Study on effect of in-situ stress ratio on vertical wellbore stability using DEM-DFN method: A case study on one of the wellbores in Persian Gulf

Wellbore instability and drilling fluid loss in fracture formation is one of the main issues in deep drilling. In order to determine an efficient drilling methodology it is necessary to investigate the effect of fracture on instability and fluid loss mechanism. In this article in order to evaluation of the vertical wellbore stability and fluid loss in fracture formation, three dimensional simulation of of a wellbore in the Persian Gulf was carried out using Discrete Fracture Network (DFN) and Distinct Element Method (DEM). Validation of the model and stability analysis of wellbore was carried out using maximum allowed movement, normalized yield zone radius criteria and caliper log data. The initial analysis of the model showed that the wellbore is in an unstable state for kakhdumi formation. In order to investigate the hydromechanical mechanism in fracture formation, drilling fluid was injected by rate of 25 BPH and viscosity of 1.08 cP to the wellbore. Slip in fractures, shear displacement and the volume of fluid loss was determined as main parameters for wellbore stability analysis. The effect of in-situ stresses ratio (σ_H/σ_h ) on instability mechanism and fluid loss was carried out based on six different scenarios for in-situ stresses ratio. By increasing in-situ stresses ratio and in an anisotropic (σ_H/σ_h =2) satat, slips and shear displacement along the discontinuity increased. In this case, for 25 BPH drilling fluid flow ratio the fluid pressure decrease along the discontinuities. The parametric study for five different fluid flow ratio showed that in (σ_H/σ_h =1.06) the fluid expansion in fracture increased. Moreover, tension failure and shear displacement decreased in low fluid flow ratio. In 5 BPH fluid flow ratio, the fluid pressure in fractures decreased compared with higher fluid flow ratio. This is because of less shear displacement and fluid expansion along fracture in lower fluid flow ratio.