Pre-Drill and Real-Time Pore Pressure Prediction: Lessons from a Sub-Salt, Deep Water Wildcat Well, Red Sea, KSA

Abstract

Saudi Aramco's first deepwater exploration well targeted a sub-salt Miocene syn-rift section located in over 2,000 ft of water and beneath 9,000 ft of halite and evaporites. Offset well information from previous shallow exploration wells was limited; therefore, calibration for pre-drill pore pressure and fracture gradient prediction (PPFG) was performed using a single shallow water well completed two months prior to spuding the well. Pre-drill PPFG predictions presented a very high degree of uncertainty, which translated into uncertainty in well design and mud weight planning. Pre-drill pore pressure prediction relied on seismic velocities extracted from a wide azimuth 3D survey and used Residual Normal Move Out (RNMO) and seismic inversion to extract velocities that were presumed to represent shale velocities. Real-time pore pressure monitoring was based on a comprehensive program that included logging while drilling (LWD), multiple look-ahead vertical seismic profiles (VSPs), velocity model updating and rapid remigration (pre-stack depth migration) around the wellbore to produce simultaneous improvements in imaging and depth estimates that were tied back to an evolving geological pore pressure model. Significant differences between the pre-drill pore pressure model and measured well pressures highlight the critical importance of the pre-stack depth migration (PSDM) velocity model and the necessity to be able to modify the seismic velocity model and calculated pore pressures in real time to provide accurate information to drilling operations. An integrated team of technical professionals from nine separate departments was required to successfully carry out this project, which resulted in the successful drilling of a deepwater well in a high overpressure -low fracture gradient environment with minimal operational downtime. Geological Setting and StratigraphyOpening of the Northern Red Sea rift began approximately 25 MaBP as the Arabian platform began to move east (25-15 MaBP) then northeast (15-0 MaBP) relative to the African craton. Initiation of the Northern Red Sea rift triggered the onset of syn-rift deposition into a series of graben and half graben basins that continues to present day. The deepwater (beyond 1,000 ft water depth) syn-rift stratigraphy consists of Oligo-Miocene sediments up to 21,000 ft thick deposited under varied depositional environments and settings that are related to the macro tectonic evolution of the rift system. In terms of

Deformation, stress, and pore pressure in an evolving suprasalt basin

“…In addition, different salt flow patterns and different driving mechanism in salt basins such as topographic differential sedimentation and salt gravity spreading may result in different patterns of stress and pore pressure in salt basins [e.g., Daudré and Cloetingh , ; Gemmer et al , ; Hudec and Jackson , ; Hudec et al , ; Jackson et al , ; Tingay et al , ]. Hence, it is difficult to fully understand stress and pore pressure in salt basins, and it becomes a challenge to predict them on drilling through salt bodies in salt basins [e.g., Brown et al , ; Rohleder et al , ; Whitson and McFadyen , ; Willson et al , ; Yu et al , ; Zhang , ; Zhang et al , ].…”