The South China YQ Basin with 15 trillion cubic meters natural gas is typical of ultra high temperature-high pressure (ultra-HTHP) with the highest bottomhole temperature (BHT) at 249°C, the maximum bottomhole pressure (BHP) at 142MPa and the extremely narrow pressure window. Therefore, there are kinds of technical challenges during drilling there. In recent years, the managed pressure drilling (MPD) has been successfully applied in the basin with risks and well cost reduced instead.

The operational designs of MPD consist of three parts: the precise calculation of drilling fluid equivalent circulating density (ECD), the optimization of operational parameters and the well control. The first part includes four models: the wellbore temperature field model, the drilling fluid equivalent static density (ESD) model, the drilling fluid rheological property model and the effects of cuttings concentration on ECD. The second part is the determination of the two key operational parameters: the mud weight (MW) and the surface backpressure (SBP). The third part is the plans of three cases: downhole accidents, equipment failures and termination conditions of MPD.

The first part includes four steps: establish the instantaneous wellbore temperature model based on the convection and thermal conductivity theory by dividing the wellbore into five areas; establish the ESD model by considering the elastic compression effect of HP and thermal expansion effect of HT; establish the drilling fluid rheological property model based on the Herschel-Buckley model by considering the effect of ultra-HTHP on dynamic shear force, consistency coefficient and liquidity index; consider the effects of cuttings concentration on ECD based on the solid-liquid two-phase flow. The ECD model is established based on above models. The second part includes two steps: determine the MW based on the critical pressure constraint principle by the operational window simulation of different well depth and fluid volume; determine the SBP of pump-on and pump-off by considering the rated operating pressure of the equipment, the calculated pressure loss and the 0~1MPa higher BHP than formation pressure. The third part includes three steps: make the emergency measures against downhole accidents by well control matrix; make the emergency measures against the failure of equipment such as rotating control device (RCD); determine the MPD termination conditions such as drilling big cracks.

The MPD is successfully applied to X gas field featuring offshore ultra-HTHP. The casing structure is optimized from 7-8 layers to 5 layers and the well is drilled in the micro pressure window of 0.01~0.02sg without accidents. Additionally, the non-productive time (NPT) decreases by 60% and the well cost is obviously reduced. Generally, the MPD yields time and cost savings for tomorrow's market.

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