STUDY ON THE INFLUENCE OF ROCK STRENGTH ANOMALIES ON THE COLLAPSE PRESSURE OF THE BOREHOLE WALLAND ITS INSTABILITY AREA DURING OIL AND GAS DEVELOPMENT

摘要

The significant anisotropy of shale is the main reason for the frequent occurrence of borehole wall instability accidents during formation drilling.The density of the drilling fluid is the main factor con-trolling the stability of the borehole. Therefore, appropriately reducing the density of the drilling fluid can increase the ROP and accelerate the effi-ciency of exploration and development. Within a controllable range, the pollution of the reservoir can be effectively prevented by reducing the density of the drilling fluid. In this paper, the influence of shale strength anisotropy on wellbore collapse pres-sure and instability area around the well has been systematically studied. The results show that the existence of weak areas of shale strength will lead to a significant increase in well wall collapse pres-sure and instability areas. And as the drilling time increases and the strength of the shale body and weak surfaces decreases, the instability of the well-bore will further deteriorate. In the later stage of drilling, the equivalent circulation density equiva-lent should be prevented from decreasing, and the displacement should be increased to clean the bot-tom hole cuttings. The study of the influence of strength anisotropy on the collapse pressure and instability area can provide guidance for the opti-mization of wellbore trajectory,the control of bot-tom hole safety pressure, the inversion of in-situ stress, and the drilling and completion operations.