Pre-Salt Geohazard Assessment and Well Trajectory Multi-objective Optimization

This paper aims to optimize well trajectories in offshore reservoir development by applying a Pareto-based multiobjective approach. The methodology seeks to minimize key factors influencing well performance and cost, including the number of drilling phases, well length, geomechanical problems, and geohazards. Extending geohazard assessment beyond the main reservoir into the overburden section ensures safer and more efficient well planning, crucial for maximizing recovery and net present value. The approach uses Pareto-based multi-objective optimization, integrating seismic data and operational constraints to design optimal well trajectories. Input data includes specialist-interpreted 3D seismic volumes, providing information on stratigraphy, faults, lithological facies, and infrastructure such as flow lines and wellheads. By assigning weights and risk certainty limits to different hazards, the objective function balances safety and efficiency. NSGA-II adjust well trajectory variables like wellhead location, kick-off point, buildup rate, slant angle, and drop-off rate to meet reservoir targets while minimizing risk and cost. Different trajectory types—vertical, directional (slant and type S), and horizontal—are considered to accommodate field requirements. Applying this methodology to a pre-salt reservoir in Brazil demonstrated significant improvements in well trajectory optimization. The approach effectively mitigated geohazards and reduced operational costs, which allows us to minimize non-productive events and wellbore instability. Enhanced well placement ensured better alignment with reservoir targets and infrastructure requirements, such as subsea equipment and pipeline installation. The method's robustness was validated through collaboration with a multidisciplinary team of experts in well allocation, trajectory design, and geomechanical analysis. This integrated strategy enabled safer, more efficient drilling operations and optimized well performance. This paper contributes innovative methodology by integrating geohazard assessment into well trajectory optimization through a Pareto-based multi-objective framework. Combining NSGA-II with weighted risk assessments offers a systematic and adaptable approach to offshore well planning. The findings provide a valuable reference for applying advanced optimization techniques in complex reservoir environments, improving drilling efficiency and operational safety across the petroleum industry.