Pore Fluid Research Articles

A Method to Identify Pore Fluids in Heterogeneous Conglomerate Reservoirs Using a Nuclear Magnetic Resonance Log with Oil-Based Mud Invasion

Summary Nuclear magnetic resonance (NMR) logging has been extensively utilized for identifying pore fluids in recent years. However, after oil-based mud (OBM) invasion, OBM filtrate partially takes the place of the original fluid in the reservoir and the morphology of the NMR T2 (transverse relaxation time) spectrum changes. OBM invasion makes it difficult to identify the formation fluid using routine NMR fluid identification methods. In this study, we took heterogeneous conglomerate formation in the northwestern Junggar Basin as a case study and carried out core experiments under four conditions (viz., water-saturated, OBM displacing water, oil-saturated, and OBM displacing oil) and simulated the states of OBM invasion into water and hydrocarbon-bearing formation. Comparative analysis finds that when the OBM invades the water layer, the movable peak of the T2 spectrum primarily reflects the bulk relaxation characteristics of OBM filtrate, whereas when the OBM invades into the oil layer, the T2 spectrum may exhibit a three-peak distribution, where the first peak mainly indicates irreducible water, the second peak reflects OBM filtrate, and the third peak primarily reflects nondisplaced oil. To facilitate fluid identification, two T2 cutoffs are adopted to divide the T2 spectrum into three segments, and combined with the T2 geometric average, a fluid identification factor (ifluid) is proposed. Finally, identification criteria for reservoir type are established on the NMR logging and drillstem test data. The field application verifies the reliability of the proposed methods. These methods realize the identification of oil layers under OBM drilling and guide the subsequent production and development of oil reservoirs.

State of the Art on Fe Precipitation in Porous Media: Hydrogeochemical Processes and Evolving Parameters

The mixing of terrestrial groundwater and seawater creates dynamic reaction zones in intertidal areas, where land-derived Fe(II) is oxidized to Fe(III) and then precipitates as Fe hydroxides at the groundwater–seawater interface. These hydrogeochemical processes contribute to the formation of iron bands at the saltwater wedge (SW) and beneath the upper saline plume (USP). This study provides a comprehensive review of physical and geochemical processes at field scale in coastal areas, explores the impact of mineral precipitation on pore structure at pore scale, and synthesizes reactive transport modeling (RTM) approaches for illustrating continuum-scale soil physio-chemical parameters during the evolution of porous media. Upon this review, knowledge gaps and research needs are identified. Additionally, challenges and opportunities are presented. Therefore, we reach the conclusion that the incorporation of observational data into a comprehensive physico-mathematical model becomes imperative for capturing the pore-scale processes in porous media and their influence on groundwater flow and solute transport at large scales. Additionally, a synergistic approach, integrating pore-scale modeling and non-invasive imaging, is equally essential for providing detailed insights into intricate fluid–pore–solid interactions for future studies, as well as facilitating the development of regional engineering-scale models and physio-chemical coupled models with diverse applications in marine science and engineering.

Effect of temperature and salt concentration of pore fluid on the behaviour of an expansive soil during drying and wetting cycles

ABSTRACT A study was conducted on the effects of temperature and salt concentration of pore fluid on the properties of an expansive soil during drying and wetting cycles. The experimental tests were performed on modified and conventional odometers at constant and variable temperatures under 10 kPa surcharge pressure. Distilled water and solutions of sodium chloride and calcium chloride with concentration of 250 g/litre were used for making samples and distilled water was used for inundating samples during the wetting stage. The results showed that the swelling potential is decreased with increasing the number of cycles for both constant and variable temperature conditions. It was revealed that the potential of swelling is a function of salt concentration of pore fluid of the sample and the effect of temperature on the potential of shrinkage and swelling is negligible. The variation of void ratio with water content is dependent on the salt concentration of pore fluid and the position of void ratio-water content curve is a function of temperature. The results of scanning electron microscopy (SEM) tests showed that the structure of soil is changed during drying and wetting cycles which results in the reduction of potential of shrinkage and swelling.

