Abstract

This paper presents the results of a two-year comprehensive effort to design, test, manufacture and implement a highpressure completion tubular for ChevronTexaco's Tahiti project. Several enabling technologies resulted, including the project-specific design of a premium seal, rotary shouldered connection to withstand absolute pressures up to 29,000 psi, metallurgical and heat treatment advances to permit manufacture of ultra-high strength tool joints (12.5% greater than the industry standard yield strength) and an industry first rotary shouldered connection testing program consisting of simultaneously applied internal and external pressures with temperature, bending, tension and compression loads. Field results from the completion operations are presented.

Introduction

Located in approximately 4,000 ft of water in the Green Canyon block of the Gulf of Mexico, ChevronTexaco's Tahiti prospect is one of the most significant oil discoveries in the history of the deepwater Gulf. Tahiti well depths are in excess of 28,000 ft creating challenging conditions for high-pressure, subsea completion operations. A major challenge was the design, testing and manufacture of a subsea completion string that would provide efficient hydraulics during the fracturing operations while insuring mechanical and pressure integrity to absolute pressures up to 29,000 psi during screenout.

Similar wells have been completed with up to four outside diameter (OD) tapered strings. Tapered strings such as this limit the treating rate during the fracture stimulation and often yield a less desirable fracture network, potentially reducing the well's production capability. To maximize Tahiti's production rates, a fracpac completion was selected to create an extensive fracture network in the highly overburdened formation. Treating rates during the fracpac were deemed critical for the greater than 17,000 psi treating pressure. A large diameter completion tubular and a new connection that could meet all loads from top to bottom were required.

Overview of Tahiti Field

The Tahiti prospect was generated using an advanced seismic depth-imaging technique proprietary to ChevronTexaco. Applying this technique to 3-D seismic data, geophysicists were able to view improved clarity of the Tahiti architecture through over two miles of salt layers. This clarity of the reservoir enabled precise positioning of the first Tahiti exploratory well.¹

In March 2002, ChevronTexaco drilled the Tahiti #1 well in 4,017 ft of water to a total depth of 28,411 ft. The Gulf of Mexico well was located approximately 190 miles southwest of New Orleans in Green Canyon Block 640. Results from the exploratory well indicated the presence of high-quality reservoir sand with a total net pay of over 400 ft.²

Following the results of the exploratory well, two appraisal wells were drilled simultaneously in Green Canyon blocks 596 and 640. The two-well appraisal program confirmed that the reservoirs in Tahiti were well developed and correlated over a three-mile distance. Results validated the hydrocarbon reservoirs found in the discovery well, with one appraisal well encountering more than 1,000 ft of net pay in high-quality sandstones.³ Though uncommon to deepwater development, simultaneous appraisal wells provided reservoir information in half the time and accelerated field development planning.⁴

The appraisal program verified ChevronTexaco's initial estimates of 400 to 500 million barrels of ultimate recoverable oil reserves, one of the most significant net pay accumulations in the history of the Gulf of Mexico. A well test of the Tahiti #1 discovery well was planned for the second quarter of 2004. To complete the discovery well, several complex challenges faced the Tahiti Well Test Team.

Completion Challenges of Tahiti

Due to the extreme depth and bottom hole pressure of the Tahiti reservoir, the completion and well test required design of numerous new equipment to successfully perforate, fracpac and flow test the Tahiti #1 discovery well. Technologies that had to be developed for the Tahiti well test are as follows:

1. Subsea test tree system

2. Perforating firing head

3. Perforating guns and shock absorbers

4. Downhole isolation production valves

5. Smart well test production flow valve