LWD Sonic Data Analysis and Applicability in South China Sea

Abstract Many drilling risks are met during exploration and evaluation stage in South China Sea, China. The narrow mud weight window and presence of high pressure formation are two main challenges in this area. Generally, geomechanics study will be carried out using information obtained from nearby wells together with the surface seismic data before drilling the well. Sonic measurement is one of the most important input data in the geomechanics analysis to provide an estimate for mud weight window and drilling risks evaluation. What's more, the real-time LWD sonic is critical to improve the accuracy of the safe mud window by calibrating the initial geomechanics model to reduce the associated drilling risks. The real-time LWD sonic data together with the pre-drill geomechanics model calibration is our solution for those challenges. Based on our experience, the former LWD sonic tool delivers good memory data but the real-time data quality is not very consistent depending on drilling noise, borehole condition, formation properties as well as other operation restrictions. In such cases, the sonic memory data was used to calibrate the mechanical earth model, the updated model will then be used as reference to drill the subsequence well. However, with unstable real-time sonic data makes it difficult to perform real-time evaluation. Having a reliable real-time sonic measurement is very important for offshore operation, especially in High Pressure High Temperature (HPHT) environment to reduce the drilling risks and save rig time. It is vital to get good quality and accurate real-time sonic data to meet our requirements. With the latest LWD sonic tool, good quality of LWD sonic data was obtained for both real-time and memory. The real-time slowness acquired matches the memory slowness very well which give us more confidence to perform real-time geomechanics analysis in the coming HPHT deep water project. This paper focuses on data quality comparison between the two generation of LWD sonic tools tested in this area, challenges and methodology in real-time data quality control as well as the applicability of the LWD sonic in South China Sea. With the confidence of the real-time measurement data quality shown in these examples, this will significantly reduce the drilling risks in our next HPHT project. This assurance also enables us to explore additional applications in our projects.