Evolution of anisotropic capillarity in unsaturated granular media within the pendular regime

While the shear behavior of granular soils is directly related to the microstructure of contacts which often leads to the coaxiality between Cauchy stress and Satake fabric tensors, it is generally accepted by the geomechanics and geotechnical engineering community that the capillary effects are isotropic. At low saturation levels, however, the pore fluid tends to form interparticle menisci that can also manifest an anisotropic structure, which may result in the development of anisotropic capillarity in wetted granular media. To study the interplay between the solid grain contacts and the liquid bridges at the micro-scales, this study adopts a coupled discrete element method that utilizes a linear contact model combined with a capillary model, and explores their effects by conducting a series of numerical experiments. The distributions of contact and capillary force orientations during the experiment are further investigated to better understand how their alignments affect the global response of the granular assembly subjected to a deviatoric loading. The results indicate that the global shear stress response is not only affected by the contact fabric but also by the network of liquid bridges, and we also observe that the particles may lose contact while the pendular menisci may not be destroyed during the elastic unloading.

An Experimental Investigation into the Role of an In Situ Microemulsion for Enhancing Oil Recovery in Tight Formations

Surfactant huff-n-puff (HnP) has been shown to be an effective protocol to improve oil recovery in tight and ultratight reservoirs. The success of surfactant HnP for enhanced oil recovery (EOR) process depends on the efficiency of the designed chemical formula, as the formation of an in situ microemulsion by surfactant injection is considered to be the most desirable condition for achieving an ultra-low interfacial tension during the HnP process. In this work, we conducted experimental studies on the mechanism of in situ microemulsion EOR in the Mahu tight oil reservoir. Salinity scan experiments were carried out to compare different surfactants with crude oil from the Mahu reservoir, starting with the assessment of surfactant micellar solutions for their ability to form microemulsions with Mahu crude oil and examining the interfacial characteristics. Subsequently, detailed micromodels representing millimeter-scale fractures, micron-scale pores, and nano-scale channels were utilized to study the imbibition and flowback of various surfactant micellar solutions. Observations of the in situ microemulsion system revealed the mechanisms behind the enhanced oil recovery, which was the emulsification’s near-miscibility effect leading to microemulsion formation and its performance under low-interfacial-tension conditions. During the injection process, notable improvements in the micro-scale pore throat heterogeneity were observed, which improved the pore fluid mobility. The flowback phase improved the channeling between the different media, promoting a uniform movement of the oil–water interface and aiding in the recovery of a significant amount of the oil phase permeability.

Investigation of kinetics and mechanisms of metallic beryllium corrosion for the management of radioactive wastes

Activated beryllium wastes are produced by the nuclear industry and have to be managed as radioactive waste during decommissioning and dismantling of nuclear facilities. One potential approach to the management of these wastes is their encapsulation and stabilization in cementitious matrices. The main issue with this conditioning is aqueous corrosion, leading to the hydrogen production and potential crack formation leading to a loss of confinement. To evaluate the suitability of different cement formulations, such as Ordinary Portland Cement (OPC) or magnesium phosphate cement for encapsulation of metallic beryllium, the corrosion behaviour of beryllium metal was investigated in solutions with different pHs. In alkaline solutions representative of OPC pore fluids (i.e. NaOH solutions with pH between 12.5 and 14), corrosion rates increase drastically with increasing pH. Investigations of the surface of the corroded beryllium samples by scanning electron microscopy indicate that pitting corrosion is the main corrosion mechanism under these conditions.Graphical abstract

A thermo-hydromechanical damage model and its application to a deep geological radioactive repository

This study conducts a numerical analysis of long-term thermo-hydromechanical (THM) processes, with a particular focus on deep geological disposal of radioactive waste. It emphasizes the modeling of damage zones resulting from excavation activities, as well as changes in pore pressure and temperature. The numerical model presented in this paper delineates the fundamental relations of thermo-poro-elasticity. It also introduces a double phase field model specifically formulated for rock materials under compressive stress, taking into account THM coupling and time-dependent behavior. The proposed model has been implemented in a finite element code and applied to numerical analysis of short and long terms responses around a horizontal borehole in the context of the French High-Level Waste (HLW) disposal concept. In particular, displacements, temperature changes, pore fluid pressure variations as well as evolution of induced damage zones are presented and analyzed for different periods including excavation, operation and post-closure. It is found that the occurrence of damage zones is clearly related to the spacing distance between two neighboring disposal boreholes. The time-dependent deformation of host rock plays an significant role in the long-term responses.

