深井超深井井底应力场

深井超深井钻井技术对加快我国石油勘探开发进程具有重要意义，深井超深井井底应力场是破岩机理研究进而提高机械钻速的基础。针对井深4500～7000 m时3种不同地应力状态下的井底岩石应力分布规律进行研究，在井底岩石力学分析的基础上，建立了考虑孔隙压力和垂直、水平最大和最小三向地应力差下的流固耦合模型，并采用数值方法进行求解。结果表明，岩石内部孔隙压力以约呈井眼径向距离的–0.055次幂减小；当井深一定时，垂直总地应力为最小地应力时，岩石所受围压最大，为中间主应力时次之，为最大地应力时最小；当地应力状态相同时，随着井深增加，岩石所受围压呈线性增加，导致岩石塑性强度增加，这是深井超深井机械钻速低的主要原因之一。深井超深井井底应力场的定量研究对深井超深井破岩机理研究和提高机械钻速具有重要的理论指导意义。

The stress field of bottom hole in deep and ultra-deep wells

The drilling technology of deep and ultra-deep wells is of great significance in speeding up petroleum exploration and development of China.Study on the stress field of bottom-hole rocks in deep and ultra-deep wells is one of the bases to investigate rock-breaking mechanisms and the enhancement of the rate of penetration(ROP).Here,we examined the stress field distribution of bottom-hole rocks under three different in-situ stress states in wells at depths from 4 500 to 7 000 meters.On the basis of mechanical analysis of well bottom-hole rocks,a fluid-solid coupling model that considers the pore pressure and differences between the maximal and the minimal values of vertical and horizontal triaxial stress was established,and a numerical method was applied to solve the model.The results indicated that the pore pressure within rocks decreased approximately by the-0.055th power of the borehole radial distance.When the well depth was constant,the confining pressure suffered by rocks decreased from the maximum to the minimum in order of the vertical total in-situ stress equal to the minimum in-situ stress,the intermediate principle stress and the maximum in-situ stress,respectively.When the in-situ stress state was constant,the confining pressure suffered by rocks increased linearly with the increase of well depth,leading to the increase of rock plasticity,which was defined as one of the main reasons why the drilling of deep and ultra-deep wells had a low rate of penetration.Finally,a couple of feasible drilling modes used for deep and ultra-deep wells were summarized.Quantitative studies on the stress field of bottom-hole rocks in deep and ultra-deep wells are of theoretical significance in guiding investigations of rock breaking mechanisms and the enhancement of the rate of penetration.