Abstract

This paper demonstrates the benefits of the application of a synergistic approach in furthering the understanding of the layered nature of the Piper Field. Core, log, FMT pressure and fluid redistribution information obtained from the eight well Piper B Redevelopment Predrilling programme are discussed. The integration of the Piper B results necessitated a reassessment of the Piper Alpha data and led to an improvement in the reservoir characterisation in terms of layer permeability definition, flow behaviour, pressure and aquifer influx. This improved understanding of the reservoir led to the definition of an initial selective completion policy for the Piper Field redevelopment. The primary objective of this policy is to reduce the potential for water override in problematic high permeability layers and thereby improve the sweep of the less prolific layers by closer targeting of injection support. Such an approach al lows improved reservoir management, greater flexibility in scheduling remedial work and is oriented towards maximising ultimate recoverable reserves by optimising the sweep in individual sand layers prior to their abandonment/isolation for water shut off purposes.

Introduction

The Piper Field is located in UK North Sea Block 15/17, approximately 110 miles north-east of Aberdeen and is situated on a shelf on the northern margin of the Witch Ground Graben (WGG) in the Outer Moray Firth (Figure 1).

Production from the Piper Field was initiated in December 1976 and injection support in January 1978 (Figure 2). A total of 55 successful development wells were drilled from the original 36 Piper Alpha slots up until the shutdown of the field following the tragedy in July 1988. At that time, the field was producing approximately 112500 STBOPD at 62 percent watercut via 27 active producers and was being supported by a combination of natural aquifer influx and by 8 active injectors (Figure 3). The cumulative field production from the three main fault blocks was 834.4 MMBBLS of oil, representing approximately 60 percent of the estimated original oil in place of 1362 MMSTB.

Following the loss of Piper Alpha and the subsequent assessment of feasibility studies, a strategy for the redevelopment of the Piper Field was defined. Redevelopment of the reservoir was initiated with the predrilling of eight wells from a subsea template during 1990 and 1991. The shutdown period afforded the opportunity to gather additional information during the predrilling program and to improve the characterisation of the Piper reservoir. The geological model was reappraised and the in-house simulation model updated to incorporate the refined characterisation. Limitations and uncertainties in the existing model which included the definition of the high permeability layers and the differential strength of attachment of the aquifer around the field were addressed. A data gathering program to evaluate the impact of fluid redistribution during the shut in period, relative permeabilities at high water saturations and residual oil saturations was also initiated.

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