Predicting sandstone water abundance using seismic dispersion attribute inversion: A case study of Yuwang coal mine, China

AbstractPredicting the water abundance of coal‐bearing strata is crucial for ensuring mining safety. However, owing to the dispersion and attenuation characteristics caused by pore fluid flow, it is difficult to estimate the water abundance of coal seam roof aquifers using seismic data. To overcome this challenge, we provide the relationship between the frequency‐dependent seismic wave velocity and water saturation based on the Chapman fracture model and the mixing fluid model. We propose a seismic dispersion attribute technique that can use dispersion information as an indicator of water abundance. Numerical experiment results show that the water saturation of the sandstone aquifer is positively correlated with the dispersion attribute. The results of low‐frequency rock physical experiments are roughly consistent with those predicted by the model for the given parameters. Using seismic dispersion attribute inversion and the frequency slice wavelet transform method, we predicted the water abundance of sandstone in the coal seam roof of the Yuwang coal mine in Yunnan Province, China. The predicted sandstone water abundance was compatible with the actual water‐rich scenario observed in well logs and downhole drilling in the study area. Therefore, the method proposed herein has the potential to quantitatively determine the water abundances of sandstone aquifers in coal seam roofs.

Thermodynamically consistent numerical modeling of immiscible two‐phase flow in poro‐viscoelastic media

AbstractNumerical modeling of immiscible two‐phase flow in deformable porous media has become increasingly significant due to its applications in oil reservoir engineering, geotechnical engineering and many others. The coupling between two‐phase flow and geomechanics gives rise to a major challenge to the development of physically consistent mathematical models and effective numerical methods. In this article, based on the concept of free energies and guided by the second law of thermodynamics, we derive a thermodynamically consistent mathematical model for immiscible two‐phase flow in poro‐viscoelastic media. The model uses the fluid and solid free energies to characterize the fluid capillarity and solid skeleton elasticity, so that it rigorously follows an energy dissipation law. The thermodynamically consistent formulation of the pore fluid pressure is naturally derived for the solid mechanical equilibrium equation. Additionally, the model ensures the mass conservation law for both fluids and solids. For numerical approximation of the model, we propose an energy stable and mass conservative numerical method. The method herein inherits the energy dissipation law through appropriate energy approaches and subtle treatments for the coupling between two phase saturations, the effective pore pressure and porosity. Using the locally conservative cell‐centered finite difference methods on staggered grids with the upwind strategies for saturations and porosity, we construct the fully discrete scheme, which has the ability to conserve the masses of both fluids and solids as well as preserve the energy dissipation law at the fully discrete level. In particular, the proposed method is an unbiased algorithm, that is, treating the wetting phase, the non‐wetting phase and the solid phase in the same way. Numerical results are also given to validate and verify the features of the proposed model and numerical method.

Geochemical characteristics of hot springs in active fault zones within the northern Sichuan-Yunnan block: Geochemical evidence for tectonic activity

