Sobhasan field is one of the oldest fields of Mehsana Asset, ONGC, India. Hydrocarbons are produced, mainly, from multi-layered reservoirs in Mehsana and Mandhali members of Kadi Formation. Being a mature field, the pressure of reservoirs has gone down to 0.7 MWE. These depleted reservoirs (0.7 MWE) are inter-bedded with pressurized shale sequences (1.18–1.20 MWE) and thick/fragile coal bands. With this type of pressure regimes, drilling of ‘S’ profile wells with conventional drilling fluids is a challenge. Recurrence of differential stuck-ups and coal-caving during drilling and ‘failure of logging’ during completion operations is a common feature. The paper presents how the application of tailor-made polyol drilling fluid along with improved drilling practices helped in mitigating the recurring complications.

The strategy to resolve such challenges required to either review the casing policy or change the profile of the well or bring in new drilling fluid system. First and second options were not feasible due to high heterogeneity in lithology and technical implications respectively. So the emphasis was given on the third option (i.e. use of KCl-PHPA-Polyol drilling fluid) along with improvement in drilling practices to resolve the prevailing problems. Extensive studies were carried out to formulate a tailor-made KCl-PHPA-Polyol drilling fluid.

The polyol drilling fluid was applied in two wells recently. Better drilling practices, while drilling coal sections, were adapted to avoid disturbance of coal sections. The apparent benefits are: saving of rig days, healthy bore hole, reduction in torque/drag, prevention of differential sticking, and smooth logging operations. The present paper is an endeavor to demonstrate how ‘polyol drilling fluid’ and ‘tailored drilling practices’ can complement each other in the completion of problematic ‘S’ profile wells smoothly. This proposal is an example of ‘Sharing Best Practices’.

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