中文摘要：

硬脆性页岩作为井壁围岩时，在循环介质流体作用下其裂缝形成的机理一直是近年来研究的热点。钻井液和页岩之间水活度差会间接影响硬脆性页岩裂缝的形成机制。通过对广义Usher模型修正，建立了活度-膨胀率-水化程度分析模型，认为岩石膨胀率低于“临界膨胀率”时处于井壁稳定性良好的非渗透水化阶段，根据其内部对应的钻井液活度范围，提出活度窗口的概念，并利用室内实验对模型进行了验证。实验结果表明，不同活度的钻井液在宏观上使页岩强度发生差异性变化，微观上对页岩内部结构具有时间敏感性影响。这一机理的提出，可以为钻井液活度设计提供更准确的配制范围，为井壁稳定提供有力保障。

英文摘要：

The failure mechanism formed under the action of circular medium lfuid around the wellbore of brittle shale has been a hot issue in recent years. The water activity difference between the drilling lfuid and shale will directly affect the formation mechanism brittle shale fracture. The model of activity-inlfation-hydration is set up ifrstly through revising generalized Usher model. For this model, it states that the wellbore is in non-osmotic hydration stage within the critical expansion ratio, and thus puts forward the concept of ac-tivity window according to the drilling lfuid activity range, and this is veriifed by indoor test. Results show that the different activity of drilling lfuid brings difference in rock strength in macro and time sensitive effect for internal structure of rock in micro. This mechanism can provide a more accurate range design for drilling lfuid activity and strong guarantee for wellbore stability.