No abstract

The design of new dual-function inhibitors simultaneously preventing hydrate formation and corrosion is a relevant issue for the oil and gas industry. The structure-property relationship for a promising class of hybrid inhibitors based on waterborne polyurethanes (WPU) was studied in this work. Variation of diethanolamines differing in the size and branching of N-substituents (methyl, n-butyl, and tert-butyl), as well as the amount of these groups, allowed the structure of polymer molecules to be preset during their synthesis. To assess the hydrate and corrosion inhibition efficiency of developed reagents pressurized rocking cells, electrochemistry and weight-loss techniques were used. A distinct effect of these variables altering the hydrophobicity of obtained compounds on their target properties was revealed. Polymers with increased content of diethanolamine fragments with n- or tert-butyl as N-substituent (WPU-6 and WPU-7, respectively) worked as dual-function inhibitors, showing nearly the same efficiency as commercial ones at low concentration (0.25 wt%), with the branched one (tert-butyl; WPU-7) turning out to be more effective as a corrosion inhibitor. Commercial kinetic hydrate inhibitor Luvicap 55 W and corrosion inhibitor Armohib CI-28 were taken as reference samples. Preliminary study reveals that WPU-6 and WPU-7 polyurethanes as well as Luvicap 55 W are all poorly biodegradable compounds; BODt/CODcr (ratio of Biochemical oxygen demand and Chemical oxygen demand) value is 0.234 and 0.294 for WPU-6 and WPU-7, respectively, compared to 0.251 for commercial kinetic hydrate inhibitor Luvicap 55 W. Since the obtained polyurethanes have a bifunctional effect and operate at low enough concentrations, their employment is expected to reduce both operating costs and environmental impact.

Gas hydrates are considered a major problem in the oil and gas transportation pipelines, and their crystallization involves nucleation and growth of hydrate crystals. Hence, developing inhibitors that can affect nucleation and growth of hydrates is essential to inhibit their formation. Acrylamide polymers are well-known hydrate inhibitors, but they show a low cloud point, causing precipitation problems for field applications. In this research, we used chitosan to synthesize chitosan-graf t-polyacrylamide (CSg-PAM) as a green and high-cloud-point kinetic hydrate inhibitor (KHIs). The inhibition performance of CS-g-PAM on nucleation and growth of methane hydrate crystals was assessed by a high-pressure autoclave and highpressure microdifferential scanning calorimeter (HP-μDSC). CS-g-PAM showed no cloud point in both deionized water and 3.5 wt % NaCl solutions, up to 100 °C. Autoclave experiments demonstrated that CS-g-PAM can increase the hydrate nucleation time 13 times (in 1 wt % sample) compared to the pure water system. According to HP-μDSC results, by adding 0.1, 0.5, and 1 wt % CS-g-PAM, the onset methane hydrate formation temperature was decreased from −12.7 °C in the pure water system to −15.8, −17.0, and −19.0 °C, respectively. Also, CS-g-PAM can change the morphology of methane hydrate crystals from solid state into a viscous foam-like slurry. These results show that the modification of acrylamide-based KHIs with natural polymers is an attractive option to improve their deposition point and change the morphology of hydrate crystals.

The development of technologies for the accelerated formation or decomposition of gas hydrates is an urgent topic. This will make it possible to utilize a gas, including associated petroleum one, into a hydrate state for its further use or to produce natural gas from hydrate-saturated sediments. In this work, the effect of water content in wide range (0.7–50 mass%) and the size of quartz sand particles (porous medium; <50 μm, 125–160 μm and unsifted sand) on the formation of methane and methane-propane hydrates at close conditions (subcooling value) has been studied. High-pressure differential scanning calorimetry and X-ray computed tomography techniques were employed to analyze the hydrate formation process and pore sizes, respectively. The exponential growth of water to hydrate conversion with a decrease in the water content due to the rise of water–gas surface available for hydrate formation was revealed. Sieving the quartz sand resulted in a significant increase in water to hydrate conversion (59% for original sand compared to more than 90% for sieved sand). It was supposed that water suction due to the capillary forces influences both methane and methane-propane hydrates formation as well with latent hydrate forming up to 60% either without a detectable heat flow or during the ice melting. This emphasizes the importance of being developed for water–gas (ice–gas) interface to effectively transform water into the hydrate state. In any case, the ice melting (presence of thawing water) may allow a higher conversion degree. Varying the water content and the sand grain size allows to control the degree of water to hydrate conversion and subcooling achieved before the hydrate formation. Taking into account experimental error, the equilibrium conditions of hydrates formation do not change in all studied cases. The data obtained can be useful in developing a method for obtaining hydrates under static conditions.

