Experimental Study of Natural Gas Hydrate Formation Kinetics and Inhibition in Brine and Water

Abstract Hydrates formed under high pressure and low-temperature pipeline conditions are a serious flow assurance challenge. Furthermore, there are uncertainties with respect to the performance of different hydrate inhibitors based on their type, hydrocarbon and brine compositions, and operating conditions. Considering the inhibiting effect of salt ions in the brine composition can benefit hydrate chemical management strategies by reducing the high quantities of inhibitors. This study experimentally evaluates the performance of kinetic and thermodynamic inhibitors and investigates the growth of hydrate under subsea pipeline pressure and temperature conditions. The effect of hydrate inhibitors was studied through a novel isothermal approach using varied brine compositions. Few studies such as Kakati et al. (2015) have focused on the inhibiting effect of multivalent ionic brine composition. Our study considers several multivalent ionic salts in formation brine and seawater including NaCl concentrations of 9.7 wt% and 2.4 wt%, compared to previous isochoric and isothermal studies relying only on one salt type. The influence of high and low salinity levels, including multivalent ionic salts, were evaluated, as they have been shown to be able to reduce the quantity of inhibitors used in hydrate chemical management strategies. The results were benchmarked against experiments using de-ionized (DI) water to capture the individual effect of the inhibitors and brine in inhibiting hydrate formation. Growth detection was also captured through image analysis to improve the understanding of hydrate kinetics in water and brine. The experimental results were evaluated according to hydrate equilibrium curves simulated using Calsep PVTsim Nova. A novel macro scale isothermal approach was applied in PVT Cell to investigate natural hydrate formation and growth in DI water and brine systems. The ability of i) a kinetic hydrate inhibitor (KHI) and ii) methanol as a thermodynamic hydrate inhibitor (THI) to inhibit hydrate formation was studied. A sudden drop in the pressure (indirect measurement) confirmed through visual observations (direct measurement) is applied to identify the hydrate formation point. Nine isothermal hydrate tests were conducted, with and without inhibitors, in DI water and brine systems. Results indicate that the KHI was best able to inhibit hydrate formation. KHI inhibited hydrate formation for more than 20 hours in the presence of formation brine at 4.8°C and 97 bar (>1400 psi). In the absence of an inhibitor, hydrate formation in water occurred during the pressurization step, at a pressure below 35 bar (500 psi). The hydrate mixture resulted in a sudden shift to darker colours upon formation, further grew, and agglomerated.