Natural hydraulic fracturing example, North Sea

Trap description

A trap in the Central Graben, North Sea, appears to have been bled dry by natural hydraulic fracturing. This trap is a low-relief salt structure with a dry hole and has less-than-sufficient strain to fracture the top seal.

Evidence

Evidence for hydraulic fracturing and seal rupture include the following:

• A fossil oil—water contact exists at the synclinal spill point of the trap. The trap was once filled with hydrocarbon, but something happened after trap fill to bleed off hydrocarbons.
• Hydrocarbon shows exist throughout the sediment column above the reservoir. These hydrocarbons are direct evidence of a breached top seal.
• The pore pressure is close to the theoretical fracture pressure. In adjacent fields, the pore pressure is significantly less than P_f.
• Cores show vertical, open extension fractures rather than the more common shear fractures found in adjacent fields. These fractures are identical to fractures produced by hydraulic fracturing.

Figure 10-44 shows that the pore pressure is close to the fracture pressure at the base of the top seal (approximately depth::1300 ft). The pore pressure is inferred from mud weight and RFT measurements (solid dots). Leak-off tests (LOT) help constrain the fracture pressure. The fracture pressure is close to the lithostatic pressure or overburden pressure.

Figure 10-44 . Copyright: Esso Exploration and Production, U.K.

See also

• Overpressure and natural hydraulic fracturing
• Natural hydraulic fracturing of top seals
• Fracture threshold in the real world
• Overpressure and hydrocarbon distribution, gulf coast

External links

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