Deepwater Exploration Breakthrough with LWD Technology in South China Sea

Abstract

Abstract For CNOOC ZhanJiang branch (CCLZ), 2014 was a milestone year where for the first time, they managed their deepwater exploration in Lingshui 17-2 (LS17-2) block independently, without the involvement of other international oil companies. CCLZ faced many drilling challenges in their six-well deepwater project, including stuck pipe, kick and mud losses due to wellbore instability, as experienced on previous five exploration wells drilled in nearby area. The occurence of these events has the potential to increase operational cost and risk. The sedimentary geology in Lingshui block is complex, and the downhole environment is challenging as the pore pressure ramps up quickly above the reservoir resulting in a very narrow mud weight window. To address these challenges and drill the wells safely, the exploration project management team decided to employ real-time geomechanics monitoring. A mechanical earth model (MEM) was constructed before drilling to define optimized wellbore stability boundaries and identify potential hazards associated with the planned trajectory. Based on the real-time logging while drilling (LWD) data, the MEM was updated and drilling parameters adjusted to keep within operational limits. Using LWD data, the pore pressure was calculated in real-time and verified against other drilling observations, such as connection gas, flow tests, drilling parameter analysis and caving analysis. The fracture gradient was calibrated against a formation integration test (FIT) at the casing shoe. These real-time updated fracture gradient and pore pressures were then applied to keep mud weight and equivalent circulating density (ECD) within the narrow safe mud weight window. Upon completion of the well, real-time pore pressures predictions were compared with actual pressure measurements in permeable sandstone intervals, and the results found to match very well. This provided the drilling team with confidence in both the real-time data quality, and the accuracy of the pore pressure prediction. The application of the measurements while drilling and the resulting reduction in uncertainty meant that the 16" casing was not required in the next four deepwater wells, saving USD8 million per well and significantly impacting the economic efficiency of this project.