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Can producing oil store carbon? Greenhouse Gas footprint of CO2EOR, offshore North Sea R Jamie Stewart, and R Stuart Haszeldine Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 19 Mar 2015 Downloaded from http://pubs.acs.org on March 20, 2015

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Environmental Science & Technology

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Can producing oil store carbon? Greenhouse Gas

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footprint of CO2EOR, offshore North Sea

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R Jamie Stewart*, R Stuart Haszeldine

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Scottish Carbon Capture & Storage,

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School of GeoSciences, University of Edinburgh,

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EH9 3JW, Scotland, UK

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*Corresponding author – email: [email protected], phone: +44 (0)131 650 5936,

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fax: +44 (0)131 650 7340

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1 ACS Paragon Plus Environment


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Abstract

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Carbon dioxide enhanced oil recovery (CO2EOR) is a proven and available technology used

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to produce incremental oil from depleted fields whilst permanently storing large tonnages

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of injected CO2. Although this technology has been used successfully onshore in North

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America and Europe, there are currently no CO2EOR projects in the United Kingdom. Here,

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we examine whether offshore CO2EOR can store more CO2 than onshore projects

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traditionally have, and whether CO2 storage can offset additional emissions produced

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through offshore operations and incremental oil production. Using a high-level Life Cycle

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system approach, we find that the largest contribution to offshore emissions is from flaring

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or venting of reproduced CH4 and CO2. These can already be greatly reduced by regulation.

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If CO2 injection is continued after oil production has been optimised, then offshore CO2EOR

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has the potential to be carbon negative - even when emissions from refining, transport and

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combustion of produced crude oil are included. The carbon intensity of oil produced can be

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just 0.056-0.062 tCO2e/bbl if flaring/venting is reduced by regulation. This compares against

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conventional Saudi oil 0.040tCO2e/bbl, or mined shale oil >0.300tCO2e/bbl.

28 Abstract Art 29 2 ACS Paragon Plus Environment

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Introduction

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Carbon Dioxide Enhanced Oil Recovery (CO2EOR) may be an option for reducing the cost of early-

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stage carbon capture and storage (CCS) as more and more projects around the European Union (EU)

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utilising saline aquifer storage are deemed too expensive and fail to go beyond Front End

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Engineering Design (FEED).

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CO2EOR is a technique that has been applied in the United States (US) since the mid 80’s to enhance

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oil recovery from mature fields.1 In the United States where large volumes of CO2 are readily

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available from natural sources, CO2 has been injected as the primary recovery agent in these EOR

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operations. In Europe however the lack in availability of low cost CO2 has meant that EOR operations

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have conventionally used water injection or methane gas injection, with currently no CO2EOR

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projects in an offshore environment.2-5 With the prospect of Carbon Capture technology developing

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across the EU however, the required volumes of CO2 may become available through anthropogenic

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sources.6 CO2EOR is regarded as an option for storing large volumes of this captured CO2 with the

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added benefit of improving the recovery rate from depleted oil fields. This oil production may hold

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potential to improve CCS project economics and get projects beyond FEED.

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However it is known from currently operating CO2EOR projects onshore that the operations and

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processes involved in CO2EOR are energy intensive and could result in significant atmospheric

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emissions.7,8 This paper presents the conclusions from a medium to high level life cycle assessment

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of CO2EOR operations, in which emissions and CO2 stored are modelled for a theoretical offshore

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North Sea project. Consideration of the uncertainties involved in operating a CO2EOR project in an

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offshore environment means a number of scenarios and uncertainty ranges are presented. Particular

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attention is paid to the flaring and venting of reproduced gases which is found to have a large

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control on the emissions associated with the production of oil through CO2EOR.



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Several other studies have attempted to quantify how the emissions from EOR operations, and from

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the final combustion of the petroleum products, relate to the total volume of CO2 stored in EOR

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projects.9-12 These studies focus on historical data from onshore developments and vary widely in

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their outcomes.

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The primary reason for the varying conclusions relates to inclusion or exclusion of the large

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contribution to emissions made by the combustion of produced petroleum products, and the

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principal of additionality. Jaramillo et al., (2009)10 who looked at 5 onshore North American CO2 EOR

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projects found that when emissions from the full system boundary, from coal mining to final product

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combustion are included, then onshore CO2 EOR projects have historically been net emitters of CO2.

