Introduction
Growing global energy demand is pushing oil and gas development toward low-grade petroleum resources with intricate reservoir conditions and great extraction difficulties. China is rich in low and ultra-low permeability reservoirs [1], but oil displacement in these reservoirs tends to be less efficient compared to their conventional counterparts due to a range of factors: heterogenous pore wall physical and chemical properties, micro- and nano-pore sizes, diverse mineral compositions and pore structures, and varying formation water salinity, which jointly induce highly complicated micromechanical behaviors at oil-water-rock interfaces, and severely affect crude oil occurrence and mobility [2-4,5]. Therefore, systematically revealing the underlying mechanisms of fluid- solid coupling in low and ultra-low permeability reservoirs has become the most important research task for developing supporting materials, breaking through technical bottlenecks, and finally enhancing crude oil recovery.
In studies aimed at enhancing displacement efficiency in low and ultra-low permeability reservoirs, previous literatures have primarily looked at oil-rock interfacial interactions from the perspectives of mineral surface wettability, adsorption-desorption behaviors, interfacial slippage, and diffusion and flow of reservoir components [6-11], offering meaningful theoretical basis and methodological guidance for understanding rock-fluid interactions. Nevertheless, given the oil-water-rock multiphase interfacial interactions ubiquitous in the exploitation of these reservoirs: formation water adsorbs onto the surface of hydrophilic minerals via capillarity and forms a water film, which, driven by ionic exchange, mineral dissolution and other physical-chemical processes, can further evolve into a “brine film” with intricate physical-chemical properties, resulting in even more intricate microscopic interfacial interactions among active components in crude oil, brine films, and rock minerals [12]. In 1999, in an experimental study on oil-water- rock interactions, Tang et al. [13] found that decreasing salinity increased crude oil recovery. In 2008, Lager et al. [14] noticed cation bridges between oil molecules and rock. In 2015, Mugele et al. [15] experimentally demonstrated how metal cations like Na+ and Ca2+ adsorbed on mineral surfaces altered oil-rock interfacial wettability. Awolayo et al. [16] summarized eight oil-water-rock interfacial interaction models: cation exchange, anion exchange, ligand exchange, protonation, water bridge, cation bridge, hydrogen bond, and van der Waals force. In 2019, Yang et al. [17] presented an experimental study on the influence of ion diffusion on crude oil migration and suggested that ions hinder oil flow in reservoirs. Although the big role of ions in oil-water-rock interfacial interactions has been extensively demonstrated [18-23] (e.g. in terms of interfacial slippage, adsorption, wettability, and effective crude oil displacement), the underlying mechanics remains an open question pending quantitative evaluation. In recent years, to address challenges facing the exploitation of low permeability reservoirs and the mobilization of remaining oil in confined space, a concept of “hydrated ion bridge” (HIB) has been proposed and examined as a potential mechanistic interpretation [24-27]. HIB operates when rich hydrated ions adsorbed on mineral surfaces connect polar groups in crude oil to mineral surfaces via a “bridge-like array”, which generates strong effects by altering interfacial adsorption energy. As research in this field is just taking off, exact interfacial interaction and disruption mechanisms of HIB still require substantial investigation for validation and refinement.
Given the nascent stage of research in this domain, the precise mechanisms of interfacial interactions and disruptions associated with HIB require extensive investigation to ensure validation and refinement.
Focusing on the HIB effect at crude oil-rock interfaces in low and ultra-low permeability reservoirs, this paper reviews the research mythology, and the formation, interaction and disruption of HIBs, discusses how HIBs influence crude oil occurrence and mobility, analyzes key challenges in studying HIBs and suggests the potential directions of future development.
1. Research methods
Low and ultra-low permeability reservoirs are generally tight, and of low porosity reflected by micro- and nano-matrix pores [28]. These reservoirs are influenced by complicated oil-water-rock multiphase interfacial interactions, including HIB which is the key driver behind crude oil adsorption onto rock mineral surfaces (Fig. 1a, 1b). The microstructure of HIB is shown in Fig. 1c. As this interaction operates primarily on a nanoscopic scale, high-precision characterization methods, including experimental characterization and molecular simulation, are essential to revealing the underlying mechanisms. It is worth noting here that HIB-related experimental characterization and molecular simulation are complementary to, rather than independent from, each other.
