Intact membrane seal leakage

Exploring for Oil and Gas Traps
Series	Treatise in Petroleum Geology
Part	Predicting the occurrence of oil and gas traps
Chapter	Predicting preservation and destruction of accumulations
Author	Alton A. Brown
Link	Web page
Store	AAPG Store

An intact membrane seal fails when the capillary pressure (created by the height of an underlying petroleum column) exceeds the seal capillary displacement pressure. This type of seal does not fracture during deformation.

Ductility and capillary displacement pressure

Fine-grained, water-wet ductile rocks will seal as long as the capillary pressure exerted on the seal (the difference between the water and the petroleum fluid pressure) is less than the capillary displacement pressure of the matrix porosity of the seal. Under these conditions, the relative permeability of the seal to petroleum is zero. The accumulation remains preserved until one of three things happens:

• The seal is ruptured or altered.
• The structure is spilled.
• The petroleum is altered.

Most old accumulations have seals of this type.

Claystones, salt, and sulfates (gypsum and anhydrite) make seals of this sort because they are ductile under most geological strain rates and confining pressures. Ductility is important; if fracturing occurs, oil can leak through the fractures without invading the matrix porosity.

Deeply buried claystones, salts, and anhydrite have capillary displacement pressures great enough to exceed the buoyancy pressure from any reasonable oil column height (hundreds to thousands of feet). Conversely, silty mudrocks, shallow-buried claystones, and argillaceous siltstones have displacement pressures low enough to allow leakage even where the petroleum column has not filled to the structural spill point.

Predicting leakage

The failure of intact membrane seals after charging is rare because the capillary displacement pressure of mudrock seals increases with compaction and burial. Intact seal failure usually results in failure to trap in the first place. Limitations on the height of the petroleum column due to intact membrane seal failure can be evaluated by laboratory capillary pressure tests in conjunction with estimates of in situ petroleum density.

See also

• Trap leakage
• Fractured membrane seal leakage
• Hydrofractured seal leakage
• Micropermeable seal leakage
• Diffusive seal leakage
• Seal failure prediction

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