Zhi Li

Gas hydrate blockage threatens pipeline safety in sour gas reservoirs, where hydrogen sulfide (H2S) accelerates hydrate formation and the conventional kinetic inhibitor poly N-vinylcaprolactam (PVCap) severely degrades. Microsecond-level molecular dynamics (MD) simulations and quantum chemical calculations were performed to investigate hydrate nucleation and growth in a 30% H2S environment using unmodified PVCap and three functionalized copolymers: PVCap-co-Apy, PVCap-co-Pip, and PVCap-co-n-butyl. Based on cage type distributions, molecular diffusion, and interaction energies, the inhibitory efficacy follows PVCap-co-Apy > PVCap-co-n-butyl > PVCap-co-Pip > PVCap. The inhibitors barely delay initial nucleation but significantly suppress postnucleation rapid growth. PVCap-co-Apy shows the best performance and a pronounced selective inhibition of 51262 cages, driven by its highly hydrophilic N-acryloylpyrrolidine group that preferentially hydrogen-bonds with water and disrupts local hydrogen-bonded networks. PVCap-co-n-butyl regulates kinetics via a hydrophobic mechanism: the n-butyl group forms local hydrophobic domains that preferentially accumulate methane rather than H2S, modifying the local guest composition and weakening the driving force for cage formation, though steric hindrance partially shields adjacent polar groups. PVCap-co-Pip shows limited efficacy because of weaker hydrogen bonding with polar molecules, and unmodified PVCap performs the poorest. The novelty of this work is 3-fold. It provides the first microsecond-scale molecular comparison of unmodified and functionally modified PVCap under identical high-H2S conditions; it resolves inhibitor action into a nucleation stage that is barely affected and a postnucleation growth stage that is strongly suppressed, a distinction that macroscopic induction-time measurements cannot make; and it identifies two distinct and independently tunable mechanisms, hydrophilic hydrogen-bond disruption and hydrophobic modification of the local guest composition, that map directly onto the choice of pendant functional group. The work indicates structure–activity relationships governed by functional group polarity and hydrophobic–hydrophilic balance, providing guidance for designing high-performance kinetic inhibitors for sour gas fields.

Hydrate-based CO2 capture and storage (HCCS) is a compact route for CO2 sequestration, but hydrate formation is still limited by demanding conditions and slow kinetics. Five natural plant powders, namely citrus peel, pu-erh tea, elm twigs, elm leaves and Poa grass, were evaluated at 275.2 K and 3.6 MPa across three liquid-to-solid ratios (RL/S = 20, 50 and 100) in a visual stirred cell. Elm twigs, elm leaves and Poa grass increased hydrate formation, reaching CO2 uptakes of 118.0, 75.9 and 94.1 v/v at RL/S = 100, respectively; citrus peel acted as a milder capacity promoter, whereas pu-erh tea inhibited formation at RL/S = 20 and 50 but approached the pure-water uptake at RL/S = 100. Hydrate films first appeared at the gas-liquid interface and climbed the reactor wall, followed by particulate growth in the bulk. The results are consistent with combined effects of soluble leachates and residual particles. Elm twigs and Poa grass merit further study as low-cost promoters for HCCS.

An AMP-PZ-EG-H2O water-reduced absorbent was developed for CO2/CH4 separation in natural gas decarbonization. Promoter screening and composition optimization identified PZ as the preferred promoter and an AMP/PZ mass ratio of 2:1 with a total AMP-PZ-EG mass fraction of 50 wt % as the best performance–viscosity compromise. The formulation exhibited a CO2/CH4 separation factor of 831, a CO2 recovery of 98.8%, and an apparent desorption enthalpy of 14.1 kJ/mol at high CO2 loading. 13C NMR and FTIR supported the coexistence of PZ-derived carbamate, AMP-associated bicarbonate, and a possible EG-related carbonate species. After three absorption–thermal regeneration cycles, 96.4% of the initial total titratable alkalinity was retained, and headspace GC-MS showed no monotonic increase in the minor peak tentatively assigned to 4,4-dimethyloxazolidine. An ELECNRTL-based Aspen Plus model predicted 96.41% CO2 removal, 1.20 mol % dry-basis CO2 in the sweet gas, and 98.91% CH4 recovery.

