Success of achieving extra well-bore stability in highly reactive shale column of 1225 mtrs, in PR#2, is attributed to adequate pre-well planning, execution of drilling plan and guidelines, and the K+Cl -Polymer Drilling fluid chemistry. Well-bore cleaning and hydraulic modeling in an integrated geo-pressure gradient and critical analysis of shear failure pressure due to chemical osmosis and fluid - shale interaction was the significant attribute on well-bore stability. Pre well modeling of well-bore stability in problematic shale - zones and hole cleaning in its deeper final phase, contributed to stable hole behavior during unexpectedly longer durations on non - drilling downtime for frequent surface equipment failures, etc. Paper critically analyzes shear failure pressure and chemical osmosis during fluid - shale interaction to improve better well construction - design and drilling fluid modeling for productive oil - field economics in entire KG Basin - India. Key conclusions drawn from such a critical analysis of water based K+Cl - K+ Lignite/PHPA Polymer mud system include:

  • Hole cleaning and hydraulic model derived in this analysis, proved to be quite useful in the pre well planning process as well as in post - well analysis on high bore-hole stability.

  • Chemical osmosis and shale - fluid interaction further demonstrated the pore pressure transmission, and geo-pressure variance, and its integration with geo-mechanical stress and shear failure pressure derived the well-bore stability modeling.

The case study presented compares original mechanical stability and hydraulic- fracturing estimates with the critical mud weight identified for successfully completing this difficult operation. Descriptions of well-bore stability and fluid losses are presented along with drilling fluid parameters used to combat them. Method used for estimating formation pore pressure, in-situ stresses and formation rock properties are also described. Additionally, possible explanations for differences between planned and actual stability estimates are explored with future recommendations for similar operations.

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