Fig. 1. Experimental device diagram
Experimental procedure :
Open the vacuum pump and empty the entire experimental loop to 0.08 MPa
to remove air from the tube. Observe whether the pressure gauge changes,
check the air tightness of the experimental circuit. Pump a
predetermined amount of tap water or coconut oil amide propyl betaine
solution into the hydrate kettle. Start magnetic centrifugal pump
stirring evenly, at the same time, start water bath and set the
experimental inlet temperature T0. Adjust the angle
between the loop experimental section and the horizontal plane, open the
circulating pump on the loop, and the liquid in the kettle enters the
loop and circulates at a certain flow rate. Open carbon dioxide
cylinders and piping intake valves and pressure regulating valves. When
the pressure in the pipeline reaches the experimental pressure and
reaches the dissolution equilibrium ( the pressure remains unchanged
within 30 min ), the gas injection process is stopped. Also ensure that
the temperature in this step is higher than the phase equilibrium
temperature of carbon dioxide hydrate. FBRM and PVM were started to
record the change of particle size before and after hydrate formation.
Get the string length distribution of particles and the microscopic
image of the experimental system ; the water bath temperature was set to
a set value T1, and the data acquisition system was
started. At the same time, the macroscopic morphology of hydrate
formation and flow was observed through the visual window. In the
cooling process of some experimental schemes, the circulating pump was
closed to simulate the shutdown environment of the actual pipeline. When
the temperature and pressure of the experimental system tend to be
stable or the experiment reaches the preset duration or when the flow
rate of the pipeline decreases to zero, the loop is blocked by the
hydrate. Close the circulating pump and cooling system. Open the pipe
drain valve to remove carbon dioxide and liquid from the pipe. Then
reinjection clean water to flush the pipe and use compressed air to
clean the pipe. Hydrate formation experiment ended, the next set of
experiments.
3. Result and discussion
In this experiment, the influence of water content on the effect of
polymer inhibitor(Composition of the oil phase is No. 10 industrial
white oil), the influence of initial flow rate on the flow
characteristics of hydrate slurry, the plugging characteristics of
hydrate slurry in inclined pipe, and the plugging characteristics and
mechanism of hydrate under the condition of shutdown and restart were
explored. The initial pressure of each group was 3.2 MPa, and the
initial temperature of each group was 12 °C. The initial flow of the
power control system of the circulating pump was changed.
3.1 Antipolymer effect under different moisture content.Polymer inhibitors disperse hydrate particles in fluid and allow hydrate
to be transported as mud. Some of the most effective and typical
Anti-agglomerants agents are quaternary ammonium salts. Usually, they
consist of ionic head groups and various hydrophobic tail groups. AA
molecules are usually the most effective in the presence of a large
number of hydrocarbon phases in the mixture, and in the so-called high
water content state, the anti-agglomeration performance decreases ;
therefore, it is very important to understand and improve the function
of polymer inhibitor under different water content conditions.
Firstly, the effect of polymer inhibitor on the flow of hydrate slurry
under the condition of 50 % and 70 % water content was compared. Under
the condition of 50 % water content, the plugging time of adding
polymer inhibitor and without polymer inhibitor was 5.1 min and 11 min,
respectively. Under the condition of 70 % water content, the plugging
time of existing polymer inhibitor and without polymer inhibitor was 20
min and 27 min, respectively. Obviously, coconut oil amide propyl
betaine can effectively play the role of anti-coagulation of hydrate in
oil-water two-phase system and prolong the flow time of hydrate slurry.
In addition, an increase in water content also leads to a decrease in
gas solubility per unit volume of oil-water emulsions during nucleation
and growth. Gas mass transfer limitation reduces the nucleation and
growth rate of hydrate, which leaded to longer blockage time. In order
to further study whether the new polymer inhibitor is limited by
moisture content, the anti-coagulation effect of polymer inhibitor under
pure water condition was studied. Under the condition of pure water, the
plugging time of adding Anti-agglomerants and without Anti-agglomerants
is 23 min and 49 min respectively. Obviously, the anti-coagulation
effect of cocoamidopropyl betaine on hydrate particles is not limited to
water content, even in pure water conditions, it can effectively enhance
the fluidity of hydrate slurry and prolong the plugging time. In
addition, with the increase of water content, the blocking time of the
system is gradually prolonged. In the oil-water system, the solubility
of gas near the hydrate formation region plays an important role in the
nucleation and growth of hydrate. The increase of water content can
significantly reduce the solubility of gas molecules in the emulsion
system. Figs 2, 3, 4 are flow curves of 50 %, 70 % and pure
water, respectively.