9th July 2019
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Lancaster is the first UK oil field to be put into production and is currently producing from two 1km long horizontal wells tied back to a floating production and off take vessel. Initial average daily production is constrained to 20,000 barrels of oil per day, however the vessel has capacity to handle 30,000 bopd. The Lancaster field is on the Rona Ridge, West of Shetland. The Rona Ridge comprises a narrow (<15km wide) upfaulted elongate ridge, ca 200Km long, of naturally fractured Neoarchean granitic-tonalitic orthogneisses of the Lewisian Gneiss Complex. The Lewisian gneiss complex is overlain with a diverse Palaeozoic to Cenozoic sedimentary succession. The primary source rocks are Kimmeridge Clay, shales located within the Faroe – Shetland Basin (Figure 1).
Figure 1. The Lancaster field in relation to the Rona Ridge, Faroe-Shetland Basin and other Hurricane assets (red boxes).
Figure 2. Nw-se cross section across the Lancaster field portraying the key elements of the petroleum system
Lancaster is a conventional reservoir in that it can be produced without artificial stimulation and the trapping mechanism requires a seal of low permeability rock. Top and side seal to the Lancaster field is provided by Late Cretaceous shales and to the south east of the field the Brynhild Fault Zone (Figure 3).The north west of the field is constrained by the block boundary. The Lancaster host rock is an igneous protolith, circa 2.7 billion years old, that supports a natural network of hydraulically conductive fractures (fracture network). The protolith has undergone episodes of regional metamorphism up to granulite facies prior to fracturing. The fracture network has then developed through a variety of deformational and diagenetic processes including cooling, faulting, hydrothermal fluid ingress and fissure development associated with rifting and near-surface diagenetic processes.
The reservoir is classified as a type 1 Fractured reservoir in that the effective porosity and permeability is provided by a fracture network with no material contribution from intergranular or intercrystalline porosity. Hurricane’s exploration model is that Lancaster forms a continuous reservoir with the contiguous discovery Halifax and that the overall trap is formed by a buried hill draped by Cretaceous mudstones and laterally sealed by the Westray and Brynhild Fault Zones (Figure 3).
Figure 3. 3D rendition of the basement top surface portraying Hurricane’s basement assets. Hurricane’s exploration model is that the Lancaster field and Halifax discovery are a single hydrocarbon accumulation.
It is acknowledged that many type one fractured reservoirs provide drilling and data acquisition challenges atypical to conventional reservoirs having intergranular porosity and permeability. The bulk of these challenges are caused by formation invasion by drilling fluids. Lancaster is no exception and the inherent challenges are compounded by Hurricane’s drilling strategy that utilises over weighted brine and a philosophy of drilling with losses. Whilst this approach achieves a safe and efficient means of drilling the resultant deep and pervasive invasion precludes the use of resistivity logs to estimate water saturation and can also, preclude the use of wireline-based pressure to establish formation pressure. Hurricane has developed formation evaluation strategies designed to overcome the limitations of deep invasion and have acquired sufficient data to model both the initial oil in place and forward production to the standards provided by PRMS guidelines delimiting 37 million barrel 2P reserve and 486 million barrel oil equivalent base case (2C) contingent resource.
The Lancaster field is currently producing through a two well early production system which is designed to access a 37 million barrel volume over the next six years and to provide early dynamic data that will help to characterise the long term production characteristics of the reservoir. This lecture will provide an insight into Lancaster’s reservoir characteristics, how Hurricane has modelled the reservoir and what data acquisition/analysis programmes are being considered for the future.
The Geological Society of London
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