When drilling in the open hole with negative density window, which occurs leakage in the upper section and collapse in the lower section, it will lead to many problems using the conventional method to seal the formation with technical casing, such as high cost,complex casing structure and small completion size. It is difficult to meet the needs of low cost and high efficiency development in the deep wells.

This paper presents a new Gas-Lift MPD drilling method based on Dual-Channel compound Drillpipe (DCP), which can solve the above problems. The DCP is composed of the 139.7mm DCP, the gas lift joint and the conventional 139.7mm drill pipe. It has two circulating channels. Channel 1 is for drilling fluid circulation and channel 2 is directly connected with the upper borehole annulus. The advantage of this method is that it can change the upper wellbore annulus into gas-liquid two-phase flow by injecting gas the channel 2 when the drilling fluid is circulating in the channel 1. It can develop different annulus ECD gradient (small to large) profile and complete complex pressure formation drilling operation in the same open hole section.

According to the design of SBX-5H well, the article uses OLGA to carry out dynamic simulation and analysis of the construction plan of the Gas-lift MPD based on Dual-Channel Drillpipe. The simulation results show that it has great advantages in reducing the number of casing layers, shortening the drilling cycle and enlarging the completion size. Firstly, the control requirements of the ECD profileare is satisfied, which is that the ECD<1.28g/cm3 in the upper section and ECD>1.32g/cm3 in the bottom of the open hole (collapse ECD=1.31g/ cm3, leakage ECD=1.29cm3) by simulating and adjusting drilling pump flow, injected gas flow rate, Dual-Channel Dirllpipe length and drilling fluid density. Secondly, the complicated drilling problem is expected to be solved,such as repeated leakage in the long open hole section. The number of casing layers can be further reduced from 4 to 3 and the drilling cycle is expected to be reduced by more than 20%. Finally, the completion well bore size will be expanded from 149.2mm to 165.1mm (enlarged 11%), which can better meet the requirements of the next production and reservoir transformation.

Through the simulation analysis, it is proved that the Gas-lift MPD technology based on Dual-Channel Drillpipe can effectively solve the problem of negative density window in deep well drilling operation. It not only reduces the drilling difficulties and the drilling cycle, but also is economical and efficient. Therefore, it has a good market prospect.

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