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

Abstract

Abstract Lack of consideration of the geomechanical interaction between salt bodies and surrounding formations has led to documented drilling failures adjacent to salt diapirs1–3, in some cases resulting in individual well abandonment costs of tens of millions of dollars. To address this issue, a three-dimensional non-linear finite element geomechanical simulation effort was initiated to analyze the in situ stress state existing in and adjacent to salt bodies before drilling as well as under producing conditions. This work leverages unique expertise in salt mechanics and computational geomechanical modeling. Non-linear finite element geomechanical models were developed for four idealized deepwater Gulf of Mexico geometries including a spherical salt body, a horizontal salt sheet, a columnar salt diapir, and a columnar salt diapir with an overlying tongue. The analyses reveal that at certain locations for specific geometries: shear stresses may be highly amplified; horizontal and vertical stresses may be significantly perturbed from their far-field values; principal stresses may not be vertical and horizontal (i.e., the vertical stress may not be the maximum stress); and anisotropy in the horizontal stresses may be induced. For some geometries, the vertical stress within and adjacent to the salt is not equal to the gravitational load; i.e., a stress-arching effect occurs. Analogously, the assumption that the horizontal stress within a salt body is equal to the lithostatic stress is shown to be incorrect sometimes. The modeling also suggests an alternative explanation for the so-called rubble zones thought to occur beneath and/or adjacent to salt diapirs, in that they may be an intrinsic consequence of the equilibrium stress field needed to satisfy the different stress states that exist within the salt body and in the non-salt surrounding formations. We demonstrate with an example how this work can enable more rigorous planning of well locations and trajectories by providing more accurate estimates of the vertical and horizontal stresses around and within salt bodies for wellbore stability analyses so as to avoid areas of potential geomechanical instability, and to enable accurate fracture gradient prediction while entering, drilling through, and exiting salt bodies. Introduction The deepwater Gulf of Mexico (GoM) is the most active deepwater region in the world, currently providing some of the greatest challenges in scope and opportunity for the industry. The region is estimated to contain undiscovered recoverable resources of at least ~13 billion boe, and is known to harbor exceptional reservoirs such as the Thunder Horse discovery at a water depth exceeding 6,000 ft and with estimated recoverable reserves of at least 1 billion boe. However, the complex salt tectonics and the extreme water and reservoir depths necessitate high development costs, and innovative technology is required to bring these fields on stream. Integral to successful economic development is a well service lifetime of 15–30 years. Many of the significant deepwater GoM objectives are subsalt, with several thousand feet of salt not being uncommon. At the edge of the industry's experience are salt sections of 10,000-ft thickness that overlie targets at depths of 25,000(30,000 ft below mudline.4,5 The cost of drilling these deepwater subsalt wells is substantial, and, in some cases, operators have been forced to sidetrack or even abandon wells after experiencing drilling difficulties, with losses running to several tens of millions of dollars.3 The zone lying immediately below the salt section is notoriously difficult, and there are well known difficulties in accurately predicting fracture gradient and pore pressure immediately upon exiting the salt body.3–5 There are necessarily two key components in assuring the considerable investment that must be made to develop these deepwater subsalt fields. First, the planning of well locations and trajectories needs to consider the large-scale geomechanical loading conditions that exist in and around massive salt bodies, and second, the well casing designs need to consider the long-term casing loading that will occur because of salt creep. In this paper we present some of our work addressing the first area that relates to the large-scale geomechanical setting, and specifically, to unique geomechanical effects associated with the presence of massive salt bodies. Our other work that addresses the second component focuses on wellbore-scale finite element modeling to assure the long-term integrity of through-salt well casings and is presented elsewhere.6,7