The geochemistry of the fluids is spatially closely related to faults and earthquakes that have been detected in the northern Sichuan-Yunnan (SC-YN) block. However, the processes and factors that control hydrothermal fluid circulation on a regional scale remain challenging. The geochemical circulation of the hot springs integrated with geological and geophysical data revealed the processes and controlling factors of the relationship between fluid circulation and tectonic activity. Our results showed that the hydrochemical type of the hot springs at the intersection of the Yushu-Ganzi fault (YS-GZF), Xianshuihe fault (XSHF) and Yunongxi fault (YNXF), Jinshajiang fault (JSJF) and Batang fault (BTF) as well as Ganzi-Litang fault (GZ-LTF) is composed of Na-HCO3 and Na-HCO3·Cl. It demonstrated higher reservoir temperature and circulation depth (5 − 7 km). The helium isotope (0.22 − 2.36 Ra) of the gas indicated that the mantle helium composition was up to 5 − 30 %. These hot springs may undergo water–rock-gas exchange reactions with deep fluids. Considering geological and geophysical background as well as previous studies, the geochemical characteristics of the hot spring at the intersections of faults in the north of the SC-YN block are thought to be controlled by the deep-driven upwelling lower crustal flows, mixing with shallow hot spring water and the temporal and spatial heterogeneity of the high strain fields, slip rates, and permeability of the fault intersections. Whereas, the transport of fluids in hot springs of fault may increase pore fluid pressure, and reduce the effective stress at the intersection of the fault zones, which can promote fault sliding and provoke seismic events as well as fault movements. It could result in the enhancement of regional tectonic activity. This study provides an important basis for seismic prediction with hot spring hydrochemistry stations.

The effects of pore structure on wave dispersion and attenuation due to squirt flow: A dynamic stress-strain simulation on a simple digital pore-crack model

Squirt flow, a phenomenon typically observed in porous cracked rocks, occurs due to the contrasting compressibility between the pores and cracks, leading to the pore pressure diffusion and dissipation of wave energy. Understanding the influence of pore structure on wave dispersion and attenuation signatures due to squirt flow is essential for interpreting seismic and sonic logging data in various fields of earth and energy sciences, such as hydrocarbon exploration, geothermal energy exploitation, and CO2 sequestration. In this study, we construct a simple digital pore-crack model with varying pore structures and use a dynamic stress-strain simulation approach to characterize wave dispersion and attenuation signatures due to squirt flow. Numerical simulation suggests that, in addition to the commonly considered parameters such as porosity, crack density, and crack aspect ratio, additional pore structure parameters, such as pore size, pore aspect ratio, crack orientation, crack length, and crack width, significantly affect the wave dispersion and attenuation signatures. Increasing the pore size leads to the pronounced enhancement of attenuation magnitude and a decrease in characteristic frequency. We demonstrate that variations in crack length have a pronounced impact on attenuation magnitude, whereas crack width is decisive in controlling the characteristic frequency. Furthermore, it is found that the saturation paths (the gas filling the pore first or gas filling the crack first) considerably affect the velocity-saturation and attenuation-saturation relationship, suggesting that the coupling effects of pore structure and fluid distribution complicate the fluid pressure diffusion and wave attenuation behaviors. The presented results offer insights for deciphering pore structure parameters using attenuation- or dissipation-related seismic attributes.

Three-Water Differential Parallel Conductivity Saturation Model of Low-Permeability Tight Oil and Gas Reservoirs

Addressing the poor performance of existing logging saturation models in low-permeability tight sandstone reservoirs and the challenges in determining model parameters, this study investigates the pore structure and fluid occurrence state of such reservoirs through petrophysical experiments and digital rock visualization simulations. The aim is to uncover new insights into fluid occurrence state and electrical conduction properties and subsequently develop a low-permeability tight sandstone reservoir saturation model with easily determinable parameters. This model is suitable for practical oilfield exploration and development applications with high evaluation accuracy. The research findings reveal that such reservoirs comprise three types of formation water: strongly bound water, weakly bound water, and free water. These types are found in non-connected micropores, poorly connected mesopores where fluid flow occurs when the pressure differential exceeds the critical value, and well-connected macropores. Furthermore, the three types of formation water demonstrate variations in their electrical conduction contributions. By inversely solving rock electrical experiment data, it was determined that for a single sample, the overall cementation index is the highest, followed by the cementation index of pore throats containing strongly bound water, and the lowest for the pore throats with free water. Building on the aforementioned insights, this study develops a parallel electrical pore cementation index term, ϕm′, to account for the differences among the three types of water and introduces a parallel electrical saturation model suitable for logging evaluation of low-permeability tight oil and gas reservoirs. This model demonstrated positive application effects in the logging evaluation of low-permeability tight gas reservoirs in a specific basin in the Chinese offshore area, thereby confirming the advantages of its application.

