Numerical investigation on environmental effect associated with gas-hydrate exploitation

Abstract

With the technology development for exploitation of natural gas hydrate, long-term production test of gas hydrate has been planned in Alaskan North Slope by U.S. and Japan. Public concerns over the potential environmental risk are also growing, especially possible methane escape to seafloor in hydrate exploitation. High-permeability conduits in shallow sediments are regarded as the most likely pathways for methane escape, as observed with methane seepages at seafloor. However, whether hydrate exploitation can trigger disastrous methane blowout along these conduits in hydrate reservoirs remains not well addressed. In this research, numerical simulations have been conducted for specifying the long-term methane behavior in the presence of a vertical high-permeability conduit in the process of gas production from a hydrate reservoir with depressurization method. In the production system, the low pressure around the production well and the relatively high pressure around the conduit work together to drive methane to the production well, resulting in less methane leak along the conduit than that before production when depressurization method is applied. Taking the base case as an example, compared with the amount of methane leak (∼7000 kg in 15 years) from seepage without production, it decreases to ∼6000 kg in production, most of which are leaked before production. More sensitivity studies suggest that whether methane leak would be arrested by the depressurization method depends on the competing processes of the attraction effect of the production well and the upward movement of methane-rich fluid along high-permeability conduits. The results are consistent with the previous observations in production tests, indicating that exploitation of natural gas hydrate can be achieved in a safe way with depressurization method.