Quantifying Velocity, Strain Rate and Stress Distribution in Coalescing Salt Sheets for Safer Drilling

weijermars, r. ; harmelen, a. van;

Reaching sub-salt hydrocarbon targets in the deeper part of the Gulf of Mexico requires drilling through a salt canopy. The suture zones in the salt canopy are potential drilling hazards due to anomalous pressure behaviour of entrapped sediments. The P´olyavector field of coalescing salt sheets inside the canopy is used to explain suture formation and distinguish between upright and inclined suture contacts. Our analytical models, based on complex potentials, provide exact solutions for multiple source flows as they compete for space when spreading into the viscous continuum of the salt canopy. The velocity gradient tensoryields the strain rate tensor, which is used to map the principal strain rate magnitude inside the canopy. Quantification of one of the principal strain rates is sufficient because the planed eformation assumption ensures the two principal strain rates are equal in magnitude (but of opposite sign); the third principal dimension can have neither strain nor deviatoric stress. Visualization of the locations where the principal stress vanishes or peaks (with highs and lows) is useful for pre-drilling plans because such peaks must be avoided and the stress-free locations provide the safer drilling sites. A case study—of the Walker Ridge region—demonstrates the practical application of our new method.