Most of mature oil fields in China have been developed for decades of years, the majority of hydrocarbons being exploited today are found in pressure depleted formations or complex and lower quality reservoirs. Underbalanced drilling (UBD) is identified as an effective way to address problems associated with conventional overbalanced drilling, but its development and application in China is still on the preliminary stage. To cost-effectively serve for the complex exploratory situation, CNPC (China National Petroleum Corp.) initiated a large-scale UBD campaign from 2007 as a major change of its economic growth pattern.

This paper is a critical summarization of the ongoing campaign, with its purpose to explain where we are and what to do next. It highlights the new developments in techniques and auxiliary equipments in: 1) the whole process UBD, in which self-made equipments like the liquefied gelling valve and casing valve were particularly introduced; 2) underbalanced horizontal drilling, which plays a more important role in developing fractured reservoirs; 3) gas drilling, which gains further enhancement in air hammer, circulated medium diversion, water production prediction and reduction, and significantly improves drilling speed in deep formations; 4) managed pressure drilling (MPD) based on UBD equipments, which more precisely controls the annular pressure profile and effectively eliminates lost circulation in highly fractured zones; 5) bottom hole pressure measurement while drilling, which provides necessary information for the design of equivalent circulating density (ECD) as well as the control of bottom hole negative pressure in UBD. Case histories show that the UBD campaign not only improves drilling speed, reduces non-productive time (NPT), protects reservoir, but also enables comprehensive characterization of the reservoir while drilling, making smaller-reserves targets viable and previous low quality formations producible. In addition, the development trend of UBD in CNPC was discussed, and recommendations were proposed.

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