Wellbore Instability of Directional Wells in Laminated and Naturally Fractured Shales

Abstract

Many wellbore instability problems, such as wellbore collapse and lost circulation, occur in laminated and naturally fractured shales. These instability problems often lead to difficulty with hole cleaning, tripping, logging, and casing running, resulting in significant nonproductive time and increased costs. In this article, the stress distribution around deviated wellbores in laminated shale and sand sequences is analyzed and a modified model based on the single-plane-of-weakness theory is developed to predict the strength of laminated formations. Chemical effects resulting from the unfavorable interaction between the drilling fluid and the formation are also considered. If a chemical imbalance exists, additional wellbore instability may be encountered, which is generally worsened in naturally fractured intervals. Traditionally, nonaqueous fluids are chosen to reduce the impacts of chemically induced wellbore instability. In addition, it is generally accepted that increasing the drilling fluid salt concentration can enhance the borehole stability. This perception needs to be revised, particularly while drilling in naturally fractured shales. Both mechanical and chemical borehole instability models were applied in a case study to evaluate the potential for wellbore instability. In particular, bedding plane-related and chemically induced instability were addressed and overcome through comprehensive modeling and the deployment of modified operational procedures.