| With the continuous improvement of China's energy demand,conventional energy has not been able to meet the needs of modern development,so the focus of exploration and development has turned to shale gas.With the development of horizontal wells and hydraulic fracturing technologies in recent years,the global exploration and development of shale gas is increasing.In recent years,China's oil companies have continuously increased their exploration and development of oil and gas in the Sichuan-Chongqing region,repeatedly breaking through new records of production,and based on actual data analysis,it shows that there is a large amount of shale gas resources in the Sichuan-Chongqing region of China.The Wufeng Formation and The Longmaxi Formation shale gas resource are more than 9 trillion cubic meters,and it has the conditions for large-scale construction capacity.The deep shale gas exploration and development technology in the Sichuan-Chongqing region has continued to mature,further strengthening the confidence in achieving the long-term development planning goals of shale gas.Based on this hot spot,this paper aims to improve the drilling efficiency and reduce the instability of the well wall collapse.The problem of the stability of the borehole wall in the Longmaxi Formation in the Sichuan-Chongqing region is studied and analyzed in order to solve the problem in the drilling engineering.The problems encountered in the instability of the borehole wall provide technical support for the future exploration and development of shale gas in the Sichuan-Chongqing region.The main work of this paper includes the following aspects:(1)The shale core samples from the Longmaxi Formation in this study area were used for indoor experiments to determine rock mechanics parameters,geostress,mineral composition,porosity,permeability,drilling fluid system and hydration shear strength.The actual data obtained are used to analyze the basic characteristics of the Longmaxi Formation and shale cores,and to lay the foundation for the stability analysis of the Longmaxi Formation strata below;(2)The form and mechanism of the wellbore instability of the Longmaxi Formation are analyzed.The mechanical analysis of the borehole stability is carried out without considering the contact between the drilling fluid and the formation shale.The collapse pressure and the fracture pressure are calculated;(3)Based on the composition and physical properties of mud shale,the internal causes of hydration were analyzed,and the hydration mechanism of shale was further studied.The traditional adsorption-diffusion equation was deduced and calculated by finite difference method,and Longmaxi was quantitatively analyzed.The variation of water content distribution around the formation wellbore with radial distance and time;(4)The finite element method is used to analyze the heat conduction problem and the thermo-mechanical coupling problem after the drilling fluid invades the Longmaxi Formation.The finite element simulation software ANSYS is used to calculate and analyze the temperature field around the wellbore caused by the heat transfer between the drilling fluid and the formation.The distribution law of time is further analyzed.The distribution of stress around the borehole wall under temperature and geostress is considered and compared with the temperature effect.The influence of thermal effect on the stability of the borehole wall is analyzed.The ANSYS simulation software was used to compare and analyze the difference between the hydration effect and the thermal effect of the Longmaxi Formation.The influence of the two on the stability of the borehole wall was quantitatively analyzed;(5)The measures and methods proposed in this paper were adopted in Well of Weiyuan 35-1 and Youdi 2 in Sichuan-Chongqing area.The application effect was obvious.The average mechanical drilling speed and the processing time of underground complex conditions in the Longmaxi Formation were greatly improved. |
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