定向井坍塌压力上限对钻井安全密度窗口的影响

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

一般钻井安全密度窗口通过计算坍塌压力与破裂压力确定,钻井液密度过低会引起井壁坍塌。实际钻井情况反映,钻井液密度超过坍塌压力上限也会造成井壁坍塌。结合井壁成像测井,分析了高密度钻井液下的井周应力状态,建立了σz'>σr'>σθ'模式的坍塌压力上限计算模型,推导得出了直井的解析表达式与定向井的数值计算方法。结果显示:不论直井或定向井,对于强度低于某一临界值的地层,以传统的破裂压力作为安全密度窗口的上限将会低估井壁失稳风险;地层强度临界值的大小取决于主地应力、孔隙压力、有效应力系数。对定向井而言,坍塌压力上限随井斜角和方位角变化的敏感性较高,沿水平最小地应力方向钻进的井眼坍塌压力上限较高;随井斜角增大,坍塌压力上限与破裂压力值逐渐趋于接近。研究结果可防范高密度钻井液引起的井壁损伤和井塌现象。

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 σz’>σr’>σθ’ 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.