Study On Borehole Stability Of Hard Brittle Mud Shale In PL7-6 Area

The wellbore stability of hard and brittle shale has always been the focus of attention.The increasing problems in oil and gas drilling have become a technical difficulty in restricting drilling speed and hindering safe and efficient exploitation in the process of oil and gas resource development.However,the research on the wellbore stability of hard and brittle formation is not comprehensive at this stage.The PL7-6 block studied in this paper is located in the south section of low uplift in the east of Bohai Sea.This structure is located in strike slip fault.The ground stress is very complex.The pressure torsion anomaly often collapses,while the mudstone is easy to hydrate and expand,resulting in block falling and shrinkage.The high frequency of jamming occurs in the drilling process,which seriously affects the drilling operation time and the geological data acquisition,and brings huge to the offshore exploration and development challenge.In view of the difficulties in drilling operation of Penglai 7-6 structure,by analyzing the instability mechanism of each well,a large number of targeted experiments are carried out to determine the law of hydration expansion and strength of the hard brittle shale in the unstable interval,the collapse period of the well wall under different drilling fluid systems,and the window of mud safety density under different well tracks,and optimize the trajectory.The results show that the orientation of the horizontal maximum principal stress of PL7-6 Oilfield is 45°to the east by North,the uniaxial compressive strength of the formation is relatively low,the strength decreases greatly after immersion,and the hydration and expansion are strong.When the density of PEM mud system is 1.35g/cm~3,the collapse period is about 5 days.According to the theoretical calculation and experimental results,it is suggested that the mud density should not be too high.If the drilling fluid intrudes into the formation,the formation strength around the well will be reduced and the stability of the fractured formation will be greatly affected.It is important to strengthen the plugging and inhibiting ability of the drilling fluid,prevent the drilling fluid and its filtrate from intruding into the fractures as much as possible,and improve the effective supporting force of the drilling fluid column on the well wall To increase the formation leakage pressure;to drill along the direction of the horizontal minimum principal stress to avoid the excessive inclination angle of the well;to determine a reasonable casing second spud in depth to avoid the shallow layer prone to hydration of the soft shale and other layers of the hard brittle shale in an open hole section,which will cause the sticking problem at one time.This set of solutions can be continuously popularized in the follow-up evaluation well and development well operation of this structure and other unstable formations.