Phase Equilibrium of Methane Hydrate in Aqueous Solutions of Polyacrylamide, Xanthan Gum, and Guar Gum

The use of water-soluble polymers as natural gas hydrate inhibitors has gained interest in recent years. Variety of polymers have been studied for their kinetic performance in methane hydrate inhibition in the past; however, thermodynamic hydrate inhibition by water-soluble polymers is not fully understood and needs further investigation. This study investigates the effect of molecular weights and concentrations of aqueous solutions of various oilfield water soluble polymers on phase equilibrium of methane hydrate. Water-soluble polymers, such as polyacrylamide (PAM), xanthan gum (XG), and guar gum (GG) with two different molecular weights and varying concentrations, have been considered for the investigations. These are as follows: PAM (Mw: 1.1 X 10(6) g/mol, PAM-1 and 1.5 X 10(5) g/mol, PAM-2), XG (Mw: 6.4 X 10(5) g/mol, XG-1 and 2.4 x 10(5) g/mol, XG-2), and GG (Mw: 1.7 X 10(6) g/mol, GG-1 and 6 X 10(5) g/mol, GG-2), with varying concentrations of 100, 200, and 500 ppm each. These are referred to as high-molecular-weight polymers (PAM-1, XG-1, and GG-1) and relatively low-molecular-weight polymers (PAM-2, XG-2, and GG-2). The experiments have been conducted in the pressure and temperature range of 8.63-5.50 MPa and 284.6-279.8 K, respectively. The results indicate that the water-soluble polymers have shown thermodynamic hydrate inhibition with an average temperature depression ranging from 0.25 to 1.05 K. The molecular weight and the concentration of polymers have been shown to affect the hydrate inhibition tendency. We have also proposed a hypothesis for hydrate inhibition based on the mobility of the polymer chain in the solution with a desired functional group in relation to nonelectrolyte/electrolyte thermodynamic hydrate inhibitors. The presented study on methane hydrate phase stability in the presence of various oilfield polymers is vital for their use in the design and development of hydrate inhibitive drilling fluids for offshore wells, hydrate-bearing formations, and studies related to the recovery of methane from hydrate-bearing sediments using polymer injection.