Stress Perturbations Adjacent to Salt Bodies in the Deepwater Gulf of Mexico

摘要

Lack of consideration of the geomechanical interactionbetween salt bodies and surrounding formations has led todocumented drilling failures adjacent to salt diapirs, in somecases resulting in individual well abandonment costs of tens ofmillions of dollars. To address this issue, a three-dimensionalnon-linear finite element geomechanical simulation effort wasinitiated to analyze the in situ stress state existing in andadjacent to salt bodies before drilling as well as underproducing conditions. This work leverages unique expertise insalt mechanics and computational geomechanical modeling.Non-linear finite element geomechanical models weredeveloped for four idealized deepwater Gulf of Mexicogeometries including a spherical salt body, a horizontal saltsheet, a columnar salt diapir, and a columnar salt diapir withan overlying tongue. The analyses reveal that at certainlocations for specific geometries: shear stresses may be highlyamplified; horizontal and vertical stresses may be significantlyperturbed from their far-field values; principal stresses maynot be vertical and horizontal (i.e., the vertical stress may notbe the maximum stress); and anisotropy in the horizontalstresses may be induced. For some geometries, the verticalstress within and adjacent to the salt is not equal to thegravitational load; i.e., a stress-arching effect occurs.Analogously, the assumption that the horizontal stress within asalt body is equal to the lithostatic stress is shown to beincorrect sometimes. The modeling also suggests analternative explanation for the so-called rubble zones thoughtto occur beneath and/or adjacent to salt diapirs, in that theymay be an intrinsic consequence of the equilibrium stress fieldneeded to satisfy the different stress states that exist within thesalt body and in the non-salt surrounding formations. Wedemonstrate with an example how this work can enable morerigorous planning of well locations and trajectories byproviding more accurate estimates of the vertical andhorizontal stresses around and within salt bodies for wellborestability analyses so as to avoid areas of potential geomechanical instability, and to enable accurate fracturegradient prediction while entering, drilling through, andexiting salt bodies.