Al-Khafji Joint Operations is developing the offshore Khafji Field in the X-Neutral Zone by drilling horizontal wells. These wells encounter shale formations namely Ahmadi Shale and Wara Shale before reaching the pay zones, and Intermediate Shale between KB-1 and KB-2 Sands. While drilling these shale formations, severe time-dependent wellbore instability problems due to shale sloughing were experienced. These problems have led to several stuck pipes and sidetracks.

A drilling fluid optimization study was conducted on seven offset wells in the field in order to evaluate the potential time-dependent wellbore stability mechanism(s) in Ahmadi Shale, Wara Shale and Intermediate Shale, to define appropriate mud type, salt type and concentration, and to develop solution and strategy to mitigate and/or manage the wellbore instability problems.

End of Well reports of selected offset wells were reviewed, and numerous time-dependent wellbore instability-related drilling events in Ahmadi Shale, Wara Shale and Intermediate Shale were identified. Both mud pressure penetration and chemical potential mechanisms were considered in the back-analysis of time-dependent wellbore instability in the shales. The analysis showed that the instability was attributed to insufficient salinity in the drilling fluid which resulted in pore pressure increase due to the mud pressure penetration mechanism being not counteracted by the chemical potential mechanism. As a consequence, the pore pressure increased near the wellbore wall which reduced the effective mud support and led to unstable wellbore condition.

Based on the drilling fluid optimization study together with input from mechanical wellbore stability analysis, recommendations on optimum drilling fluid design were made for drilling Ahmadi Shale and Wara Shale which were successfully applied in a planned horizontal well in the field.

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