Zinc Subsea Production System: An Overview
Authors: Bednar J.M. 
Published: December 2007

Exxon Co. U.S.A.'s Zinc development, in 1,460 ft of water in the Gulf of Mexico, uses a subsea production system to produce gas to a host platform 6miles away. The 10-slot subsea template, designed specifically to accommodate the soft soil conditions at the Zinc site, supports the non-through-flowline(non-TFL) wells, which are controlled by means of an electrohydraulic control system. This paper describes key features of the development.

Introduction

The Zinc development is a gas field in the Gulf of Mexico, approximately 50miles south of Grand Isle, LA (Fig. 1). Designated as the Mississippi Canyon(MC) Block 354 Unit, the field comprises Blocks MC 354, MC 355, MC 398, and MC399. For production, a 10-slot subsea template is positioned on Block MC 355and wells are directionally drilled to the designated bottomhole locations. Water depth at the template site is 1,460 ft. Exxon is the sole operator of the development.

The host platform is Exxon's Alabaster platform, located on a knoll in 468ft of water, approximately 6 miles to the west in Block MC 397 (Fig. 2).Production from Zinc is transported to Alabaster through two 8-in. and one4-in. flowlines. The subsea wells are non-TFL and are controlled by means of an electrohydraulic control system.

In defining the Zinc development system, we strived to use industry-proven designs and technology wherever possible and practical. To ensure reliability, an active quality assurance program was implemented through all phases of the project, from design and fabrication to installation and start up. Emphasis was placed on factory acceptance testing to ensure that delivered equipment would meet the designated performance standards properly. An 8-month system integration test then was conducted to ensure that the individual components and subsystems would perform satisfactorily as a total system when they were joined. Installation, drilling, and operations personnel were actively involved to ensure a smooth transition to the end users. Production was initiated in the summer of 1993, with individual wells producing at up to 19 MMcf/D gas.

The Zinc subsea production system is Exxon's first commercial subsea template in the Gulf of Mexico. It also is the industry's largest multiwell satellite subsea system in the U.S. gulf.

Development History

Productive gas was first proved on the site in 1977, when Exxon and Amoco drilled a joint discovery well in Block MC 354. After purchasing the Amoco leases in 1978, Exxon drilled an additional five exploration wells to define reserve limits.

In the early 1980's, all development efforts became focused on defining and evaluating the site soil conditions. The upper 2,000 ft of soil at the location is highly under consolidated, posing significant design constraints on the foundation system for the subsea wells. To understand these soil conditions better, Exxon conducted an extensive soil investigation from 1979 to 1983. The investigation included a high-resolution geophysical survey, a soil boring to2,000 ft, an acoustic monitoring of seafloor motion over a 1 1/2 year period, and analytical studies.

These studies confirmed theories regarding the canyon's geologic history and provided data on how the fill material would behave relative to loads from the subsea installation. Soil creep was predicted to be less than 0.5 in./yr at the mudline owing to the gradual slope of the canyon floor; however, the driving forces likely to cause mud slides were not present. Collectively, the four studies supported the overall conclusion that the soil conditions at the site were difficult but manageable with proper foundation design.

Further development planning, design studies, and gas-marketing activities continued through the 1980's, and Exxon announced final plans to develop Zincin April 1990. The subsea template was installed in late 1992, and drilling began in April 1993. Fig. 3 shows a simplified project schedule.

Reservoir Characteristics

The primary Zinc reservoir is located at a depth of about 7,000 ft subsea. Other production horizons are at depths up to 9,500 ft subsea. Initial bottomhole pressure (BHP) in the primary reservoir is about 3,200 psi, while the deeper reservoirs have BHP's up to about 4,300 psi. Bottomhole temperatures are approximately 150 and 200 F for the primary and deeper reservoirs, respectively. The Zinc wells were brought on production initially in July 1993;drilling and production activities then continued simultaneously.

Production is typically sweet gas with low condensate. No H2S and only traces of CO2 are present. Production from individual wells was expected to be about 15 MMcf/D gas, but rates as high as 19 MMcf/D have been sustained. Field life is expected to be more than 20 years.

Six wells have been included in the Zinc development plan. The additional four slots on the template are provided in the event that additional wells are required to produce the primary reservoir, potentially to capture additional reserves from other secondary production horizons, and to serve as spares. The three initial wells all produce from the primary reservoir. Of the second three wells (to be drilled later), one is planned to produce the primary reservoir and the other two are planned to produce the deeper reservoirs.

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