Fig. 1. Oil-water-rock interfacial interactions and the molecular structure of HIB in low and ultra-low permeability reservoirs. |
Experimental data such as crude oil components and the composition of rock minerals are critical inputs for constructing an accurate molecular model of oil-rock interaction. Molecular simulation reveals the micro-dynamic processes that are often unobservable in experiments, offering theoretical insights and mechanistic explanations for experimental observations. Together, they constitute a mutually validating and deepening research pathway.
1.1. Experimental characterization
Atomic force microscopy (AFM), X-ray absorption spectroscopy (XAS), neutron scattering, infrared spectroscopy, and Raman spectroscopy directly analyze the microstructures and dynamic behaviors of hydrated ions. Contact angle measurement quantitatively evaluates the variation in rock surface wettability. These analyses provide important experimental basis for understanding the formation and operation of HIB. AFM directly measures the interfacial force between hydrated ions and rock surface or polar components in crude oil using functionalized probes. For instance, Hu et al. [26] modified an AFM probe tip with carboxyl groups (—COOH) and measured the force between the probe and a quartz substrate in brine environments containing NaCl, CaCl2, or AlCl3 (at a concentration of 0.1 mol/L). With these measurements, they assessed the micro-mechanical strengths of different HIBs involving Na+, Ca2+, and Al3+. AFM also characterizes the distribution of hydrated ions on mineral surfaces and their nano-scale impacts on interfacial morphology. Peng et al. [29] obtained high-resolution images of metal cation hydrates at interfaces using a scan tunneling microscope (STM) and non-contact AFM system. X-ray absorption spectroscopy (XAS) and neutron scattering examine local structural features (e.g. hydration number and hydration radius) and kinetic behaviors of hydrated ions at the molecular scale [30]. Infrared spectroscopy and Raman spectroscopy output information on chemical bonds between hydrated ions and polar molecules in crude oil by analyzing molecular vibration patterns. Contact angle measurement gives an indirect description of how HIBs modulate the spreading at oil-water-rock interfaces by assessing changes in rock surface wettability [31]. So far, although experimental characterization of oil-water-rock interfacial interactions has extended to a sub-microscopic scale, direct observation and precise interpretation still face tremendous challenges. Nano- to subnano-interactions expose experiments to environmental interference. The measurement accuracy is also limited by instruments' capacity, making it hard to capture the real-life physical-chemical processes of ionic exchange and mineral dissolution in real time or to directly observe the flowing of crude oil in nanopores. Hence experimental characterization alone can hardly reveal the mechanisms of HIBs. Molecular simulation is needed to further explore the micro-dynamics of HIBs.
1.2. Molecular simulation
Molecular simulation is a popular approach to understanding microscopic interaction between oil and rock. By precisely tracking the motion, interaction and energy variation of every particle from the atomic/molecular perspective and simulating the same at a time step on the order of 1×10−15 s, this new approach effectively captures nanosecond to sub-microsecond physical-chemical processes, such as the formation and disruption of HIB, and the dynamic arrangements and trajectories of water molecules and ions. It also enables precise control of the initial conditions and environmental variables like temperature, pressure, and chemical composition, saving both time and money [24]. Some of the most commonly used molecular simulation techniques include molecular dynamics simulation, Monte Carlo simulation, dissipative particle simulation, and first-principles calculation. When HIB is concerned, first-principles calculation [25] and molecular dynamics simulation [26-27] predominate.
First-principles calculation examines the interactions between atomic nuclei and electrons by approximately solving the Schrödinger equation based on quantum mechanics principles. This method applies to a range of purposes: to determine the stable spatial configuration of HIB through structural optimization [25,32]; to reveal the electronic interactions and bonding characteristics among minerals, ions, water molecules, and crude oil components by analyzing electron density and charge transfer [25,33]; to confirm the interaction sites and strengths of HIBs by evaluating the adsorption of crude oil molecules onto mineral surfaces [25]; and to simulate how crude oil molecules detach from rock surfaces [25].
