Borehole Stability of Multilateral Junctions

Abstract

Abstract The number of multi-lateral wells are increasing in the North Sea area. Potential for improved accessibility and recovery has led to a significant interest both from oil companies and the service industry. With improved technology, these multi-lateral branches will be key elements in the development of the so called "smart" wells. Several multi-lateral wells recently drilled in the North Sea have experienced problems. One well had severe problems as the casing inside the branch deformed into the main wellbore, making both branches unavailable for reentry. Clearly, the formation at the junction had collapsed, forcing the casing to deform. A rock mechanics study was undertaken. The hole geometry above the junction was a circular hole, which became oval at the junction, and then split into two adjacent boreholes below the junction. Since the solution to this problem was not found in the literature, a new mathematical model had to be developed to model these geometries. This model was coupled to a stress model and a failure model for the rock. The paper will present the complete models. It was found that both the critical fracturing pressure and the collapse pressure changed as the hole geometry changed from a circular hole. The paper present a field case using the new models. The study showed that the stress concentration increased as the hole became oval at the junction. It was found that the oval geometry created more severe conditions both for mechanical hole collapse and fracturing. It was also found that at certain borehole pressures, this geometric effect diminished. One main conclusion of the study is that the allowable mud weight window is smaller at the junction itself compared to the holes above and below. The fracturing pressure is lower, and, the critical collapse pressure is higher in the junction than outside the junction. Therefore, the optimal mud weight is the most important condition for trouble-free drilling of a junction. The optimal stress conditions for the geometries will be defined in the paper, and based on these, the paper will present optimal mud weight selection criteria to drill multi-lateral junctions.