exercise provided key recommendations toward better scaling approaches for estimating reliable De and Kd parameters under in‐situ conditions.

Characterization of Pore Structures and Implications for Flow Transport Property of Tight Reservoirs: A Case Study of the Lucaogou Formation, Jimsar Sag, Junggar Basin, Northwestern China

Quantitate characterization of pore structures is fundamental to elucidate fluid flow in the porous media. Pore structures of the Lucaogou Formation in the Jimsar Sag were investigated using petrography, mercury intrusion capillary porosimetry (MICP) and X-ray computed tomography (X-ray μ-CT). MICP analyses demonstrate that the pore topological structure is characterized by segmented fractal dimensions. Fractal dimension of small pores (r < Rapex) ranges from 2.05 to 2.37, whereas fractal dimension of large pores (r > Rapex) varies from 2.91 to 5.44, indicating that fractal theory is inappropriate for the topological characterization of large pores using MICP. Pore volume of tight reservoirs ranges over nine orders of magnitude (10−1–108 μm3), which follows a power-law distribution. Fractal dimensions of pores larger than a lower bound vary from 1.66 to 2.32. Their consistence with MICP results suggests that it is an appropriate indicator for the complex and heterogeneous pore network. Larger connected pores are primary conductive pathways regardless of lithologies. The storage capacity depends largely on pore complexity and heterogeneity, which is negatively correlated with fractal dimension of pore network. The less heterogeneous the pore network is, the higher storage capability it would have; however, the effect of pore network heterogeneity on the transport capability is much more complicated.

Fluid transport through heterogeneous pore matrices: Multiscale simulation approaches

Fluids confined in nanopores exhibit several unique structural and dynamical characteristics that affect a number of applications in industry as well as natural phenomena. Understanding and predicting the complex fluid behavior under nano-confinement is therefore of key importance, and both experimental and computational approaches have been employed toward this goal. It is now feasible to employ both simulations and theoretical methods, the results of which can be validated by cutting-edge experimental quantification. Nevertheless, predicting fluid transport through heterogeneous pore networks at a scale large enough to be relevant for practical applications remains elusive because one should account for a variety of fluid–rock interactions, a wide range of confined fluid states, as well as pore-edge effects and the existence of preferential pathways, which, together with many other phenomena, affect the results. The aim of this Review is to overview the significance of molecular phenomena on fluid transport in nanoporous media, the capability and shortcomings of both molecular and continuum fluid modeling approaches, and recent progress in multiscale modeling of fluid transport. In our interpretation, a multiscale approach couples a molecular picture for fluid interactions with solid surfaces at the single nanopore level with hierarchical transport analysis through realistic heterogeneous pore networks to balance physical accuracy with computational expense. When possible, comparison against experiments is provided as a guiding roadmap for selecting the appropriate computational methods. The appropriateness of an approach is certainly related to the final application of interest, as different sectors will require different levels of precision in the predictions.

Development of nanocomposite collagen/ HA/β-TCP scaffolds with tailored gradient porosity and permeability using vitamin E.

The production of the biomimetic scaffolds with well-designed porosity parameters is a critical and challenging factor in biomaterials processing. The porosity parameters (i.e., pore size, pore shape, and distribution pattern) impact scaffold permeability, proteins/cells infiltration, and angiogenesis. This study introduced a new approach for the production of gradient porous nanocomposite scaffolds with controllable porosity and permeability using basic biomaterials of collagen and nanobiphasic calcium phosphate (nBCP) powder consisting of nano HA/β-TCP. A modified freeze-drying method (i.e., variables; collagen/nBCP ratio and quenching rates) was integrated for the first time with the chemical foaming method with the use of vitamin E as a potential surfactant and porogen. Vitamin E successfully increased the range of pore size, pore interconnection, and scaffold permeability. Further control of collagen/nBCP ratios and quenching rates allowed modulation of the pore morphology, total porosity, and the surface roughness of the scaffold. Scaffolds produced using vitamin E with collagen/nBCP ratio of 92/8% at -80°C quenching rate displayed a multimodal heterogeneous pore network with a wide range of pore sizes of mostly round/oval and polygonal pore morphology. Furthermore, these scaffolds revealed a more consistent gradient porous network with peripheral large pores-that gradually become smaller toward scaffold central-that produced a significantly higher permeability and better support of initial cellular performances. Accordingly, considering the various potentials of vitamin E, this study would provide promising insight into the production of smart and customized scaffolds for regenerative and therapeutic applications.

