CURLEW PROJECT - "An Elegant Solution"
Authors: Hand Austin F. 
Published: December 2007

Curlew, are complex oil/gas condensate fields, with considerable reserves uncertainty, located more than fifty kilometres from the nearest infrastructure. Field development required an innovative approach in order to establish a sound business case. The selected development solution, of a contractor leased and operated Floating Production Storage and Offtake facility, with a minimum lease of four years and an option to extend year on year provided that solution. This approach addresses the need to manage the uncertainty in recoverable reserves in an area outwith existing heartland, whilst at the same time working with industry to deliver a fit for purpose facility in the shortest possible time.

Introduction

The Curlew area development is located about 220 kilometres east of Aberdeen and is centered on the Curlew D reservoir in block 29/7 in the Western Central Graben. The field was discovered by an exploration well in the second quarter of 1994. This well encountered a rich high productivity gas-condensate reservoir in Fulmar sands. Notwithstanding large volumetric uncertainty, Curlew D is by far the most significant prospect within a thirty kilometre radius.

Curlew B, an under saturated oil in Fulmar reservoir located four kilometres to the west-south west in the same block, was discovered in 1991. It is believed to share an aquifer with Curlew D, suggesting that they should be produced at the same time to avoid loss of recovery.

Prior to the discovery of Curlew D, development plans were long term, with Curlew B notionally developed in 2005 via the Anasuria re-located from Teal/Guillemot. The D discovery presented an opportunity for near term development.

To complete the picture for the Curlew area, the adjacent fields A and C were also considered for development. Curlew A discovered in 1977, consists of under saturated oil in Rogaland and Forties sands with scope for recovery of 15 MMboe in the expectation case. A high well count would be required, four producers and three water injectors, due to the highly faulted and compartmentalised nature of the reservoir. thereby rendering it uneconomic.

The Curlew C field, discovered in 1993, is thought to have considerable volumes of oil in place - 90 MMboe, however it is a chalk reservoir to which a recovery factor of less than ten percent has been assigned. No development plans have been formulated but a sub-surface reservoir study has been undertaken. (FIG 1).

Reserves Uncertainty

Curlew B contains 53 MMbbls of light oil and 39 Bscf of associated gas in place and it is estimated that 16 MMbbls of oil and 12 Bscf gas can be economically recovered. Curlew D contains approximately 642 Bscf gas and 167 MMbbls of oil/condensate in place. There is large volumetric uncertainty attached to the Curlew D reserves, in the expectation case recoverable reserves are believed to be 111 MMboe and in the high case 135 MMboe, in the low case only 78 MMboe will be recovered. The main contributors to this reserves uncertainty are, hydrocarbon water contact, aquifer response and condensate recovery efficiency. Whilst development drilling could resolve the former, only one to two years production history can resolve aquifer response and condensate recovery factors. Other contributors to uncertainty are reservoir heterogeneity and the drainage area of the producers. This was a major factor in pursuing an early production strategy and in determining the selected development option.

Reservoir Appraisal and Development

Curlew B has been developed using one dual-lateral well with two horizontal branches. The first lateral was an exploratory branch drilled into the crest of the northern fault block to evaluate the presence of hydrocarbons and subsequently to drain the northern accumulation if required. The northern block was found to be water bearing and this branch of the well was subsequently abandoned. P. 197^