Burwicz, Ewa B. , Ruepke, Lars and Wallmann, Klaus (2012) A new numerical reaction-transport model of marine gas hydrate deposits. [Invited talk] In: 11. International Conference of Gas in Marine Sediments (GIMS11). , 04-07.09.2012, Nice, France .

Abstract

Introduction
We have developed a new multi 1-D numerical model to investigate and understand the processes of gas hydrate formation and dissolution in anoxic marine sediments under a wide range of conditions. By this reaction-transport model we are able to investigate a various aspects of gas hydrate dynamics: sediment compaction which results in expulsion of pore fluids containing various chemical species, reduction in porosity and permeability of the sediment matrix due to hydrate formation, time-resolved evolution of pressure and temperature regimes, multiphase flow of compressible pore fluids, gas hydrate, and a free gas, thermal-blanketing effect due to vigorous sedimentation of cold impermeable layers, gas hydrate dissolution as a response to a slowing down sedimentation, and the effects of salinity variations on the thickness of the Gas Hydrate Stability Zone (GHSZ).

Numerical model
The reaction-transport model contains various chemical compounds (solid organic carbon, dissolved in pore water methane, dissolved inorganic carbon, dissolved sulfates, gas hydrates, and free methane gas). We consider a reference frame which extends from the seafloor to the bottom of the GHSZ (defined as a combination of pressure, temperature, and salinity conditions) plus 50m of Free Gas Zone lying directly beneath. However, the upper part of sediment column (10 cm) is not considered in the model due to strong bioturbation processes which might potentially have an impact on the gradients of dissolved chemical species.
Initially, the system is filled by compressible pore fluids of a given salinity (consistent with a value at the sediment-water interface). As the upper boundary conditions, we have applied constant concentrations of dissolved methane, dissolved inorganic carbon, and sulfate according to the mean values in the ocean.
At the beginning of each time-step, a new sediment layer is deposited at the top of sediment column according to a given sedimentation rate, lithological type, and initial porosity at the surface.
Transport processes have been split into the advection and diffusion part and solved separately for every chemical compound. Multiphase flow of dissolved chemical species and free gas phase has been solved by finite-volumes method according to the Darcy’s law. Molecular diffusion of dissolved species is controlled by changes in concentration gradients and has been solved by finite-elements method.
Reaction module contains kinetically controlled rates of methanogenesis, sulfate reduction, methane oxidation, and POC degradation. POC decay via microbial sulfate reduction takes place until the dissolved sulfate pool in ambient pore waters is depleted. Below the sulfate penetration depth, POC is microbially decomposed into methane and CO2. Upward diffusing dissolved methane is consumed by anaerobic oxidation within the sulfate-methane transition zone. This reaction module has been evaluated previously by Wallmann et al., 2006, Marquardt et al., 2010, and Burwicz et al., 2011.

Applications
Dynamic un-steady state compaction allows us to investigate gas hydrate formation and dissolution in terms of changing parameters (e.g. sedimentation rate or permeability of deposited sediments). By depositing sediment layers of a different grain size (‘sandwich-like’ scenario), we have observed that lithology of potential hydrate-bearing layers (e.g. coarse-grained sands vs. shales) results in preferential hydrate accumulation in the first ones which stays in agreement with field observations.
We have also investigated the effect of slowing down sedimentation rates on gas hydrate dissolution. We have concluded that slow deposition of sediment layers at the top of sediment column and, as a result, a decrease in POC input in time, result in undersaturated in CH4 pore waters causing hydrate destabilization. This scenario clearly shows the importance of constraining a time-resolved sedimentation history in gas hydrate simulations which are coupled with climate models.
By depositing thick layers of cold low-permeable sediments on top of the column, we have investigated the temperature variations within sediments, known as ‘thermal blanketing’ effect, which has an impact on previously formed hydrates.

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