CLENNELL, M. BEN
School of Earth Sciences, University of Leeds, Leeds, LS2 9JT, UK ([email protected])
Abstract: Gas Migration Mechanisms in Fine-Grained Sediments
The movement of methane through the first kilometre of a subaquatic sediment column is a subset of the "Migration Problem" defined by: (1) Low solubility of methane in water, (2) Low singlephase permeability of fine-grained sediments, (3) Low efficiency of chemical diffusion, (4) High capillary entry pressure of gas, and (5) Low 2-phase relative permeability.
Increasing methane solubility aids advection of methane dissolved in water, while gradients in solubility enhance the diffusive flux. Small pores in a fine-grained sediment can lead to supersaturation of methane in the aqueous phase through the very high capillary pressures of tiny bubbles. The appearance of clathrate hydrate as a stable phase lowers solubility and provides a sink for methane that drives more flow.
We can learn from other sciences. Publications about concrete describe accelerated diffusive flux when water-saturated and bubble-containing pores exist together in the same porous matrix (catenary diffusion). Experiments in nuclear waste disposal suggest that pressurised gas tends to create its own permeability through argillaceous sediment. Building pressure will widen pore throats and connect up pathways until the point of yield or fracture of the sediment matrix is reached, and a burst of gas is vented.
New developments in mathematical physics have broadened the scope of percolation theory. Gas migration is a critical ly-I im ited form of invasion percolation. Gas flow is characterized by a nonlinear or chaotic response to pressure and stress changes, leading to irregular surges in, or self-organised oscillation of, the flux. Sometimes classical chaotic behaviour (period doubling, etc.) is manifest. It may be difficult to differentiate between mechanical valving (fracture dilation) or capillary breakthrough as the driving mechanism: both produce a similar system response.
AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California