Summary
As a good low-dose hydrate inhibitor, the anti-agglomeration agent can keep the generated hydrate particles well dispersed, and avoid the plugging caused by a large amount of hydrate aggregation. However, the application of most Anti-agglomerants is severely limited by the moisture content in the system, that is, they cannot play a role under high moisture content. The purpose of this work is to explore the effect of new anti-aggregation agent ( coconut oil amide propyl betaine ) on the flow stability of oil-water emulsion system under different water content and flow rate conditions, and confirm that the new anti-aggregation agent under high water content and even pure water conditions can still play an anti-coagulation role, so that hydrates can form stable and mobile mud. In addition, in order to explore the parameter changes in the system caused by the transient changes ( shutdown and start ) in the flow system and the flow characteristics of hydrate in the non-flat pipeline, the flow characteristics of hydrate slurry with inclined pipe section are explored, and the changes in the flow characteristics of hydrate with shutdown and restart in the actual production process are explored by stopping and restarting the equipment. The conclusion can provide theoretical support for hydrate anti-agglomeration in high water content system and hydrate slurry flow in unconventional pipe closure and restart system.
Introduction
Natural gas hydrate, also known as methane hydrate, is a solid compound with ice-like appearance but different crystal structures formed by natural gas and water in a non-stoichiometric (Baek et al. 2017; Dong et al. 2020; Wu et al. 2021). In 1934, Hammerschmid et al. found that hydrate was one of the reasons for blockage of natural gas pipeline mining equipment, and its formation temperature was far above ice. The initial stage of natural gas exploitation usually has a high moisture content. The full contact of air flow and free water in the pipe provides a large number of nucleation sites, and hydrates are formed in the pipe when the temperature and pressure conditions are appropriate. In recent years, with the gradual transfer of oil and gas exploitation from inland to deep sea areas, the possibility of hydrate formation is further enlarged under the environment of high pressure and low temperature, and the possibility of hydrate accumulation and pipe blockage is also increased. Once the gas well is stopped or the pipeline is blocked, the transportation accident is caused, which is easy to cause equipment damage and large economic losses, and even cause casualties. In the past 10 years, hydrate has become an important issue in the field of flow security. Controlling the risk of hydrate formation and blockage in pipelines and ensuring the safety of deep-sea oil and gas flow have become an urgent problem for researchers in various countries.
In the past, some engineering methods that were ineffective or costly were often used to prevent or inhibit the formation of hydrates, such as dehydration and adding thermodynamic inhibitors. In order to save the economic cost of natural gas hydrate risk management and improve engineering efficiency, low-dose hydrate inhibitors ( kinetic inhibitors and anticoagulants ) have been found and widely used in hydrate blockage prevention and control. Kinetic inhibitors are usually water-soluble polymers, and the existence of hundreds of ppm kinetic inhibitors can change the intrinsic kinetic characteristics of hydrate growth, prolong the induction period required for nucleation, so that the fluid can flow smoothly in a certain time. However, the application of kinetic inhibitors is affected by the environment. When the undercooling is too high, kinetic inhibitors will lose their effectiveness. Therefore, scholars have developed compound inhibitors of various kinetic inhibitors and synergistic agents to avoid this shortcoming.
On the contrary, the inhibitor is not involved in the nucleation and growth of hydrate, so it can play a role at high undercooling. AA can keep the generated hydrate particles in good dispersion, and avoid the plugging caused by a large amount of hydrate accumulation. Therefore AA usually plays a role in the management and control of hydrate particles. Quaternized ammonium salt ( QAS ) is the most common Anti-agglomerants. Quaternized ammonium salt ( QAS ) with many commercial applications has been used for hydrate Anti-agglomerants, and it has been proved that it can withstand high undercooling. The single-tailed quaternary ammonium salt QAS contains a hydrophobic tail group of 10 – 14 carbon atoms with ammonium head groups and anions . Due to the higher enthalpy of the hydrophilic group of the inhibitor, the hydrogen bond formed between the hydrate and AA is more solid. Therefore, anti-aggregation in oil-water coexistence system will make oil-water phase emulsification, resulting in water phase dispersed in the oil phase in the form of water droplets. Gas hydrates formed on the surface of water droplets will be solubilized in the microemulsion and thus difficult to aggregate. Because of this, the application of most Anti-agglomerants is severely limited by the moisture content in the system, that is, they cannot play a role under high moisture content. In addition, QAS has limitations such as toxicity and low biodegradability.
