Research On Stability Technology Of Continental Stratum Wall In Yuanba Area, Northeastern Sichuan Basin

The instable continental strata in Yuanba area of northeast Sichuan is mainly concentrated at the top of Shaximiao formation to Xujiahe formation; three main instable reasons for wellbore instability are:air drilling vibration, shale expansion and dispersion caused by filtration invasion during the process of gas-liquid conversion and further cause wellbore collapse, and unstable wellbore mechanics caused by underbalanced liquid drilling. The main phenomenon is wellbore spalling and collapse. The content of shales in continental strata is more than 70%. The shales in Penglai town formation to upper Shaximiao formation are pure, brittle and hard, while the shales in the lower Shaximiao formation to the fifth member of Xujiahe formation are soft and have strong water absorption and plasticity. Meanwhile, the upper Shaximiao formation to Ziliujing formation contains shales and the well-developed lamellation, which could be peeled into lamellar shales. Through clay mineral analysis, electron microscope scanning and linear expansion, and dispersion experiment, we know that the shales in Yuanba area belong to hard and brittle shales, content of illite is generally higher than 50%, have low expansion, weak dispersion, microfracture development and strong stress sensitivity. For the borehole instability phenomenon of continental strata in Yuanba area of northeast Sichuan, should actively adjust the density of drilling fluid according to the formation pressure change, the under voltage value shall not be too high during underbalanced drilling process, and the density of drilling fluid should be increased 0.1-0.2g/cm3 on the basis of current situation. At the same time, priority is given to improve the sealing and anti-collapse ability of drilling fluid, and take into account of inhibition. Fluid plugging performance could be further improved through introducing key treating agents such as cationic emulsified asphalt. By means of experiments such as surface tension, optimize and form the full oil-based preflush formulation and process, the wellbore instability during gas-liquid conversion could be controlled. By field application of oil-based preflush in wells of YB10-1H and YL601H, the average redressing time for test well is about 31h, is greatly reduced than that of non-test wells of 73.5h. The anti-collapse drilling fluid technology focus on plugging and density control, what has been successfully applied in wells of YB10-1H and YL601H and has obvious effect on stabilizing borehole. The average rate of hole diameter enlargement of third interval of well YB10-1H is only 5.5%, far below the design requirement of 12%. Through the successful application in the field, ultimately form a set of wellbore stability technology of continental strata fit for Yuanba area of northeast Sichuan, and meet the requirements of drilling.