Abstract: Ultra-High Pressure Implications for Prospectivity and Drilling

Nicholson, P. H. and Cayley, G. T. - Esso E&P, UK Ltd.

Ultra high pressures are encountered in the deep Central graben of the UK North Sea. This paper will look at the controls on these pressures and the implications for drilling and prospectivity in the context of a well drilled by the Shell/Esso UK North Sea partnership. Due to significant pressure problems, this well took one year and required 2 sidetracks to reach its objective. The objective contained excellent quality Jurassic reservoir with a residual bitumen stain. Ultra high pressures and elevated temperatures cracking oil to gas had caused hydraulic failure of the top seal. The consequent vertical leakage of hydrocarbons and pressure resulted in the charging of thin porous chalk zones in the overlying fine-grained top seal. It was these elevated shallow pressures which resulted in the significant drilling problems encountered in the well.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil