Study on Wellbore Instability Mechanism of Continental Shale Reservoir in Northeastern Sichuan Basin

Shale reservoirs in the Qianfoya continental facies shale reservoirs in northeastern Sichuan collapsed seriously, leading to severe difficulties in drilling a well successfully. To deal with this problem, a systematic analysis of instability characteristics and laboratory experimental evaluation studies were carried out. The analyses of the field data show that the instability mainly occurs in the dark-gray shale layer of the Qianyi member of the Qianfoya Formation, which is characterized by an irregular instability cycle and a coexistence of collapse and lost circulation. Drilling fluids currently used cannot effectively maintain the stability of this easy-to-collapse formation. Laboratory studies have shown that there are significant differences in the wellbore stability among different layers of the Qianyi member of the Qianfoya Formation. Among them, the dark-gray shale layer in the formation has a high content of clay minerals and certain degree of hydration swelling and amphiphilic wetting characteristics. The Qianyi member has strong bedding and the organic matter scratch sliding mirror surface et al. (weak structural surfaces) in the shales are extremely developed. The cementation strength of the shales is weak, resulting in natural breaking of the formation and significant reduction in the rock mechanical properties. Under the action of the drilling fluid and drill string disturbances, the shales peel off and cave in, causing wellbore instability and collapse. This study has presented technical countermeasures for safe and successful drilling, such as avoiding the formations that are prone to collapse, providing references for horizontal well drilling for continental shale oil and gas in the future.