Microscopic occurrence and distribution of oil and water in situ shale: Evidence from nuclear magnetic resonance

Characterizing the microscopic occurrence and distribution of in-situ pore water and oil is crucial for resource estimation and development method selection of shale oil. In this paper, a series of nuclear magnetic resonance (NMR) experiments were conducted on shales from the Gulong Sag, Songliao Basin, China, at AR, WR-AR, WOR-AR, Dry, SO, and WR states. In-situ pore water and oil were reconstructed after WOR-AR. An improved T1–T2 pattern for shale oil reservoirs comprising water and oil was proposed to classify and quantitatively detect pore fluids at different occurrence states. The total and free oil contents derived from NMR T1–T2 spectra at AR states were found to correlate well with those from multistage Rock-Eval. Moreover, the NMR-calculated total and free oil are generally larger than those measured from multistage Rock-Eval, whereas adsorbed oil is the opposite, which implies that adsorbed, bound, and movable oils in shale pores can be accurately and quantitatively detected via NMR, without absorbed hydrocarbons in kerogen. As per the NMR T2 and T1–T2 spectra at WOR-AR state, the microdistributions of in-situ pore water and oil were clearly demonstrated. Adsorbed, bound, and movable oils primarily occur in the micropores (<100 nm), mesopores (100–1000 nm), and macropores (>1000 nm), respectively, whereas capillary-bound water is primarily correlated with micropores. Thus, the microscopic occurrence and distribution of adsorbed oil are remarkably affected by pore water, followed by bound oil, and movable oil is hardly affected. This study would be helpful in further understanding the microscopic occurrence characteristics of pore fluids in-situ shale oil reservoirs.

Data-driven modal analysis of turbulent momentum exchange and heat transfer in composite porous fluid systems

This paper investigates the dynamics governing turbulent momentum exchange and heat transfer between pore flow within porous media and the turbulent flow passing over it. Employing high-fidelity pore-scale large eddy simulation, our investigation explores the fundamental mechanisms driving these phenomena. Modal analysis based on snapshot proper orthogonal decomposition (POD) is employed to quantify the modes of interaction between porous and non-porous regions, providing a comprehensive understanding of the underlying processes. Spatial and temporal modes reveal the existence of localized flow structures at the pore scale, contributing to time-varying patterns of information exchange. At the commencement of the porous block, the mean flow (Mode = 0) from the porous to non-porous region is the dominant mechanism in momentum exchange and heat transfer. This mode facilitates convective heat transfer from the porous to the non-porous region through upward and forward flow movements, showcasing positive flow leakage. In addition to the mean flow, the turbulent flux inherent in alternate POD modes (Mode ≠ 0) plays a substantial role in information propagation, influencing diverse directions. Spatial modes, complemented by statistical analysis, uncover a significant likelihood of observing negative vertical velocity values in the wake of the porous ligaments at the porous-fluid interface, indicative of negative flow leakage. This negative flow leakage precisely corresponds to the local penetration of fluid from the non-porous region into the porous region. Furthermore, our study reveals that information exchange via turbulence fluctuations manifests through complex outward and inward interactions in regions characterized by substantial positive flow leakage. Notably, these regions exhibit a distinct tendency for high-momentum streamwise-oriented flow to migrate outward from the porous region into the non-porous region (outward interactions). Conversely, inward interactions arise in these regions when the instantaneous magnitude of positive flow leakage is smaller than the mean value of positive flow leakage, emphasizing the pulsating nature of positive flow leakage. Finally, the distribution of the Nusselt number highlights that more than 60% of total heat transfer occurs within the initial one-third of the porous block length. Significantly, a notable portion of the porous ligaments experiences insufficient cooling due to positive flow leakage, underlining the critical implications of these findings for the understanding of turbulent momentum exchange and heat transfer in a composite porous-fluid system.

