Exploring to drill through the strata with different lithologies and pressure systems in one stage: A case study on the side-tracking Well AT2X1 in the Yangshuiwu buried hill belt

Yangshuiwu buried hill belt is located in the Langgu Sag of the Bohai Bay Basin, and it suffers the problems of deep reservoir, complex drilled strata, long well construction cycle and high drilling cost. A series of studies were carried out to optimize the casing program and shorten the well construction cycle of the wells in the Yangshuiwu buried hill belt. After the formation pressure profile was analyzed and the circulation loss in the Ordovician buried-hill carbonate rocks and the borehole instability mechanisms in the Carboniferous-Permian complex strata were studied, the borehole stabilization technology for sensitive mud shale, circulation loss technology for buried-hill carbonate rocks and the liner cementing technology for ultra high-temperature deep wells were formed gradually. Taking the side-tracking Well AT2X1 as an opportunity, the drilling fluid with lower density was used to drill through the Carboniferous-Permian complex strata and the Ordovician buried-hill carbonate reservoirs smoothly in one stage, which provides the beneficial trial for the subsequent simplification of four-stage casing program into the three-stage one. The successful side-tracking of Well AT2X1 provides the technical reference for drilling the internal buried hill in this area, penetrating the Fujunshan Formation inside the buried hill in one stage and solving the mudstone collapse problem of Xuzhuang Formation, Maozhuang Formation and Mantou Formation inside the buried hill.