This paper describes a real-time case study to prevent mud loss and blowouts while drilling a high-pressure gas well in Tarim basin, China. The complex geological structure, high tectonic stresses, and overpressured- and fractured-reservoir formations in the field present a huge challenge to drilling. Of the seven wells drilled in the field in 2005, two did not reach target depths, four experienced huge mud loss, and the other experienced a blowout, resulting in lost control of the well. In early 2006, PetroChina teamed up with Schlumberger and Petroleum University of China to form a collaborative technical group to develop a better understanding of mud-loss and blowout mechanisms. The key component of the study was to establish a mechanical Earth model (MEM) based on offset well data before drilling, update the model using downhole monitoring and logging data during drilling, and predict a safe mud-weight window in real time.
Real-time prediction of a safe mud-weight window with annular-pressure monitoring helped ensure that downhole annular pressure was maintained within the safe mud-weight window during drilling and tripping. The study resulted in a x17 reduction in mud loss and x10 reduction in total nonproductive time (NPT), which is mainly because of mud-loss reduction and elimination of an extra casing. A better understanding of mud-loss/blowout mechanisms was achieved, and guidelines for preventing mud loss/blowouts specific for this gas field were developed.