Abstract

The Snapper field, which is located in Bass Strait, offshore south-eastern Australia, contains a large gas reservoir, the "N-1", with an 8 metre "pancake" oil leg. While snapper Was developed primarily as a gas field, initial work has concentrated on maximizing oil recovery prior to eventual large scale gas production. Phase I development was undertaken during 1981-1983 with fourteen N-1 oil completions coming on-stream. Encouraged by the successful performance of these wells, a second drilling phase was undertaken during 1986-87, resulting in nine new oil completions. some gas production has been required to meet gas market demand. This production, from gas completions and gas coning in oil wells, has caused the oil zone to steadily rise. Over the next few years, workovers in many of the wells will extend production as the column continues to rise.

This paper examines the highly variable performance of the mature Phase I wells and the early performance of the Phase II wells. This will assist other operators in planning similar, thin oil developments.

Introduction

The Snapper field is located 30 kilometres off the Victorian coast of south-eastern Australia in the waters of Bass Strait (Figure 11. It is one of eleven oil and gas fields which have been developed by Esso Australia Ltd and BHP Petroleum International Pty Ltd. Three of these fields, Barracouta, Marlin and Snapper are primarily gas, supplying the Victorian gas market. The Snapper "N-1" reservoir (Figure 2) is currently assessed to contain 3.2 Tcf of original gas-in-place underlain by a thin "pancake" type oil column of 4 to 8 metres gross thickness. Several small, deeper oil reservoirs were discovered during development drilling and are also being produced. This discussion is restricted to the N-1 oil zone.

Snapper was discovered by the Snapper-1 exploration well in 1968 and delineated by Snapper-2 in 1969 and Snapper-3 in 1970 (Figure 2). Construction of the 27 conductor Snapper 'A' platform did not commence until 1978 because Barracouta and Marlin were able to meet gas market demand through the 1970's. Although Snapper gas was required to meet a growing market demand, particularly in the peak winter months, it was recognised in the planning stages that full gas deliverability was not required until the 1990's. Initial development could therefore concentrate on the oil leg allowing for later conversion of depleted oil wells to gas service to meet gas market demand.

Development drilling occurred in two phases. During Phase I development, 21 Wells were drilled over the period 1981-83, of which fourteen were completed as N-1 oil producers. This was followed by the delineation and exploration wells, Snapper-4, Snapper-5 and Snapper-6 which were drilled to better define the N-1 oil leg to the south-west of the field and to look for additional, deeper reservoirs to justify a second platform development. However, the results were disappointing and a second platform is not currently considered viable. The excellent sand quality and reservoir continuity of the N-1 gas reservoir means that the gas should be effectively drained from the single, crestally located platform.

With the successful performance of the Phase I N-1 Oil wells and with the potential of making further deep discoveries, Phase II development drilling was undertaken during 1986-87. The remaining six conductors were used for N-1 oil targets and deeper exploration targets, and five of the Phase I wells were redrilled to new N-1 oil targets, resulting in nine new N-1 oil completions. In 1986, the first direct evidence of oil column movement due to gas cap withdrawals became available from a cased hole log in one of the Phase I wells. This column rise was confirmed by open hole logs in the Phase II wells. Activity over the next few years will concentrate on wireline recompletions of wet wells to maximize recovery from the slowly rising oil column prior to the start of high gas production expected around the mid-1990's.

RESERVOIR DESCRIPTION

The Gippsland Basin fields consist of sandstone reservoirs contained within the Latrobe Group sediments of Upper Cretaceous to Eocene age.

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