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.