OOS 26-5 - Overpressure, fluid venting and sub-seafloor drilling: How can we preserve and build required expertise to respond to large oil vents in U.S. waters?

Wednesday, August 10, 2011: 2:50 PM
16B, Austin Convention Center
David Mohrig and Peter B. Flemings, Jackson School of Geosciences, The University of Texas at Austin, Austin, TX

Natural and drilling-induced venting of hydrocarbons at the seafloor is caused by the development of over-pressured sedimentary deposits.  These over-pressures develop when muddy deposits are unable to expel excess pore water during their burial. We will show that in the Gulf of Mexico these over-pressures can start near the seafloor and continue to great depth.  These over-pressured mudstones can prevent encapsulated sandy deposits from compressing as they are buried.  The net result is a significant amount of potential energy stored in the subsurface. Breaching of one of these sand-rich reservoirs/aquifers can therefore result in sustained high-pressure venting of both hydrocarbons and water, in many cases all the way to the sea floor.  We will discuss the very large size of many sandy reservoirs/aquifers buried beneath the Gulf of Mexico sea floor and describe the system properties, both natural and drilling induced, that can result in blowouts, the uncontrolled flow of hydrocarbons and water through a vent.  We will also summarize the aspects of buried sediment deposits that control the rate, duration and total volume of vented subsurface fluid.


Timely response to large oil vents requires the building of core knowledge in the driving forces controlling subsea venting at the federal, state and community levels.  This core knowledge must include an understanding of the connections between elevated pore pressures and rock fracturing, behavior of multiphase fluid systems (gas, oil and water), and basic well design.  Academic and government scientists have very little experience with riser drilling; the industry technique used to protect against over-pressured sediment deposits and the sudden expulsion of fluid.  Quantitative models can be used to predict hydrocarbon entrapment, design safe drilling programs, and illuminate large scale flow behavior.  We propose that the grand challenge in pore pressure and stress prediction is the incorporation of realistic deposit and structural geometries into numerical models in order to accurately define stress perturbations that may induce blowout under conditions not predicted by simple burial scenarios.

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