Molecular dynamics simulation examines microscopic behaviors and evolution by solving the equation of motion of atoms in a molecular system over time. Compared to first-principles calculation with a limited calculation capability, molecular dynamics simulation can cover larger spatial and temporal scales, so it is applicable for studying the spatial configuration, bridge morphology, stability and bonding energy of HIBs, and their influences on interfacial properties [24]. It's also used to assess the adsorption and desorption behaviors and how HIBs change the wettability of oil-water-rock interfaces [12], and investigate the impacts of different types of ions on the formation of HIBs and how HIBs enhance crude oil displacement efficiency [26-27].
2. Micro-mechanism of HIBs
In the early stage of reservoir formation, reservoir rocks are primarily in contact with formation water, so the mineral surfaces tend to water-wet at a certain salinity. As crude oil migrates and accumulates in the reservoir, HIBs are formed between the polar components in crude oil (e.g. carboxylic acids and phenols) and the mineral surfaces via hydrated ions. During production, under the strong interaction of HIBs, crude oil occurs over rock surfaces as continuous or discontinuous adsorbed oil films, making it hard to be detached and mobilized effectively. Understanding the formation, interaction strength, and disruption of HIBs is crucial to improving the oil displacement efficiency in low and ultra-low permeability reservoirs.
2.1. Formation mechanism
Water is a polar molecule due to its V-shaped structure, where the center of positive charge from the two hydrogen atoms does not coincide with the center of negative charge from the oxygen atom, resulting in a non-uniform distribution of charge. When metal cations (e.g. Na+, Ca2+) dissolve in water, water molecules become one or more structurally ordered, tightly bonded hydration shells around the ions via strong ion-dipole interactions, giving rise to what we know as hydrated ions [30]. As illustrated by the microstructure diagram in Fig. 1c, hydrated ions adsorb on mineral surfaces by connecting to mineral surfaces at one end via water molecules on the shell surface, and to polar components in crude oil at the other end, producing a “bridge” that significantly enhances the interaction between crude oil and rock surface.
Based on the structural analysis above, the formation of HIBs is primarily influenced by mineral type, ion type, and polar groups in crude oil, and involves microscopic processes like electrostatic interaction, hydrogen bonding, and van der Waals force. In an aqueous setting, reservoir minerals (e.g. quartz, feldspar, clay) typically carry net charges due to crystal structure breakage, ion dissociation, or isomorphism substitution. For instance, the surface of silicate minerals like quartz (SiO2) usually carries negative charges due to partial dissociation of silanol groups into ≡Si—O−, while hydrated metal ions inherently carry positive charges. Under neutral conditions, polar components in crude oil—such as carboxylic acid—carry negative charges due to partial ionization of the groups. The electrostatic interaction between these charges is one of the key drivers behind the formation of HIBs, and secures the stability among polar components in crude oil, ions, water molecules, and mineral surfaces [34]. Furthermore, water molecules connect silanol groups or ≡Si—O− on rock surfaces with carboxylic groups in crude oil via hydrogen bonding. On the one hand, this maintains the ordered structure and stability of water molecules in the interfacial region [35-36]; on the other hand, when hydrated ions interact with polar groups in crude oil, the interfacial water molecules can indirectly link them as a hydrogen bonding bridge. During the formation of HIBs, van der Waals force works as a ubiquitous non-specific, attractive force widely present among mineral surfaces, hydrated ion layers, and polar molecules in crude oil, preparing the stage for close interfacial contact. However, compared to electrostatic interaction and hydrogen bonding, van der Waals force is weaker and operates over a shorter distance.
