Research on 3D spatial distribution features of drilling fluid pollution using CT technology

The understanding on spatial distribution features of oil/gas reservoir pollution by conventional liquid drilling technique is a concern of many scholars. The authors used high-energy, high-precision CT instrument to explore the 3D spatial distribution features of full-diameter core after being polluted by drilling fluid, established the corresponding test method and the means of quantitatively determined the filtration depth of drilling fluid, and obtained the 3D spatial distribution diagram of drilling fluid pollution under various conditions. Besides, the authors analyzed the microscopic features of porous and fractured reservoir drilling fluid pollution with the help of scanning electron microscope and energy spectrum. The research shows that the drilling fluid filtration depth mainly occurs at 11 mm to 24 mm from the core end face. After being polluted by drilling fluid, the core from porous and fractured reservoirs shows ‘pot-bottom’ feature of polluted space. And from fractured reservoir cores, it can be observed that drilling fluid has penetrated the whole fracture. Pressure difference and fracture conductivity are the main factors restricting drilling fluid from invading the matrix pore depth. The introduced particles in fractures are mainly barite grains and salt crystals and distributed in regional filter cake and aggregated grains. The introduced particles in matrix porous reservoirs are mainly salt crystals and distributed in the forms of settlement on pore faces, pore filling and throat plugging. The research findings provide an essential understanding on the mechanism of drilling fluid pollution features in porous and fractured reservoirs and provide a new research method for indoor experimental evaluation.