Research On The Mechanism Of Borehole Instability In Gas-bearing Sandstone Gas Drilling

Drilling deep gas-bearing sandstone layers with air drilling technology can effectively protect reservoirs and prevent circulation lose.When air drilling is used to uncover high-pressure gas production layers,a great negative pressure difference will be formed between the wellbore and the formation,the natural gas is produced at a very high rate,and a strong fluid-solid coupling occurs in the near-wellbore formation,causing formation pore pressure and ground stress change drastically in a short period of time.It has become one of the key problems that restrict the application of air drilling technology.In this paper,the wellbore instability problem induced by sandstone air drilling is analyzed.The wellbore-formation transient coupling seepage is studied,the dynamic formation stress and the dynamic collapse pressure of the gas production process are studied,and a comprehensive evaluation method for wellbore stability in the high-speed gas production process of high-pressure gas layer was established,and the gas wellbore stability evaluation model was improved.The main research results were as follows:(1)The wellbore gas flow equation is established by the mass conservation and momentum conservation equations.The transient seepage model of the matrix production layer and the fracture production layer is established by the seepage continuous equation and the seepage motion equation.Coupling the transient flow model of the wellbore and the formation according to the internal and external boundary conditions.The influence of formation parameters on the wellbore-formation coupling seepage is studied according to the coupled model.(2)During the formation gas production process,take the changes of pore pressure and drag force into account,the effective stress model of the gas-bearing sandstone is established.The acoustic emission triaxial failure experiment is conducted in order to verify this model.The influence of the morphology of the formation fracture on the effective stress of the formation after gas production was analyzed by finite element software.(3)Through the analysis of the longitudinal and transverse wave experiments and logging data,the dynamic change model of formation collapse pressure during gas production is deduced based on the shear failure criterion.According to the model,the stability of the borehole wall after gas production from gas-bearing sandstone is preliminarily evaluated,and the influence of the angle and azimuth angle of the well,the presence or absence of cracks in the gas-bearing sandstone,the permeability of the fracture and the pore pressure of the formation on the collapse pressure of the formation are analyzed.The research in this paper is of great significance for revealing the wellbore instability mechanism of air-drilled gas-bearing sandstone and the safety control of gas drilling in the production layer.