The bole enlargement is very serious in the sandstone formation if the hole is exposed to mud for long time. The changes of Young's modulus, Poisson's ratio., cohesion and friction angle of the sandstone are studied by experiments that the cores are immersed in mud for different time. While me stresses around the wellbore are studied, that the changes of Young's modulus.、 Poisson's ratio are considered. Base on these, the model of collapse pressure is established. Thought the experiments and calculation, the conclusions are got as follows: The Young's modulus, cohesion and friction angle are reduced and the Poisson's ratio enlarged more as the time that cores are immersed in mud is longer; the stresses are changed with the changes of Young's modulus., Poisson's ratio; the collapse pressure is rested with the changed stresses and cohesion and friction angle; the collapse pressure of wellbore will be reduced first and then enlarged.%长时间裸露在钻井液中的砂岩地层扩径严重,通过研究钻井液侵入对井周围岩弹性模量、泊松比、黏聚力及内摩擦角等力学性能的影响,开展了井周围岩坍塌压力的研究.研究了钻井液侵入后井周应力场变化规律,并在此基础上建立了考虑钻井液侵入影响的井壁坍塌模型.通过实验与模型计算可以看出,钻井液滤失,将会在地层周围形成与原地层岩石力学特性不同的侵入区.钻井液的侵入导致侵入区弹性模量、黏聚力与内摩擦角减小,而泊松比增加,随着侵入时间的增加,弹性模量、黏聚力与内摩擦角减小越多,而泊松比上升也增加；岩石力学参数的变化与侵入时间呈现线性关系；由于弹性模量、泊松比的变化,井周应力重新分布,井壁的垮塌决定于此时的井周应力与岩石强度；井眼开挖后,井壁坍塌压力往往是先减小后增大.
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