2.2. Interaction strength
The interaction strength of HIBs is affected by ion type and concentration, reservoir solution environment, rock minerals, and polar components in crude oil. More specifically: (1) Ion type and concentration are the main determinants for HIB interaction. Cui et al. [25] performed quasi-static pulling simulation based on first-principles calculation to observe how polar oil molecules (valeric acid) are detached from quartz surfaces where they adsorb, and used the peak value from the force-displacement curve to evaluate the interaction strength of HIBs. The results indicated that divalent HIBs like Ca2+ have much higher interaction strength than monovalent counterparts like Na+. Hu et al. [26] measured the interaction strength of hydrated Na+, Ca2+, and Al3+ ion bridges through AFM experiments. They found that the force between an ATM probe tip functionalized with carboxyl groups and quartz increased with cation concentration, and under the same conditions, cation concentration made a greater difference in CaCl2 than in either AlCl3 or NaCl solution. Hu et al. [37] experimentally demonstrated that the interaction strength of Ca2+ bridges can be as high as 0.48 nN, markedly higher than that of Al3+ (0.36 nN) and Na+ (0.11 nN) bridges; Ca2+ bridges also operate across a broader range than Na+ and Al3+ bridges. (2) The behavior of HIBs directly relates to solution environment. In quartz sandstone, for instance, the pH level of the solution affects the protonation degree of siloxane groups on the quartz surface [21], and alters the polarity of the rock mineral surface, accordingly the interaction strength of HIBs. (3) The interaction strength of HIBs varies from one rock mineral to another. For instance, stable hydration structures are more likely to form in carbonate minerals like calcite (CaCO3) since their Ca2+ possesses high valence and high charge density [38-39]. (4) Hu et al. [26,37] measured the adhesion strength between characteristic groups in typical crude oil components (methyl, sulfonic, and carboxyl groups) and different hydrophilic minerals (quartz, albite and orthoclase) in deionized water and in simulated formation water using AFM, and confirmed polar components in crude oil and high-concentration cations as key contributors to strong adhesion between crude oil and rock.
2.3. Disruption mechanism
The disruption mechanism of HIBs represents a core scientific issue for enhancing crude oil displacement efficiency in low and ultra-low permeability reservoirs. As the shear force associated with fluid flow serves as an external contributor to disrupting HIBs, AFM experiment and molecular simulation usually involve applying a given level of pulling force to the bridges to reveal their disruption mechanism [26-27]. In order to obtain an intuitive understanding, Cui et al. [25] visualized oil-water-rock interfacial interactions using the interaction region indicator (IRI) method and compared the results with the simulation of a water bridge system to reveal the role of ions on interfacial forces. In a water bridge system, the interactions at both the quartz-water and water-valeric acid, which represents polar molecules in crude oil interfaces were hydrogen bonding in the initial adsorption stage. As the water-valeric acid distance increased, the interaction region between quartz and water remained blue all the time, suggesting that hydrogen bonding persists. The interaction region between water and valeric acid turned from blue to green, signifying the breakage of hydrogen bonds and the transform to van der Waals interaction (Fig. 2a). In an HIB system, the interactions between quartz and ions, and between ions and water showed consistently strong attraction, governed primarily by electrostatic force. The hydrogen bonding between water and valeric acid gradually diminished to zero, indicating the breakage of hydrogen bonds and the transform to van der Waals interaction (Fig. 2b).
Fig. 2. Visualized presentation of interaction types, sites, and strengths of water bridge (a) and HIB (b) during disruption [25]. |
A simulation study of disrupting HIBs [25] found the hydrogen bonds between polar molecules and hydrated ions are the sites from which HIBs can be disrupted. The study employed a non-covalent interactions (NCI) index to describe the change in interaction strength at different interaction sites in HIBs (e.g. water-valeric acid, ion-water, quartz-ion). NCI typically takes a negative value, and a larger absolute value indicates a stronger attraction at the interaction site [25] (Fig. 3). Crude oil molecules adsorb onto rock mineral surfaces via HIBs. Due to the low stiffness of hydrogen bonds, as the distance between crude oil molecules and rock surface increases under external forces, the hydrogen bonding between crude oil molecules and water molecules will attenuate rapidly, leading to ultimate bridge disruption. When pulled, the bridge structure undergoes tremendous changes. For example, during the disruption of a divalent HIB, the bridge structure keeps relaxing to the current-state minimum energy. This process is accompanied by atomic reconfiguration analogous to the pre-disruption yielding of a plastic material, which significantly increases the difficulty of crude oil detachment from rock surface [25].
Fig. 3. NCI variation curves of HIB [25]. |
The disruption of hydrogen bonds in HIBs is critical to the gradual detachment of crude oil molecules from rock surfaces. In field displacement practice, a combination of low-salinity water flooding and chemical flooding with specifically designed materials can be employed to disrupt HIBs, weaken the adsorption of crude oil on rock surfaces and improve the crude oil detachment efficiency in low and ultra-low permeability reservoirs.
