SIMULATING THE SEAFLOOR: LARGE VOLUME EXPERIMENTAL ANALYSES OF GAS HYDRATE FORMATION AND DISSOCIATION IN SEDIMENT

Gas hydrates are widespread in ocean sediments and areas of permafrost and may contain many times more carbon than exists in Earth's atmosphere today. Hydrates are potentially large sources of natural gas for human energy needs, directly affect seafloor stability, and play roles in the global carbon cycle and climate change through the release or sequestering of greenhouse gases. However, very little data exists on how hydrates form in nature or where they are likely to accumulate in sediment.

Novel meso-scale (10-50 liter) experiments to closely monitor the formation and dissociation of methane hydrates in sediment on a centimeter scale are underway at Oak Ridge National Laboratory's Gas Hydrates Laboratory. Experiments are completed inside the Seafloor Process Simulator (SPS), a 72-L Hastelloy C-22 pressure vessel, which is temperature-controlled inside an explosion-proof cold room. Temperature and strain within the sediment is monitored by a Luna Distributed Sensing System, a fiber optics-based system with data-gathering gratings every 1 centimeter. Total fiber length for any given experiment is at least 4.5 meters. Hydrate formation was observed spatially along three fiber planes within the sediment volume. Hydrate formation and dissociation were first observed along the walls of the chamber, moving inwards through the sediment. Hydrate was observed to form faster and more plentifully near the vessel wall, with formation and dissociation inception times increasing toward the center at rates of 5.5 cm min-1 and 84.4 cm min-1, respectively. Preliminary results also demonstrate the self-limiting effects of hydrate formation. The exothermic reaction raises local temperatures, pushing the system out of the methane hydrate stability field (MHSF), causing hydrate formation to cease before complete transformation of methane and water into hydrate. Subsequent hydrate formation was observed once temperatures dropped to within the MHSF. The results of these experiments can be applied to hydrate formation models as well as natural gas production scenarios from sediment hosted hydrate deposits.