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.