Polyol Drilling Fluid: A Novel Solution to Curb Recurring Drilling and Completion Problems in Mature Sobhasan Field, Mehsana Asset, ONGC, India

Abstract

Abstract 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’. Introduction Mehsana Asset is one of the highest oil producing onshore assets of ‘Oil and Natural Gas Corporation’, India. Annual oil production of this asset is to the tune of 2.40 MMT. Sobhasan field has been contributing substantially to the total annual oil production of Mehsana Asset for over 35 years. Multilayered sandstone bodies in ‘Mehsana’ and ‘Mandhali’ members of Kadi Formation act as reservoirs. Being a mature field, the pressure of reservoirs has gone down to 0.7–0.8 MWE. These multilayered depleted reservoirs are inter-bedded with fragile coal bands (20–40 meter thick bands in Mehsana Member) and pressurized shale/siltstone sequences. Both the members have highly heterogeneous lithologies. Above Mehsana Member occurs reactive/pressurized ‘Upper Tongue’ shale sequence. Upper Tongue shale sequence is overlain by Kalol Formation with thick/fragile coal bands and sub-hydrostatic sandstone layers (Table 1).