Molecular dynamics simulation was employed to reveal the influence mechanism of H2S on the hydrate inhibition performance of two typical kinetic hydrate inhibitors (i.e., polyvinylpyrrolidone (PVP) and polyvinylcaprolactam (PVCap)). The results indicate that the presence of hydrogen sulfide significantly weakens the inhibitory performance of PVP and PVCap on methane hydrates, and in the growth stage, PVCap appeared to lose its inhibitory effect in our simulations. Because hydrogen sulfide itself promotes hydrate formation, it weakens the performance of the inhibitors primarily by altering the bulk hydrate-forming environment. Hydrogen sulfide and water molecules form transient “pseudo-cyclic” complexes. This structure weaken the adsorption of methane molecules on the inhibitor surface and promote the migration of methane to the hydrate clusters, which may contribute to the self-assembly and growth of hydrates. The simulations also showed a greater reduction in the inhibition performance of PVCap than of PVP. However, condensed-phase analysis indicates that this difference does not result from direct interactions between H2S and the inhibitors. The oxygen-containing functional groups remain coordinated by water molecules, and H2S is not enriched around the hydrophobic groups in either inhibitor system. Instead, the reduced inhibition performance is primarily associated with the influence of H2S on the bulk hydrate-forming environment, including enhanced methane mobility. Together, these effects lead to a significant reduction in the effectiveness of kinetic hydrate inhibitors in H2S-containing systems and provide a molecular-level explanation for the interference caused by H2S. These conclusions are based on a limited number of simulation trajectories and should therefore be regarded as representative of the dominant trends.

This study investigates the microscopic oil recovery characteristics of natural depletion and liquid huff-n-puff in ultralow-permeability conglomerate reservoirs of the Mahu Oilfield. A series of experiments were conducted on conglomerate cores to simulate natural depletion, followed by three rounds of liquid huff-n-puff, utilizing nuclear magnetic resonance (NMR) to quantitatively characterize oil recovery and mobile pore-throat radius threshold. The results demonstrate that natural depletion yields limited recovery, with average recoveries of 9.9%, 8.9%, and 4.8% for cores of high, medium, and low permeability, respectively. In contrast, liquid huff-n-puff significantly enhances recovery, with surfactant A solution achieving the highest incremental recovery (24%) due to its superior interfacial tension reduction capability (0.03 mN/m). Surfactant B solution and simulated formation water increased the recovery by 17% and 14%, respectively. During natural depletion, the oil distinctly mobile pore-throat radii ranged from 0.27 to 1.7 μm for dual-peak pore structures (Types I–II) and from 0.04 to 0.09 μm for single-peak cores (Type III). Posthuff-n-puff, these thresholds decreased, indicating enhanced oil drainage from smaller pores. Notably, surfactant injection does not impede water imbibition in nanosubmicron pores, highlighting synergistic effects between interfacial tension reduction and capillary imbibition. Surfactant huff-n-puff is most effective for dual-peak cores with micrometer-scale pores, whereas imbibition dominates recovery in nanopore-dominated systems. Additionally, the first huff-n-puff round contributed the highest recovery increment, with diminishing returns in subsequent rounds. This work provides the first quantitative analysis of surfactant huff-n-puff efficacy postdepletion in conglomerate reservoirs, offering practical guidance for enhancing recovery in similar unconventional systems.

Sour natural gas holds significant economic value, but its pipeline transportation is challenged by H₂S accelerating natural gas hydrate nucleation/growth and exacerbating pipeline blockages under low temperature. Through molecular dynamics simulations, our study reveals that H₂S promotes nanobubble dissolution and increases local guest molecule concentration, thereby shortening induction time and accelerating nucleation. However, high H₂S concentrations inhibit stable cage growth by disrupting water's hydrogen-bond network, causing the decomposition of unstable/large cages. Simultaneously, H₂S alters hydrate cage composition: reducing H₂S unstable/large cages while significantly increasing stable 5¹²6² cages, thus promoting Structure I (sI) hydrate formation. Mechanistically, H₂S acts as a hydrogen atom donor to form hydrogen-bond-like interactions with water, stabilizing the H-bond network and repairing lattice defects─collectively accelerating hydrate nucleation and growth. These findings provide a fundamental understanding that could advance research on sour natural gas transportation.