The experience of drilling the horizontal and subhorizontal wells in the Achimov deposits interval demonstrates a high risk of complications and accidents. This relates to the complexity of geological factors, anomalously high reservoir pressure and narrow safe density interval. At the present time, there is no consensus on the reasons of borehole wall disturbances in the Achimov rock interval. Some researchers consider a mechanical impact of drill pipes on borehole walls, while the others reckon among the main factors the tripping of borehole walls, their physical and chemical interaction with the flush liquid, the mechanical impact of drilling tool and the stress state of the rocks. The friction forces between the rock particles are also emphasized as the reason of borehole wall disturbances. One of the reasons for the strength degradation of clay material is the process drill mud penetration, which causes the emergence of the disjoining pressure in the rock and the borehole wall disturbance. Thus, notwithstanding a major number of researches related to the borehole wall strengthening in the Achimov deposits interval, this issue remains unresolved and requires a further research.
The article presents the plan of actions reducing the risk of complications and accidents while drilling the horizontal wells at Lutseyakhoye field. The first stage is geomechanical modelling of borehole wall stability, which allows to define a window of drill mud safe density throughout the whole wellbore. The second stage is updating the hydraulic program defining the borehole cleaning from drill cuttings and the selection of the best rheological parameters. The third stage is working out drilling mud composition to drill in the Achimov deposit intervals. While assessing the non-productive drilling time (NPT) for the Achimov deposits of the adjoining fields, it was established that the average NPT is 23 days. The complex approach to accident-free drilling of horizontal wells made it possible to reduce NPT based on geological factors to 3 days.
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