P. Seligmann,^ SPE, Conoco (UK) Ltd.

Abstract

The Jupiter fields comprise a number of Rotliegendes gas fields in blocks 49/16, 49/17s and 49/22 of the UKCS and are situated about 85 miles east of the Conoco gas terminal at Theddlethorpe (Figure 1). The fields are operated by Conoco in partnership with BP, Mobil and Statoil. This paper covers the recent development of two of the Jupiter fields; Ganymede and Callisto and highlights the effective use of a multi company, multi discipline team of experienced reservoir scientists using deterministic, probabilistic and decision tree analysis where appropriate to define and quantity the uncertainty in the IGIP, deliverability and reserves of the two fields. This led to a data acquisition and analysis plan to resolve some of the uncertainty, and the implementation of a reservoir management plan to produce a cost effective development. The paper illustrates with field data the impact of resolving uncertainty and thereby reducing the P10/P90 (upside/downside) ratio for reserves by 50% following the development drilling. The learnings from the development drilling programme and the first months of production are reported.

Introduction

Phase 1 of the Jupiter area development consists of a ten slot not-normally-manned platform for the Ganymede field and a one well subsea tieback from Callisto to the Ganymede platform. As shown in Figure 2 considerable uncertainty regarding reserves and deliverability existed at the time of project approval, the Jupiter team was challenged to develop innovative methods for maximising the value of the development while minimising risk. The aim of the reservoir management plan was to maximise deliverability and reserves while reducing forecasting uncertainty. The reduction in forecasting uncertainty was important as the Jupiter field's gas has been sold individually by each field owner, and where lifting rights are managed by a gas balancing agreement.

Development History

Although the Jupiter fields were discovered in the early 1970's, they were not developed. They were considered to be uneconomic as both deliverability and reserves were uncertain. Reservoir quality in the Ganymede and Europa discovery wells was poor with air permeabilities in the 0.1 to 1 md range. Flow tests yielded only ca. 5 – 10 MMscf/D. It was not until gas market the UK began to open up that the Jupiter field owners began to revisit the concept of developing the Jupiter fields. A parallel exploration and appraisal programme revealed upside potential not only in the Ganymede field, but also in the Callisto and Thebe fields where the discovery wells identified rock properties significantly greater then that of the early 1970 wells. Permeabilities were in the 10–100 md range and well test rates in excess of 40 MMscf/D were achieved.

Following an initial review of the data available, a decision was made to defer development of the Europa and Thebe reservoirs as part of Phase 1. Even at this stage it was clearly obvious that the uncertainties in deliverability (Europa) and reserves (Thebe) in these two fields were too large to include in any cost effective near term development. An extended test of a horizontal well was proposed for the Europa field in order to quantify the uncertainty in deliverability before milking a development decision.

Phase 1 thus consisted of Ganymede and Callisto. The Phase 1 development drilling programme was further planned and prioritised with the following guiding principles:

  • Minimise the largest uncertainties first

  • Optimise the overall drilling programme with regard to reserves, deliverability and costs.

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