A fully coupled thermo-poro-elastic model predicting the stability of wellbore in deep-sea drilling. Part B: Sensitivity analysis

Abstract

A good understanding of the mechanical behavior around the wellbore is important for predicting wellbore stability in deep-sea drilling (DSD) for oil and gas production. In Part A of the companion papers, a fully coupled thermo-poro-elastic (THM) model has been developed by combining fluid circulation in the wellbore and rock deformation in the reservoir to predict the mechanical behaviors of a wellbore/reservoir system during DSD, and the analytical solution is obtained in Laplace space. In this paper, a comparison is made first to show the difference between land drilling (LD) and DSD. A sensitivity analysis is carried out to study the effect of different parameters, i.e. drilling depth, rock permeability, density and injection rate of drilling fluid, on the mechanical behaviors of wellbore/reservoir system. The results show that (1) The injection rate plays a dominant role in affecting the BHT. In the case studied, the BHT decreases by around 9 °C when the injection rate increases by 4 kg/s (3.64 × 10−3 m3/s). This will lead to a large temperature decrease after long time circulation. The injection rate does not have a significant effect on the pressure evolution; (2) Due to existence of the geothermal gradient, when the drilling depth varies, the change of temperature and pressure in the rock formation is approximately linearly proportional to the change in the drilling depth. The larger the drilling depth is, the larger the absolute value of the displacement and failure risk at the wellbore; (3) When the rock permeability varies from k = 4 × 10−8 D to 4 × 10−5 D, the hoop stress and failure index (FI) distribution change at small time; (4) Too large and too small fluid densities may cause wellbore instability; (5) The DSD model is degenerated into the LD model when the depth of seawater approaches 0 m. The difference in the temperature and pressure changes between the two models are relatively small. While the magnitude of radial and hoop stresses in LD is larger than that in DSD due to the in-situ stress fields and the failure index in LD is lower than that in DSD around the wellbore, but larger in the far field.