3. Influence of HIBs on crude oil occurrence and mobility
A nano-scale “point” process as it is, HIB governs the macroscopic “areal” properties of rock-oil interfacial interaction by means of synergistic effects, distribution characteristics, and spatial configuration, thus affecting the crude oil occurrence and mobility in low and ultra-low permeability reservoirs. More specifically, at a microscopic level, HIBs do not exist as isolated individuals, but a dense array of numerous ions adsorbed on mineral surfaces. Each bridge can be seen as a local “anchoring point” that fixes polar components in crude oil on rock surfaces via electrostatic interaction and hydrogen bonding. When the areal density of the “anchoring points” is high enough, a microscopically “continuous adsorption network” takes shape on the rock surface. Polar components in crude oil spread and connect via these dense anchoring points, resulting in a stable, continuous absorbed oil firm over the rock surface. In this sense, microscopically observed rock-oil interfacial properties, including wettability, adhesion, and difficulty of crude oil detachment can essentially be considered as a statistical average and macroscopic collection of numerous HIB anchoring points at a microscopic level. While the action of a bridge represents the strong interaction at one “point”, the collective behavior of numerous bridges ultimately determines the bonding strength across the rock-oil interface. Discussing how HIBs affect crude oil occurrence and mobility helps understand HIBs at both microscopic and macroscopic levels.
3.1. Crude oil occurrence
HIBs affect crude oil occurrence by strengthening crude oil adsorption and altering rock-oil interfacial wettability. Studies have shown that brine films with different pH levels determine the adsorption of counterions on reservoir rock surfaces, ultimately affecting the adsorption of crude oil molecules at interfaces, by modulating the chemical group composition and the protonation/deprotonation of organic acids and bases on reservoir mineral surfaces [9,17]. Hence in low and ultra-low permeability reservoirs, ions in formation water can alter the charge characteristics on rock surfaces; particularly, metal cations can neutralize negative charges on quartz and other minerals, leading to strong adsorption of polar oil molecules onto rock surfaces [18-19]. Additionally, affected by ion concentration, high-salinity brine can cause extensive adsorption of hydrated ions on the surfaces of minerals like calcite, greatly strengthening crude oil adsorption [20].
By adsorbing on mineral surfaces, hydrated ions alter interfacial properties and affect wettability. For instance, Na+ in brine films can form Na-naphthenate precipitates on calcite surfaces, making them more oil-wet [24]. By measuring oil-water interfacial tension and performing AFM and Zeta potential analysis, Sun et al. [40] demonstrated the ability of ions to transform oil-wet rock surfaces toward more water-wet. Zhang et al. [41] confirmed that Ca2+ and SO42− in seawater enhance the hydrophilicity of chalk, accordingly the spontaneous imbibition capacity of water into oil-bearing matrix under capillary force. Contact angle measurements in different solution environments revealed that monovalent ions make a smaller difference to surface wettability compared to divalent ions [42], although a study by Liu et al. [43] found that in a basic environment, NaCl and KCl solutions containing monovalent ions altered the rock surface wettability more remarkably than CaCl2 and Na2SO4 solutions and concluded ions do not alter rock surface wettability unless in a basic environment. Also, using different salt solution combinations enables differentiated wettability modulation. For example, adjusting the mixing ratio of NaCl-sulfate solutions effectively enhances rock hydrophilicity [44].
3.2. Crude oil mobility
HIB strengthens the adsorption of crude oil on rock surfaces, and increases the threshold and flow resistance of crude oil, ultimately reducing macroscopic displacement efficiency. To tackle this issue, Hong et al. [12] performed molecular dynamic simulation to examine crude oil occurrence and flow in kaolinite nanopores under HIB effects and explored the influences of four types of brine films (NaCl, CaCl2, MgCl2, MgSO4) on crude oil flow. At the same pressure gradient, compared to the ion-free water film system, crude oil in the brine film systems flew at much lower rates, suggesting that the interfacial interaction between ions in water film and polar components in crude oil significantly limits crude oil mobility in nano-pores, and this restriction is more pronounced in divalent ion systems than in monovalent ones. This observation has been supported by the nonlinearity of the rate-pressure gradient curves (Fig. 4a): HIB affects the initiation of crude oil in nano-pores. So far, most studies around the threshold pressure gradient for crude oil flow in low and ultra-low permeability reservoirs have relied on core flooding experiments to characterize the flow behavior of crude oil [45], but no consideration has been given to cases where both oil and brine exist. By analyzing the simulation results of Hong et al. [12] based on the determination method for threshold pressure gradient in the study of Cui et al. [46], it can be assumed that the threshold pressure gradients for crude oil under different HIB effects follow the order MgSO4≥MgCl2≥CaCl2>NaCl, concurring with the experimental and simulation observation that divalent HIBs act more strongly than the monovalent counterpart.
