致密砂岩地层气体钻井井眼稳定性试验研究

Test Research on Tight Sandstone Wellbore Stability During Gas Drilling

为明确气体钻井过程中致密砂岩地层井壁失稳的机理,基于能量耗散原理,利用三轴压缩试验,研究了气体钻井中致密砂岩地层井壁失稳的机理。通过分析三轴压缩试验结果得知:砂岩的能量演化可分为弹性能稳定聚集阶段、耗散能缓慢聚集阶段、弹性能释放且耗散能快速聚集阶段;随着围压降低,破坏砂岩结构所需耗散能峰值极限呈指数下降,而弹性储能峰值极限呈线性下降。随着加载速率增大,破坏砂岩结构所需耗散能先减小后增大,存在临界加载速率;砂岩耗散能转化速率与围压和加载速率呈正相关,较高的砂岩耗散能转化速率引起黏聚力弱化、摩擦强化。气体钻井速度过快将引起井壁失稳区域增大,而且在钻遇高压地层时更严重,因此,适当地降低钻井速度,给予地层充分泄压时间,有利于气体钻井过程中保持井壁的稳定性。研究结果对于优化气体钻井速度具有重要作用。

In light of the energy dissipation principle,the mechanism of wellbore instability in a tight sandstone formation during gas drilling was studied through triaxial compression tests.The results of triaxial compression tests were analyzed,and it was found that the energy evolution process of sandstone includes three stages,i.e.,the stable accumulation of elastic energy,the slow accumulation of dissipated energy,and elastic energy release with rapid accumulation of dissipated energy.With the decrease in confining pressure,the limit of dissipated energy required to destroy sandstone structure decreased exponentially,while that of the stored elastic energy declined linearly.As the loading rate was enhanced,the dissipated energy required to destroy the sandstone structure first decreased and then increased,with the appearance of a critical loading rate.The conversion rate of dissipated energy of sandstone was positively correlated with confining pressure and loading rate,and a high conversion rate caused the weakening of cohesion and the strengthening of friction.Too fast gas drilling enlarged the wellbore instability area,this was more distinct when drilling high-pressure formations.Therefore,appropriately reducing the drilling speed while giving sufficient pressure relief time to formations is conducive to maintaining wellbore stability during gas drilling.The research results are of great significance for optimizing gas drilling speed.