Published by the DTI Oil & Gas Directorate for the reservoir engineering and IOR community in the UK.
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Potential UKCS CO₂ Retention Capacity from IOR Projects

Eugene Balbinski (eugene.balbinski@ecl-winfrith.com) of ECL Technology Ltd summarises work on evaluating the potential capacity for CO2 sequestration in UKCS fields.  The study was undertaken for the DTI SHARP Programme and presented at the DTI's recent IOR Research Dissemination Seminar (Authors: Matthew Goodfield and Claire Woods)

Tertiary CO₂ injection in onshore North American fields is a successful IOR technique, where the CO₂ is sourced from natural CO₂ reservoirs. Concern over greenhouse gas emissions has renewed interest in the potential for CO₂ flooding in the UKCS. CO₂ would be captured onshore, e.g. from power station emissions, and injected into UKCS reservoirs to improve recovery and leave CO₂ trapped in the reservoirs at the end of project life. The overall UKCS potential for CO₂ injection has been evaluated based on estimates of CO₂ IOR potential from a systematic screening exercise of all UKCS fields in the early 1990s.

The current UKCS IOR potential is in the region of 600 MMSTB (range 350-850 MMSTB) from WAG schemes and 1100 MMSTB (range 800-1400 MMSTB) for GSGI schemes. The potential decreases with increasing time as fewer fields remain on production. The net CO₂ retention capacity for the WAG schemes is currently around 150 million tonnes (2700 Bscf), whereas that for GSGI schemes is around 550 million tonnes (10000 Bscf). These results reflect the superior CO₂ utilisation efficiency in WAG schemes compared to GSGI projects.

Cumulative UKCS WAG IOR Potential for CO₂ Injection
There are currently around 60 potential WAG projects and significantly fewer GSGI opportunities. Despite this, the CO₂ retention potential from GSGI injection is approximately a factor of 3 larger than that for WAG injection, because the projects are generally bigger.

A WAG project could be implemented at any time between the present and the pre-COP deadline, because the well pattern for water-alternating-gas injection is consistent with that for water flooding. In contrast, the well pattern for GSGI is fundamentally different to that for water flooding.  The ‘window of opportunity’ for the start of each GSGI project might, therefore, be taken as a limited period around the COP date. An example GSGI CO₂ usage profile based on this principle shows a peak capacity of approximately 35 million tonnes per year, corresponding to 7% of the total UK CO₂ emissions for 1999.

Cumulative UKCS GSGI IOR Potential for CO₂ Injection
It may not be necessary to maintain miscibility in a GSGI project.  Dropping the reservoir pressure before injection commences potentially improves gravity stability and the net gas utilisation efficiency. Several fields might benefit from this type of pressure management. If the CO₂ from a GSGI project had re-sale value for use in future IOR projects, the field could be depressurised or dump-flooded with water at the end of life and the CO₂ could be recovered via wells at the top of the structure.

WAG schemes typically end with a water post-flush, but it might be more attractive to leave as much CO₂ as possible in the formation if sequestration had commercial value. The overall CO₂ sequestration capacity of WAG projects might be increased by a factor of two if this strategy was adopted.

Use this link to access the full paper:

www.dti-sharp.co.uk/dissemination/openreports/UKCS_cap_paper_02.pdf

Potential UKCS CO₂ retention capacity from IOR projects, Matthew Goodfield and Claire Woods DTI Improved Oil Recovery Dissemination Seminar, Aberdeen, June 2002.

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