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Gas migration mechanisms from accumulation to surface

1994; Volume: 41; Linguagem: Inglês

10.37570/bgsd-1995-41-03

2245-7070

C. J. Clayton, Stephen Hay,

Methane Hydrates and Related Phenomena

The driving force for gas migration is buoyancy, aided in some cases by sediment overpressure gradients (pressures above hydrostatic). Resistance to migration is controlled by the-capillary entry pressure into the pore network of the overlying sediments, or by their resistance to fracturing. It can be shown that migration mechanisms vary with depth. Below the upper few hundred metres gas migrates by invading the pore network of the sediment whereas at shallower depths fracturing may occur preferentially. Thus even though gas appears to seep into the water column from fractures it will be more dispersed below the sediment surface. In addition, gas shows a tendency to "pond" in near surface sediments, which can be explained as a function of sediment compaction with depth and decreasing gas-water interfacial tension. Approximate calculations of seal leakage rates are possible using a modified Darcy flow equation. For capillary failure of a mudstone seal at 3 km depth, gas fluxes are in the order of .0015 m3/m2/year at ambient pressure or 0.23 m3/m2/year at surface conditions. Flow rates from fractured seals depend on the amount of time the fracture remains open. They vary from a value similar to that for capillary failure if the fracture is open for an average of one minute per year, to five or six orders of magnitude faster. The calculated capillary failure fluxes are comparable to measured surface gas seepage fluxes.