Surfactants have been reported as the most efficient gas hydrate promoters (GHPs) for gas storage and transportation; however, slow kinetics of nucleation and growth of hydrate crystals and foam formation during hydrate dissociation severely impact their applications. Here, a new class of chemical additives based on ethylenediaminetetraacetic acid bisamides was developed to control methane hydrate formation for gas storage and flow assurance applications. Synthesized molecules contain both polar fragments (carboxyl and amide groups) and hydrophobic alkyl groups with different sizes and branching. The obtained results revealed that bisamides with short alkyl chains (n-propyl and isopropyl) promoted the formation of methane hydrate and significantly reduced foam stability during hydrate decomposition compared to sodium dodecyl sulfate (SDS). Moreover, by increasing the length of the alkyl substituent up to propyl, the nucleation time increased. However, the conversion of gas to hydrate escalated remarkably. A transition from promotion to inhibition properties is observed with a further increase in the alkyl chain from propyl to butyl. Nevertheless, bisamides with hexyl groups showed surfactant properties, which is responsible for their poor promotion efficiency. In addition, the studied compounds practically do not form foam and are less toxic compared to SDS as a wellknown GHP. The results of this study can be useful for the design and development of effective additives for gas storage and flow assurance applications.

The efficiency of corrosion inhibition for waterborne polyurethane based on N-tert-butyl diethanolamine (tB-WPU) is investigated using different techniques. Corrosion weight loss, open circuit potential experiments, electrochemical impedance spectroscopy, and potentiodynamic polarization measurements show that both a commercial reagent and a polyurethane-based inhibitor prevent corrosion at increasing temperature to 50 °C. At 75 °C, the activity of both reagents is reduced. In stirring conditions, the effectiveness of acid corrosion inhibition (25 °C, 500 ppm) drops abruptly from 89.5% to 60.7%, which is related presumably to the complexity of binding the polymer molecules to the metal surface. As follows from thermodynamic calculations, the adsorption of tB-WPU on the metal surface in 2M HCl can be treated as a physisorption. Model quantum–chemical calculations support the experimental studies and elucidate the nature of steel surface–inhibitor molecule chemical bond, which is realized mainly by carboxyl and amino groups. It is concluded that WPUs can be considered as a perspective alternative to commercial oilfield reagents due to their versatility.

High toxicity and huge foaming are two severe challenges for gas storage strategies based on promoting the gas hydrate formation using surfactants. The present study used castor oil as an eco-friendly resource to develop novel biosurfactants for methane storage. Transmission and scanning electron microscopy, dynamic light scattering, and interfacial tension measurements revealed the surfactant properties of sulfonated castor oil (SCO). In addition, a high-pressure autoclave and a microdifferential scanning calorimeter test unveiled SCO as an effective kinetic hydrate promoter. The results showed that SCO significantly enhanced the rate of methane hydrate formation. A maximum of 76% water-tohydrate conversion was observed in 0.1 wt % SCO solution under stirring conditions. Pure water, 0.1 wt % SCO, and 0.1 wt % sodium dodecyl sulfate (SDS) solutions allowed 50% conversion to be achieved for 329, 39, and 27 min, respectively. This made the castor oil-based reagent as effective as the well-known kinetic hydrate promoter SDS. Furthermore, the SCO solution's foam ratio and stability were 8.25 and 2.75 times lower than those of SDS. Additionally, SCO showed a more favorable safety profile for humans and the environment as it is toxic to animals in higher concentrations than SDS according to in vivo studies. In addition, a combination of high-pressure DSC, low-temperature powder Xray diffractometry, and visual analysis of hydrate samples depending on the temperature mode, promoter type, and its concentration revealed that SCO and SDS enhanced hydrate growth by different mechanisms. The loose hydrate mass was squeezed out toward the gas phase in both cases. However, in the case of SDS, the hydrate traditionally climbed the cell walls while SCO seemed to change the wall wettability, which led to the transfer of water into the reaction zone along with the forming hydrate crystals and the domed shape of the hydrate. These findings provide reliable evidence to synthesize efficient and environmentally friendly reagents based on castor oil to improve methane storage in the clathrate hydrate.

No abstract