2 The table below identifies the key stratigraphic formations and geological roc

Question

2 The table below identifies the key stratigraphic formations and geological rock types that a well being drilled in the Southem North Sea will encounter. Using you knowledge and judgement: (a) Identify the common problems associated with drilling each of these [10 marks] formations in the Southem North Sea b) Suggest oil rig operations and drilling solutions to the geological problems [10 marks] associated with all these formations Geological Age Formation type Tertiary Claystones, silrstones& occasional shelly limestones Late Cretaceous Chalk Group Chalk with flint bands Early Cretaceous: Red Cha Cromer Knoll Group Speeton Clay Jurassic: Humber Group West Sole Group Lias Grounp Minor Limestone Triassic Winterton Keuper Anhydrite Haisboroush Group Evaporite Series: halite, anhydrite & shales Bunter Sandstone&shale;, Brockelschiefer shale & Triassic Bacton Group Piatten Dolomite Limestones, Halire, Anhvdrite Zechstein Group Reservoir Rock) Coal and shale Carboniferous: Coal Measure

Explanation / Answer

(a) The various drilling problem that can be encountered in the mentioned succession are:

The tertiary claystone and the Jurassic clay stone can cause problem if water based mud is used to drill in those formations as the clay can become unstable and the hole can collapse or the drill string can stuck due to swelling of clay formation

Since there is seal of salt diapirs (Zeckstein group of Permian time) above the reservoir rock hence there can be high possibilities of high pressure associated with dolomites, severe washouts, movement associated with salt layers.

(b) The remedies that should be done while drilling this succession are:

We should use conductor casing in the shallow zone to protect the wall of the hole from caving in.

We should use intermediate casing for Jurassic claystone and Triassic evaporites series.

We can go for deep kick off profile (directional drilling) so that we won’t get any effect of the evaporite seal/cap rock.

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