Study on Electrical Characteristics and Formation Mechanism of Micro-Ion Capacitor in Shale Pore and Fracture Structure

Due to the micro/nanoscale and intricate pore structure, poor connectivity, complex pathways, the presence of microfractures, and the coexistence of organic and inorganic pores, shale and other tight reservoirs exhibit increasingly complex conductive characteristics. This paper mainly studied the electrical properties of shale based on experimental test data, including scanning electron microscope (SEM) thin-section and microtomography CT (μ-CT) images, and analyzed the shale pore type, pore structure characteristics, and development of fracture. Then, the distribution of pyrite, content, and graphitization of organics and their influence on the electrical properties are discussed. Furthermore, the double electrical layer and zeta potential in the shale zone were discussed in depth. The results revealed that, within the content of pyrite, organics, and its graphitization, the vitrinite maturity are inversely proportional to shale resistivity. It was also found that in the presence of an external electromagnetic field, the fluid in shale pores is subjected to the combined strength of pore pressure and external field potential difference. Thus, its response equation should be an improved Navier-Stokes equation, which considers pore pressure, zeta potential, and Coulomb force. When shale is subjected to an external electromagnetic field, due to the complex pores structure and organic and inorganic minerals, it will represent more of a dielectric-like property than electricity. So, it will form special microscopic ionic capacitors, which are different from common plate capacitors. There are three special kinds of microscopic ionic capacitors, they are (I) the intergranular pore microscopic ionic capacitor model, (II) the particle with isolated pore microscopic ionic capacitor model, and (III) the pyrite or graphite or other organics microscopic ionic capacitor model. Finally, the characteristics of microscopic ion capacitors are summarized: irregular polar area and varying distance between poles, varying charges with time, and salinity of the formation water.

Determining the pore structure and fluid characteristics of different ranks of coal using nuclear magnetic resonance

The microstructure of coal reservoirs and their fluid-endowment characteristics are important indicators for evaluating coalbed methane during mining. This study aimed to investigate the influence mechanism of the nanopore structure of different coal ranks on the fluid distribution characteristics. Low-field nuclear magnetic resonance (NMR), centrifugal force, and heat treatment tests were conducted. The results indicated that as the degree of coal metamorphism increases, micro-transition pores increase significantly, mesopores–macropores decrease, and the nonhomogeneity of the pores increases. The optimum centrifugal forces for low-, middle-, and high-rank coals were 1.68, 1.08, and 1.08 MPa, respectively. The corresponding mean T2C1 (first T2 cutoff) values were measured as 8.01, 1.61, and 5.09 ms, respectively, while the mean T2C2 (second T2 cutoff) values were 0.23, 0.26, and 1.25 ms, respectively. Pore sizes less than 11.6 nm indicate unrecoverable fluid, those higher than 88 nm indicate free fluid, and the rest (11.6–88 nm) indicate capillary bound fluid. As the degree of coal metamorphism increases, free fluid saturation decreases, capillary bound fluid saturation increases and then decreases, and unrecoverable fluid saturation increases.

Temporal Seismic Velocity Changes Associated With the Mw 6.1, May 2008 Ölfus Doublet, South Iceland: A Joint Interpretation From dv/v and GPS

AbstractIn South Iceland, populated and agricultural areas are at risk of earthquakes due to their location within the South Iceland Seismic Zone (SISZ). In 2008, two moderate‐sized earthquakes (M5.8 and M5.9) occurred in Ölfus, the western end of this highly active transform zone. We analyze temporal seismic velocity variations (dv/v) related to the Ölfus earthquake doublet, using cross‐correlations of ambient noise in the frequency range of 0.1–3.0 Hz. The two mainshocks decrease the average velocity by 0.8% at the nearest stations. The co‐seismic changes are most noticeable from 0.7 to 1.7 Hz and affect a 40 km wide region. We present a first‐time comparison of dv/v to crustal deformation, seismicity, co‐seismic volumetric stress changes and reported PGA distribution for the Ölfus doublet. Ground accelerations caused by mainshocks at intermediate distances suggest that strong shaking‐related damage may contribute to the co‐seismic dv/v decrease. A rapid velocity increase (0.3%) in a month after the co‐seismic drop indicates crustal rock healing. We find 3‐months of post‐seismic decorrelation, followed by a nearly permanent velocity decrease (0.2%) confined to a shallow layer (1 km) until the end of the observation period. Afterslip and pore fluid effects in the near‐source region are likely to influence post‐seismic dv/v. We demonstrate that seismic interferometry can contribute to future fault‐zone monitoring operations in the SISZ by detecting small changes in velocity.

