Deep-Ocean Sequestration of Excess Produced Gas to Mitigate Greenhouse Gas Emissions from Offshore Flaring.

Many offshore petroleum platforms employ flaring (open combustion) to dispose of excess produced gas, thereby emitting greenhouse gases to the atmosphere. We investigate deep-ocean sequestration of excess produced gas, named Seawater Injection of Natural Gas (SWING), to mitigate routine flaring at deepwater platforms. We evaluate 435 SWING scenarios using well-validated models of pipe transport, multiphase plume dynamics, gas/liquid transitions, and aqueous dissolution of natural gas bubbles and liquid condensate droplets. We identify SWING scenarios that entrap the produced gas in the deep ocean, where it would rapidly biodegrade and subsequently ventilate to the atmosphere as CO2 over decades to centuries. For example, a simulated injection of 10 MMSCFD (million standard ft3 d-1) of heavy produced gas at 1000 m depth yields dissolution of 95% of the injected hydrocarbon mass at ≥500 m depth, including 100% of the methane, assuming a median bubble diameter of 2 mm after jet breakup. Compared to flaring, this SWING scenario reduces CO2-equivalent emissions to the atmosphere by 260 kilotons year-1, representing a ≥10-fold decrease, analogous to removing CO2 emissions from 57,000 US passenger vehicles. This scenario reduces contributions of dissolved inorganic carbon to the surface ocean by ≥6-fold, mitigating this input to surface ocean acidification.