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Attard, M., Amoco (UK) Exploration Co. SPE Member

Abstract

This paper discusses the occurrence of annulus pressures in the North West Hutton oilfield, offshore UK, and how the problems associated with these pressures have been addressed. Several wells have experienced annulus pressures in this field, where production is exclusively via gas lift. The causes of annulus pressures are discussed in the context of the mechanical configuration of the wells. An evaluation of the safety aspects and concerns associated with these pressures is then presented.

Annulus pressure monitoring and operating procedures have been developed for use by offshore production personnel. These procedures contain a list of criteria which, when exceeded, require some type of action to remedy the problem in the well. Some of these criteria are based on a maximum pressure limitation for each well annulus. The derivation and application of these pressures is explained. The final section describes the changes made to the drilling and completion design of wells, in order to prevent or minimise the occurrence of annulus pressures.

Introduction

The North West Hutton oilfield, operated by Amoco (UK) Exploration Company ("Amoex") is located in the East Shetland Basin offshore UK, approximately 300 miles (483 km) north east of Aberdeen in licence block 211/27. Water depth is 475 ft (145 m). The reservoir is of Middle Jurassic Brent sandstone, and lies at an average depth of 11,500 ft (3 500 m) subsea. It is characterised by a series of tilted fault blocks and extreme lateral and vertical heterogeneity in its five major sand units.

The field was discovered in 1975 and commenced production in April 1983 from seven pre-drilled wells, following the installation of a 40-slot fixed steel platform with two drilling rigs. Peak oil production of 86,000 BOPD (13 672 m3/d) was achieved the next month. Initially, reservoir pressure and oil production declined rapidly, and water injection was initiated in February 1984 to arrest this decline. In October 1984, gas lift was commenced to increase production rates, and by the end of 1985 all producing wells were on continuous gas lift. The current production rate from the field is approximately 20,000 BOPD (3 180 m3/d), and a continuous infill drilling program is currently underway to increase reserves and maintain production from the field.

HISTORICAL OCCURRENCE OF ANNULUS PRESSURES

The procedures for drilling and completing wells are designed such that no pressure should be seen on any well annulus. The exception is in gas-lifted wells, where gas is injected under pressure into the tubing-production casing annulus. Thermal expansion of the tubing and casing when wells are first placed on production may cause pressure to build up in one or more annuli; however these pressures should not recur once they are bled off and the well is in a normal production mode.

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