Gas hydrates are nonstoichiometric crystalline compounds that form when guest molecules are incorporated in host cages formed by water molecules through hydrogen bonding. Their formation in offshore flowlines transporting hydrocarbons has been a serious concern in oil and gas industry because it may cause the blockage of fluid flow. The industrial practice to prevent the formation of hydrates has been the injection of thermodynamic hydrate inhibitors (THIs) such as methanol and mono-ethylene glycol to shift the hydrate equilibrium conditions toward higher pressure and lower temperature, such that the hydrocarbon fluids inside flowlines will not experience the hydrate formation. However as production from offshore oil and gas wells moves into colder and deeper region, alternative materials have been developed to delay hydrate formation kinetically or preventing the agglomeration of hydrate particles. Recently, it was revealed that the addition of MEG in under-inhibition condition was able to prevent the agglomeration and deposition of hydrate particles in the water + decane mixture. However, the production of oil and gas may accompany the produced water where the salinity of produced water may vary from a few milligrams per litre to 300,000 mg/l (~30 wt%). In this study, we investigated the hydrate equilibrium conditions in the presence of both MEG and NaCl while changing the concentration of NaCl from 3.5 to 20 wt%. Then the formation kinetics of gas hydrate was investigated as well for MEG and NaCl solutions. The hydrate dissociation enthalpy of aqueous solution with MEG and NaCl was calculated from this work experimental data by using Clauisius-Clapeyron equation. The enthalpy of hydrate dissociation was increased linearly with increasing the NaCl concentration.
Experimental results showed that when 10 wt% NaCl was added to 20 wt% MEG solution, no hydrate formation was observed for at least 10 hours. Also the obtained gas consumption data showed the formation of hydrate was delayed significantly, i.e. kinetic inhibition effect, while the torque changes suggested prevention of the agglomeration of hydrate particles, i.e. anti-agglomeration effect, in the presence of both MEG and NaCl. Initial catastrophic growth rate of MEG 20 wt% and NaCl 3.5wt% solution was 4.8 times lower than that of MEG 20 wt% solution. These results indicate that the formation of hydrate can be inhibited kinetically and the blockage formation can be prevented in the presence of MEG and NaCl.
See more of this Group/Topical: Topical 9: 4th International Conference on Upstream Engineering and Flow Assurance