Abstract

Drilling in Gulf of Suez is difficult due to wellbore instability, lost circulation and time dependent shale stabilization related problems. Drilling troubles in this region generally originates from high earth stresses and abundant natural fractures and faults associated with this tectonically active region. Presence of salts and evaporates with depleting reservoir pressure of a mature field aggravate the problem. Operators in this region always experienced drilling difficulties which sometime leads to well abandonment and costs over-runs in millions of dollars. Planning for the present well was the first attempt to drill a horizontal section in the porous and oil bearing Asl sand. Therefore, the main objective of the current study was to assess the risk of wellbore instability which may occur while drilling and recommend the remedial action plan for any risk encountered.

Wellbore stability analysis has a major impact on the well design and planning the orientation of trajectory for safe and stable well and successful drilling operation. Traditional drilling practices based on pore-pressure of the reservoir and fracture gradient does not necessarily proved successful especially drilling horizontal wells. The failure criterion works in a completely different way for a horizontal well compared to a vertical well in the vicinity. Therefore, a safe vertical offset well inevitably never assures that the identical drilling design and practice will safely drill a horizontal well. The stress distribution and direction works differently for a vertical and a deviated or horizontal well. A rule of thumb is that the drilling gets more and more difficult with decreasing width of safe and stable mud window as the well becomes deviated and the situation worsen as the well turns horizontal. Adding to this complexity the direction of the trajectory with respect to in situ stress distribution and variation poses a major role in drilling a safe horizontal well.

The case history presents a geomechanical risk analysis for a planned horizontal dual lateral well. The study is based on stress regime and well failure with the significance of choosing proper mud weight and drilling parameters using a proper mechanical earth model from a nearby offset well. It includes an assessment of the major risks expected during drilling the horizontal section and also indicates magnitude of failure that can happen while drilling, based on the trajectory sensitivity analysis. The planned well was drilled without any wellbore stability related problem. The present study suggests the importance and benefits of a proper well stability study while drilling a horizontal well in a tectonically disturbed area.

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