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The operation experience of horizontal segment well in the southern Sichuanarea within the hard brittle shale layer of Longmaxi formation demonstratesthat the high-density oil-based drilling fluid (HDOF) induced down-hole workingcondition, which is typically high-temperature, high-pressure andsolid-phase-dominant, will directly impact the stability of the wellbore wall.Focusing on the impact of bottom-hole osmotic pressure and solid phase contentand a series of density-reduction field tests, we found some helpfulcountermeasures. In this paper, we study and discuss the impact of high osmoticpressure and operation measures, and provide details of the field tests whichindicate that maintaining a higher osmotic pressure at the bottom-hole canreduce the formation pore pressure of the bottom hole, widen the fluid densitywindow, and maintain the stability of the wellbore wall. By improving the HDOFformula, such as using new treatment agents (e.g. polymer modified nano-sealingagent, ultra-micro barite weight additive) and optimizing the solid particlesize distribution of HDOF, we can effectively address the down-hole problems.The field test of "pressure-controlled drilling + density reduction" inhorizontal drilling operations is therefore proved to be capable of reducingthe complexity of down-hole faults, improving the rate of penetration, andsaving the overall cost.

Field Tests of High-Density Oil-Based Drilling Fluid Application in Horizontal Segment