However, scholars have never stopped their research and development on the applicability of Anti-agglomerants agents in different systems and new green Anti-agglomerants agents. Sun et al. (Sun et al. 2013) developed a new low-dose surfactant ( 0.2 wt. % ) and found that it can play an anti-coagulation role in any water content system, even in pure water systems without oil-in-water emulsions. Based on the experimental results, they obtained a new anti-coagulation mechanism of emulsion-free hydrates based on micelle equilibrium. Phan et al. (Phan et al. 2021) aimed to accurately predict and design the molecular structure and properties of the Anti-agglomerants agent by simulation method. They compared the kinetic simulation data with the experimental data of micromechanical force measurement and obtained good consistency. The results showed that the entropy and solvent free energy of AA and its molecular orientation at the rehydrate-oil interface greatly determined the Anti-agglomerants performance of AA. Gao et al. (Gao et al. 2009) found through experiments that the increase of salt concentration of brine in the high water content system would make the performance of the polymer inhibitor step-by-step increase. Firoozabadi et al. (Firoozabadi et al. 2014) found that in the system containing carbon dioxide and other acidic components, the Anti-agglomerants agent will lose its original effect. By adding a small amount of sodium hydroxide and eliminating foaming oil, good synergistic benefits can be achieved. Zhao et al. (Zhao et al. 2016) found that the addition of lithium hydroxide was more effective than the traditional sodium hydroxide for the Anti-agglomerants, and the dosage was greatly reduced. They suggested that there was a complex synergy between sodium chloride and AA, and an increase in salt concentration would significantly reduce the use of basic chemicals. Li et al. (Li et al. 2018) studied the anti-agglomeration performance of different concentrations of Anti-agglomerants on cyclopentane hydrate through micromechanical force measurement device, and proposed the mechanism of Anti-agglomerants on hydrate at different concentrations. Dong et al. (Dong et al. 2018) reported the effect of water content and sodium chloride concentration on the effectiveness of AA in the presence of cosurfactant ( span80 ). The results showed that sodium chloride decreased the anti-aggregation effect of the compound system when the water content was 10 %, while it was promoted when the water content was 20 – 30 %. NaCl had no significant effect on AA performance at 50 % moisture content. In the case of 80 – 100 % moisture content, the water-in-oil emulsion with hydrate as continuous phase needs higher salt concentration to promote anti-agglomeration.
At present, the research on the hydrate management strategy for steady flow has gradually matured, but the research on the transient situation ( shut-down and start-up ) in the flow system is very scarce. During the shut-in period, the temperature is rapidly cooled due to the static fluid, so the environment may reach the hydrate formation conditions during this period. When restarted, due to the sudden increase in the flow rate, the full mixing of oil, gas and water leads to the increase of the nucleation site of the hydrate, which may lead to the explosive growth of the hydrate and cause plugging. The shutdown caused by various factors in practice is uncontrollable, which is the most worrying and important threat to hydrate formation. Therefore, it is urgent to explore hydrate formation and flow characteristics under shutdown and restart conditions. Some studies have been devoted to the flow characteristics of transient hydrate slurry and the influence of rheological properties and Anti-agglomerants on the plugging characteristics of hydrate during restart (Zhang et al. 2021; Kakitani et al. 2019; Kakitani et al. 2022; Shuard et al. 2017; Sohn et al. 2017). Shi et al. (Shi et al. 2018) carried out a series of experiments on the shutdown and restart of carbon dioxide hydrate in water-dominated system. The results show that the sudden restart after the first shutdown will lead to explosive hydrate formation and irreversible blockage. Liu et al. (Liu et al. 2021) explored the visualized high-pressure flow loop to explore the plugging characteristics of hydrate shut-down and restart. The results showed that shut-down and restart would lead to continuous decrease of system temperature and acceleration of hydrate accumulation. With the extension of shut-down time, the plugging risk of hydrate was further increased, and low liquid loading would accelerate the deposition process of hydrate. Yan et al. (Yan et al. 2014) carried out a long-term shutdown test ( 2 hours, 4 hours, 8 hours ) in restart under the action of AA can be safe flow, shutdown restart experiments show that hydrate slurry has obvious shear thinning behavior.
The purpose of this work is to explore the effect of new anti-aggregation agent ( coconut oil amide propyl betaine ) on the flow stability of oil-water emulsion system under different water content and flow rate conditions, and confirm that the new anti-aggregation agent under high water content and even pure water conditions can still play an anti-coagulation role, so that hydrates can form stable and mobile mud. The flow characteristics of hydrate slurry with inclined pipe section were explored, and the change of flow characteristics of hydrate with shutdown and restart in the actual production process was explored by stopping and restarting the equipment. The conclusion can provide theoretical support for hydrate anti-agglomeration in high water content system and hydrate slurry flow in unconventional pipe closure and restart system.
2. Experimental materials and procedures
Experimental device : The detailed information of the self-made hydrate flow loop experimental device has been reported in previous literature. In short, the system consists of an intake system, a liquid intake system, a cooling system and a data acquisition system. Piston metering pump ( 30L / h; 50MPa ) can be pumped into the hydrate kettle, circulating pump ( 1400rpm ; 50 Hz ) can be used to drive the fluid in the kettle into the loop and make the liquid flowing into the loop circulate. The total length of the loop test section is 30 m, the inner diameter is 22 mm, the total volume is 45 L, and the design pressure of the pipeline is 16 MPa. The whole test section and the hydrate kettle are wrapped by the cooling jacket. The coolant pumped by the cooling system can cool the loop, and the temperature control range is - 10 - 40 °C. In addition, pressure sensors, differential pressure sensors, temperature sensors, gas mass flowmeters, liquid mass flowmeters, focused beam reflection measurement ( FBRM ) and particle video microscope ( PVM ) are equipped to detect the temperature and pressure state of fluid in the loop and the macroscopic and microscopic flow characteristics. Fig. 1 is the experimental device diagram.