Stress, borehole stability, and hydrocarbon leakage in the northern North Sea

Knowledge of the state of stress in hydrocarbon reservoirs is critical for maintaining wellbore stability and understanding hydrocarbon migration and leakage. The principal goals of this dissertation are to constrain the in-situ stress tensor using drilling-induced tensile fractures and wellbore breakouts; to determine optimal trajectories for wellbore stability; and to evaluate potential hydrocarbon leakage and migration pathways in the context of in-situ stress and pore pressure. Four fields in the northern North Sea are examined.; In the Visund field, stress analysis using drilling-induced tensile fractures indicates the stresses are consistently orientated and magnitudes of the principal stresses are consistent with observed earthquakes. Analysis of wellbore stability shows that horizontal wells drilled in the direction of the maximum horizontal stress are the most successful. Observations of gas leakage along a normal fault which is optimally oriented for reverse failure in the current stress field are correlated with predictions of leakage.; In Field 1, Field 2, and Field 3 stress analysis indicates the stresses are consistently oriented and the stress magnitudes are consistent with observed earthquakes. Observations of gas leakage and migration in the Field 3, and observations of large pore pressure differences across faults in Field 1 and Field 2, are correlated with predictions of leakage and sealing. Predictions of probable leakage and migration in the Field can be matched with actual observations of up-fault leakage as well as observations of along fault migration to the south. Similarly, predictions of a large sealing potential in the Field 1 and Field 2 are matched with observations of quite large pore pressure differences across faults which are poorly oriented for frictional failure in the current stress field.; Pore pressures and hydrocarbon column heights in individual fields appear to be controlled by the potential for reservoir-bounding faults to slip. Fields located in areas with high differential stresses and faults well-oriented for frictional failure tend to have small hydrocarbon column heights, while those with lower stresses and poorly-oriented faults tend to have higher column heights.