Hydrofractured seal leakage
| Exploring for Oil and Gas Traps | |
| |
| Series | Treatise in Petroleum Geology |
|---|---|
| Part | Critical elements of the trap |
| Chapter | Predicting preservation and destruction of accumulations |
| Author | Alton A. Brown |
| Link | Web page |
| Store | AAPG Store |
In settings with extreme overpressure, pore-water pressure approaches the pressure required for natural hydraulic fracturing. If the petroleum column is thick enough, the sum of the capillary pressure and fluid pressure can equal or exceed the pressure needed to fracture the rock. The result is natural hydraulic fracturing: the seal becomes hydrofractured and the petroleum leaks.
Characteristics[edit]
Unlike other fractured seals, hydrofractures remain open only as long as pore pressure exceeds fracture pressure. Once the total pressure drops, the fracture closes. The petroleum column height remains approximately in equilibrium with the difference between the water pressure and the fracture pressure.
Hydraulic fracture seal failure affects all rock types, but the fracture gradient is a function of rock type and basin. Usually hydraulic fracturing limits the petroleum column height during charging instead of destroying accumulations after charging. This occurs where accumulations are charged during times of peak geopressure so the trap capacity is minimal. Charging occurs during times of peak geopressure because both charging and geopressure are a response to high sedimentation and heating rates.
Predicting leakage in deep accumulations[edit]
Hydrofractured seal leakage limits the thickness of a petroleum column whether the seal fails during or after charging. Leakage in deeply buried accumulations occurs only where geopressure is close to the fracture gradient (hard geopressures). Hard geopressures are characteristic of shale-dominated basins that have undergone recent rapid subsidence.
We can use downhole fluid pressure analysis techniques (e.g., [1] to evaluate hydrofractured seal failure for an area. Leak-off tests estimate the fracture gradient, and mud weight, well logs, or seismic data approximate the fluid pressure gradients. Because gas and condensate have much lower densities than oil, gas columns are more likely to have hydraulic failure than oil columns of the same height in similar settings.
Predicting leakage in shallow accumulations[edit]
Hydrofractured seals also leak in shallow accumulations of normal water pressure. At depths < 1000 ft, the absolute magnitude of the difference between water pressure and geostatic pressure is relatively small—on the order of several hundred pounds per square inch. Exceptionally thick (1000–2000 ft) columns of gas or oil have a capillary pressure equal to or greater than this difference, so hydrofracturing may occur.
The potential for shallow hydrofractured leakage is best evaluated from a pressure-depth diagram, where
- Water pressure gradient is estimated from water salinity.
- Geostatic pressure gradient is estimated from density logs or porosity trend.
- Petroleum density is estimated from the gas-oil ratio (GOR), API gravity, temperature, and pressure.
See also[edit]
- Trap leakage
- Intact membrane seal leakage
- Fractured membrane seal leakage
- Micropermeable seal leakage
- Diffusive seal leakage
- Seal failure prediction
References[edit]
- ↑ Caillet, G., 1993, The caprock of the Snorre field, Norway: a possible leakage by hydraulic fracturing: Marine and Petroleum Geology, vol. 10, p. 42–50., 10., 1016/0264-8172(93)90098-D
External links[edit]
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