gas hydrate - CCOP

where R POC is the POC degradation rate, C (DIC) is the concentration of dissolved inorganic
carbon (CO 3 + HCO 3 + CO 2 ) in the considered depth interval, C (CH 4 ) is the ambient
methane concentration in pore waters, k x is an age-dependent kinetic constant, POC is the
POC concentration and KC is a Monod constant describing the inhibition of POC degradation
by DIC and CH 4 . The age effects on POC degradation are considered using the approach
introduced by (Middelburg, 1989). Ages were calculated from sediment depth and burial rate
assuming an initial age of 1000 years. The rate law predicts that the microbial degradation of
organic matter is inhibited by metabolites accumulating in adjacent pore fluids since the
Gibb’s free energy available for the microbial metabolism is reduced in the presence of high
concentrations of reaction products.
Dissolved methane is produced via microbial organic matter degradation after sulfate has
been depleted by microbial sulfate reduction (Wallmann et. al., 2006). Gas hydrate is
precipitated from the pore solution when the concentration of dissolved methane calculated in
the model surpasses the solubility of gas hydrates (C SAT ).
R
GH
⎛ C(CH ⎞
4
)
= k ⋅
⎜ −1
⎟
GH
⎝ CSAT
⎠
We use a kinetic approach to simulate hydrate precipitation and dissolution (Hensen and
Wallmann, 2005; Torres et al., 2004) and a modified Pitzer-approach for the calculation of
hydrate solubility (Tishchenko et al., 2005). The kinetic constant for hydrate precipitation is
set to a large value (k GH = 2 x 10 -2 wt-% yr -1 ) to prevent over-saturation with respect to gas
hydrate. Hydrates are allowed to dissolve in under-saturated pore solutions applying a
corresponding kinetic constant of k GHD = 2 x 10 -2 wt-% yr -1 :
R
GHD
= k
GHD
⎛ C(CH ⋅
⎜1
−
⎝ CSAT
4
) ⎞
⎟ ⋅ G GH
⎠
( )
Hydrate concentrations (G(GH)) are initially calculated in wt-% and are subsequently
converted into percent of pore volume filled by hydrate considering the density of dry
sediments (d S = 2.5 g cm -3 ) and gas hydrates (d GH = 0.916 g cm -3 ).
Boundary conditions for dissolved species applied at the base of the model column have a
strong effect on the amount of hydrate formed in the model sediments. We apply constant
gradient conditions for SO 4 , DIC, and CH 4 :
δC
δx
δC
δx
( n) = ( n −1)
where the concentration gradient in the last interval of the model column (n) is assumed to
have the same value as the gradient in the overlying depth interval (n-1). With this approach,
methane and DIC continuously produced in deeper sediment sections situated below the base
of the model column (410 m) are allowed to enter the model domain via molecular diffusion.
New Energy Resources in the CCOP Region - Gas Hydrates and Coalbed Methane 13