允许适度坍塌的水合物储层最低钻井液密度

Lower limit of safe mud weight window with considering moderate borehole collapse

深水海域的天然气水合物储层通常埋藏较浅、压实较弱、强度较低,钻井过程中钻井液的扰动极易导致水合物的分解和井眼的坍塌。基于此,文中建立了有关水合物储层中井眼稳定性的二维流-固-热耦合数值模拟模型,研究了相同钻井液温度和不同钻井液密度条件下的水合物分解及井眼坍塌状况,根据不同钻井液密度下的井眼扩大率模拟结果,确定了不同井眼扩大率下的钻井液密度下限。结果表明:钻井液密度越低,剧烈的钻井液扰动作用导致的近井壁地带天然气水合物的分解就越严重,井眼的坍塌状况也越糟糕。当使用温度为21.25℃的钻井液进行水合物储层钻井作业时,如果井眼扩大率为2.5%,则最低钻井液等效密度为1.116 g/cm3;而当井眼扩大率提高到10.0%时,最低钻井液等效密度则降低到1.024g/cm3。可以得出结论:随着对井眼稳定性要求的提高,最低钻井液密度会在合适的范围内不断增大。

Natural gas hydrates in deep-water area are usually buried in shallow reservoirs with low reservoir compaction and strength. The disturbance of drilling fluid to hydrate reservoirs during the drilling operation may easily result in the vigorous dissociation of natural gas hydrates and borehole collapse. In this paper, a 2D finite element model that coupled seepage, deformation and heat transfer was established, and the hydrate dissociation and the resulting borehole collapse caused by disturbance of drilling fluid were both investigated when the fluid density was different. Besides, the minimum mud density was determined with considering the acceptable borehole enlargement rate. The investigation results show that hydrate dissociation and consequent collapse of borehole become severe with the decrease of the drilling fluid density. The minimum mud density decreases from 1.116 to 1.024 g/cm3 , if the acceptable borehole enlargement rate is increased from 2.5% to 10.0% when the fluid temperature is 21.25 ℃. Therefore, it is concluded that the minimum drilling fluid density could be increased in the reasonable range if the requirement for borehole stability is high.