Pore and Fracture Pressures prediction – new geomechanic approach in Deepwater Salt Overthrusts, Case histories from Gulf of Mexico

Abstract

Along this active exploration belt, applying the conventional effective stress methods and algorithms, where maximum stress (S1) is vertical would lead to unintended and unrealistic results. In the frontier active thrust belt of the Gulf of Mexico, the unique geomechanical setting of S1 as the lateral salt creep, and the minimum (S3) as the OB greatly impact the formation geopressure framework. The buoyancy of thick salt produces two different pressure gradients above and below the salt. Moreover, the inclusion of rafted sediments in the salt and the plowing rubble zone at the salt base substantially affect the pore and fracture pressures (PP-FP) profiles. These proceeding geological setting were the foundation for the conceptual framework. Building an alternative pre-drilling prediction numerical model based on this anomalous geomechanical settings and the lack of adequate seismic velocity is a challenge. All the available measured or pertained PP-FP data from key wells were tabulated. Prediction models were established by correlating the populated database and generating the empirical algorithm for each data gather. A substantial discrepancy between above and below the salt where high pressure gradient (PG) in the sediment above the salt and slow PG development below the salt. A considerable regressive pressure (average 2 ppg) in both PP-FP subsalt section. The PP within the salt is contingent on the presence of sediment inclusions and a substantial FP drop in the rubble zone leads to extensive loss of mud circulation. The trend lines of each data gather led to generating two depth dependent equations for the PP-FP above and within the salt and two others for the subsalt. The prediction models were validated against blind data set. Before drilling, this model establishes the PP-FP vs. sediment subsea depth in an abnormal geomechanical setting and the lack of coherent seismic velocity for PP -FP prediction.