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They assumed that oil produced through CO2EOR is additional to the global system and therefore

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emissions from the combustion of the final petroleum products should be included in the study,

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which resulted in overall net emissions.10,11 Other studies such as Faltison and Gunter., (2011)12 who

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have analysed emissions and CO2 stored at 8 onshore US CO2EOR fields, argue that oil produced

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through CO2EOR will displace oil produced through other sources and emissions from final product

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combustion should therefore not be included.12

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In this work, the net carbon balance of CO2EOR is examined with results from both including and

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excluding emissions from end product combustion. To date only one study by Hertwich et al.,

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(2008)13 has attempted to model offshore EOR in the North Sea to quantify emissions from the EOR

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process. Although they model a theoretical development in a similar fashion to this study only one

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CO2 storage scenario was considered within the EOR modelling. Hertwich et al., (2008)13 also did not

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consider emissions from flaring/venting of recycled CO2 and CH4 which we believe may be of

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significant importance to the carbon balance of a CO2EOR project.

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Alongside quantifying the emissions specifically related to operating a CO2 EOR project offshore this

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paper aims to assess the carbon budget performance of a realistic offshore CO2EOR operation with

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regard to both incremental oil produced and CO2 stored for two EOR scenarios. A common way to 4 ACS Paragon Plus Environment

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assess the storage performance of an operation is to assess how many tonnes of CO2 are stored per

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barrel of incremental oil produced. Established onshore projects have stored 0.160-

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0.300tCO2/bbl8,10,11 (3.0-5.7 Mscf/STB). However a number of studies have estimated that CO2EOR

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operations may store between 0.300-0.600tCO2/bbl (5.7-11.3Mscf/STB) if optimised to store

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CO2.9,13,14 This study highlights that gross storage rates (not incorporating emissions) at North Sea

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CO2EOR developments may be significantly higher than storage rates seen at traditional onshore

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CO2EOR developments.

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injection beyond the time period needed to optimise oil recovery may be needed to give a North Sea

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CO2EOR project a negative carbon balance.

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Although it is technically useful to estimate the storage factor (CO2 stored/bbl of oil produced) and

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net carbon balance of a project, it must be noted that the assessment of CO2 stored in the carbon

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balance of a CO2EOR development is based on a number of important assumptions. The most

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fundamental of these assumptions is the counterfactual that CO2 which is being stored in the EOR

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operation would otherwise have entered the atmosphere. This assumption allows the CO2 storage

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element of an operation to be seen to offset the operational emissions and end use emissions of the

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produced crude (not including emissions from CO2 capture plant and resource mining etc.). If

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however the CO2 that is stored within the EOR development would have otherwise been captured

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and stored using pure CO2 storage (no EOR) then the carbon balance calculation should not

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incorporate the CO2 stored in the EOR development. In this study the former of the two scenarios is

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used to estimate the net carbon balance. This issue will be examined further in the discussion

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section of this paper.

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It is also proposed by a number of studies10,15 that an effective way of assessing the climate benefits

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of CO2EOR is to estimate the emissions associated with the production of every barrel of oil

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produced (tCO2e/bbl). This can then be compared to production of oil from other sources. This paper

Interestingly however the study also highlights that CO2 import and



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presents estimations of the carbon intensity of oil produced through CO2EOR in the UK North Sea for

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a number of different EOR scenarios.

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Designing the system boundary

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To complete a life cycle assessment of CO2EOR operations, a system boundary must be drawn which

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identifies the processes, activities and materials used within the operation. Once these activities

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have been identified the emissions associated with each process flow can be modelled.

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Although CO2EOR relies on many associated activities such as resource mining for construction of a

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power plant, carbon capture at a power plant, CO2 transport, crude oil refining and crude oil

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consumption, these processes are all common to either CCS with aquifer storage or conventional

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crude oil production. The production operations which relate to processes and activities that take

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place at the production platform are however unique to CO2EOR projects. For this reason a number

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of studies have attempted to address this phase of the CO2EOR chain.7,8,13 Advanced Resources

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International (2010)14 found that in traditional onshore projects, upstream CO2EOR operations are

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dominated by three energy demanding processes. CO2 compression has the largest contribution to

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energy demand and therefore associated emissions. Although variable from project to project, gas

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separation and artificial lifting also significantly contribute to the energy demands of CO2EOR

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operations.14 Hertwich et al., (2008)13 found that the emissions associated with these energy

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intensive processes were largely controlled by whether equipment was powered by gas/diesel

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turbines or connected to a larger electricity grid. This paper also presents the results from a ‘gate to

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gate’ LCA where modelling focused on the offshore production operations, but also incorporates

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flaring/venting emissions and fugitive emissions. The results from this work may be integrated with

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a broader ‘cradle to grave’ life cycle assessment (LCA) for the full chain as diplayed below in figure 1.