Path and Slip Dependent Behavior of Shallow Subduction Shear Zones During Fluid Overpressure

AbstractElevated pore fluid pressure is proposed to contribute to slow earthquakes along shallow subduction plate boundaries. However, the processes that create high fluid pressure, disequilibrium compaction and dehydration reactions, lead to different effective stress paths in fault rocks. These paths are predicted by granular mechanics frameworks to lead to different strengths and deformation modes, yet granular mechanics do not predict their effects on fault stability. To evaluate the role of fluid overpressure on shallow megathrust strength and slip behavior, we conducted triaxial shear experiments on chlorite and celadonite rich gouge layers. Experiments were conducted at constant temperature (130 and 100°C), confining pressure (130 and 140 MPa), and pore fluid pressures (between 10 and 120 MPa). Fluid overpressure due to disequilibrium compaction was simulated by increasing confining and pore fluid pressure synchronously without exceeding the target effective pressure, whereas overpressure due to dehydration reactions was simulated by first loading the sample to a target isotropic effective pressure and then increasing pore fluid pressure to reduce the effective pressure. We find that the effects of fluid pressure and stress path on the mechanical behavior of the chlorite and celadonite gouges can generally be described using the critical state soil mechanics (CSSM) framework. However, path effects are more pronounced and persist to greater displacements in chlorite because its microstructure is more influenced by stress path. Due to its effects on microstructure, the stress path also imparts greater control on the rate‐dependence of chlorite strength, which is not predicted by CSSM.

Study on the Flow Behavior of Gas and Water in Fractured Tight Gas Reservoirs Considering Matrix Imbibition Using the Digital Core Method

Tight gas reservoirs possess unique pore structures and fluid flow mechanisms. Delving into the flow and imbibition mechanisms of water in fractured tight gas reservoirs is crucial for understanding and enhancing the development efficiency of such reservoirs. The flow of water in fractured tight gas reservoirs encompasses the flow within fractures and the imbibition flow within the matrix. However, conventional methods typically separate these two types of flow for study, failing to accurately reflect the true flow characteristics of water. In this study, micro-CT imaging techniques were utilized to evaluate the impact of matrix absorption and to examine water movement in fractured tight gas deposits. Water flooding experiments were conducted on tight sandstone cores with different fracture morphologies. Micro-CT scanning was performed on the cores after water injection and subsequent static conditions, simulating the process of water displacement gas in fractures and the displacement of gas in matrix pores by water through imbibition under reservoir conditions. Changes in gas–water distribution within fractures were observed, and the impact of fracture morphology on water displacement recovery was analyzed. Additionally, the recovery rates of fractures and matrix imbibition at different displacement stages were studied, along with the depth of water infiltration into the matrix along fracture walls. The insights gained from this investigation enhance our comprehension of the dynamics of fluid movement within tight gas deposits, laying a scientific foundation for crafting targeted development plans and boosting operational efficiency in such environments.

Morphology Characteristics of the Liquid–Vapour Interface in Porous Media

The evolution of the liquid–vapour interface plays a crucial role in multiphase flow, heat and mass transfer, and fluid phase change in porous media. A thorough investigation of the interface under varying degrees of saturation is necessary and crucial to fully understanding the key mechanism of soil water evaporation. The pore voids and fluids are characterized using X-ray microtomography and image processing. Salt solutions usually replace pure water for better contrast and image development. Machine learning algorithms were employed to identify and extract the different phase and their interface accurately. Then, variations in the geometrical and topological features of the interface at varying saturation during evaporation were analysed to quantitatively describe the connectivity of the liquid phase and the morphological change in the liquid–vapour interface. Topological analysis reveals that normalized Euler characteristic numbers quantify the complementary connectivity of liquid and vapour phase. The curvatures of the liquid–vapour interface of the samples under various saturations classify the liquid–air interface curvature of samples under various saturations for quantitatively describing the migration progress and quantity distribution of typical interface along with drying.