Pressure and Stress Prediction Using Seismic Velocities, 3D Geomechanical Models and the Full Stress Tensor: Mad Dog Field, Deepwater GoM

Abstract

ABSTRACT: We discuss a 3D case study for pressure and stress prediction in the Mad Dog field, deepwater GoM. We employ a novel approach that couples seismic velocities with geomechanical models and the full stress tensor (including lateral and deviatoric stresses; FES workflow). We show that steeply inclined salt faces increase the stress ratio in wall rocks. We further show that below salt overhangs, the stress state remains practically uniaxial. We discuss that the approach predicts stress perturbations that result from the 3D salt geometry and extend km away from salt. We extract pressure and stress along a well and show that the geomechanical prediction of least principal stress is systematically lower than the uniaxial estimate but comparable to field measurements. This is because the 3D model captures the extensional effect of the anticline structure in the area. Overall, we demonstrate that the 3D FES models incorporate the complexity of salt loading not captured by any VES approach and can provide pressure and stress estimates along any existing or planned well paths. 1. INTRODUCTION Pre-drill pore pressure and stress prediction is crucial for borehole stability, planning of safe and economic well trajectories and design of casing plans (Dodson, 2004; Flemings, 2021; Zhang, 2013). It is also a key input in the exploration stage to determine the integrity of reservoir seals. Pressure-prediction workflows are commonly based on the vertical effective stress (Zhang, 2013). Such workflows have been successfully established because, in sedimentary basins, strain is uniaxial and sedimentation drives loading and overpressure generation (Bowers, 1995; Osborne & Swarbrick, 1997). In the vertical effective stress (VES) workflow, the vertical effective stress across the volume of interest in calculated by coupling the seismic velocity with a velocity – vertical effective stress relationship, for example that of Bowers (1995): (equation) where (equation) is the vertical effective stress, V is a velocity measurement (e.g., seismic or sonic), V0 the velocity of water, and A & B empirical coefficients calibrated using well measurements at a regional well (see Appendix A for nomenclature). Pore pressure, uVES, is then estimated as the difference between the vertical total stress (σv) and vertical effective stress: (equation) The vertical total stress is usually assumed to be equal to the overburden stress.