负压钻井最小钻井液密度的确定

DETERMINATION OF MINIMUM DENSITY OF THE DRILLING FLUID FOR UNDERBALANCE PRESSURE DRILLING

在负压钻井过程中，地层流体不断流入井内。如果负压钻井的钻井液密度过低，造成过大的井底负压，大量的地层流体会流入井内。这不仅会给涌出地层流体的地面处理带来困难，还会造成储层垮塌，导致储层的毁灭性伤害。因此，必须确定合理的钻井液密度以避免地层坍塌。井眼形成后井筒周围的应力将伴随地层流体的流入而重新分布，由于地层岩石的力学特性不同，井筒周围应力的重新分布可能是弹性的，也可能是弹塑性的，并且与地层流体的流速密切相关。从分析井筒周围的应力分布入手，以地层不发生坍塌为条件，导出了负压钻井所需的最小钻井液密度；对影响最小钻井液密度的主要因素进行了分析，指出地层的胶结强度和原始地层压力是影响最小钻井液密度的主要因素。

In the process of underbalance pressure drilling,formation fluid continuously flows into borehole.If the density of the drilling fluid for underbalance pressure drilling is unduly small to cause an unduly high negative bottom hole pressure,a great quantity of formation fluid will flow into the borehole,which will not only bring difficulty to the surface treatment of the gushed formation fluid,but also cause reservoir to be collapsed,leading to a disastrous damage to the reservoir.Therefore,a reasonable drilling fluid density must be determined to avoid formation collapse.After the borehole being formed,the stresses around the borehole will be redistributed along with the entering of formation fluid.Due to different rock mechanical properties the redistribution of the stresses around the borehole may be elastic,also plastic,being closely related to the flowing velocity of formation fluid.Starting with the distribution of the stresses around the borehole and by taking the formations being not collapsed as a condition,the minimum drilling fluid density necessary for underbalance pressure drilling is derived,the main factors influencing minimum drilling fluid density are analyzed and it is pointed out that the formation cementation strength and original formation pressure are the main factors influencing minimum drilling fluid density.