Abstract

This paper describes an unusual well design that has been implemented in a mature onshore oil field to minimise the cost of development drilling, improve well performance and utilise existing infrastructure. Horizontal wells were proposed to maximise oil recovery from the thin remaining oil column in the Kenmore Field, which has a strong bottom water drive. The challenge was to design horizontal wells that cost no more than low cost vertical wells. The drilling project was completed in January 2000, with the lessons learnt widely applicable.

Introduction

The Kenmore Oil Field in the Eromanga Basin, western Queensland, was discovered in 1985, with 22 wells drilled during the next 14 years. A common-oil water contact in the Hutton Formation and overlying, laterally variable Birkhead Formation was originally at 1231.7 m TVD subsea. Most of the wells are vertical, with 7" production casing and jet-pumping completions. A thin remaining oil column and strong bottom water drive in the Hutton Sandstone results in significant water production via coning into vertical wells, with water cut averaging 94% from the 14 wells remaining on production at the end of 1999.

In early 1999, two infill vertical development wells were drilled at low cost and these proved to be economically successful. Further targets were identified, and horizontal wells of at least 50 m horizontal length were shown by reservoir simulation to improve productivity and oil recovery. With a risk of encountering poor quality reservoir, and uncertainty in the height of the remaining oil column, a new vertical well cost of A$720,000 (completed with a jet pump and tied in) was set as the target for horizontal well design.

A scope study commenced in July 1999, targeting a drilling date in December 1999.

Geological Setting

The Kenmore oil field is located ~20km east of the township of Eromanga, Queensland, and is covered by PL 32, a production licence within ATP 269P (Figure 1). The field is one of a series of regional NW-SE trending Jurassic four-way dip structural traps (Figure 2), within the western sector of the Cooper-Eromanga Basin. The basin is Australia's largest onshore hydrocarbon province, where nearly 100 oil fields and 200 gas fields have been discovered. The largest gas field is Moomba/Big Lake (4 trillion cubic feet of original-gas-in-place) in South Australia. The largest oil field is Jackson (100 million barrels of original-oil-in-place), south-west of ATP 269P.

Production at Kenmore is from fluvial sandstones of the Jurassic Birkhead Formation, Hutton Sandstone, and basal Jurassic formation (Figure 3), which have histories of high oil productivity in this region. The Kenmore Field has yielded over 9.5 MMstb of oil from total expected ultimate recoverable reserves of ~12 MMstb in the Birkhead/Hutton reservoir system since coming on line in 1985.

The Birkhead Formation comprises volcano-lithic, fine to medium grained sandstones and common interbedded mudrocks, with the lower boundary transitional on a basin-wide scale. The Hutton Sandstone comprises widespread, fine to coarse grained, quartzose sheet sand and minor mudrocks, deposited in a low sinuosity fluvial environment.

The current PL 32 joint venture consortium comprises Oil Company of Australia Ltd (as operator), Petromin NL, and Beach Petroleum NL.

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