Oil & Gas - Maximising Recovery Programme (formerly SHARP) IOR Views

Background

Sand injectites are increasingly being recognized to be of far greater significance in the context of hydrocarbon exploration and production worldwide. Within the UKCS remobilization of deepwater Tertiary reservoirs is now recognized in a large number of Fields (e.g. Alba, Forth-Harding, Gryphon, and Leadon) and in Norway well known examples include Balder, Grane, and Jotun (http://www.abdn.ac.uk/~wpg010/isghome.php).

The increased recognition stems partly from greater awareness of diagnostic features on seismic and in core. Increased resolution of 3-D seismic data, for example, allows features associated with sand injection to be identified, both in producing fields and as potential exploration targets (earlier this year Marathon and their partner Lundin drilled the first ever deliberate exploration well into an injectite complex, Norway 24/9-7, discovering a >100m oil column). However, at present there is no systematic approach taken to reservoir modelling of the geometries of remobilized sand bodies because seismic resolution is incapable of defining intrusive geometries with sufficient detail.

Many of the challenges associated with the producibility of remobilized sandstones are related to understanding their rock properties – are they sand-prone? – what is the N/G? – what is the lateral continuity? – how well are sand bodies connected? – how can the sand bodies be represented in geomodels? - can pre-drill facies prediction be improved? All these issues are intractable using subsurface data (seismic, borehole and dynamic data) in isolation; however, by acquiring appropriate outcrop data and applying these data to subsurface modelling, we believe that more robust prediction will be possible.

As development proceeds there is substantial evidence that a failure to recognize the importance of sand injection may compromise the optimization of recovery. As sand injectites often extend 10s to 100s meters above “conventional” top reservoir maps (as picked on seismic) substantial reserves may be overlooked. In this context, top reservoir seals are often much higher in the stratigraphy than originally conceived, which leads to reconsideration of hydrocarbon migration pathways and the integrity of top seals, for example.

Figure 1: Oblique view in GeoViz of system of conical sandstone intrusions each with ~1km diameter in the Chestnut Field, UKCS   (Huuse et al. 2005).

Aims of the Research

The aims of the research are:

1. To synthesise the vast amount of data already accumulated from reservoirs where the effects of remobilisation are well known.
2. To define classes of reservoir scale sandstone intrusion (e.g. conical sheets and wings, with seismic identification criteria for each class (e.g. Figures 1 and 2).
3. To define core recognition criteria, with guides to separating injection and remobilization facies and discrimination from mass flow deposits.
4. To define textural (poroperm) and petrophysical differences between injectite facies and depositional facies to aid in log interpretation and reservoir modelling.
5. To compile detailed descriptions of outcrop analogues of the UKCS remobilized sandstone reservoirs to aid in reservoir modelling (dimensions, continuity, connectivity, N/G etc).

 

Figure. 2: Wireline log to seismic calibration for intrusive sandstones of reservoir scale: Chestnut Field UKCS from Huuse et al. (2005). These show the typically complex geometries involved in remobilized and intrusive reservoirs.

Workflow

Our current sponsor group (DTI, Marathon, Total, DONG and Lundin) has provided support for a three year programme. The project will consist of two main strands: a desk study based on published/released well and seismic data, and a primary research investigation of outcrop analogues in SE France, California, Greenland, Tunisia and South Africa. The outcrop analogues have been selected on the basis that they come closest to the context and scale of the remobilized systems in the UKCS Tertiary reservoirs. Outcrop geometry and physical properties descriptions will be input into seismic modelling programme to produce reference sets for comparison with subsurface examples.

We will construct static reservoir models, with seismic analogues, and aim to up-scale these into dynamic models. Given the likely diversity of sponsor geo-modelling procedures and requirements we intend generating generic templates that are amenable to integration into any modelling package.

Deliverables

Annual sponsor meetings/workshops will be convened to monitor progress and to facilitate networking between sponsors. A summary classification of geometries and statistical data will be presented as an outcrop characterisation atlas. A fully illustrated database and classification of sand injectites will be presented together with comparisons of subsurface occurrences of similar features. Particular attention will be given to the spatial distribution of injectites, variations in their volumetric distribution and their geometry, and the relationship to depositional and tectonic setting.

A workflow for integrating outcrop data into seismic, and static and dynamic reservoir models will be provided together with the results of selected case studies. All appropriate outcrop data will be presented as 3-D geomodels.

Expected Benefits and Impacts

A number of economically important reservoirs in the UK have been substantially remobilized from their original depositional configuration. Understanding the processes and products of this remobilization is fundamental to the optimum development of these fields. This project will fill a substantial gap in current knowledge of these reservoirs through integrating outcrop analogues with subsurface case studies. Our research will clarify the mechanisms by which remobilisation occurs, thus providing a physically rooted methodology for modelling these complex reservoirs. We will also provide actualistic models based on outcrop reservoir characterization to constrain reservoir models.

Selected Publications by the Principal Investigators

Duranti, D. & Hurst, A. 2004. Fluidisation and injection in the deep-water sandstones of the Eocene Alba Formation (UK North Sea). Sedimentology, 51, 503-529..

Duranti, D., Hurst, A., Bell, C. Goves, S. & Hanson, R. 2002. Injected and remobilised sands from the Alba Field (Eocene, UKCS): core and wireline log characteristics. Petroleum Geoscience, 8, 99-107.

Hurst, A. Cartwright, J.A. & Duranti, D. 2003. Fluidisation structures in sandstone produced by upward injection through a sealing lithology. In: van Rensbergen, P., Hillis, R. R., Maltman, A. J. & Morley, C. K. (eds) Subsurface sediment mobilization. Geological Society, London, Special Publications, 216, 123-137.

Hurst, A., Cartwright, J.A., Duranti, D., Huuse, M. & Nelson, M. 2005. Sand injectites: an emerging global play in deep-water clastic environments. In: Doré, A.G. & Vining, B. (eds) Petroleum Geology of NW Europe: Proceedings of the 6th Conference, Geological Society, London, in press.

Hurst, A., Cartwright, J., Huuse, M., Jonk, R., Schwab, A., Duranti, D. & Cronin, B. 2003. Significance of large-scale sand injectites as long-term fluid conduits: evidence from seismic data. Geofluids 3, 263-274.

Huuse, M., Duranti, D., Steinsland, N., Guargena, C. G., Prat, P., Holm, K., Cartwright, J. A. & Hurst, A. 2004. Seismic characteristics of large-scale sandstone intrusions in the Paleogene of the South Viking Graben, UK and Norwegian North Sea. In: Davies R. J., Cartwright J., Stewart S. A, Underhill, J. R., Lappin M. (eds) 3D Seismic Technology: Application to the Exploration of Sedimentary Basins. Geological Society, London, Memoir, 29, 263-277.

Huuse, M., J. Cartwright, R, Gras, R., and A. Hurst, 2005a, Km-scale sandstone intrusions in the Eocene of the Outer Moray Firth (UK North Sea): migration paths, reservoirs, and potential drilling hazards, in A. G. Doré, and B. Vining, eds, Petroleum Geology of NW Europe: Proceedings of the 6th Conference, Geological Society, London, 1577-1594.