Geomechanical modelling for analysis of directional drilling techniques in the course of construction of underwater passages

The paper presents engineering solutions utilized in directional drilling as well as the impact they have on construction of underwater passages. This is exemplified by an accident development scenario of passing through gravel and cobble soils. Following the model of stress-strain state of soils composing the wellbore and based on fluid dynamics model pertaining to drillingmud filtration, potentially hazardous process failures have been analyzed.
Materials and methods
Geomechanical and fluid dynamics modelling of submerged crossing.
Results
The reviewed model of underwater passage suggests that poor stability of gravelly sand at the interface of semi-firm clay is determined by a number of factors, including application of drilling mud, the composition of which enables formation of suspension with high sealing and low filtration capacity. Wellbore collapse and increased pulling capacity of the drilling rig resulted from the contact of the pipeline with accumulated cobble soil on the lower edge of the well and further bumping of the pipeline against the well arch. Geomechanical modelling of underwater passage and fluid dynamics modelling data gives grounds for assertion that saturation of gravelly sand with highly structured drilling fluid (plasticity index ? 1300 s-1) helps to reduce soil porosity and increase its strength. This ensures stability of the borehole while drilling which is proven by underwater passages constructed in similar geotechnical conditions.
Сonclusions
Elaboration of geomechanical model of underwater passage and fluid dynamics model of filtration enables to predict problems arising in the course of construction of underwater passages with directional drilling. It also helps to find engineering solutions to prevent such problems.