Understanding the Geomechanical Challenges and Risk Mitigation in Sichuan Shale Gas Drilling, China

The drilling of shale gas development wells in the Sichuan Basin has been problematic, with stuck pipe and fluid loss events reported in many wells. These events hindered the development efforts as operators whose aim is to reduce drilling costs and increase drilling speed are faced with developing drilling strategies for more cost effective well delivery. As such, it becomes critical to understand the key mechanisms of borehole failures and fluid losses during drilling of the laterals so that proper mud weights and mud designs can be formulated to overcome the drilling challenges. To optimize the entire drilling process from different angles, an integrated approach is required to combine the knowledge and expertise from different disciplines. A robust geomechanical model and detailed diagnostics of the borehole instability related problems are key and form the basis for the drilling risk management. To build a robust geomechanical model, several representative areas in the Sichuan Shale Gas play were reviewed using a consistent approach. A general understanding of the in-situ stress conditions and rock mechanical properties of the Longmaxi Shale Gas reservoir was developed by combing data and knowledge in the different areas. Using the geomechanical model, a series of newly drilled horizontal wells was also reviewed so that the main causes of stuck pipes and fluid losses can be determined. Based on the geomechanical model and drilling experiences review, risks that could potentially cause non-productive time (NPT) during drilling of the planned wells were postulated and listed, and each risk was then assessed in detail so that a thorough understanding of the risk factors can be achieved from different drilling perspectives. Consequently, a multi-disciplinary mitigating solution was proposed in order to help with reducing the occurrence of any borehole instability-related problems. The integrated drilling optimization plan was executed successfully during the drilling of the horizontal wells in the Sichuan Basin. The increase in understanding of the geomechanical issues in the Sichuan Shale Gas drilling indicated that common trends in shale characteristics are present although it is widely accepted that shale types and stress conditions are different in each field. With the geomechanical models being calibrated from information in different areas, uncertainties are reduced and the robust models in turn provide solid foundations for risk identification and mitigation, leading to successful and economical drilling of the long laterals. However, it is a long and continuous learning process in order to apply this integrated approach effectively. To achieve continuous improvement, the geomechanical model and mitigating solutions need to be refined regularly following the drilling process while further information and knowledge are being acquired in the Sichuan Shale Gas area.