With the rapid development of Under-balanced Drilling (UBD) technologies, more and more people come to realize that the UBD is one of the most effective drilling technologies in reservoir exploration. However, the conventional Underbalanced Drilling Technology has its own faultiness. Take tripping pipe and liner for instance, we have to operate the well killed for well-control safety. The kill weight mud (KWM) will increase Bottom Hole Pressure (BHP) and make the down hole overbalanced. This will decline the effectiveness of reservoir protection in UBD technology.

The present paper aims to provide a new type drilling tool, named Downhole Blowout Preventer (DBOP), developed by Xinjiang Drilling Technology Research Institute of Western Drilling Engineering Corporation of CNPC, which could achieve a wellbore full-procedure underbalanced effectively. There’re some advantages in DBOP, such as hydraulic control, reliable gas/ liquid sealing property, pressure real-time transmission of valve, etc. Moreover, the DBOP can save drilling cost enormously and guarantee the behavior of barren bore safely.

The DBOP tripping in wellbore as one of intermediate casing, is in open condition during the running casing procedure. When tripping occurs, the DBOP, which has a valve, will isolate the wellbore into two sections (upside section and underside section) by closing its valve. For the sake of well isolation, the DBOP has perfect sealing property to isolation fluids which may include drilling mud and formation fluids. During the procedure of coming out of the hole, when the bit arrives at the upside section, the DBOP will close its valve timely; while, during the procedure of going in the hole, the DBOP's valve will be unclosed when the bit closes to the DBOP.

Because of DBOP's coordinating with Rotating Blowout Preventer (RBOP), the full-procedure underbalanced drilling achieves efficiently.

The Downhole Blowout Preventer (DBOP) has been used in some oilfields successfully, such as Xinjiang Oilfield, Huabei Oilfield, Sichuan Oilfield, etc. The effectiveness of DBOP has been proven to be efficient and successful.

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