层理性泥页岩大斜度井井壁稳定性研究

Borehole Stability of Highly-deviated Well in Bedding Shale

层理性泥页岩的井壁失稳是制约深部油气钻井成功的关键问题之一。由于强度的各向异性,钻进大斜度井时更容易发生严重的井壁失稳问题。根据多孔介质弹性力学理论,建立了大斜度井井周应力状态,采用双重坐标转换,得到了井周集中应力在层理面上的分布。层理弱面和岩石基体均采用莫尔-库仑破坏准则,建立了层理性泥页岩大斜度井井壁稳定分析方法,综合考虑了层理面产状、井斜方位和地应力等因素的影响。研究结果表明,对特定产状的层理面而言,井斜角和方位角对井壁稳定性有重要影响。当井斜角超过一定角度后,坍塌压力急剧升高,井壁失稳风险增大,维持井壁稳定需要的钻井液密度较高,向水平最大主应力方向钻进大斜度井时坍塌压力最高,井壁失稳风险最大,应尽量避免井眼方位与水平最大主应力方向一致。

Borehole instability in bedding shale is one of the main factors restricting the success of the deep oil and gas drilling. Due to the anisotropy of the strength, the serious instability problems occurred easily especially in highly-deviated well drilling. According to the Porous medium elastic mechanics theory, the stress state surrounding the highly-deviated well was established, using double coordinate conversion, the concentrate stress distribution on the bedding plane was obtained, the Mohr-coulomb failure criterion was selected both for the bedding plane and rock matrix, the analyze method of borehole stability for highly-deviated well was established, comprehensive considering the attitudes of the bedding plane, well inclination and azimuth, in-situ stress and so on. The results showed that, for the bedding plane with specific attitude, the deviation and azimuth angle has important influence on the borehole stability. When the deviation angle is higher than the specific angle, the collapse pressure rises rapidly, the instability risk increases, the drilling fluid density to keep borehole stability was higher, the collapse pressure of the highly-deviated well towards the direction of maximum horizontal in-situ stress is highest, the instability risk is biggest, the drilling should avoid towards the direction.