Wellbore Stability Analysis in the South China Sea: A Case Study

Abstract

Abstract The paper describes a case study in the South China Sea in which drilling problems were successfully addressed through a wellbore stability analysis. The wellbore stability analysis started from understanding of the drilling history of offset wells. A specially designed drilling event review plot, which categorizes and organizes all drilling problems in a graphical display, was used as to illustrate drilling and borehole failure history for the two offset wells in the study area. A mechanical earth model was constructed by following a systematical workflow with emphasis on reducing uncertainty of the in-situ stresses. The synthetic borehole failure image was used to provide an excellent visualization of borehole failure forecast, which made it easily understandable even for geomechanics non-experts and was comparable to the borehole failure observations from caliper logs and borehole image data. A drilling map was constructed to provide a user-friendly presentation of the outcome of the wellbore stability analysis. This improved the communication among all drilling stakeholders. Based on the results provided from the analysis, engineers revised the setting depth for 9 5/8-in. casing to reduce drilling risk. Post-drilling analysis showed that there was substantial improvement in the subsequent drilling performance for the planned well compared to that for the previous wells. This paper illustrates that incorporating wellbore stability analysis as part of the well planning process allows the drilling team to optimize the casing design and drilling mud weight to minimize wellbore instability and improve drilling performance.