Tahiti: Development-Strategy Assessment

Abstract

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper SPE 100656, "Tahiti: Development-Strategy Assessment Using Design-of-Experiments and Response-Surface Methods," by P.E. Carreras, SPE, S.E. Turner, SPE, and G. Wilkinson, SPE, Chevron, prepared for the 2006 SPE Western Regional/AAPG Pacific Section/GSA Cordilleran Section Joint Meeting, Anchorage, 8-10 May. The Tahiti field in the deepwater Gulf of Mexico (GOM) is a three-way anticlinal structure trapped against salt. The discovery well was drilled in 2002, and two appraisal wells were drilled soon afterward. Because of significant uncertainties remaining after appraisal, probabilistic methods were used to assess development alternatives. A classical experimental-design method was applied, and reasonable reservoir-simulation models were designed and validated. The proposed workflow enhances use of experimental design by combining the technique with response-surface methods and rational engineering judgement. Introduction Tahiti field is in more than 4,000 ft of water in the Green Canyon area of the GOM, approximately 190 miles southwest of New Orleans. The field was discovered in April 2002 with the drilling of Well GC 640-1. Results from the exploratory well indicated the presence of high-quality reservoir sand with more than 400 ft of total net pay distributed in three main Miocene turbidite sheet sands at depths ranging from 24,000 to 27,000 ft true vertical depth. In 2003, two appraisal wells were drilled simultaneously. 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 GOM. The field will be developed in phases. The first development phase was sanctioned in August 2005 by Tahiti joint-venture partners Chevron, Statoil, and Shell at a cost of more than U.S. $1.8 billion. Total capital costs for the project are anticipated to be approximately U.S. $3.5 billion. The field is expected to come on line in mid-2008. Because of significant uncertainties, probabilistic methods were used to assess development alternatives during Phases 2 and 3 of Chevron's project-management process. The main purpose of this study, part of Phase 3 (Develop Preferred Alternatives), was to assess a mature predevelopment strategy. Reservoir Description The Tahiti prospect was identified by use of advanced subsalt 3D-seismic imaging technology, along with geologic-data interpretation to identify the presence of reservoir-sand development, hydrocarbon charge, and migration into a geologic trap. The trap tested by the discovery well is a three-way anticlinal closure truncated against a salt feeder/weld system buried beneath an 11,000-ft-thick subsalt canopy (Fig. 1). On the basis of seismic data and appraisal wells, the trap is estimated to extend approximately 5 miles in a north-to-south direction and 1.5 miles in a west-to-east direction. Main reservoirs are Lower to Middle Miocene. DOE Methodology Design of experiments (DOE) is a methodology that reduces the number of reservoir-simulation runs that must be generated to develop equations for response variables. The response variables usually are economics parameters such as oil recovery or net present value (NPV). DOE provides not only a systematic framework for unbiased decisions but also technical continuity to the study. The understanding of field dynamics increases as the project evolves. The DOE workflow applied in this study comprises nine steps.