Experimental Study on the Adaptability of Plugging Drilling Fluids to Wellbore Stability in Hard Brittle Shale Formations

[Image: see text] The problem of wellbore stability in hard brittle shale formations is an important research topic in the exploration and development of shale gas. To solve this problem, the adaptability of the plugging drilling fluid to wellbore stability in the hard brittle shale of the tertiary Dongying formation in Bohai Bay Basin, China, was investigated. The results show that the clay content of the hard brittle shale in the study block is as high as 39.2% on average, with great possibility for hydration. The pore structure in the shale is dominated by micron-scale fractures and pores. A dense structure was formed on the surface of the shale after being immersed in plugging drilling fluid, and the matrix permeability of the shale was reduced by 91.1% and the fracture permeability by 98.7%. The water content increment of the shale after immersion was merely 0.75%, which reduced the probability of hydration greatly. Compared with the field-inhibitive drilling fluid, the plugging drilling fluid improved the uniaxial compressive strength of shale by 28%, which is more conducive to maintaining the wellbore stability. The seepage stress aggravates the risk of wellbore instability, while the hydration stress does not, but both increase the risk of rock instability at positions away from the well wall. The plugging drilling fluid affects the seepage stress and hydration stress by reducing the shale permeability and water content. With the decrease of permeability and water content, the potential instability zone of a wellbore becomes smaller.