Time-dependent borehole stability in hard-brittle shale
硬脆性页岩中随时间变化的钻孔稳定性
硬脆性頁岩における時間依存のボアホールの安定性
딱딱한 셰일에서 시간에 따른 시추공 안정성
Estabilidad del pozo en función del tiempo en lutitas duras y frágiles
Stabilité du forage en fonction du temps dans le schiste dur et cassant
Зависящая от времени стабильность ствола скважины в твердом хрупком сланце
Chuan-Liang Yan 闫传梁 ¹ ², Lei-Feng Dong 董磊峰 ², Kai Zhao 赵凯 ³. Yuan-Fang Cheng 程远方 ¹ ², Xiao-Rong Li 李晓蓉 ⁴, Jin-Gen Deng 邓金根 ⁴, Zhen-Qi Li ², Yong Chen 陈勇 ²
¹ Key Laboratory of Unconventional Oil & Gas Development (China University of Petroleum (East China)), Ministry of Education, Qingdao, 266580, Shandong, China
中国 山东 青岛 中国石油大学（华东） 非常规油气开发教育部重点实验室
² School of Petroleum Engineering, China University of Petroleum (East China), Qingdao, 266580, Shandong, China
中国 青岛 中国石油大学（华东）石油工程学院
³ College of Petroleum Engineering, Xi’an Shiyou University, Xi'an, 710065, Shaanxi, China
中国 西安 西安石油大学石油工程学院
⁴ State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum (Beijing), Beijing, 102249, China
中国 北京 中国石油大学（北京）油气资源与探测国家重点实验室
Rock damage appears in brittle shale even prior to peak stress (i.e., before failure) due to the occurrence of microcracks in these rocks. In this work, a coupled hydromechanical model was built by incorporating the mechanical and fluid seepage induced stresses around a wellbore during drilling. The borehole instability mechanism of hard-brittle shale was studied. The results show that even if a well were simply drilled into a hard-brittle shale formation, the formation around the borehole can be subjected to rock damage.
The maximum failure ratio of the formation around the borehole increases with drilling time. A lower drilling fluid density corresponds to a faster increase in the failure ratio of the borehole with time and a shorter period of borehole collapse. When the initial drilling fluid density is too low, serious rock damage occurs in the formation around the borehole. Even though a high-density drilling fluid is used after drilling, long-term borehole stability is difficult to maintain. While drilling in hard-brittle shale, drilling fluid with a proper density should be used rather than increasing the density of the drilling fluid only after borehole collapse occurs, which is more favorable for maintaining long-term borehole stability.
