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Investigations into the causes of high formation pressures in deep drilling in western Greece

Mavromatidis A, Kelesidis Vasilis

Πλήρης Εγγραφή

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Περίληψη

This research discusses the occurrences of high formation pressures during the last time there was deep drilling through thick evaporitic (mainly anhydrite) sections in northwestern Greece, near Ioannina. The stratigraphy of the area is composed of Jurassic to Tertiary carbonates underlain by Triassic evaporites. The well structure was a sub-evaporite high, with uncertainty as to the nature of the interval beneath the Triassic evaporites. The hydrocarbon play system depended on the geological model applied. The most likely model comprised carbonate reservoir of Cretaceous-Eocene age, sealed by the Oligo-Miocene Flysch and over-thrusted Triassic evaporites. The reservoir was thought to be sourced from underlying Mesozoic source rock intervals. The alternate, less likely model comprised of Permo-Triassic carbonate-dominated reservoir, sealed by Triassic evaporites and probably sourced from Triassic intervals. During drilling at the depth of ~3,900 m in the evaporitic section, high pressures were encountered creating significant problems and the well was killed. The well was sidetracked at shallower depth (~2,900 m) but at ~3,600 m, high formation pressures were encountered again. The well was found flowing and finally it was shut, plugged and abandoned. Possible causes of the high pressures could be: flysch overpressure laying beneath the evaporites which could somehow charge dolomitic lenses within the evaporates via faults; intra evaporite overpressure due to earlier isolation of carbonate lenses/rafts within anhydrite while they were at greater depths, followed by uplift of those lenses to the present (shallower) depths; and/or the result of formation water migration from under laying potential oil-gas bearing formations which has accumulated due to the evaporites seal. The results are cross-analyzed and we explore similarities and differences with similar situations that have been reported in other regions of the world where in similarly lithological environments (e.g. high pressures in or below evaporites), high pressure phenomena have also been reported. The difficulties in predicting high formation pressures while drilling thick evaporite sections are presented and suggestions are made for possible actions to be taken, as implemented in similar situations, in order to overcome these difficulties for future drilling activity, which must continue because viable plays may exist underneath the evaporites.

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