11th February 2020
The Finlaggan Field is located 200km northeast of Aberdeen, 17km southwest of the Britannia Field, and will be developed via a two well subsea tieback to Britannia with first gas in Q4 2020 (Figure 1). Finlaggan was discovered by ConocoPhillips in 2005/6 by wells, 21/5a-6z and 6x, which encountered gas condensate in a layered section of Lower Cretaceous turbidites of the Britannia Sandstone Formation. Data obtained in these wells included a mixed set of logs, core, pressures and samples, although the dataset was hindered by poor depth control and lack of seismic calibration. In 2011 ConocoPhillips relinquished the discovery to focus on shallower accumulations in the area.
The Finlaggan trap is a two way structurally enhanced stratigraphic closure against the Rattray High (Figure 2), a Middle Jurassic volcanic-cored high that formed a positive feature from Jurassic into Chalk times. The southern boundary of the trap is formed by a long lived west to east orientated strike slip fault that extends across the Rattray High.
In 2015 Zennor acquired the 21/5c block and in 2016 drilled a vertical appraisal well, 21/5c-7, to establish the southern extent of the field. A walkaway VSP was run to provide seismic ties to the key reservoir units and give important clarity on trap geometry. The sidetrack, 21/5c-7z, proved reservoir deliverability, flowing at an equipment constrained rate of 40 mmscf/d and 3150 bbls/d.
The Britannia sandstone comprises a series of high density turbidites which were deposited rapidly and are typified by various dewatering features. The sands are fine grained and contain significant detrital clay in places. They are interbedded with pervasive marine shales, tuffs and occasional debris flows. The individual sands are assumed to be sourced through the regionally extensive “Kopervick Fairway” via fill and spill of the minibasins linked to the Halibut Horst and influenced by the Rattray High. Additionally, an important secondary input from the Britannia area to the north is recognised in Finlaggan, where a biostratigraphically constrained chemostratigraphic study identifies key differences between sands. Notably the recognition of the distinctive chemical signature of the main reservoir unit which is depleted in K-Spar relative to the other sands.
The sub-regional understanding of the depositional system which links the sand architecture into the offsetting fields has provided insights into the geometries of sand pinchout onto the Rattray High, the connectivity of the key reservoir units, and the differential depletion that has occurred in the play fairway. In conjunction a seismic reprocessing project generated a dual azimuth preSDM volume and inversion via GRT processing, that has allowed Zennor to re-map the field (Figure 3). This formed the basis for a series of static models that quantify the effect of key variables including reservoir (quality, pinch out, connectivity), faulting and HWC.
In 2018 Zennor selected the Britannia Field as the export route and consequently took FID for the field. The development well programme executed in 2018/2019 was planned to address the various scenarios, mitigating against some of the risks highlighted at FID.
The first development well included a pilot to confirm reservoir presence and quality in the previously unpenetrated northern part of the field. Resistivity modelling updated using this pilot provided insight on the expected tool responses in these low contrast sand and shale sections. Consequently the lateral drilled off this pilot was successfully geosteered using Haliburton’s Earthstar tool which maintained the well in the key sand, the trajectory was then allowed to drop to penetrate deeper reservoir units. Wellsite stratigraphic techniques provided confirmation.
The second development well in the southern part of the field was designed to drill the reservoir section across a zone where a significant fault mapped at BCU was thought to compartmentalise the field (Figure 4). A number of scenarios were envisaged to pre-plan the well design for drilling across the area of this fault in order to accommodate the uncertain throw. However, the well drilled successfully across the anticipated fault entirely within the main reservoir indicating that this fault mapped at BCU was early and had been buried by the Valhall and lower Britannia reservoirs. Subsequent un-faulted upper reservoirs were deposited over the top of this infilled topography. The well was successfully geosteered to penetrate all of the targeted reservoir units, providing an along hole net pay of >1700ft. In addition, the Earthstar tool provided key information on reservoir thickness and geometry, allowing improved definition of post-well sand body mapping.
In 1Q 2019 the two development wells were each cleaned up with equipment constrained rates of 12mboe/d with gas left over perforations. The remapping that has followed post development drilling has produced new static and dynamic models though post drill recoverable volumes are within 5% of pre-drill estimates. Field commissioning and first gas are scheduled for 4Q 2020.
Figure 1: Finlaggan Field location map
Figure 2: Finlaggan Field Net Hydrocarbon Thickness Map superimposed upon Depth to Top Reservoir, illustrating, FWL, Pinch out, position of wells and lines of section.
Figure 3: A NW-SE seismic line from the Dual Azimuth GRT PSDM (Quadrature trace) through the 21/5a-6Z and 6X wells. For location see Figure 2.
Figure 4: Section along the well path of the second development well 21/5c-F2X, with the wells displayed on the Seismic. Location shown on Figure 2
This event is kindly sponsored by Zennor Petroleum Limited
The Geological Society of London
+44 (0)20 7434 9944