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.