Influence of upper limit of directional well collapse pressure on safe drilling density window

Generally, the safe drilling density window is determined by calculating collapse pressure and fracturing pressure. Low drilling fluid density will cause wellbore collapse. The actual drilling conditions show that when the density of drilling fluid exceeds the upper limit of collapse pressure, it will also cause wellbore collapse. Based on borehole imaging logging, the stress state around the well under high drilling fluid density was analyzed, and the upper limit calculation model of collapse pressure in σ_z' > σ_r' > σ_θ' mode was established. The analytical expression of vertical wells and the numerical calculation method of directional wells were deduced. The results show that the risk of wellbore instability will be underestimated when the traditional fracture pressure is used as the upper limit of the safe drilling density window for formations whose strength is lower than a certain critical value, whether in vertical or directional wells, and that the critical value of formation strength depends on the principal crustal stress, pore pressure and effective stress coefficient. For directional wells, the upper limit of collapse pressure is sensitive to the variation of deviation angles and azimuth angles, and the upper limit of collapse pressure of a wellbore is higher when it is drilled along the direction of minimum horizontal in-situ stress. With the increase of the deviation angle, the upper limit of the collapse pressure and the value of fracturing pressure tend to approach each other. The research results can prevent wellbore damage and collapse caused by high density drilling fluid.