Hydrogen Sulphide Scavengers: Recent Experience in a Major North Sea Field
Authors: Wilson D.R. 
Published: December 2007

A number of wells in the Scott Field produce relatively high levels of hydrogen sulphide gas (H2S). The H2S is partially removed from the export gas before it enters the export pipeline system. Offshore experience on Scott has shown that directly injected liquid non-regenerable scavenger remove H2S from the gas with significant operational, environmental and cost benefits compared to a zinc oxide (ZnO) removal method. Following field trials, scavenger was implemented as the sole H2S removal method. This has reduced the chemical cost of export gas H2S treatment on Scott by over 70%, significantly cutting the overall field operating expenditure.

Introduction

The Scott Field is located in Blocks 15/21a and 15/22 of the U.K. Sector of the North Sea, some 110 miles North East of Aberdeen on the Scottish East Coast (Fig. 1). The field is operated by Amerada Hess Limited and came onstream in September 1993. Average production for 1995 was 184,000 BOPD oil exported via the Forties Pipeline System and 97 mmscfd gas exported via the SAGE (Scottish Area Gas Evaluation) pipeline system. Maximum daily oil and gas production has exceeded 220,000 BOPD and 120 mmscfd respectively. Over 165 million barrels of crude and 81 billion scf of gas have now been exported from Scott since start-up.

The field produces from an Upper Jurassic sandstone reservoir that comprises several fault blocks and heterolithic layers. A wide range of reservoir fluid compositions are present and a number of the 20 production wells in the field produce H2S levels exceeding 150 ppm vol. in the separator gas phase.

H2S is a naturally occurring gas found within oil and gas reservoirs. The gas is highly toxic, flammable and corrosive to certain materials. Produced gas containing high levels of H2S requires chemical treatment to remove or convert the gas to meet export pipeline specifications.

This paper reviews the background, laboratory and field testing of liquid chemical H2S removal methods on the Scott platform.

System Description

Scott Process System.

The Development currently includes 9 subsea oil production wells, 11 platform oil producers and 17 subsea injection wells.

The Scott Field platform facilities comprise two separate jacket structures in around 140 metre water depth linked by two bridges. The UQ jacket supports the accommodation and utilities modules including the water injection pumps and risers, control room and helideck; the DP jacket supports the drilling rig module, separation and gas compression facilities, wellheads, and subsea production well and export pipeline risers (Fig. 2).

Hydrocarbons from the production wells are processed through a single-train, three-stage oil, gas and water separation system. A dual train gas compression system separates and recycles NGLs from the produced gas, dries the gas by glycol dehydration and exports gas to the SAGE system (Fig. 3).

ZnO H2S Removal System.

The early start-up of Scott gas export in October 1993 required a temporary tight specification for the H2S content. The export gas was therefore routed through a single ZnO filled vessel located downstream of the high pressure gas compressor.

The H2S reacts with ZnO to form zinc sulphide (ZnS) and water. The ZnS remains as a solid within the vessel while the water is removed as a vapour along with the gas. A by-pass around the vessel can be adjusted to obtain the required overall specification.

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