Jingsheng Lu

Gas invasion into liquid-saturated hydrophilic porous media is governed by interfacial, wetting, viscous, and pore-scale effects that control pathway selection and residual liquid trapping. However, how liquid composition influences gas-pathway establishment, postbreakthrough redistribution, and final liquid retention remains insufficiently understood. Here, nitrogen displacement was visualized in a hydrophilic glass microfluidic network initially saturated with 12 liquid systems, including water, 3.5 wt % NaCl brine, and formulations containing ethanol, polyethylene glycol (PEG), sodium dodecyl sulfate (SDS), or sodium dodecylbenzenesulfonate (SDBS). Breakthrough time, liquid-saturation evolution, final residual liquid saturation, stabilization time, and final gas morphology were quantified. Three invasion modes were identified: preferential-channel flow, dendritic expansion, and network-like expansion. Breakthrough times ranged from 1 to 67 s, showing that rapid formation of a spanning gas pathway did not necessarily correspond to extensive liquid removal. PEG system W5 showed the highest final residual liquid saturation (∼0.42), whereas SDBS system W11 showed the lowest (∼0.07), corresponding to a final gas occupancy of ∼0.93. SDS system W8 and SDBS system W10 both exhibited high final gas occupancy together with relatively short stabilization times. Lower measured air–liquid surface tension was associated with broader branched invasion in some systems, while final liquid removal likely reflected coupled effects of viscosity, wettability, and pore-structure heterogeneity. Film-like residual liquid and locally isolated gas regions further showed that extensive gas occupation did not necessarily ensure complete connectivity among the optically resolved gas-filled regions. These results reveal a decoupling between breakthrough dynamics and final liquid removal and provide mechanistic pore-scale insight relevant to evaluating water-block mitigation in hydrate-bearing sediments.

During natural gas hydrate production in the Shenhu area of the South China Sea, water released from hydrate dissociation coupled with formation water, may induce water-locking, which blocks gas flow pathways and restricts productivity. To elucidate the mechanisms and potential remediation strategies, this study employed low-field NMR to systematically investigate the spatiotemporal evolution of water-locking in argillaceous silt hydrate reservoirs at initial water saturations of 50%, 70%, 90%, and 100%, and evaluated three antiwaterblocking agents (fluorocarbon types I and II, and nonionic surfactant III). Results show that water-locking predominantly occurs in micropores (T2 < 10 ms, pore size < 1 μm), with intensity increasing exponentially with saturation through four stages (end-face, mid-end combined, wide-area, and full-domain water-locking) identifying 70% saturation as the critical range for the tested material. Agents I and II perform effectively at low to moderate saturations, but their efficacy is severely limited at saturations ≥90% and under fully saturated conditions. Displacement pressure synergistically enhances agent performance, yet pressure alone is insufficient. This study clarifies the evolution mechanisms and application boundaries of chemical remediation, offering theoretical guidance for water-locking prevention and efficient hydrate development in the South China Sea.

As a representative metal oxide, ZnO nanoparticles combine nanoscale dimensions with tunable amphoteric surface properties and have shown considerable potential for foam stabilization. However, the molecular mechanism underlying this stabilization, particularly the role of surface protonation, remains unclear. Using an SDS foam film as the model system, molecular dynamics simulations (MD) were used to characterize interfacial hydration and surfactant adsorption, while steered molecular dynamics (SMD) simulations reproduced compression-driven drainage at P2 and lateral rupture of the solid bridge at P1. The results show that surface protonation governs foam-film behavior by jointly regulating nanoparticle hydration and surfactant adsorption. The protonated ZnO surface formed the densest hydration layer, with a surface O–water O coordination number of 3.817 within 0.35 nm, while simultaneously exhibiting the strongest adsorption of SDS headgroups. These effects produced a compact ZnO–SDS solid bridge maintained primarily by electrostatic attraction, supplemented by hydrogen bonding and broadly distributed van der Waals interactions, with steric repulsion confined to localized close contacts. The coupled hydration and adsorption structure impeded pressure-driven water migration during compression, reducing the apparent diffusion coefficient at P2 by 29.0% and maintaining the lowest water mobility at elevated temperatures. Decreasing surface protonation shifted the dominant stabilization pathway. Neutral ZnO primarily acted as a physical barrier to local film thinning, delaying rupture relative to the pure SDS film. Deprotonated ZnO weakened direct sulfate adsorption but induced pronounced Na⁺ counterion accumulation, whose electrostatic screening effect, together with the ordered SDS layer, generated considerable collective interfacial resistance. ZnO therefore stabilizes SDS foam films through the cooperation of solid-bridge reinforcement, hydration-mediated water retention, restricted drainage, and counterion-mediated interfacial organization. These findings clarify the molecular basis of ZnO-stabilized foam films and provide theoretical guidance for developing high-performance foam fluids for industrial applications.

