Reservoir Modeling Simulation of CH4 Production

ID No. 43

Presentation Style Poster (6/15/16)

Category of Presentation Reservoir Modeling

Simulation of CH4 Production through the Depressurization Method from Gas Hydrate in Nankai Trough using Non-equilibrium Approach Khadijeh Qorbani, and Bjørn Kvamme University of Bergen The amount of natural gas reserves trapped in Natural Gas Hydrates (NGHs) exceed by far the total amount of conve ntional fossil fuel explored until today. NGHs are formed from various hydrate former phases (i.e. gas, aqueous solution, and adsorbed phases). As a result, due to Gibbs phase rule and the combined first and second laws of thermodynamics NGHs cannot reach thermodynamic equilibrium in real reservoir conditions. Local temperature and pressure are given by hydrodynamics and geothermal-gradients so number of independent thermodynamic variables exceed number of variables known by conservation-laws and equilibrium constraints. Gas from biogenic degradation of organic material dominates NGHs deposits. Therefore they are almost pure CH4-hydrates.
The non-equilibrium nature of hydrates in porous media is also a global concern since methane is leaking from dissociating hydrates through contact with undersaturated water through fractures and faults. Methane that leaks into the oceans will add to the carbon fluxes in the ocean and CH4 that eventually reach the atmosphere directly from hydrates in permafrost or from oceans is a climate issue. Even though CH4 has a shorter lifetime than CO2 in the atmosphere is a more aggressive greenhouse gas.
Lack of reliable field-data makes it difficult to predict the production-potential, as well as safety of CH4-production from NGHs. Simulations can be used to investigate CH4production through different scenarios. Most hydrate simulators within academia and industry treat hydrate phase transitions as an equilibrium process and those which employ the kinetic approach utilize simple laboratory model in their simulations. Furthermore, it is typical to utilize a limited thermodynamic description where only temperature and pressure projections are considered. Another common simplification is to assume a single route for the hydrate phase transitions. However, the non-equilibrium nature of hydrate indicates a need for proper kinetic models to describe hydrate in the reservoir with respect to thermodynamics variables as well as considering all possible phase transition routes. 
 The RetrasoCodeBright (RCB) hydrate simulator was extended to model CH4-hydrate dissociation towards CH4 gas and CH4 into the aqueous phase and water. Two routes of CH4- hydrate were added as pseudo-minerals. Phase transitions were treated as non-equilibrium processes under constraints of mass and heat fluxes. CH4-hydrate phase transitions were resolved by looking at supersaturation and undersaturation with respect to thermodynamics variables. Hydrate instability due to undersaturation of CH4 in the contacting water phase was also considered. A non-equilibrium in-house thermodynamic package was combined with RCB to account for competing phase transitions through Gibb’s free energy minimization. The energy differences were calculated from variations in chemical potentials of hydrate and hydrate formers. Mass transport, heat transport and non-equilibrium thermodynamic effects were implemented through classical nucleation theory to model the kinetic rate of hydrate phase transitions.
In this work, we utilized an in-house extension of RCB, taking into account two possible routs of hydrate phase transitions (using non-equilibrium thermodynamics) to perform a case study using geological data extracted from field studies of gas hydrate in Nankai Trough area where we analyzed CH4-production using the depressurization method.

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