硅酸盐钻井液降低易失稳地层钻井液密度可行性分析

珠江口盆地古近系地层在钻井过程中，多口井出现坍塌、卡钻、起下钻遇阻等井壁失稳问题，现场通过增加钻井密度解决垮塌问题，但又出现压差卡钻、钻井时效低的问题。从黏土矿物含量分析、扫描电镜、滚动回收率及膨胀性实验入手，分析珠江口盆地古近系地层井壁失稳原因。古近系地层为微裂缝发育，在钻井过程中井壁周围应力发生变化及钻井液侵入，引起井壁坍塌，继而出现卡钻、起下钻遇阻等问题。为了解决以上问题，结合硅酸盐钻井液抑制性强、封堵性好、可增强岩石强度的特点，降低地层坍塌压力。对硅酸盐水基防塌钻井液进行优化及性能评价，结果表明，其性能参数满足钻井要求，具有提高岩石内聚力、抗压能力的特点，同时与古近系已用PLUS-KCl钻井液、油基钻井液相比，优化的硅酸盐水基防塌钻井液浸泡后岩石具有抗压强度高、内聚力高等特点，其内聚力高达11.7 MPa，是PLUS-KCl钻井液的1.9倍，是油基钻井液的1.5倍，可见在保证井壁稳定的前提下，用硅酸盐钻井液降低地层坍塌压力是可行的。 

Analysis on the Feasibility of Reducing Drilling Fluid Density with Silicate Drilling Fluid in Drilling Unstable Formations

Drilling operations in the Pearl River Mouth Basin have been face with downhole troubles in the Paleogene System, such as borehole wall collapsing, pipe sticking and difficulties in tripping the drill string. Addressing these problems with increased drilling fluid density resulted in other problems such asdifferential pipe sticking and low time efficiency. The causes of borehole wall collapse in drilling the Paleogene System in the Pearl River Mouth Basin were analyzed through clay content analysis, SEM experiment, hot rolling test and core swelling test. The Paleogene System is developed with micro fractures, formation stress change due to drilling operation and invasion of mud filtrate into the micro fractures cause borehole wall collapse, followed by pipe sticking and difficulties in tripping the drill string. To address these problems, silicate drilling fluid was adopted to drill the Paleogene formation. Silicate drilling fluid has good inhibitive capacity and plugging capacity. It helps enhance the strengths of the rocks and reduce the collapse pressure of the formation drilled, thereby reducing the density of the drilling fluid to balance the formation pressure. Performance evaluation of an optimized silicate drilling fluid showed that its properties satisfy the requirements of drilling operations. It helps increase the cohesion and compressive capacity of rocks. Compared with the PLUS-KCl drilling fluid and oil base drilling fluid that have been already used in drilling the Paleogene formation, the optimized silicate drilling fluid performed much better; rocks immersed in the optimized silicate drilling fluid had higher compressive strength and cohesion, the cohesion was tested to be 11.7 MPa, 1.9 times of the cohesion of the rock samples immersed in PLUS-KCl drilling fluid, or 1.5 times of the cohesion of the rock samples immersed in oil base drilling fluid. These experimental results proved that silicate drilling fluid can be used to reduce the density of the drilling fluid while maintaining the borehole wall in stable condition.