Regional Trends in Undercompaction as a Predictor of Shallow Water Flow and Overpressure in the Deepwater Gulf of Mexico

Shallow water flow and overpressure, arising primarily from rapid sedimentation rates generated by the Mississippi River depocenter, represent major drilling hazards in the deep water Gulf of Mexico. Based on data from geotechnical wells, drilled in several different prospects in the central deepwater Gulf of Mexico, Ostermeier et al (2001) report that where significant overpressures are present, they begin at or close to the mudline, and that regional trends in overpressures correlate with the incidence and severity of shallow water flow. For shallow sediments in the deepwater Gulf of Mexico, compaction disequilibrium is the most important cause of overpressure. For sediment to compact, pore water must be expelled. If the sedimentation is too rapid compared to the time for fluid flow, the pore fluid will become overpressured and will support part of the overburden load. As a result, the porosity of the sediment will be higher and the velocity of elastic waves will be lower than in normally pressured sediments. Overpressures resulting from compaction disequilibrium may therefore be predicted, given sufficiently accurate seismic velocities (Sayers et al., 2002). In this paper, all available checkshots released by the MMS for the entire Gulf of Mexico are inverted for velocity versus depth below mudline, then kriged to populate a 3D Mechanical Earth Model with both velocity and expected uncertainty. The 3D velocity cube thus obtained is used to infer the regional variation in undercompaction of shallow sediments and overpressure. By applying a threshold to the predicted kriging error, a map of undercompaction and overpressure can be restricted to areas of greater reliability. The results of this analysis are expected to find wide application in the drilling of safe and economic deepwater wells. References: Ostermeier, R.M., Pelletier, J.H., Winkler, C.D. and Nicholson, J.W. (2002) Trends in shallow sediment pore pressures - Deepwater Gulf of Mexico, SPE/IADC 67772. Sayers, C.M., Johnson, G.M. and Denyer, G. (2002) Pre-drill Pore Pressure Prediction Using Seismic Data, Geophysics, 67, 1286-1292.