Trends in Shallow Sediment Pore Pressures – Deepwater Gulf of Mexico

Abstract

Abstract As part of the effort to understand the problem of shallow water flow (SWF) in the Gulf of Mexico (GOM), and to obtain data and develop design criteria for offshore production structures, geotechnical wells were drilled in several different prospects. These prospects cover a 200+ mile swath across the central deepwater GOM where SWF control problems have been experienced. In these wells, pore pressure measurements, as well as core and other in-situ measurements, were taken in the deepwater shallow sediments. Key facts and observations from these measurements include the following: A majority of the pore pressure measurements were made in low permeability clay rich material, predominant in the shallow sediments. Some measurements were proximal to more permeable sand and silt zones. A few measurements were made in sandy/silty intervals.Where significant over-pressures were present they were found to begin at or very near the mudline, and to increase more-or-less linearly with depth below mudline (BML).Over-pressures in the shallow sediments of the central deepwater GOM are due primarily to rapid sedimentation rates generated by the Mississippi River depocenter. This is supported by the fact that measured over-pressures exhibit regional trends generally consistent with sedimentation rate.Regional trends in over-pressures are also correlative with drilling experience and the incidence and severity of shallow water flow (SWF) occurrences.The degree of over-pressuring is consistent with sediment porosity based on core measurements, i.e., higher over-pressures are associated with higher porosities.There are exceptions to these general regional trends. Most notable is that over-pressures in shallow permeable sand or silt zones may not be in equilibrium with their bounding shales. Both pressure depletion and hyper-pressuring have been observed in the geotechnical-well pore pressure measurements and in drilling operations. Thus well location (up dip, down dip), faulting, fracturing and other factors need to be considered in predicting pore pressures in these permeable zones.