Multiphase flow phenomena

The low solubility of methane in water generally makes movement of methane via advection and diffusion very inefficient. Far more methane can be transported if free phase gas is in motion. In very coarse sediments, fractures and other conduits bubbles of methane may be entrained in the flow, adding to the advective flux. Microbubble transport along fractures and bedding planes is considered by some to be a major mechanism of gas migration (e.g. Neglia 1979, Saunders et al. 1999) but we note that in general small bubbles will evanesce or coalesce. Microbubbles are most important for short travel distances in the shallow subsurface (Klusman 1993). In all other cases, gas must overcome capillary resistance if it is to move through the sediment.

Less dense fluids can rise spontaneously through denser fluids because there is a net buoyant force as a consequence of a difference in hydrostatic pressures. The greater is the vertical extent of the lighter fluid column (however thin it may be; thin columns are called stringers), the stronger is this pressure difference to drive flow. Buoyancy gradients of gas in water or brine vary from about 4 to 11 kPa per meter (Schowalter 1979; Hunt 1997). Consider the case where free phase gas occurs in a sand layer, but its upwards motion under buoyancy is impeded by a finer layer of mud (Fig. 5).

Figure 5. Buoyancy and capillary equilibrium. Pressure of the less dense gas phase Pg is higher than that of the water Pw when the phases are in hydrostatic and capillary equilibrium. The pressure difference generated by the buoyant gas column of height L relative the hyrostatic pressure gradient in the continuous water phase (E) is supported by the capillary resistance of small pores in the top seal and/or fault seal (A). A gas filled fracture penetrating the seal (C) will have a higher internal pressure than a water filled fracture (B) by an amount proportional to the difference in the densities of the phase and to the column height.

Figure 5. Buoyancy and capillary equilibrium. Pressure of the less dense gas phase Pg is higher than that of the water Pw when the phases are in hydrostatic and capillary equilibrium. The pressure difference generated by the buoyant gas column of height L relative the hyrostatic pressure gradient in the continuous water phase (E) is supported by the capillary resistance of small pores in the top seal and/or fault seal (A). A gas filled fracture penetrating the seal (C) will have a higher internal pressure than a water filled fracture (B) by an amount proportional to the difference in the densities of the phase and to the column height.

In order to penetrate further, the pressure difference across the gas/water interface at the top of the sand layer must exceed the capillary entry pressure of the mud. Say this mud has a characteristic pore entry radius of r, while the pore radius of the sand is R. By combining the equations for the hydrostatic pressure in gas and water columns, and the Young-Laplace equation, we can arrive at the classic criterion for the maximum height of the gas column; Lc that can be trapped:

(e.g., Schowalter 1979; Watts 1987). The curvature 1/R is so much smaller than 1/r that it can be neglected. Usually cos 6 is unity for gas and water, and so we have an equation in which the driving force is a density contrast and the resistance is related to the inverse of the pore size of the mud layer:

The maximum pressure that a seal can withstand ranges from hundreds of kPa for clayey silts to tens of MPa for mudrocks. With the fluid densities and surface tension given above, we can calculate from (8) that a mud with an entry radius of 100 nm can support a gas column nearly two kilometres high! The efficiency of capillary trapping explains the wide distribution of trapped shallow gas where methane is sourced biogenically or from greater depths.

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