Study On Fracture Mechanism And Wellbore Initiation Pressure Of Ultra-deep Fractured Sandstone In Tarim Oilfield

The reservoir in the Keshen block of the Tarim Oilfield is buried in deep depths,with high temperature and high stress and high pore pressure.At the same time,there are different degrees of filling cracks,these factors lead to complex deformation and fracture mechanisms of rocks,it is prone to well leakage,and it is difficult to determine the crack initiation pressure,causing drilling fluid loss during drilling or high pressure at the wellhead in the fracturing process,and it is difficult to construct,which seriously restricts safe and efficient drilling and completion.Due to the special engineering geological conditions,there are few studies on the fracture mechanism of ultra-deep sandstone reservoirs with filling fractures.This paper systematically studies the characteristics of deformation and failure of ultra-deep tight sandstone with fractures under high temperature and high pressure conditions,analyzes the rock fracture mechanism and main controlling factors,and establishes the failure criteria of sandstone with fractures under high temperature and high pressure.Based on this,a wellbore initiation pressure prediction model considering formation characteristics is established and field verification is carried out.Specific research content includes:(1)High temperature and high pressure test of sandstone with fracturesHigh-temperature and high-pressure rock triaxial tester RTR-1500 is used to systematically perform triaxial compression tests of sandstone with fractures under high temperature and high pressure conditions.The influences of fracture dip,temperature,confining pressure on the mechanical parameters and deformation failure modes of sandstone with fractures are analyzed.It is found that sandstone with fractures performs shear fracture mode along the fracture surface under high temperature and high pressure and show obvious plastic characteristics.(2)Study on deformation and failure criteria of sandstone with fracturesBased on experiments,the variation law of the cohesion and internal friction dip of sandstone with fractures under high temperature and high pressure conditions is analyzed,and the empirical formula for the cohesion and internal friction dip of ultra-deep reservoir in the Keshen block is established.Based on this,the modified Mohr-Coulomb criterion for sandstone reservoirs with fractures is obtained.(3)Study on hydraulic fracture initiation pressure in ultra-deep fractured formationsAccording to the model including hydraulic fracture initiation mode and fracture initiation pressure calculation formula in the fractured formation,and the cohesion and internal friction dip of the fracture adhesive weak surface are considered,combined with the failure criterion under the influence of temperature,the fracturing initiation identification method for the ultra-deep sandstone reservoirs with filling fractures is given,and field verification of the initiation pressure calculation is carried out.