| Popis: |
Deepwater areas offshore the Gulf of Mexico, Angola, and Brazil contain vast oil and gas deposits that underlie thick salt formations. Drilling wells to recover these resources can be challenging because of extreme water and reservoir depths, adverse environmental conditions, and complex tectonics. In particular, stress changes can occur in the vicinity of salt diapirs that are of sufficient magnitude to affect fracture gradient and wellbore stability, and the vast majority of difficult drilling events associated with salt diapirs in the deepwater Gulf of Mexico are in fact associated with the salt – sediment interface. In prior work, non-linear finite element analyes were used to identify the style and magnitude of near-salt stress changes for several idealized diapir geometries. While providing insights, the analyses for idealized diapir geometries remain essentially unvalidated. The work reported here is intended to provide key validation for the conceptual model and numerical methodology developed to predict near-salt stress changes. In 2002, Conoco drilled the Spa prospect, Walker Ridge 285 #1, in the Gulf of Mexico to a depth of 29,452 feet MD / 29434 feet TVD, penetrating a nearly ten thousand foot salt section. While the potential for reduced fracture gradient below salt was recognized in well planning, pre-drill pore pressure and fracture gradient estimates were nevertheless based on seismic velocities in the adjacent abyssal basin. The sub-salt section was difficult to drill, and several formation integrity tests were performed. We developed basin-scale finite element models to represent the geometry of the salt diapir and performed non-linear geomechanical simulations to predict the stress changes in and around the diapir. The simulations predict stress perturbations that vary spatially in accord with the complex and irregular salt geometry. Significant perturbations are predicted adjacent to the salt diapir, and the models predict the reduction in fracture gradient observed sub-salt (nearly 3 ppg). This work thus serves to validate non-linear finite element stress analysis techniques that can be applied to reduce risks when drilling through massive salt diapirs. Besides providing a geomechanical basis for understanding sub-salt fracture gradient reductions, this work also demonstrates the potential for significant stress distortions associated with topography along the top of salt. |