Fig. 4. Influence of HIBs on crude oil mobility in nanopores [12]. |
By comparing the influences of cations and anions in brine films on crude oil flow, Hong et al. [12] found that cations inhibit crude oil flow more strongly, since polar molecules in crude oil form more hydrogen bonds with HIBs, further confirming that weakening hydrogen bonding is critical to improving the displacement efficiency in low and ultra-low permeability reservoirs. HIBs strengthen oil-rock interfacial interaction, and greatly affect crude oil mobility. Hence modifying water chemistry (e.g. low salinity) would be a feasible way to disrupt HIBs, and reduce the adhesion across the interface to a certain extent, and finally effectively improve crude oil mobility and recovery.
Comprehensive analysis indicates that disrupting HIBs effectively improves the oil displacement efficiency in low and ultra-low permeability reservoirs, especially in a low-salinity water flooding context. This can be achieved by optimizing the type and concentration of ions in injected water and replacing high-valence cations adsorbed on rock surfaces with low-valence ions like Na+ through ion exchange to directly disrupt the stability of HIBs. The simulation by Hong et al. [12] revealed that monovalent ions whose concentration is higher than divalent ions can effectively replace divalent ions at oil-water-rock interfaces (Fig. 4b), thus weakening the interaction strength of HIBs and the adsorptive force acting on crude oil molecules.
Some commonly accepted mechanisms of low-salinity water flooding include: reduced ionic strength leads to diffuse double-layer expansion, which facilitates crude oil desorption by enhancing electrostatic repulsion; low-salinity water is frequently associated with an elevated pH value, which further increases the negative charge density on mineral surfaces and weakens oil film adhesion; ion adsorption alters mineral surface wettability; osmotic effects enhance the penetration pressure of reservoir fluids and accordingly the displacement pressure, ultimately improving displacement efficiency [14,22,47]. Molecular dynamic simulation by Fang et al. [48] demonstrated that reducing the salinity of reservoir formation water increases brine film thickness. This degrades the interfacial binding to crude oil molecules, enhances the mobility of liquid molecules at interfaces and causes oil droplets in nanopores to desorb. Under these assumptions, ideal displacement can be achieved by adding the salt solution with only monovalent cations in injected water. However, according to some experimental results, adding both monovalent and divalent ions in injected water is better than adding only monovalent ions for enhancing displacement efficiency. This observation is obviously contradictory with the theoretical explanation of multi-component ion exchange mechanism [49]. Understanding the mechanism of HIBs at oil-water-rock interfaces is central to develop the HIB-related technology.
4. Challenges and prospects
4.1. Challenges
Despite increasing attention to and valuable fundamental findings on HIBs in both academic and engineering communities, systematic challenges persist across three dimensions: research methodology, scale integration, and geological complexity.
(1) The dynamic evolution process of HIBs remains inadequately elucidated. Applicable in-situ characterization techniques (e.g. AFM) still have limitations in spatiotemporal resolution. They can hardly capture the dynamic responses of HIBs to temperature, pressure and fluid chemistry variations in real time and with high precision, resulting in a lack of micro-dynamic description of the entire formation-stability-breakage cycle. Even molecular simulation finds it hard to accurately track and quantify the dynamic evolution of HIBs (e.g. exchange with water molecules, reconfiguration and instantaneous breakage of bridge structures), preventing further understanding of key controlling mechanisms.
(2) A discontinuity exists in cross-scale spatiotemporal modeling and prediction. On a spatial scale, although molecular simulation can reveal interfacial interactions at an atomic/electronic scale, as the results can hardly be upscaled to a pore or core scale in an effectively way, it cannot measure the actual influence of HIBs on macroscopic displacement efficiency. On a temporal scale, molecular simulation is typically limited to a nanosecond range, which is vastly different from a years-long timescale of field displacement. Full timescale coverage and effective integration of dynamic processes have not achieved yet.
