Analysis and countermeasures of drilling challenges in complex ultra-deep exploration wells in thrust structural belt with multiple salt layers

Abstract

The deep and ultra-deep zones of Bokhtar oilfield in Tajikistan are the critical blocks for future exploration and development, which are imbedded between two salt layers, accompany with four reservoir and cap assemblages of middle and upper Jurassic, Lower Cretaceous, Upper Cretaceous and Paleogene. The reservoir and cap assemblages of pre-salt carbonate and salt-gypsum cap have great potential. The complicated geological structure, uncertain formation and non-developed pre-salt formations made the critical challenges for drilling operation, including: (1) The difficult casing program design caused by peripheral compression and thrust of the basin, the complicated geological structure, the large geological difference above and below the salt layer, the unfaithful prediction of multiple sets of salt layer, great formation pressure uncertainty, and many sealing points; (2) The upper salt formations are featured by developed thrust belt and large dip angle, the contradiction between deviation prevention and drilling acceleration in high steep structure is prominent, and the wellbore quality is difficult to control; (3) Due to the mud loss in upper salt layer, loss coexisted with blow out in the salt layer and inaccurate prediction of salt bottom, the drilling complexes such as mud loss, overflow and well collapse are occurred frequently; (4) Ultra-deep well depth (over 7000m), HTHP (180°C,140MPa), multiple salt layers and long open hole section made high requirements on high temperature resistance of drilling fluid, cementing slurry and downhole tools, as well as high drilling risk and cost. It is necessary to carry out the drilling difficulties investigation of ultra-deep exploration well under the consideration of pertinence, safety, applicability and economy, then formed the suitable drilling engineering plan for Bokhtar Block through the optimization of domestic and foreign advanced and mature deep and ultra-deep well technologies, which is used to provide the technical support for the first pre-salt ultra-deep exploration well in this Block to be drilled successfully.