Mechanism and control measures of wellbore instability of large-sized boreholes in ultra-deep wells in western Sichuan Basin
In the Sichuan Basin,there are multiple formations developed vertically,and the geological structures are complex.When deep to ultra-deep wells are drilled through the formations in the upper part,fractures are encountered and the hole size is large,imposing the operations to high risks of complexities such as wellbore collapse.In order to clarify the instability mechanism of ultra-deep large-sized boreholes,core samples from the Shaximiao Formation in the upper interval of ultra-deep well in the Shuangyushi Block of western Sichuan Basin are conducted with XRD diffraction,scanning electron microscopy and direct shear test to analyze the mineral composition and bedding structure characteristics and test the relationship between water content and weak plane strength.Then,a calculation model of collapse pressure of fractured formation with consideration of hole size is established based on drilling fluid filtration towards fractures,pressure distribution within fractures,and stress distribution around the wellbore.Finally,the mechanism of wellbore instability in the Shaximiao Formation is analyzed,and the solutions to prevent wellbore instability are proposed.The following results are obtained.First,the Shaximiao Formation is bedding mud shale with developed microfractures,and Ø444.5 mm large-sized borehole has a significant impact on the formation collapse pressure,so the size effect shall not be ignored.Second,due to the presence of fractures,blindly increasing the density of drilling fluid to stabilize the wellbore may cause more serious collapse.Third,initial formation pressure and fracture dip have varying degrees of influence on the collapse pressure.Increasing the viscosity of drilling fluid is favorable for the decrease of collapse pressure,but it is not significant in practical application.Fourth,improving the quality of mud cake and reducing the filtration rate of drilling fluid are important and reasonable methods to ensure the wellbore stability in fractured formations.In conclusion,the understanding on this mechanism reveals the wellbore instability mechanism of large-sized boreholes in fractured formations under the action of multi-field coupling,which provides theoretical reference for the prevention and control of wellbore instability of large-sized boreholes like deep and ultra-deep wells in the process of drilling in the Sichuan Basin.
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