Underbalanced drilling (UBD) has gained strong momentum in recent years because of a number of advantages of the technology, including reduced formation damage and minimized lost circulation. UBD is mainly applied in exploratory wells in China, especially in deeper gas exploratory wells. Due to the inaccuracy of pore pressure and the complex nature of water, oil, gas, and solid multiphase flow in the UBD system, it is very hard to remain the UBD condition. Furthermore, conventional MWD or LWD can’t be used in multiphase flow annulus to measure the downhole pressure. Although EMWD is an ideal tool in UBD, its cost is too high. There have been many calculation algorithms or mechanistic models about multi-phase in UBD, but it is still not easy to gain accurate results. In this paper, a new tool is introduced, which was designed to measure the annulus pressure near bit during UBD, and the data is analyzed after round trip to gain the actual value of downhole pressure. Compared with the data about pore pressure gained through logging, the actual underbalanced pressure differential can be acquired. This is an effective approach in deeper gas exploratory UBD, especially in Daqing oil field, in which the change of deeper pore pressure isn’t obvious. Two deeper gas exploratory wells have been completed successfully by using this method with the tool. The design, manufacture and testing of the tool, as well as field problems encountered are reviewed in this paper.

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