鄂尔多斯盆地陆相中生界页岩气储层井壁失稳机理研究

Research on borehole wall instability mechanism of Mesozoic shale gas reservoir in Ordos Basin

鄂尔多斯盆地陆相中生界页岩气储层地质复杂,钻井过程中易发生井壁失稳。通过对鄂尔多斯盆地延长陆相中生界页岩气储层理化性质、岩石力学特性以及钻井液与地层之间的相互作用这三方面分析,对延长陆相中生界页岩气储层井壁失稳的主导机理进行研究,结果发现钻井液沿张开的页岩气储层的层面和裂缝面渗流,引起井周附加应力,导致液柱压力对井壁的有效支撑作用降低,削弱地层强度,是造成井壁坍塌的主要原因,当井斜角大于50°时,大斜度井、水平井沿着水平最小地应力方位钻进,井壁坍塌压力最大,不利于井眼稳定。因此,在陆相页岩气储层钻井过程中必须强化钻井液的封堵性,合理优化水平段方位,为页岩气井钻井工程设计提供了参考依据。

Mesozoic shale gas reservoir in Ordos Basin is very complex and in which borehole wall instability occurs easily. By ana- lyzing the formation characteristic, the rock mechanical property and the interaction between drilling fluid and the formation of shale gas reservoir in Ordos Basin, the main mechanism of the borehole wall instability is researched to find the main reason of borehole wall caving in shale gas reservoir drilling. It is because the infiltration of drilling fluid along the layers and fractures of the shale gas reservoir, which causes the additional stress around the wells, reduces the support to borehole wall of fluid column pres- sure and weakens the formation strength. When the deviation angle is greater than 50°, the caving pressure which is bad for the wellbore stability reaches its maximum when drilling along the direction with the minimum crustal stress of the high angle deviated wells and horizontal wells. Therefore, We must improve the sealing properties of the drilling fluid and optimum seeking azimuth of horizontal section in shale gas reservoir drilling. The results provide a reference for the drilling design in gas shale.