Fractured 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 |
Fractured membrane seals fail when the capillary pressure created by the height of an underlying petroleum column exceeds the displacement pressure of the fracture porosity in the seal.
Characteristics
Many rock types with high capillary displacement pressures rarely seal petroleum accumulations. These rocks are often characterized by brittle behavior, i.e., they fracture easily at geological strain rates and confining pressures.
Fracture apertures may be much larger than matrix pore apertures in fine-grained rocks, so it is easier for petroleum to invade the fractures than the matrix porosity. In fact, many fractured rocks with open fractures have apertures so wide that they generally cannot seal economically thick petroleum columns.
The seal capacity of brittle, fine-grained rocks is not confirmed by capillary-pressure tests because seal failure is through the fractures, not the matrix pore network.
Lithologies
Brittle, fine-grained rocks—cherts, clay-free limestones and dolomites, and well-sorted siltstones—almost never seal. Intermediate lithologies such as calcareous or siliceous shales, argillaceous siltstones, and argillaceous or anhydritic limestones can seal where the rocks have not been excessively strained, such as in stratigraphic traps or broad, gentle folds. Even relatively intact, thick seals may be fractured at the high strain rates and total strains in some disturbed belts.
Predicting leakage
Leakage due to fractured membrane seal failure is most likely to destroy an accumulation when the seal is deformed after charging. In general, the tighter the folding and the faster the deformation rate, the more likely a given seal lithology will fail by fracture. The more brittle the lithology, the more likely seal failure will occur with deformation. Thick seals are less likely to fail by fracturing than thin seals of similar lithology at the same level of deformation because deformation is less likely to form an open fracture pathway as the pathway lengthens.
Seal response to deformation can be evaluated empirically by examining nearby accumulations or outcrops in similar tectonic settings.
See also
- Trap leakage
- Intact membrane seal leakage
- Hydrofractured seal leakage
- Micropermeable seal leakage
- Diffusive seal leakage
- Seal failure prediction
External links
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