Drill-in fluid loss mechanisms in brittle gas shale: A case study in the Longmaxi Formation, Sichuan Basin, China

Abstract

Brittle gas shale consists of a high content of brittle mineral and developed natural fractures and has become the focus of petroleum industry in recent years. Lost circulation may easily occur due to the long soaking time and an overbalanced pressure difference during the drill-in process of the horizontal long section. The dynamic fracture width simulation induced by drill-in fluid invasion, plugging zone pressure containment test, friction coefficient test between lost circulation materials and shale surface, oil based drill-in fluid filtrate immersion test were conducted to investigate the drill-in fluid loss mechanisms in the brittle Longmaxi Shale in the Sichuan Basin of China. Results show that the fracture width at the fracture mouth increased from 2 μm to 239 μm for the two mutually perpendicular fractures connecting to the wellbore due to drill-in fluid invasion. Effective plugging for dynamic fractures was failed to be achieved for the original drill-in fluid. Fracture width increase induced by drill-in fluid invasion is the major cause of plugging failure of brittle gas shale. Instability of the brittle shale induced by oil phase lubrication and high pH drill-in fluid erosion further exacerbates lost circulation problems. Optimized LCMs considering the dynamic apertures of fractures should be selected and added into drill-in fluid to rapidly seal pre-exiting fractures and induced fractures for lost circulation control in brittle shale. The results serve as a guide for the drill-in fluid design of shale gas in brittle gas shale formation.