Kuwait Oil Company (KOC) was planning further development of the Sabriyah (SA) and Raudhatain (RA) fields by drilling a number of horizontal wells. Both fields presented considerable challenges that included, among other factors, drilling through several problematic shales. The major drilling issues experienced in the shales comprised of tight spots, packs-off, etc. To optimize the drilling process and minimize the risks while penetrating the shales, a decision to conduct a geomechanical study was made. The geomechanical study was focused not only on improvement of the mud weight program (mechanical wellbore stability analysis was conducted) but also on drilling fluid optimization as well.

The drilling fluid optimization analysis was carried out in order to evaluate the potential time-dependent wellbore instability mechanism(s) in the Ahmadi, Wara and Middle Burgan shale formations in the two fields, to define salt concentration for nominated mud types, and to develop solution and strategy to mitigate and/or manage the wellbore instability problems.

Based on the time-dependent shale stability analysis, recommendations were made for drilling the Ahmadi, Wara and Middle Burgan shale formations for water-based mud (WBM) and oil-based mud (OBM). Mud salinities were provided for several combinations of designed mud weight and evaluated breakout mud weight.

The following conclusions were drawn based on the geomechanical study:

  • Water activity of the Ahmadi, Wara and Middle Burgan shale formations was moderately high. Hence, it was feasible to select an optimum drilling fluid design (whereby the mud pressure penetration was fully counteracted by the chemical potential mechanism) for WBM and OBM for the shale formations, even though overbalance pressure in the formations was relatively high. The Ahmadi, Wara and Middle Burgan shale formations could have been drilled using the designed mud weight programme and salinities for the two mud types without significant time-dependent wellbore instability problems in the analyzed planned wells.

  • If the recommendations on mud weight and/or salinities were not fully followed, the shale formations could deteriorate with time due to either mud pressure penetration mechanism being not fully counteracted by chemical potential mechanism or over-dehydration. Increase in drag that could indicate formation deterioration should be established based on drilling parameters and rig capabilities.

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