Abstract: Reservoir Geology and Enhanced Oil Recovery screening in Draugen
[For presentation at the Norsk Geologisk Forening Jan 8-10 Jan 2007]
Grant, D.¹, Carnell, A.², Harvey, C.¹, Ravnås, R.¹, Cook, A.¹, Jansen, R.¹
1) A/S Norske Shell, Tankveien 1, 4098 Tananger, Norway
2) Shell International Exploration and Production, Seafield House, Aberdeen, Scotland
Over the past thirteen years Draugen field’s waterflood
has successfully and predictably recovered 99 million m3
of oil through six platform based wells. Historic field
development has focussed on managing this waterflood
(with particular emphasis on 4D seismic) and realising
new oil through infill drilling; four subsea wells being
added in 2002. Initiation of the A/S Norske Shell - Statoil
integrated "Halten Value Chain" carbon capture project
has seen a refocused effort on full field reservoir
description and analysis on Draugen in light of a
potential revision of the primary recovery mechanism.
Currently in the early Identify and Assess maturation
stage, the key objective of this renewed reservoir
investigation is to understand the range of possible
recoverable volumes through significant field
redevelopment, with tertiary recovery, especially CO2
injection, being one of the key recovery mechanisms
evaluated.
The envisaged Halten Value Chain involves CO2 capture
from a planned power station at Tjeldbergodden and its
transportation offshore for injection into Draugen as an
EOR agent. Located in 270m of water some 150km north
west of the Tjeldbergodden on the mid Norway coast,
Draugen is not only geographically well placed as a
reception point for offshore CO2 injection but with a
rising watercut (54% at 01/10/06), is also at a particularly
suitable lifecycle point for significant redevelopment.
The construction of detailed 3D geocelluar models that
honour all of the available data is considered to be a
critical element in evaluating the range of recoverable
volumes for the level of investment proposed. A ground
up approach has been taken during the construction of
these 3D models integrating geophysical, petrophysical,
geological and engineering data to give the greater degree
of detail and understanding that is needed to fully realise
remaining in-field potential. This paper will outline and
discuss the key geological components of these formative
evaluations.
Draugen field comprises two separate reservoirs (the
Garn and Rogn Formations) which straddle the Middle to
Upper Jurassic (Bajocian to Volgian). Approximately
88% of the 210MSm3 STOIIP is contained in the
Kimmerigian to Volgian Rogn Fm. Geological
characterisation of these units has focussed on structural
evolution, depositional setting and reservoir architecture
with feedback loops between each of the disciplines
helping to refine the conceptual and detailed models.
Significantly improved structural understanding resulted
from interpretation of the high resolution 3D seismic
survey acquired in 2004, where hanging wall thickening
and syn-sedimentary fault block rotation can be seen on
the large Jurassic structural elements.
The redevelopment opportunity afforded a re-assessment
and integration of all the previous biostratigraphic and
sedimentological data, into a consistent correlation
framework hung around key flooding events.
Recognition and correlation of key flooding events based
on distribution analysis of palynological events has led to
a revised two fold subdivision of the Rogn Fm. Locally
this is capped by a transgressive lag sourced from subaerial exposure and erosion of local block tops,
significantly impacting reservoir quality but with an
uncertain distribution. These palynological event derived
biozones have been used to revise the relative timing and
focus of sedimentation as well as help fine tune the
depositional setting. A westerly sediment supply feeding
very proximal and localised deltaic shoreline bodies
focussed in topographic depressions is the currently
preferred Rogn depositional model.
The Bajocian Garn Fm is an easterly derived fluviodeltaic unit, which in Draugen can be divided into four
palynologically derived biozones. These units show a
varying degree of fluvial, deltaic and marine storm wave
influence which in turn impact the resultant reservoir and
aquifer architecture. Oxfordian drowning of the Garn Fm
was followed by deposition of the Lower Spekk Fm intra
reservoir organic-rich marine shale. The later Middle
Volgian flooding mirrors this as the Rogn Fm is
drowned, with varying rates of transgression being
interpreted from the well data.
Lastly, with thirteen years of production history and three
time-lapse 3D seismic surveys a further level of property
distribution control can be introduced to the geocelluar
model construction along with feedback from the static dynamic modelling iteration loop.