gas hydrate - CCOP

Modeling of Gas Hydrate Formation in Marine Sediments
Klaus Wallmann
IFM-GEOMAR, Wischhofstrasse 1-3, 24148 Kiel, Germany
ABSTRACT: A transport-reaction model is presented and applied to simulate the
formation of gas hydrates in marine sediments. The modeling confirms that
microbial methane production within the hydrate stability zone is usually too slow
to produce significant amounts of hydrate. Economically valuable methane hydrate
deposits with high concentrations of hydrate are, however, formed by upward
methane migration.
Keywords: marine gas hydrates, methane, fluid flow, numerical modeling, pore
water, sediments
INTRODUCTION
Gas hydrates are widespread in marine sediments accumulating at the slope and rise of
continental margins. They occur as finely disseminated solids but also as massive layers often
associated with sandy sediment horizons. Only the massive layers are of economic
importance since they could be exploited at reasonable costs (Makogon et al., 2005). The
accumulation of hydrates in discrete and massive layers clearly shows that methane is
transported to these layers either as free gas or in dissolved form via fluid flow. Methane may
either be formed within the hydrate stability zone (HSZ) by microbial decay of organic matter
at low temperatures or may be produced in deeper sedimentary strata at elevated temperatures.
Recent modeling studies clearly showed that microbial methane production within the HSZ
proceeds very slowly so that the resulting methane concentrations are too small to produce
massive hydrate deposits (Wallmann et al., 2006). Strong chloride enrichments in pore fluids
from Hydrate Ridge (Haeckel et al., 2004; Torres et al., 2004) and independent constraints on
fluid flow rates at Blake Ridge (Wallmann et al., 2006) clearly show that massive hydrate
layers found at these sites have been formed by the ascent of gas bubbles originated from
deeper sediment layers located below the HSZ. In this paper we present the modeling
approach used for the simulation of hydrate formation in marine sediments and additional
modeling results confirming that methane ascent is the most important pathway for the
formation of economically important hydrate deposits.
MODELING APPROACH
A numerical transport-reaction model was developed and applied to simulate the degradation
of particulate organic carbon (POC), the microbial methane production and the precipitation
of methane hydrates in anoxic marine sediments. The model calculates the concentration -
depth profiles of 2 solid species (particulate organic carbon, gas hydrate) and 3 dissolved
species (sulfate, methane, dissolved inorganic carbon). Major processes considered in the
model are POC degradation via sulfate reduction, methanogenesis and the anaerobic oxidation
of methane (AOM).
New Energy Resources in the CCOP Region - Gas Hydrates and Coalbed Methane 11