CH/CH2 Group Clusters Doping Methane Hydrate Cages.

Methane hydrate is a crystalline compound with methane molecules as guest species trapped in host water cages. In this study, we detected methane hydrate with water cages doped by (Caromatic-H)5 clusters, (Caromatic-H)6 clusters, and (3Caliphatic-H2 + 2H2O) clusters using current spectroscopic techniques and differential scanning calorimetry (DSC). Methane molecules are trapped in the doped cages with type sI forming in nanoscale silica gel pores. The relative quantity ratio of host carbon to guest carbon in the doped hydrate sample reaches approximately 3.58. Methane hydrate doped by CH/CH2 group clusters greatly improves the ability of the hydrate unit cell to store methane and increases the stability of methane hydrate. Fast proton diffusion in the doped methane hydrate was confirmed. The results of this study will provide efficient and energy saving technical support for disruptive changes in hydrate storage and transportation of methane gas technology with a doped and dense solid phase.

Investigation on Hydrate Formation and Growth Characteristics in Dissolved Asphaltene-Containing Water-In-Oil Emulsion.

Clarifying the effect of asphaltene on hydrate formation and growth is of great significance to the operation safety in deepwater petroleum fields. To investigate the influence of low-concentration dissolved asphaltenes on the formation kinetics and growth process of hydrates in water-in-oil emulsions, experiments with asphaltene concentrations ranging from 50 to 1000 ppm were carried out using a high-pressure visual reactor. At a low concentration, the adsorption of asphaltene monomers on the oil-water interface or nanoaggregates in the bulk barely affected the nucleation of hydrate and the induction time of hydrate formation. However, it would hinder the microscopic mass transfer process and heat transfer process between gas molecules and then mitigate the initial rate of hydrate formation. Therefore, the dissolved asphaltenes could not be used as antiagglomerants (AAs) to efficiently inhibit the aggregation of hydrate particles at low concentrations under our experimental conditions, causing extensive hydrate agglomeration and deposition in the reactor.

A New Model for Calculation of Arrest Toughness in the Fracture Process of the Supercritical CO2 Pipeline.

A new model based on a decompression wave prediction model and an improved BTC model has been developed to investigate the arrest toughness in the fracture process of the supercritical CO2 pipeline. The comparison of the decompression wave velocity and the fracture propagation velocity was carried out to identify whether the pipe can prevent fracture propagation relying on its own toughness. If not, the minimum Charpy V-notch energy and the minimum wall thickness of steel pipes required for arrest fracture can be calculated using the improved BTC model. The results show that the working conditions with an initial pressure for the fracture of 11.7 MPa and a temperature of 323.15 K are the most difficult conditions to stop the fracture. The minimum wall thickness calculated only according to the strength design cannot meet the toughness requirements for ductile fracture arrest in the most difficult conditions in some cases. Then, the minimum wall thickness of the supercritical CO2 pipeline required for ductile fracture arrest in these cases will be obtained. For instance, the minimum wall thicknesses of X65, X70, and X80 steel pipes for fracture arrest with a pipe diameter of 610 mm at a design pressure of 13.2 MPa are 17.28, 14.58, and 12.81 mm, respectively, and when the pipe diameter is 1016 mm at a design pressure of 20.4 MPa, the minimum wall thicknesses of X70 and X80 pipes can meet the requirements of arrest toughness. The model established in this study can quickly and accurately calculate the minimum wall thickness and minimum Charpy energy required to stop fracture in the supercritical CO2 pipeline, which is suitable for engineering applications. The findings of this study can help in better understanding of the fracture process of supercritical CO2 pipelines.

Investigation on Hydrate Growth at the Oil-Water Interface: In the Presence of Wax and Surfactant.

Natural gas hydrates can readily form in deep-water-oil production processes and pose a great threat to subsea pipeline flow assurance. The usage of surfactants and hydrate antiagglomerants is a common strategy to prevent hydrate hazards. In water/wax-containing oil systems, hydrate coexisting with wax could lead to more complex and risky transportation conditions. Moreover, the effectiveness of surfactants and hydrate antiagglomerants in the presence of wax should be further evaluated. In this work, for the purpose of investigating how wax and surfactants could affect hydrate growth at the oil-water interface, a series of microexperiments was conducted in an atmospheric visual cell where the nucleation and growth of hydrates took place on a water droplet surrounded by wax-containing oils. On the basis of the experimental phenomena observed using a microscope, the formation of a hydrate shell by lateral growth, the collapse of a water droplet after hydrate initial formation, and the formation of hollow-conical hydrate crystals were identified. These experimental phenomena were closely related to the concentration of wax and surfactant used in each case. In addition, it was shown that the effectiveness of the surfactant could be weakened by wax molecules. Moreover, there existed a critical wax content above which the effectiveness of the surfactant was greatly reduced and the critical wax content gradually increased with increasing surfactant concentration. This work could provide guidance for hydrate management in wax-containing systems.

Investigation on Hydrate Growth at the Oil-Water Interface: In the Presence of Wax and Kinetic Hydrate Inhibitor.

In oil industry, the coexistence of hydrate and wax can result in a severe challenge to subsea flow assurance. In order to study the effects of wax on hydrate growth at the oil-water interface, a series of microexperiments were conducted in a self-made reactor, where hydrates gradually nucleated and grew on the surface of a water droplet immersed in wax-containing oil. According to the micro-observations, hydrate shells formed at the oil-water interface in the absence of kinetic hydrate inhibitor (KHI). The roughness and growth rate of hydrate shells were analyzed, and the effects of wax were investigated. In addition, vertical growth of the hydrate shell was observed in the presence of wax, and a mechanism was proposed for illustration. In the presence of KHI, small hydrate crystals formed separately at the oil-water interface instead of hydrate shells. The presence of KHI reduced the growth rate of hydrates and changed the wettability of hydrates. Moreover, the presence of wax showed no obvious effect on the effectiveness of KHI under experimental conditions.