Tahiti: Assessment of Uncertainty in a Deepwater Reservoir Using Design of Experiments

Abstract

Abstract Tahiti prospect in deepwater Gulf of Mexico is a three-way anticlinal structure trapped against salt, with primary pay sands ranging from 24,000 to 27,000 ft TVD. The field contains several hydrocarbon-bearing turbidite sands. The discovery well was drilled in 2002, and two appraisal wells were drilled soon afterwards. Due to significant uncertainties remaining after appraisal, probabilistic methods were used to assess development plan alternatives and reserves. The primary purpose of this paper is to show the integrated earth modeling and reservoir simulation workflow, which incorporates uncertainty analysis and experimental design. The first step of the reservoir simulation workflow was the identification of the uncontrollable parameters that might influence performance predictions. These uncontrollable parameters included the earth model, water-oil contact location, faulting and compartmentalization, reservoir anisotropy, aquifer support, relative permeability curves, pore volume compressibility, rock compaction and dilation, fluid characterization, skin factor and well pressure drawdown. The analysis of available information from Tahiti field and analog fields with simple statistical techniques allowed the unbiased estimation of low, medium and high values of each uncontrollable parameter. The second step was the application of design of experiments to evaluate the impact of each uncertainty on reservoir performance. Monte Carlo simulation was used to estimate P10, P50 and P90 oil recovery and discounted oil recovery for each reservoir. Finally, we developed reasonable P10, P50 and P90 reservoir simulation models that incorporate the full range of each significant reservoir uncertainty. Probabilistic production forecasts for primary depletion and waterflood strategies supported project decisions like number of pre-drills, total well count, bottom-hole locations, waterflood strategies and facility capacity. Reservoir simulation results also supported proved reserves estimation and economic evaluation of the project.