Integrated Approach for the Wellbore Instability Analysis of a High-pressure, High-temperature Field in the South China Sea

Abstract

Wellbore instability and lost circulation were frequently encountered while drilling the first batch of exploration wells in a low-permeability, high-pressure, high-temperature (HPHT) gas field in the South China Sea. These problems are attributed to very narrow drilling mud weight windows resulting from the HPHT reservoir conditions. This study focuses on combining and leveraging geophysical data, rock mechanics testing data, and well logging data to predict wellbore instability and lost circulation under HPHT conditions. Full-scale core samples were acquired from HPHT reservoirs of the field, and uniaxial and triaxial rock mechanics tests were performed to determine the mechanical properties of the rock. The influence of heat transfer between the wellbore and the formation, temperature field, mechanical properties, in situ stresses, collapse pressure, and fracture pressure were investigated. According to the results, changes in wellbore temperature generate thermal stress, which may aggravate wellbore instability. The wellbore temperature decreased by 50–60°C during the mud circulation process. Furthermore, the safe mud weight window, which was originally narrow because of the HPHT conditions, became narrower under the influence of temperature change. The use of multi-source data and the integrated model will significantly improve the accuracy of wellbore instability and lost circulation prediction in HPHT fields.