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Shunbei oilfield is located in the Midwest of Tarim Basin. Since 2018 many wells in the block have encountered broken formation, directional wells wellbore collapsed seriously, resulting drilling sidewells, with a huge loss of drilling costs. The influencing factors of igneous formation stability include internal and external factors. The internal cause refers to the formation instability caused by the strength anisotropy and strong in-situ stress nonuniformity of igneous formation. The external cause refers to various factors that cause the change of the external environment of the formation caused by drilling engineering, including the physical and chemical interaction between the drilling fluid and the formation, well trajectory and mechanical disturbance of the drilling string, etc. Based on the analysis of the influencing factors of wellbore stability in igneous rock, the technical countermeasures of wellbore stability in igneous rock formation are put forward and field tests are carried out. In the Permian fractured formation of Shunbei oil field,the idea of “prevention first, combination of prevention and blocking” is adhered to the first principles. Low density and low viscosity drilling fluid with low displacement is adopted to prevent the occurrence of lost circulation. The wellbore of the igneous rock intrusion in santamu formation of Ordovician is easy to collapse. By adopting special sealing structure and strengthening plugging and anti-collapse, combined with slurry plugging and well washing and refining engineering measures, the safe drilling of the igneous rock can be ensured.

Basic geological background, drilling fluid formulation optimization and field application were carried out aiming at the technical problems of multiple blocks in Shunbei Oilfield when collapsing strata and collapsing and blocking frequently. The lithology of the fracture zone is non-clay minerals. After the drilling fluid or filtrate is eroded, the self-developed pore network is easily pressurized and connected, which reduces the adhesion of the bedding interface, which in turn induces large-scale spalling and slump stuck. Based on the mechanism of the instability of the borehole wall, a submicron-micron (0.1-100μm) comprehensive and effective plugging is proposed to prevent drilling fluid or filtrate from invading the formation pores and fractures around the well, and at the same time ensure that the drilling fluid has an efficient rock-carrying solution. The technical achievements carried out on-site tests at Shunbei No. 5, based on the original Sikai high-temperature polysulfonate drilling fluid, optimized the indoor formulation, and developed a wellbore stable drilling fluid suitable for broken formations. During the application process, there were no drilling accidents such as block blockage, which was the shortest record of drilling time in the reservoir section of the block.

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