The number of multilateral wells drilled in the Niger Delta all geared towards increasing recovery as well as other field development objectives. However, a major challenge facing the successful drilling of these special trajectory wells is hole instability. The cost of instability according to some operators can be as high as $ 600MM per Year. Wellbore instability poses a unique threat in multilateral wells such that the tendency for collapse or fracture increases at the bore hole junctions. Casing deformation, difficulty in through-put of drilling tools and equipments, to mention but a few, are among the major problems associated with this instability.
This research revisits the issue of wellbore stability with emphasis on multilateral wells. A modified failure model is developed by including a stress concentration factor based on the junction configurations. The Mogi criterion was applied to account for intermediate stress and predict the minimum mud pressure below which shear failure will occur. A risk and uncertainty factor is attached to this model for sensitive model parameters including stress concentration factors, and lateral bore entry angle.
Results from two case studies presented in this work showed that collapse gradient prediction by Mogi based model were more reliable than those of Mohr based model. In case 1, a minimum mud weight of 0.52 psi/ft was predicted to be sufficient to maintain stability. A mud weight of 0.53 was used to drill the lateral section and hole pack offs were encountered at build angle of 30°. However, the Mogi prediction was 0.57 and sufficiently prevented hole collapse. In case 2, the Mogi predictions are all well above the actual mud weight used as against the underestimation with Mohr's.