基于井壁稳定控制建立井眼轨迹优化预测模型

Establishment of a prediction model for the borehole trajactory optimization based on controlling wellbore stability

通过线性规划最优解理论及井壁剪切破坏条件,建立了井眼轨迹优化的控制方程及提出相应的数值求取方法。研究表明:当地应力大小关系满足控制方程的临界条件时,正断层作用应力区域钻直井或沿水平最小地应力方位钻水平井时,两者井壁稳定性相同,该情形也发生在逆断层作用应力区域钻直井或沿水平最大地应力方位钻水平井,此外,走滑断层作用应力区域,水平井沿水平最大或最小地应力方位钻进时,其井壁稳定性也存在相同的可能性;另外,当水平最大地应力与上覆岩层压力比一定时,水平地应力比值(σh/σH)越大,正断层作用应力区域沿水平最小地应力方位,及逆断层作用应力区域沿水平最大地应力方位钻进的最优井斜角越小,反之,走滑断层作用应力区域,水平井的最优钻进方位与水平最大地应力方位夹角越大。研究成果有助于指导现场施工高效钻进。

Based on the linear programming optimal solution theory and the shear failure criterion of wellbore,the present study establishes the governing equations of borehole trajectory optimization,and also proposes the corresponding numerical seeking method.The research discovers that when the magnitudes of in-situ stresses satisfy the critical condition of governing equations,the wellbore stability of vertical well is equal to that of drilling horizontal well in the direction of the minimum horizontal in-situ stress under the normal faulting stress region.However,as regard to the reverse faulting stress region,the wellbore stability of vertical well is identical to that of drilling horizontal well in the direction of the maximum horizontal in-situ stress.In addition,under the strike-slip faulting stress region,the wellbore stability condition concerning horizontal well where drilling in the direction of the maximum horizontal in-situ stress may be similar to that of the minimum horizontal in-situ stress.Besides,provided the ratio of the maximum horizontal in-situ stress to the overburden pressure holds constant,the higher ratio of the minimum to the maximum horizontal in-situ stress（σh/σH）indicates that the optimum inclination angle is smaller where drilling along the direction of the minimum horizontal in-situ stress under the normal faulting stress region whereas the direction of the maximum horizontal in-situ stress under the reverse faulting stress region.Contrarily,whenσh/σHis smaller,the angle between the optimum drilling azimuth and the maximum horizontal in-situ stress direction where drilling horizontal well would become larger under the strike-slip faulting region.This research is helpful for improving the efficiency of drilling process in field.