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This study includes the emissions associated with aspects of the construction process but focuses

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on the operational phase. Emissions associated with site evaluation and characterisation,


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construction, closure and post closure monitoring have been found by Dilmore8 to contribute less

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than 1% of green house gases emitted in association with CO2EOR activity. The remaining 99% can

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be attributed to the operational phase of the project.

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The CO2EOR operation modelled is based on a theoretical oil field development that has experienced

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secondary production (water flood). Therefore much of the infrastructure that would be required to

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develop a green field development is already in place. Only processes and infrastructure associated

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with incremental tertiary production are recognised within this study. For the same reason oil

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production included in this study relates only to incremental oil produced through CO2EOR activities.

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It is however recognised that base oil (non EOR oil) production may continue throughout the lifetime

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of the tertiary EOR operation. Although LCA’s are often compiled to cover many environmental

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impacts along a production chain, such as global warming potential, human toxicity, land use,

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pollutants and acidification, this study only accounts for the large volume greenhouse gases (see

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figure 1).

139 140 Figure 1 – Above- The CCS full production chain. This study will focus on the offshore production operations which are unique to a CO2EOR operation.

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Below- Simplified overview of the system boundary included in the study. CO2(r) and CH4 (r) relate to recycled gases. Input CO2 relates to CO2 imported from a carbon capture plant and input CH4 relates to natural gas that is imported to power production equipment. Only gases that are within the red box are included in this study.ACS Paragon Plus Environment


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Overview of the EOR Process

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The inputs to the CO2EOR system included within the system boundary are CO2 and CH4. Pure dense

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phase CO2 from an anthropogenic source is delivered to the platform through a dedicated CO2

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pipeline. A separate pipeline delivers CH4 to be utilised as a fuel gas to power offshore operations.

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Within this study the source and transport distance of the anthropogenic CO2 do not affect the CO2

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operations and therefore these parameters are excluded from this analysis. When transporting CO2

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by pipeline the CO2 is normally compressed to a pressure (+9.6 MPa) where it will be in a

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supercritical phase. This allows larger volumes of CO2 to be transported in a smaller diameter pipe.

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When delivered at the platform CO2 may have to be recompressed by additional CO2 pumps on the

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platform to the required injection pressure.16

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In the modelled scenarios, CO2 is injected continuously into the reservoir. Although established

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projects have injected water at specific intervals to sweep the reservoir, this study presumes no

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water injection is undertaken. For a time CO2 injection commences and all injected CO2 is stored

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without instantaneous enhanced oil production. After a period incremental oil production will occur

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due to the increase of pressure in the reservoir as a result of CO2 injection. However oil production

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does not solely increase due to the increase in pressure. At pressures above what is termed the

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minimum miscibility pressure (MMP) CO2 reduces oil viscosity, increasing oil mobility.17 This

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production, which is known as miscible CO2 displacement, results in dissolved miscible CO2 being

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produced at the production well, alongside some free phase CO2, after a period of time. The volume

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of CO2 reproduced at the production wells will increase over time until the gas recycle capacity of

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the facility is reached. It must be made clear that although the retention of injected CO2 will not be

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100% as CO2 recycling will inevitably occur, with historically around 50% of injected CO2 being


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retained in the reservoir,18 almost all of the imported CO2 (minus CO2 lost through fugitive emissions

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and flaring/venting) will be stored in reservoir. After CO2 breakthrough a three phase mixture of oil

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(with associated CH4), CO2 and water is produced at the platform. Using a number of processes the

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crude oil and water is separated from the CH4 and CO2 which is now in a gaseous phase. The crude

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oil can then be exported, and the water treated and disposed overboard. The operator can then

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choose to either re-inject the CH4 / CO2 mixture alongside the fresh CO2 delivered to the platform, or

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separate the CH4 from the CO2 to be used as a fuel gas or for export. A fourth phase of solid

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asphaltene precipitate may also be present.19

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In this process of CO2 injection and recycling, emissions to atmosphere come from a number of

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sources. Emissions of CO2 are released from production process equipment, which are assumed to

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be powered by gas turbines. Fugitive emissions of CO2 and CH4 relate to unintentional leaks from

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valves and seals. Emissions of CO2 and CH4 occur when produced gas is flared or vented during upset

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operating conditions or for maintenance. In some cases fugitive emissions can be used also to

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describe emissions from flaring or venting. Within this work they are regarded as separate entities.

