Hole stability and differential sticking are main challenges while drilling through commingle shale & sand sequences. Conventional mud systems cannot ensure wellbore stability and sustain high overbalance, which led to the need of use ‘customized fluids’.

Traditionally, Oil-Based Mud (OBM) have been used while drilling these formations to avoid high NPT hours. It was necessary to identify an alternative fluid's solution to replace Oil-Based Mud with Water Based Mud capable to provide good borehole stability.

Using High Performance Water Base Mud (HPWBM) to drill Zubair Formation was the first ever application in KOC. A customized drilling fluid system was designed using a blend of Synthetic Copolymer, and Organic Clay inhibitors combined with sized particles. This effectively stabilizes hydratable and dispersible shale, plug pore throats and minimizes fluid invasion, as confirmed by particle / permeability plugging tests and Linear Swell Meter tests under hole conditions, overcoming below challenges.

  • Perform sampling in Zubair formation successfully for 1st time in KOC with WBM

  • Improve hole stability through stressed shale formations

  • Hole stable under extended conditions (total loss condition and multiple trips on elevator)

  • Sustain high overbalance and mitigate differential sticking tendency

Drilling, logging, running and liner run and cementing was successfully completed in the commingle section without any incident. There was no NPT related to well-bore instability or differential sticking tendency reported. MDT sampling was at an advantage due to faster cleanup time in WBM, sample quality was very good with >95% oil in sample recovered. This paper will present the success of the HPWBM utilized to comingle Zubair shale with case histories for reference.


Most operators in the world are more keen in use of non-aqueous fluid (NAF) over water based mud systems to drill challenging wells. NAF system has been considered technically superior over conventional water based muds in the areas of well-bore stability, inhibition, lubricity, rate of penetration, cuttings condition and differential sticking tendency.

As the industry strives to achieve improved environmental solutions, the introduction of HPWBM creates the possibility of same performance where OBM's is typically used.

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