Study On Borehole Instability In Sandstone Reservoir Under Hydro-mechanical Coupled Condition

Natural gas has been one of the most potential and widely used clean energy sources in the world at present.China is a big producer and consumer of oil and gas resources.China has placed the development and utilization of oil and gas resources in a strategic position of national energy development.Tight sandstone gas has been one of the important natural gas resources for the rapid development in China.Drilling engineering is an important link in the process of exploration and exploitation of oil and gas resources.With the rapid development of oil and gas engineering,drilling engineering gradually develops to the deep formation.The phenomenon of borehole wall instability and jamming of drilling tools becomes a common problem in drilling engineering.Therefore,it is of great significance to study the problem of borehole instability in sandstone reservoir.Drilling engineering is faced with complex geological environment such as high in-situ stress and high osmotic pressure.A large number of researchers have carried out researches on the mechanism of borehole breakout and failure,however,there are still many problems that have not been solved,and the research on borehole breakout under hydraulic coupling action is not deep enough.It is urgent to study the influence of borehole sizes,in-situ stress distribution,mud weight,fractures in surrounding rock on borehole instability under the action of hydraulic coupling to ensure the stability of drilling engineering.Laboratory tests and discrete element numerical simulation will be combined to carry out research on the above problems.The main research contents are as follows:(1)Physical and mechanical parameters of red sandstone from Zigong,Sichuan were obtained through laboratory tests.Darcy seepage test was carried out.There is a negative exponential relationship between permeability and confining pressure of red sandstone.The permeability of red sandstone increases greatly after the specimen is damaged by axial compression.The permeability of red sandstone increases with the increase of permeability pressure difference(2)Borehole breakout tests were carried out on thick-walled red sandstone cylinder specimens with different pore sizes under hydrostatic pressure.With the hole diameter decreasing,the critical hydrostatic pressure increases significantly,and the shear failure ratio increases.The borehole breakout starts from the axial center of the sample,extends to both ends,and eventually evolves into a spindle shape.The lamellar failure occurs around the borehole,and the lamellar thin plate spall continuously and finally presents a "V" shaped failure.(3)The stress distribution formula of borehole surrounding rock considering seepage action was deduced,and the safe window of mud weight can be obtained by Mohr-Coulomb(M-C)criterion and tensile failure criterion.(4)We use PFC2D program and improve the pipe-network algorithm.The quantitative relationship among stress field,particle contact state and pipeline opening is established,which can truly reflect the influence between stress field and seepage field.By comparing results of Darcy seepage test,the reliability of the improved pipe network flow algorithm is verified,and it is further applied to the analysis of borehole stability under the action of hydraulic coupling.(5)The discrete element model of borehole instability is established.After the microscopic parameter calibration and the reliability verification of the model,the evolution process of borehole breakout under different conditions was studied.The simulation results show that the failure may not occur first on the wellbore wall,but at a certain distance from the wellbore wall.The influences of borehole diameter,far-field stress distribution,mud weight and single joint fissure and other factors on borehole breakout pattern and critical hydrostatic pressure were systematically analyzed.