(3) The reproducibility of a real-world geological environment is insufficient in laboratory. Experiments and simulations are mostly performed on one-mineral models, but real-world reservoirs are typically complexes with multiple minerals. Significant differences in surface charge properties, ion adsorption selectivity, and micro-morphology among different minerals directly affect the spatial distribution and stability of HIBs. The jointing influence of temperature, stress, chemical and flow fields in real-world reservoirs is extremely important, but systematic exploration remains a blank in most studies, significantly limiting their predictive accuracy for real-world geological settings.
4.2. Prospects
To cope with these challenges, further efforts could seek breakthroughs in the following three aspects to advance the theoretical development and engineering application of HIBs in a more systematic way.
(1) A simulation strategy shall be developed, incorporating in-situ dynamic experimental characterization with AI-assisted simulation. To reveal the dynamic evolution of HIBs, in-situ experimental techniques with higher spatiotemporal resolutions, such as microfluidic chip with high-speed atomic force microscopy, shall be developed to facilitate real-time visualization and quantitative measurement of structural changes of HIBs during oil displacement. AI (Artificial Intelligence) should be introduced so that machine learning force fields and enhanced sampling molecular simulation can be leveraged to accurately capture the dynamics of transient processes like water molecule exchange and ion coordination reconfiguration, thus obtaining a full view of the microscopic controlling mechanisms behind the dynamic evolution of HIBs.
(2) A framework shall be established for cross-scale model fusion and upscaling prediction. To channel molecular scale simulation toward reservoir scale prediction, a multiscale modeling system coupling structure and process should be constructed. In the spatial dimension, machine learning-based upscaling methods should be developed to establish parameter transfer and mapping mechanisms spanning from microscopic interfacial structures to pore networks and even macroscopic flow models, based on insights on molecular dynamics and first-principles calculation, and thereby enable quantitative prediction of the influence of HIBs on oil displacement efficiency. In the temporal dimension, key parameters (e.g. interaction strength) as a function of time, obtained from decoupled molecular simulation, should be integrated into macroscopic flow models as initial or boundary conditions. It is recommendable to develop enhanced sampling molecular dynamics methods to effectively predict the dynamic evolution and macroscopic effects of HIBs, achieving effective upscaling from nanosecond to field scale.
(3) In-depth studies on HIBs shall be conducted under the jointing influences of complex mineral system and multiple physical fields. To improve model prediction for real-world geological settings, in terms of experiment, a microfluidic simulation platform for complex, multi-mineral surfaces should be developed to investigate the influences of mineral combination and microscopic heterogeneity on the spatial distribution of HIBs; and in terms of simulation, a fully coupled numerical platform integrating thermal, flow, stress and chemical fields should be constructed to reveal the stability evolution of HIBs. Besides, HIB prediction models considering geological complexity should be built to provide theoretical basis and technical support for field application.
5. Conclusions
The efficient development of low and ultra-low permeability reservoirs represents a major challenge to energy exploitation. HIBs, as important modulators on oil-water-rock interfaces, make great differences to crude oil occurrence and displacement efficiency. So far, studies around HIBs have primarily relied on experimental characterization and molecular simulation. The formation of HIBs is primarily affected by mineral type, ion type, and polar groups in crude oil. Microscopic interactions involved in this process include electrostatic interaction, hydrogen bonding, and van der Waals forces. The hydrogen bonds between polar molecules in crude oil and hydrated ions serve as the main sites for bridge disruption. The interaction strength of HIBs is collectively modulated by ion type and concentration, reservoir solution environment, rock type, and polar components in crude oil, thus affecting crude oil occurrence and mobility.
Despite all valuable findings reported on HIBs, challenges persist across three dimensions: research methodology, scale integration, and geological complexity. More specifically, the dynamic evolution of HIBs remains inadequately elucidated; a discontinuity exists in cross- scale spatiotemporal modeling and prediction; and the reproducibility of real-world geological environments in experimental settings is insufficient. Future efforts shall make breakthroughs to (1) in-situ dynamic experimental characterization and machine learning-augmented simulation strategy; (2) a framework for cross-scale model fusion and upscaling prediction; and (3) in-depth study on HIBs under the jointing influences of complex mineral system and multiple physical fields.