Abstract: The high stress of rocks in deep or ultra-deep wells is one of the major reasons for quick bit wear and low rate of penetration.Therefore,high-pressure water jet technology is widely used in the field of well drilling engineering because it can greatly improve the rate of penetration.According to the traditional theory of rock breaking by high-pressure water jet,it is considered that water jet can play a role in rock cleaning and auxiliary rock breaking,but generally little attention has been paid to the impact of the slotting by water jet on the distribution of downhole rock stress state.Further,in combination with the combined rock breaking process of drilling bit and high-pressure water jet,this paper proposes a method for stress release during downhole slotting by high-pressure water jet.Based on Biot's theory of poroelastic mechanics,a physical model of deep formation has been established to analyze the distribution patter of effective rock stress and the release degree of slotting stress at different well depths,in-situ stress states and drilling fluid differential pressures.Based on the mechanical properties of granite at different confining pressures,a user-defined function of compressive rock strength is used to analyze the influence law of downhole slotting on the rock strength distribution characteristics.The research results indicate that the stress release efficiency constantly rises as the well the depth increases,the effective stress release efficiency along the axis of well bottom is above 70%,and that in the peripheral area of the bottom hole is about 50%;the rise of drilling fluid pressure has lowered the stress release efficiency,and the stress release efficient along the axis of well bottom is greatly affected by drilling fluid pressure;the water-jet slotting has significantly reduced the strength of rocks along the axis of well bottom,the release efficiency can reach above 50%,and the strength release efficiency of rocks around the well bottom is around 30%;the existence of downhole slotting has released the effective rock stress and can push the concentrated areas of downhole rock stress away from the cutting surface,resulting in the lower stress value of the drilling zone.This has reduced the rock strength to a certain extent,thus improving the penetration rate.

Key words: deep or ultra-deep wells, high-pressure water jet, slotting, stress release, poroelasticity, downhole stress field

摘要： 深井/超深井岩石应力大造成的岩石强度升高是导致钻头磨损快、钻井效率低的主要原因之一。高压水射流技术由于可以大幅度提高机械钻速而被广泛应用于钻井工程领域。传统高压水射流破岩理论认为水射流能够起到清岩和辅助破岩的作用,但水射流形成的凹槽对井底岩石应力状态分布的影响一直没有引起足够的重视。结合钻头与高压水射流联合破岩过程,提出了高压水射流井底切槽应力卸载方法。基于Biot多孔弹性力学理论,建立了深部岩层物理模型,分析了不同井深、地应力状态、井底压差条件下岩石有效应力的分布形态和切槽应力的卸载程度;结合不同围压条件下花岗岩的力学特性,使用自定义岩石抗压强度函数分析了井底切槽对岩石强度分布特征的影响规律。研究结果表明:随着井深的增加,应力卸载效率不断升高,井底轴线处有效应力卸载效率在70 %以上,井底周围有效应力卸载效率约为50 %;钻井液压力的升高减小了应力卸载效率,且井底轴线处应力卸载效率受钻井液压力的影响较大;水射流切槽显著降低了井底轴线处岩石的强度,卸载效率达50 %以上,井底周围岩石强度卸载效率约为30 %;井底凹槽的存在释放了岩石的有效应力,并将井底岩石应力集中于区域推离切削面,从而降低了钻头作用区域的应力值,在一定程度上降低了岩石的强度,进而提高机械钻速。

关键词: 深井/超深井, 高压水射流, 切槽, 应力卸载, 多孔弹性, 井底应力场