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Methods

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Model design

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The study models an ‘anchor field’ development that would be the first CO2EOR project in an area,

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with a field big enough to accommodate the CO2 supply from a commercial size carbon capture

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plant.20 The theoretical field modelled here is designed to accept CO2 from a full scale carbon

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capture project with a gross power output of around 1GW and CO2 output of around 5Mt per

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annum. The CO2 supply of around 5Mt/yr is here considered to be required to allow EOR project

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economics to be justifiable. Two scenarios have been developed which represent CO2EOR operations

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with varying goals. EOR case 1 represents a scenario where the CO2 supply (5Mt/yr) is diverted to

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another field after 10 years and only the injection of recycled CO2 continues for a further 10 years.



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For economic reasons, this may be the most likely scenario for an operator focused on oil

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production, as continuation of CO2 import for a further period is here not modelled to increase the

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oil production profile (100MMbbl in both cases). EOR case 2 represents a scenario where CO2 is

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continuously supplied to the field for 20 years and is injected alongside recycled CO2. This scenario

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represents an injection strategy optimised for CO2 storage. The design assumptions such as recycle

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rate, percentage of reproduced gas flared/vented and percentage of CH4 in the reproduced gas

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stream remain constant in both scenarios.

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To allow estimates of both injected gas volumes and produced gas and fluids for each EOR scenario

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to be made, simplified reservoir numerical models were constructed in Microsoft Excel (See figures

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S1 and S2 in the Supporting Information). These models allow predictions of inputs and outputs

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within the system boundary on an annual basis. The primary input to the model is based on

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importing and injecting 5Mt (minus fugitive emissions) of CO2 annually with 100MMbbl of

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incremental oil being produced over the twenty year period. EOR case 1 is based on an oil

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production rate of 2bbl of incremental oil produced per tonne of CO2 imported. As discussed later in

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the paper other studies have found utilisation rates of between 1.7-6.5 bbl of incremental /tonne of

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CO2 imported.12,21,22 Here a rate of 2bbl incremental / tonne of CO2 imported is used as a

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conservative estimate. Although an oil production rate is estimated over the 20 year period it is here

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modelled to have no control on operational emissions as they are based on gas recycling rates and

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cumulative oil production. As described above EOR case 2 is based on importing and injecting 5Mt of

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CO2 for a further 10 years. However it is modelled here that because incremental oil production is

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already at its maximum, supplementary CO2 import and injection of CO2 will not lead to an increase

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in oil production. Details of the equations and assumptions used in the model for the two EOR cases

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can be found in the Supporting Information (Section S1).

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The basic outputs of the model which allow emission estimations to be made are annual volume of

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CH4 produced and the annual volume of CO2 recycled. The volume of CO2 recycled is based on a


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maximum recycle rate of 600mmscfd which is reached over a period of 5 years and then remains

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constant for the remainder of the 20 year time frame. The ratio of CH4 to CO2 in the reproduced gas

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was modelled to increase from 100% CH4 to 10% CH4 over a period of 5 years, with CO2 making up

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the remaining 90%. This assumption is based on work by Goodyear et al., (2011)16 who suggest that

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CO2 may make up 90mol% of the gas in the produced gas stream. An important assumption made

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within the models is that recycled volumes are constant in both EOR cases even though volumes of

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injected CO2 over the 20 year project life are different. This is based on the assumption that the

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maximum recycle rate will be reached and sustained in both EOR cases, and is not a direct function

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of injected CO2 volumes. Although these models were constructed through time, the results

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presented in this paper focus predominantly on the cumulative CO2 and CH4 emissions and CO2

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storage at the end of the 20 year project life. These outputs of volumes of recycled CO2 and CH4 are

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then be used to calculate emissions for each EOR case using formulas described in the Supporting

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Information (Section S2).

