A novel model for wellbore stability analysis during reservoir depletion

- This model considers depletion effects on wellbore trajectory.
- The model is used in a gas condensate reservoir.
- Optimum wellbore trajectory is obtained for drilling/production operations to avoid collapse and sand production.
- The model determines critical times in which the optimum wellbore trajectory will change.
- Sand production is maximum when the upper reservoir portion reaches to the critical time while deeper parts do not reach yet.

It is not common to build a geomechanical model for depleted reservoirs as logging and coring are costly and time consuming in such reservoirs. On the other hand, in regular analysis of wellbore stability, the effect of time is completely ignored. As a result, with time there will be some errors in the evaluation of wellbore stability in depleted reservoirs. In order to determine the optimum wellbore trajectory during the reservoir life, it is necessary to have a model which can estimate the rock properties based on the first full logging suite and also consider the depletion effect. In this study, a novel model is proposed which combines a mechanical earth model, borehole circumferential stresses, Mogi-Coulomb failure criteria and simulation of pore pressure variation near wellbore to determine the optimum well trajectory during drilling and production in a condensate offshore reservoir. As a main output from the application of the new model, it was found that the most stable wellbore trajectory changed after 18-27 years of production. This critical output from this study raises the need to consider the possibility of changing the designed well trajectory over the life of the reservoir to maintain wellbore stability and optimize the drilling and production operations.