Fracture mechanics analysis of the North West Fault Block of the Prudhoe Bay Field

Abstract Fracture mechanics analyses using the boundary element program FROCK have been used to determine the changes in the stress directions and magnitudes caused by the major faults around the North West Fault Block (NWFB) of the Prudhoe Bay Field. These changes from the regional stresses can have important implications on the direction and magnitude of flow in the faults, the fractures, and the rock mass as a whole, and therefore impact production. The rock stress distribution is also very important in the design of drilling operations. The analyses show that the faults can considerably modify the stress directions and magnitudes in NWFB. Larger faults have larger influences on stress and have a larger zone of influence. For example, the Niakuk Fault, although most parts of which are far from NWFB, has considerable influence on the stresses in NWFB. Fault details can also have important effects on the stresses in the areas close to the faults. Even though the regional stresses are compressive, fault openings and small normal stresses on the fault surfaces can result from the interaction of fault sliding movements. These can cause increased conductivities of the fault segments being opened or stress-relieved. Stress direction predictions from the fracture mechanics and four wellbore breakout analyses have been compared. The differences in the predictions range from 5 to 33°.