Spectrum of pore types and networks in the deep Cambrian to Lower Ordovician dolostones in Tarim Basin, China

Abstract The complex tectonic activities and multi-stage diagenetic modifications lead to the formation of a heterogeneous pore network spectrum ranging from macropores (centimeter-size) to nanometer-size pores ( Three types of matrix dolomites (dolomicrite, microbial dolomite and crystalline dolomites) and two types of dolomite cements (saddle dolomite or crystalline dolomites) are recognized in the Cambrian to Lower Ordovician dolostones. The pore systems include open vugs (enlarged dissolution pores), fabric dissolution pores, intercrystal (dissolution) pores, micro-fractures, as well as intracrystal pores, forming dual or multiple pore networks. The NMR T2 spectra are mainly characterized by bi-modal distributions and mostly right-skewed. The very low T2 components ( 1000 ms) represent the vuggy pores with pore sizes spanning from hundreds of μm to thousands of microns. The transition zones (100 ms

Estimating permeability in shales and other heterogeneous porous media: Deterministic vs. stochastic investigations

Abstract With increasing global energy demands, unconventional formations, such as shale rocks, are becoming an important source of natural gas. Extensive efforts focus on understanding the complex behavior of fluids (including their transport in the sub-surface) to maximize natural gas yields. Shale rocks are mudstones made up of organic and inorganic constituents of varying pore sizes (1-500 nm). With cutting-edge imaging technologies, detailed structural and chemical description of shale rocks can be obtained at different length scales. Using this knowledge to assess macroscopic properties, such as fluid permeability, remains challenging. Direct experimental measurements of permeability supply answers, but at elevated costs of time and resources. To complement these, computer simulations are widely available; however, they employ significant approximations, and a reliable methodology to estimate permeability in heterogeneous pore networks remains elusive. For this study, permeability predictions obtained by implementing two deterministic methods and one stochastic approach, using a kinetic Monte Carlo algorithm, are compared. This analysis focuses on the effects resulting from pore size distribution, the impact of micro- and macropores, and the effects of anisotropy (induced or naturally occurring) on the predicted matrix permeability. While considering multiple case studies, recommendations are provided on the optimal conditions under which each method can be used. Finally, a stochastic analysis is performed to estimate the permeability of an Eagle Ford shale sample using the kinetic Monte Carlo algorithm. Successful comparisons against experimental data demonstrate the appeal of the stochastic approach proposed.

A semi-experimental procedure for the estimation of permeability of microfluidic pore network

Microfluidic porous media systems are used for various applications ranging from chemical molecule detection to enhanced oil recovery studies. Absolute permeability data of the microfluidic porous media are important for those applications. However, it is a significant challenge to measure the permeability due to the difficulty in accurately measuring the ultra-low pressure drop across the pore network. This article presents a semi-experimental procedure to estimate the permeability of a microfluidic pore network. The total pressure drop across the porous media chip (ΔPchip) at a given flow rate of a single-phase liquid was obtained from the difference in the inlet pressures at the microfluidic pump with and without the pore network chip connected. The pressure drops in the inlet (ΔPinlet channel) and outlet (ΔPoutlet channel) channels of the pore network are estimated using the hydraulic resistance equation for Poiseuille flow in a wide rectangular cross section. Then the pressure drop across the pore network of the chip (ΔPpore network) is obtained by subtracting (ΔPinlet channel + ΔPoutlet channel) from ΔPchip. Subsequently the permeability of the pore network is calculated using the Darcy's law. •The proposed method is applicable for both homogenous and heterogeneous pore networks.•This method does not require a differential pressure sensor across the microfluidic chip.•This method eliminates the possibility of gas entrapment that can affect the permeability measurement.

Impact of hydrophobic surfaces on capillary wetting

Abstract The dynamic wetting behavior as first step in reconstitution of heterogeneous model food powders was investigated in the present work. Therefore, we studied the capillary rise of water into single pores and pore systems containing hydrophilic and hydrophobic walls. A two-wall setup was developed to realize the capillary rise in a single gap containing walls with two different contact angles but a constant pore size. The equilibrium penetration height was measured and compared to calculated heights using advancing contact angles inserted in a model equation. Wetting experiments in a Washburn setup were performed with respect to the investigation of water penetration into a heterogeneous pore network which leads to a variable pore size during the rise. In order to determine the effect of contact angle, three different glass materials at varied levels of hydrophobicity were prepared. Furthermore, a focus was put on understanding the impact of the pore network on the wetting kinetics by mixing two size fractions of glass beads. The equilibrium heights determined in heterogeneous single gaps fit well with the calculated heights by inserting advancing contact angles into the model equation. The results of the powder systems show a significant impact on the wetting with an increasing hydrophobic fraction, while an increased hydrophobic contact angle further increased the wetting time only at higher quantities of the hydrophobic component in the sample. Furthermore, our findings emphasize the complexity of wetting into a system with heterogeneity in pore size and contact angle.

