Mechanism of Fluid Shale Interaction and Construction of Drilling Fluid System in Marine Land Transitional Shale Reservoirs

China possesses abundant shale geological reserves and a high concentration of resources in the coastal transition zone. However, drilling operations in this area are susceptible to wall collapse and instability, creating significant risks. This study investigates the mineral composition and microstructure of reservoir rocks through drilling core observation, electron microscopy, CT scanning, and X-ray diffraction analysis. The findings reveal that the shale gas reservoir is predominantly composed of quartz and clay minerals, with a clay mineral content of 45.7%. Notably, montmorillonite is absent, while kaolinite represents 35%, and the illite/montmorillonite mixed layer accounts for 26% of the clay minerals. The rock samples exhibit nano-scale pore development, micro-crack formation with widths in the micron range, and interconnected narrow cracks. Analysis of alterations in interlayer spacing, surface tension, linear expansion rate, and crack expansion after rock-fluid interaction unveils that shale in the coastal transition zone solely undergoes surface hydration, offering insights into the mechanisms of rock-fluid interaction in shale gas reservoirs. Consequently, a high-performance water-based drilling fluid system specifically designed for the coastal transition zone shale gas reservoir is formulated, encompassing the selection of water-based drilling fluid inhibitors, plugging agents, and lubricants. The system is subsequently subject to comprehensive laboratory evaluation, which substantiates its exceptional performance in terms of conventional properties, inhibitory effects, sealing capabilities, and lubrication. It demonstrates a temperature tolerance of up to 100 ℃, experiences a fluid loss of merely 6 mL under high-temperature and high-pressure conditions, exhibits a shale swelling rate of 1.03%, possesses an overall drilling fluid lubrication coefficient of less than 0.15, reduces API filtration loss by 40% compared to the base slurry after 30 min, and effectively seals micro-cracks in the reservoir formations. Additionally, the system demonstrates a low level of biotoxicity, with an EC₅₀ value of 37,260 mg/L. It can meet the requirements of drilling fluid performance for transitional shale gas drilling operations between sea and land, and has been applied on site with good sealing and anti-collapse effects. This research addressing the considerable technical challenge of wall instability in the coastal transition zone shale wells.