Material Balance: A Powerful Tool for Understanding The Early Performance of The Schiehallion Field

Abstract

Abstract The value of using simple spreadsheet material balance models to gain understanding of field dynamic performance in advance of more complex field simulation has been well proven in the Schiehallion field, West of Shetland. This paper substantiates this claim by describing why this ‘low tech’ approach was considered necessary, how it was applied, and how it has helped. Schiehallion came on production in July 1998, and it soon became apparent that dynamic performance differed from that predicted prior to development. An explanation had to be sought in both the static and production data. Simulation predictions were clearly incorrect, and reservoir understanding was necessary in order to correct the model. Material balance was used because of its simplicity and the rapidity with which indicative results could be obtained. The field was subdivided into regions based on the producers and their supporting injectors and material balance models were constructed for each one. These were matched to continuous bottom hole pressure data collected from permanently installed gauges, when the wells were not flowing, or flowing with sufficiently low rates that well losses could be neglected. The models were used to test possible interpretations of the dynamic performance. The paper describes and illustrates these simple models, how they were used, and the results they achieved. As a result, matching of the simulation model was made easier, injection strategy could be formulated with more certainty and infill options started to emerge. The simple models have stood the test of time well, but the need for a more sophisticated tool is becoming more evident. Background The Schiehallion field lies in water depths of up to 500m and is situated some 200km west of the Shetland Islands. Schiehallion was discovered in 1993 and appraised by six more wells. These demonstrated that the net oil sand could be accurately predicted from seismic data and an Extended Well Test (E.W.T.) demonstrated that good production rates (>18mbd) could be obtained and suggested that the reservoir was well connected over a wide area. The Schiehallion Palaeocene turbidite reservoir sands (T34, T31a and T31b) lie at a depth of 1800–2064m (~1800–2000ms TWT). The producing wells are placed horizontally in the 10–50m thick sand bodies to ensure that 300–1000m of net rock is contacted so that the wells produce at sufficiently high rate. The initial reservoir pressure is close to bubble point. Near vertical water injectors are employed to provide sweep and maintain pressure to avoid excessive gas breakout. Effective pressure maintenance relies upon the presence of good hydraulic connectivity existing between injectors and producers. The variety of sedimentology in the turbidite package gives rise to a wide range of producer-injector communication. The producing and injecting wells are drilled from subsea drill centres. First oil was brought through flowlines from these drill centres into the purpose built Schiehallion F.P.S.O. (Floating Production Storage and Offloading) vessel in late July 1998. All producers have functioning bottom hole and wellhead pressure-temperature gauges. The injection wells all have functioning wellhead pressure-temperature gauges.