The formation of clathrate hydrates in pipelines is potentially threatening to exploration and gas transportation in the petroleum industry. To reduce such risks, various surfactants have been explored as anti-agglomerants to prevent the aggregation of hydrate particles. However, the anti-agglomeration mechanisms are not yet fully understood. In this study, modified atomic force microscopy is first developed to investigate the effects of two surfactants, namely, 1-naphthaleneacetic acid and dodecylbenzenesulfonic acid, on the surface of a tetrahydrofuran (THF) hydrate. The surfactants have remarkable effects in terms of changing the grain size and decreasing the grain boundary depth and surface roughness of the THF hydrate. In addition, the surfactants reduce the quasi-liquid layer (QLL) thickness of the THF hydrate and the adhesion forces between the hydrate and the microsphere probe. This phenomenon may be caused by the changes induced by surfactant molecules on the water–guest molecule structure near the gas/water interface. Thus, hydrate growth is enhanced in the QLL. The adhesion forces decrease linearly with QLL thickness after adding the surfactants. These findings indicate that surfactants reduce the adhesion forces by reducing the QLL thickness at higher temperatures. The effects of surfactants on the QLL thickness and surface morphology of the hydrate are investigated, providing critical information on the cohesive behaviors among hydrate particles or between hydrate particles with other materials. Our work provides new insights into the underlying mechanism of how surfactants prevent hydrate aggregation, which is crucial in hydrate-related safety management.
Abstract The invasion of drilling fluids during well drilling through gas hydrate‐bearing sediments may seriously damage gas hydrate stability and distort well‐logging identification and evaluation in exploration and production of gas hydrate reservoirs. Adding nanoparticles into drilling fluids can be an efficient method to reduce fluid invasion. However, nanoparticles may induce hydrate formation in wells, which will block annulus and lead to safety accidents. Therefore, suitable nanoparticles used for hydrate drilling should be clarified first. This study addressed this issue by experimentally investigating the influence of hydrophilic and hydrophobic nano‐CaCO 3 on CH 4 hydrate formation in a dynamic system. We performed a series of experiments by using nano‐CaCO 3 (1.0–6.0 wt%) with different particle sizes (20, 70, and 700 nm) at 3.0 °C and 6.0 MPa. The macroscopic kinetic parameters of hydrate formation were obtained. The results show that hydrophobic nano‐CaCO 3 particles promote hydrate formation, while hydrophilic ones can inhibit hydrate formation at certain particle sizes and concentrations. This is mainly due to the different surface wettability, resulting in the different distribution of water and gas molecules in fluids. The hydrophilic nano‐CaCO 3 with the particle size of 20 nm and addition of 3.0 wt% has the strongest inhibition effect under the given experimental conditions. In comparison with ultrapure water, the induction time is increased by about 38%, while the formation amount and rate are decreased by about 13% and 18%, respectively. This work will provide valuable ideas and references for the design of deepwater drilling fluid using nanoparticles, and also provide insight into revealing the formation and evolution mechanism of hydrate deposits at the micropore or even nanopore scale.
Abstract The distributions of lipids in surface and subsurface sediments from the northern South China Sea were determined. The n ‐alkanes were in bimodal distribution that is characterized by a centre at n ‐C 16 – n ‐C 20 with maximum at C 18 (or C 19 ) and n ‐C 27 – n ‐C 31 as well as at C 29 (or C 31 ). The short‐chain alkanes suffered from significant losses due to their slow deposition in the water column, and their presence with a slight even carbon predominance in shallow seafloor sediments was ascribed mainly to the direct input from the benthos. The long‐chain alkanes with odd predominance indicate transportion of terrigenous organic matter. Immature hopanoid biomarkers reflect the intense microbial activity for bacteria–derived organic matter and the gradual increase of maturity with burial depth. Abundant n ‐fatty acid methyl esters ( n ‐FAMEs) that are in distributions coincident with fatty acids were detected in all samples. We proposed that the observed FAMEs originated from the methyl esterification of fatty acids; methanol production by methanotrophs and methanogenic archaea related to the anaerobic oxidation of methane, and sulfate reduction provided an O–methyl donor for methylation of fatty acids. The CH 4 released from hydrate dissociation at oxygen isotope stage II of Cores ZD3 and ZS5, which had been confirmed by the occurrence of negative δ 13 C excursion and spherical pyrite aggregates, could have accelerated the above process and thus maximized the relative content of FAMEs at ZD3‐2 (400–420 cm depth) and ZS5‐2 (241–291 cm depth).
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