Transport mechanisms in heterogeneous pore networks: from molecular to meso-scale observations

Transport mechanisms in heterogeneous pore networks: from molecular to meso-scale observations

Relative permeability prediction considering complex pore geometry and wetting characteristics in carbonate reservoirs

The measurement of relative permeability in carbonate rocks is very uncertain because of the complex pore system. Several equations for the relative permeability have been previously developed. However, most equations assume a single pore system that cannot be applied accurately. This study presents a relative permeability estimation method that considers capillary pressure, contact angle, pore size distribution, and residual oil saturation with respect to the heterogeneous pore network. As results, the capillary pressure was observed to have different tendencies for macropore and micropore. Also, the pore size distribution index of the macropore system was greater than the micropore system. For the macropore, water has a higher relative permeability than oil; a small reduction in oil saturation can easily flow water. In contrast, the micropore tends to follow the conventional relative permeability curve. Therefore, stable water displacement was not assured, leading to an early breakthrough for the hetero...

4-D imaging of sub-second dynamics in pore-scale processes using real-time synchrotron X-ray tomography

Abstract. A variable volume flow cell has been integrated with state-of-the-art ultra-high-speed synchrotron X-ray tomography imaging. The combination allows the first real-time (sub-second) capture of dynamic pore (micron)-scale fluid transport processes in 4-D (3-D + time). With 3-D data volumes acquired at up to 20 Hz, we perform in situ experiments that capture high-frequency pore-scale dynamics in 5–25 mm diameter samples with voxel (3-D equivalent of a pixel) resolutions of 2.5 to 3.8 µm. The data are free from motion artefacts and can be spatially registered or collected in the same orientation, making them suitable for detailed quantitative analysis of the dynamic fluid distribution pathways and processes. The methods presented here are capable of capturing a wide range of high-frequency nonequilibrium pore-scale processes including wetting, dilution, mixing, and reaction phenomena, without sacrificing significant spatial resolution. As well as fast streaming (continuous acquisition) at 20 Hz, they also allow larger-scale and longer-term experimental runs to be sampled intermittently at lower frequency (time-lapse imaging), benefiting from fast image acquisition rates to prevent motion blur in highly dynamic systems. This marks a major technical breakthrough for quantification of high-frequency pore-scale processes: processes that are critical for developing and validating more accurate multiscale flow models through spatially and temporally heterogeneous pore networks.

Matrix diffusion and sorption of Cs+, Na+, I– and HTO in granodiorite: Laboratory-scale results and their extrapolation to the in situ condition

Matrix diffusion and sorption are important processes controlling radionuclide transport in crystalline rocks. Such processes are typically studied in the laboratory using borehole core samples however there is still much uncertainty on the changes to rock transport properties during coring and decompression. It is therefore important to show how such laboratory-based results compare with in situ conditions. This paper focuses on laboratory-scale mechanistic understanding and how this can be extrapolated to in situ conditions as part of the Long Term Diffusion (LTD) project at the Grimsel Test Site, Switzerland. Diffusion and sorption of (137)Cs(+), (22)Na(+), (125)I(-) and tritiated water (HTO) in Grimsel granodiorite were studied using through-diffusion and batch sorption experiments. Effective diffusivities (De) of these tracers showed typical cation excess and anion exclusion effects and their salinity dependence, although the extent of these effects varied due to the heterogeneous pore networks in the crystalline rock samples. Rock capacity factors (α) and distribution coefficients (Kd) for Cs(+) and Na(+) were found to be sensitive to porewater salinity. Through-diffusion experiments indicated dual depth profiles for Cs(+) and Na(+) which could be explained by a near-surface Kd increment. A microscopic analysis indicated that this is caused by high porosity and sorption capacities in disturbed biotite minerals on the surface of the samples. The Kd values derived from the dual profiles are likely to correspond to Kd dependence on the grain sizes of crushed samples in the batch sorption experiments. The results of the in situ LTD experiments were interpreted reasonably well by using transport parameters derived from laboratory data and extrapolating them to in situ conditions. These comparative experimental and modelling studies provided a way to extrapolate from laboratory scale to in situ condition. It is well known that the difference in porosity between laboratory and in situ conditions is a key factor to scale laboratory-derived De to in situ conditions. We also show that cation excess diffusion is likely to be a key mechanism in crystalline rocks and that high Kd in the disturbed surfaces is critically important to evaluate transport in both laboratory and in situ tests.