倾斜盐岩层段井眼蠕变缩径分析

文章用盐岩蠕变的非线性有限元法建立了具有倾斜盐岩夹层的地层有限元模型。研究了新疆某油气田钻遇的盐岩层段钻井液密度、地层倾角与井眼蠕变缩径的关系。研究表明，增大钻井液密度，可有效地减缓蠕变缩径。地层倾角对井眼蠕变缩径的影响较复杂，对于上覆岩层压力大于水平应力的情况下，井眼初始蠕变量随地层倾角增大而增大。地层倾角对井眼后期蠕变（稳态蠕变）缩径的影响不仅与倾角大小有关，还与蠕变应力差有关。当蠕变应力差较小时，即钻井液密度较大时，蠕变速率随地层倾角增大而减小。当蠕变应力差较大时，对于该井蠕变岩层段，地层倾角小于15°时，蠕变速率随地层倾角增大而增大，地层倾角大于15°时，蠕变速率随地层倾角增大而减小。该研究结果为盐岩段井眼稳定性分析和提供合理的钻井液密度具有指导意义。

ANALYSIS OF WELLBORE CREEP SHRINKAGE IN INCLINED SALT FORMATION

Finite element models for formations with inclined saline rock interbeds were set up by use of nonlinear finite element method for saline rock creep deformation. The relationship of drilling fluid density and formation dip with borehole undergauge creep deformation encountered in salt beds in Xinjiang Oilfield drilling was studied. The study shows that increasing drilling fluid density can slower undergauge creep effectively. The effect of formation dip on borehole undergauge creep is quite complex. When the overburden pressure is larger than horizontal stress, the initial borehole creep deformation will increase as dip angle increases. The effects of formation dip on late stage borehole creep, namely steady undergauge creep, are not only related with dip angle, but also related with differential creep stress. When the differential creep stress is low, namely when the drilling fluid density is high, the creep speed will decrease as dip angle increases. However, if the differential creep stress is high, when dip angle is less than 15°, the creep speed of creeping formations will increase as dip angle increases, and the creep speed will decrease as dip angle increases when angle of dip is higher than 15°. The study results in this paper will give guidance on stability analysis of borehole in salt beds and help to choose reasonable drilling fluid density.