| Summary: | The operational experience of a horizontal segment well within the hard brittle shale layer of Longmaxi Formation in the southern Sichuan area demonstrates that the high-density oil-based drilling fluid (HDOF) induced down-hole working conditions(high-temperature, high-pressure, and solid-phase-dominance)directly impact the stability of the wellbore wall. Focusing on the impact of bottom-hole osmotic pressure and solid phase content and a series of density-reduction field tests, we identified suitable countermeasures. In this paper, we discuss the impact of high osmotic pressure and operational measures, and provide details of the field tests, which indicate that maintaining a higher osmotic pressure at the bottom-hole can reduce the formation pore pressure of the bottom-hole, widen the fluid density window, and maintain the stability of the wellbore wall. By improving the HDOF formula, such as using new treatment agents (e.g., polymer-modified nano-sealing agent and ultra-micro barite weight additive) and optimizing the solid particle size distribution of HDOF, we can effectively address the down-hole problems. The field test of “pressure-controlled drilling + density reduction” in horizontal drilling operations was found to reduce the complexity of down-hole faults, improve the rate of penetration, and reduce overall costs.
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