Carbon dioxide hydrate formation in porous media under dynamic conditions

Injecting carbon dioxide (CO₂) into subsea water zones where the in situ temperatures are below the hydrate-forming temperature of CO₂ has been recently proposed to lock CO₂ inside the water zones in solid hydrate form. It is a common concern that CO₂ may form hydrates during the injection period that will reduce well injectivity. CO₂ injection into sandstone cores under simulated subsea temperatures of 2°C and 3°C was investigated in this study. Experimental result shows that, at 2°C temperature, flowing CO₂ at Darcy velocity 0.033 cm/s begins to form hydrate in the sandstone core at about 3.06 MPa (450 psi), which is much higher than the minimum required pressure of 1.5 MPa (220 psi) for CO₂ to form hydrate in static condition. The pressure ratio is 450/220 = 2.05. At 3°C temperature, flowing CO₂ at Darcy velocity 0.045 cm/s begins to form hydrate in sandstone core at about 3.67 MPa (540 psi), which is much higher than the minimum required pressure of 1.87 MPa (275 psi) for CO₂ to form hydrate in static conditions. The pressure ratio is 540/275 = 1.96. The reason why the required minimum pressure for CO₂ to form hydrates in dynamic conditions is about double the required hydrate-forming pressure in static conditions is not fully understood. It is speculated that the shear rate effect of flowing fluids should slow down the growth of hydrate crystals or break down hydrate films, resulting in delayed formation of bulk CO₂ hydrates. More investigations in this area are needed in the future.