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

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. Introduction Tahiti prospect is located in Green Canyon blocks 596, 597, 640 and 641, about 190 miles southwest of New Orleans in deepwater Gulf of Mexico as shown in Fig. 1. The field was discovered in April 20021 with the drilling of well GC 640 #1 in slightly more than 4,000 ft of water. Chevron is the operator and holds a 58% working interest. Statoil holds a 25% interest, and Shell holds a 17% interest (Total E&P USA, Inc. purchased Shell's working interest effective January 20062). The field is expected to come on-line in mid-2008. Results from the exploratory well indicated the presence of high quality reservoir sand with total net pay of over 400 ft essentially distributed in three main Miocene turbidite sheet sands at depths ranging from 24,000 to 27,000 ft TVD. A downdip sidetrack and an updip sidetrack encountered net pays of 75 ft and 400 ft, respectively. In 2003 two appraisal wells were drilled simultaneously using two rigs. Each rig drilled a vertical well with an updip sidetrack. One of the appraisals encountered more than 1,000 ft of net pay in high-quality sandstones, confirming one of the most significant net pay accumulations in the history of the deepwater Gulf of Mexico.3–5 Consequently, the field was delineated from three surface locations and seven reservoir penetrations. The main sands were identified as M-21A and M-21B. For simplicity, they are referred as M-21. The drilling of two appraisal wells simultaneously is unusual, particularly in deepwater, but the benefit is that key reservoir information is obtained in half the time, allowing Chevron to begin development planning for the field sooner and ultimately shortening the time to first production.