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Input Data

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Specicfic design related data was acquired after communication with a CO2EOR developer. For the

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modelled anchor project a dedicated gas supply pipeline is used to supply fuel for gas turbines. A gas

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turbine emission factor of 610g/kwhr was used.23 Since there are no current CO2EOR operations in

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the United Kingdom Continental Shelf (UKCS), assumptions have been made about the processes

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required for the modelled anchor project, and their associated energy requirements. Details of

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assumptions can be found in the Supplementary Information (Table S1). This study estimates that

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the power demand for additional CO2 infrastructure lies between 18-22kwhr/tonne CO2 recyled. This

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is slightly lower but broadly similar to the energy use figures stated by the Pembina Institute (2013)22

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who suggest a range of 35-120 kwhr/tonne CO2 recycled.

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A large percentage of the modelled operations energy demand comes from the recycle process

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(66%) (Table S1). Here the produced CO2 + CH4 mix is separated from the produced crude and brine



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and recompressed for injection. Additional CO2 pumps may also be required to increase the pressure

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of delivered CO2 to the required injection pressure (additional compression – Table S1). Fuel gas

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separation, although included in Table S1 is not included in the modelled scenarios. Here it is

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assumed that the CAPEX and module weight of gas treatment units, with current technology, is too

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high to be justified for an offshore environment, and therefore produced hydrocarbon gases will be

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re-injected.16

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required as after CO2 breakthrough the wells will ‘auto-lift’.16 This is the same assumption used by

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Hertwich et al. (2008)13.

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(Table S1) from steel required to build a new bridge linked platform are also incorporated and are

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displayed in the results section of this paper.

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Flaring / venting & fugitive emissions

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In this study an average figure for the venting/flaring of reproduced gas (CO2 + CH4) was calculated

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using flaring rates from UK North Sea oil fields in 2011 where on average 3.5% of produced gas was

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flared or vented, due to maintenance and safety reasons.24 Emissions associated with flaring and

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venting were calculated by applying this percentage to the volume of annually reproduced gas

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predicted in the Excel models described previously. This is a parameter that a number of other

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studies have disregarded when modelling CO2EOR.10-14 Due to the uncertainty in this value a range of

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flaring/venting rates of 1-5% is here used (See Supporting Information section S3 for further details

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on this parameter). The control of this parameter is also discussed in the Discussion section of this

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paper.

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Little data is currently available relating to fugitive emissions from CO2EOR projects. In Dilmore8 a

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range of 0-1% loss of purchased CO2 is assumed. Personal communication with US operators also

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revealed that an estimated 1-2% of purchased CO2 is lost to fugitive emissions. Here, total fugitive

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CO2 emissions are modelled to be 1% of CO2 imported to the platform. Given the large uncertainty of

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this parameter a range of 0.5-5% is used to give lower and upper estimates.

For simplification, all the modelled scenarios assume that ‘artificial lift’ is not

Emissions associated with drilling new wells and embedded emissions


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Crude transport, refining and combustion

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To quantify the emissions associated with the incremental oil production, a number of emission

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factors were extracted from the literature. Downstream emissions from crude oil production can be

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broadly grouped into three catagories; transport, refining and combustion. An emission factor of

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0.004 (0.0011-0.0119) tCO2/bbl was used for crude oil transport,14,25 0.03(0.027-0.05) tCO2/bbl for

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crude oil refining10,14,21,26,27 and 0.431 (0.428-0.433) tCO2/bbl for final product combustion.27 For

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further details and justifcation of the ranges used for each emission factor please see the Supporting

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Information (section S4).

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Results

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Cumulative emissions

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Total cumulative emissions for the 20 year project life time for both EOR cases are displayed in table

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1. Total emissions from EOR case 1 accumulate to 13.5 (5.2-20.5) MtCO2e over the 20 year life of the

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project. Total cumulative emissions from EOR case 2 although larger at 13.7 (5.5-23.1) MtCO2e, are

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relatively similar considering case 2 represents an additional injection of around 5MtCO2 per year for

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ten years. This similarity is due to the largest contribution of emissions arising from the recycle

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process which remains constant in both cases.

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As can be seen in both EOR scenarios venting of CO2 and CH4 has the largest contribution (~80%) to

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greenhouse gas emissions over the 20 year life time of the project. This is due to the high levels of

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CO2 in the produced gas stream preventing ignition as discussed in the Supporting Information.

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Emissions associated with drilling new wells, working over old wells and for the manufacturing of a

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new bridge link platform, which are assumed to all occur prior to CO2 injection, have a very small

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contribution to overall emissions at

Greenhouse Gas Footprint of CO2EOR, Offshore ... - ACS Publications

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