Water-lock effects can impair gas transport in low-permeability hydrate-bearing clayey-silt sediments. However, current screening strategies for candidate mitigation formulations rely primarily on macroscopic measurements and provide limited insight into pore-scale liquid invasion and gas trapping. Previous microfluidic studies have focused mainly on fundamental two-phase displacement behavior, whereas systematic pore-scale frameworks for comparing multiple candidate water-lock mitigation formulations remain limited.In this study, microfluidic visualization and quantitative image analysis were used to compare the invasion behavior of 12 solution systems, including deionized water, simulated seawater, and alcohol-, polymer-, and surfactant-based formulations, in an initially gas-saturated pore network. The A–B transport time (tAB) ranged from 26 to 161 s, whereas the final residual gas saturation (Sgr) ranged from 0.118 to 0.682. A short tAB did not necessarily correspond to low Sgr because several rapidly advancing formulations propagated predominantly along preferential pathways and bypassed substantial portions of the pore space. In contrast, more stable displacement fronts were associated with broader pore-scale sweep and lower residual gas trapping, suggesting that displacement-front stability is closely associated with the final gas distribution and liquid occupancy. Among the tested systems, the C4 formulation, containing 10 wt % polyethylene glycol (PEG), 3.5 wt % NaCl, and 86.5 wt % deionized water, provided the most favorable balance between final liquid-phase occupancy and stabilization time. The 20 wt % ethanol formulation (C3) achieved the lowest Sgr but required a longer stabilization time. These results indicate that formulation screening should jointly consider A–B transport time, displacement-front stability, and final liquid-phase occupancy rather than use propagation rate as the sole criterion. The proposed framework provides a pore-scale basis for the preliminary screening of candidate water-lock mitigation formulations, although further validation in hydrate-bearing clayey-silt sediments is required.

Hydraulic fracturing has been recognized as a practical means of enhancing production from natural gas hydrate reservoirs (NGHRs). However, during depressurization-based production, hydrate dissociation-induced skeleton weakening and production-driven fines migration can cause proppant embedment and fracture conductivity loss. A DEM-CFD framework was developed in this work to capture hydrate dissociation-induced cementation degradation together with proppant embedment and fines invasion. The effects of depressurization magnitude, gravel-proppant/GHBS-particle median size ratio (GSR), and proppant concentration on fracture closure and conductivity evolution were systematically investigated, and single-factor comparisons were used to distinguish the relative contributions of embedment and clogging. The results show that hydrate dissociation-induced cementation weakening is the key prerequisite for rapid conductivity damage. As the pressure differential increased from 1 to 12 MPa, the conductivity damage factor increased to 3.73 times its initial value, and fracture conductivity decreased by 95.81%, with the dominant damage mode shifting from slight embedment to the synergistic effect of severe embedment and near-fracture clogging. A larger GSR improved absolute fracture conductivity but also increased the relative risk of conductivity damage. Higher proppant concentration enhanced fracture aperture and conductivity, although the improvement showed clear diminishing returns and increased fines retention risk. Single-factor analysis further showed that, under hydrate dissociation conditions, closure stress is the primary controlling factor for conductivity damage, whereas flow velocity mainly affects conductivity deterioration by regulating the contribution of clogging. These findings provide theoretical support for proppant design and long-term fracture conductivity maintenance in NGHR stimulation.

Accurate determination of heavy metals in marine hydrate-associated muds is crucial for tracing methane seepage, yet it faces challenges from complex matrices. This study developed a matrix-matched microwave digestion ICP-OES method. By comparing XRF spectral profiles and statistical tests, the feasibility of using soil certified reference materials to simulate marine mud matrices was demonstrated, thereby optimizing digestion parameters (power/temperature) and ICP-OES spectral line selection. Method validation revealed detection limits of 0.0004 – 0.0105 mg/L for Cu, Zn, Pb, and Cr, with spike recoveries ranging from 95.5% – 103.7%. Accuracy was further verified using soil reference materials (GSS-4/5) and comparative t-tests with ICP-MS data. This efficient and reliable method provides a practical analytical tool for geochemical exploration of marine gas hydrates.