山前高陡构造节理围岩的井壁失稳机制研究

 我国西部山前的高陡构造油气蕴藏丰富，但剧烈构造运动形成的节理岩体在钻井过程中容易出现井壁失稳问题，严重制约了勘探开发效益的提高。深入研究节理岩体井壁失稳机制对解决这一难题具有重要意义。结合中国天山山脉南缘霍尔果斯构造的地质情况，利用RFPA2D-flow软件建立山前构造节理岩体的井眼模型，探讨了井眼周围应力场和渗流场的特点，模拟了井壁围岩失稳坍塌的过程，讨论了稳定井壁的对策。井壁周围的应力集中出现在水平最小主应力方位上，而渗流则主要发生在水平最大主应力方位的节理面上，随着渗流作用节理面上流体压力逐渐升高，并导致围岩在节理面上发生拉张破坏而坍塌，井壁坍塌主要出现在水平最大主应力方位上。这一结果合理解释双井径测井曲线上井眼长轴发生翻转的现象。研究结果表明，降低节理面的渗流速度能够明显地减轻节理围岩的失稳，说明加强钻井液封堵性能对稳定井壁具有针对性；过低的井眼液柱压力会导致水平最小主应力方位上井壁围岩剪切破坏，然而过高的井眼液柱压力却对水平最大主应力方位上节理围岩的稳定产生不利影响，说明控制泥浆密度上限是必要的。研究成果对山前高陡构造钻井的井壁稳定问题具有参考价值。

STUDY ON WELLBORE COLLAPSE MECHANISM IN JOINTED ROCK MASSES OF HIGH-DIP-STRUCTURES BEFORE MOUNTAINS

There exist abundant oil and gas in the hip-dip structures before mountains in the West China. But borehole caving takes place frequently during drilling in the section of fractured rock masses generated by violent tectonic movements，which severely restricts the profit of exploration and development. This paper sets up the model of borehole drilled in the highly fractured rock masses according to geological situation in the Huoerguosi structure before Tianshan mountains using the software RFPA-flow. The characteristics of stress and seepage around borehole are inquired and the process of wellbore collapse is simulated，also the countermeasures are discussed. The research indicates that stress concentrates around wellbore in the direction of the minor principal stress，however，seepage occurs chiefly on the joint planes around borehole in the direction of the minor principal stress，and water head rises on the joint planes along with seepage process，which results in the collapse of surrounding fractured rock masses. The result that collapse occurs in the direction of the maximum principal stress，showing a reasonable explanation to the complex phenomenon that long axis of borehole exists in the direction of the maximum principal stress on the dip log. The research also indicates that borehole collapse is easy when seepage velocity on the joint planes is decreased，which confirms that improving the plugging performance of drilling fluid is the countermeasure. Lower water head in the hole may cause collapse of rock masses around hole in the direction of the maximum principal stress，however，over-high water head in the hole can also cause collapse of fractured rock masses in the direction of the maximum principal stress，which clarifies that controlling the upper limit of drilling fluid weight is also a necessary countermeasure. The research and result may offer reference for drilling before mountains.