Application of underbalanced drilling technology in ultra-deep horizontal well (TP127H well) of carbonate reservoirs

The Ordovician carbonate reservoir in Tuoputai Area of Tahe Oilfield, the safety pressure window is narrow, the reservoir is dominated by caves and fractures and it is easy to cause lost circulation or kicking. TP127H well is an ultra-deep carbonate horizontal well in this area, which is drilled to the depth of 6914.39 meter with leakage of 1 538.67 m3. Several dissolved caverns were met during horizontal drilling. To drill to the design depth successfully, under-balanced drilling is performed for this well. This paper discusses the feasibility of under-balanced drilling of this well; analyzes the density of drilling fluid, wellhead control pressure and bottom hole pressure in the under-balanced drilling; and the achievements are applied in under-balanced drilling of TP127H well. The results of application show that the technology is able to solve problem of lost circulation of TP127H well and also to provide references for drilling in the similar reservoirs.