Study On The Interaction Mechanism Between Drilling Fluid And Hard Brittle Shale

In the global oil and gas exploration and development process,hard brittle shale formations are commonly encountered.As oil and gas exploration progresses to complex strata,the problem of wellbore instability caused by the interaction between hard brittle shale and drilling fluid frequently occurs,which seriously restricts the economical and efficient completion of drilling.However,the current research on the interaction of hard brittle shale and drilling fluid is rare.Based on the pressure transfer experiments,drilling fluid soaking experiments,Brazilian splitting experiments to study the intrusion patterns of drilling fluids in hard brittle shale and two types crack propagation models are established to analyze the influence of the interaction between drilling fluid and hard brittle shale on different types of crack propagations.The research indicates that:(1)the drilling fluid has three intrusion modes related to the crack characteristics inside the hard brittle shale.(2)Under the condition of stress free constraint,it will form I type crack after contact with the hard brittle shaleIt is found that the water-rock interaction after the contact between drilling fluid and hard brittle shale can increase the stress intensity factor at the crack tip and reduce the fracture toughness of the rock.The wettability of drilling fluid,interfacial tension and fracture geometry can affect the propagation of I type fracture.(3)From the analysis of the stress failure of the rock around the well,it is pointed out that the failure form of hard brittle shale in deep strata is more tended to shear failure and the fracture type should be ?-? composite crack.It is shown that pore pressure,hydration stress,crack geometry and friction coefficient will affect the propagation of ?-? composite crack.