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Cawley, Stephen1, Tim Watts1
(1) BP-Amoco, Aberdeen, United Kingdom

ABSTRACT: Integrated Pore Pressure Prediction for the HPHT Rhum Gas-Condensate Field, Block 3/29, UKCS

Optimising well design for HPHT field development is a zero-tolerance HSE issue. We describe an integrated project using offset well drilling data, petrophysical log interpretation/modelling, seismic velocity analysis and semi-regional scale 3D fluid flow basin modelling to provide pore pressure assurance for well planning on the Rhum Field in the North Sea, UKCS.
Sonic and velocity log-derived porosity trends suggest overpressures within Tertiary shale sequences. Hence recent wells have tended to be drilled with heavier mudweights than earlier discovery wells. A zone of apparent low density/high sonic travel times commonly occurs in the early Tertiary in this area of the North Sea leading to over estimation of pore pressure despite adjacent normally pressured sands. Conversely calculated velocity/porosity-derived pressure estimates at depth within the HPHT zones in the Lower Cretaceous and upper Jurassic often fall well below the actual measured values from test data.
Analysis of seismic velocities in the Rhum area suggest normal shale compaction for most of the Tertiary overburden and significant overcompaction in certain lithologies. To help resolve this, a 3D dynamic fluid flow basin model was built to provide extra pore pressure assurance for development well locations.
The results from the offset drilling data, new seismic velocity analysis and 3D fluid flow modelling were similar to each other, yielding a pressure envelope within which casing and mudweight programmes consistent with the discovery well could be planne

 

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.