Abstract

Lost circulation has been a great challenge to improving drilling efficiency. This is especially true for drilling depleted formations or deepwater drilling where the safe mud-weight window is much narrower. Wellbore strengthening is a new concept that has been leading us to understand the root causes of lost circulation and to develop solutions. One recent solution involves the so called "stress cage" and the application of particulate lost circulation materials in drilling fluids to prevent losses. Inspired by the field successes achieved, this study explores further understanding of this technology and wellbore strengthening in general.

This study begins with an investigation of wellbore weakening factors. Analysis of the governing equations for near wellbore stresses along with consideration of the drilling phenomena and observations has found that the presence of hydraulically conductive cracks can be the most important cause of a weak wellbore. This key factor is also responsible for frequently encountered lost circulation when drilling depleted formations. Through this understanding, investigation of wellbore strengthening has been developed with a boundary element analysis method under conditions such as sealing and/or propping a hydraulically conductive crack. The stress analysis first confirms that wellbore strengthening can be achieved by either sealing or propping a crack. Then with different setups and configurations, formation properties, wellbore pressure, and wellbore configurations are investigated in terms of tangential stress along the wellbore, fracture width, and fracture stability.

Laboratory results obtained through joint industry projects for understanding lost circulation are revisited and field results are summarized to further support this study.