Geomechanical Modeling and Drilling Challenges in Deepwater Salt Thrust Belt, Case Histories from GOM

Abstract Drilling wildcats along the overthrust salt belt exploration trend in the Gulf of Mexico (GOM) is a challenging mission. At the early stages of exploring for oil and gas in this profitable salt thrust belt during the 1990's, drilling to the objective prospective traps suffered multiple hurdles to accommodate for the creeping salt mass. Excessive loss of circulations at the exit boundary between the salt and the subsalt sediments was the dominant challenging operation. Spotting lost circulation material associated with reducing the mud weight was not enough to recover the damaged bore-hole. Sidetracks and bypasses that increases the drilling cost became the norm. Maneuvering the mud pressure within the open bore-hole between the pore (PP) and fracture (FP) pressures within the deepwater narrow drilling window need to be systematically done based on the adequate geomechanical modeling for this unique geological setting. The presence of mobile creeping salt marching toward the Abyss modifies and alter the conventional geomechanical – pore and fracture pressure coupling where vertical overburden sediment load (OB) acting as the principal stress (S1). Therefore, applying the conventional effective stress formation pressure prediction methods, where the principal stress is the vertical load OB can lead to serious mishaps. The lack of cohesive seismic velocity due to the salt perturbation added uncertainty to any predrilling velocity – pressure prediction transformation modeling. Reviewing the geological evolution of the salt – sediments interaction and the physical properties of each major units since the salt advancement up till reaching the final destination in the Abyss at the Sigsbee Escarpment is the back-bone of this new endeavor to predict PP-FP window. In addition, all the petrophysical logging data, pressure measurements and drilling events were tabulated and correlated to the unique geological development in this area. Depth dependent numerical equations were established as a PP-FP prediction method.