Shallow Water Flow Geopressures Arising From Confined Subsurface Slumps

Abstract

Abstract Elevated pore pressure within 1000-2500 feet of the seafloor (SWFG or Shallow Water Flow Geopressure) affects the casing program of many wells in the deepwater (>1000 feet water depth) Gulf of Mexico, the Nile delta, and other offshore drilling provinces. When SWFG sands are not properly isolated, a sustained (months in duration) brine flow may occur around the surface casing, leading to buckling of the casing and a large deposit of sand carried to the seafloor by the flow. For a SWFG blowout in the deepwater Gulf of Mexico, cased hole formation test data show the sands involved are sealed by an overlying marine transgressive Sangamon (100,000 Ma) age shale. Even though the four involved sands are separated by less than 60 feet of shale, the sands are also in pressure isolation from each other. Laboratory work on a sample of the sand carried to the seafloor shows large (> 1 p.u.) porosity loss could occur with finite strain cyles at low stress. Using these measured data, variation of seismic response of a flow sand before and during a SWFG well bore failure event has been tentative correlated to porosity losses of 1-2 p.u. A hypothesis is formed for finite strain and porosity loss along a critical stress (at shear failure) path for sand slumping during the shallow water flow event in the GB 515 #1 well. Porosity loss within the isolated or sealed SWFG sand could cause increases in pore pressure above existing levels.