Impact of seafloor drilling on methane seepage—enlightenments from natural gas hydrate drilling site GMGS2-16, northern South China Sea

There are a few countries in the world, which have conducted gas hydrate exploration and test production. However, the knowledge about the impact of seafloor drilling on methane seepage is limited. This study conducted two post seafloor investigations on the GMGS2-16 drilling site of the second gas hydrate drilling expedition (GMGS2). The first investigation consisted of four ROV dives, including one dive before drilling, one dive during drilling and two dives after drilling. The first dive did not find any seafloor gas emission. However, during the last two dives, massive gas bubble emission was observed from the abandoned well. The second investigation was conducted with multibeam echosounder 18 months after the well GMGS2-16A was completed. Flare-shaped high amplitude backscatter was observed in the water column, indicating methane leakage from the seafloor. The seismic profile showed that the free gas was distributed beneath the hydrate-bearing sediment. The seafloor drilling penetrated the hydrate-bearing sediment layer and built up the pathway between seafloor and free gas reservoir, which led to seafloor methane seepage. The water column multibeam data shows that gas flares disappeared at around 650 mbsl, indicating that methane gas was totally dissolved in the seawater. The impact of drilling-induced methane seepage on the atmosphere is quite limited. The methane leakage will stop when the pathways are closed by wall collapse and hydrate deposited later.