European carbon storage resource requirements of climate change mitigation targets 2

35 As a part of climate change mitigation plans in Europe, CO2 storage scenarios have been reported for 36 the United Kingdom and the European Union with injection rates reaching 75 – 330 MtCO2 yr by 2050. 37 However, these plans are not constrained by geological properties or growth rates with precedent in 38 the hydrocarbon industry. We use logistic models to identify growth trajectories and the associated 39 storage resource base consistent with European targets. All of the targets represent ambitious growth, 40 requiring average annual growth in injection rates of 9% – 15% from 2030-2050. Modelled plans are 41 not constrained by CO2 storage availability and can be accommodated by the resources of offshore UK 42 or Norway alone. Only if the resource base is significantly less, around 10% of current estimates, does 43 storage availability limit mitigation plans. We further demonstrate the use of the models to define 2050 44 rate targets within conservative bounds of both growth rate and storage resource needs. 45


INTRODUCTION 67
Very large-scale carbon capture and geological storage (CCS) may be needed to mitigate 68 climate change 1,2,3,4,5,6,7 . Assessments of technological pathways available for limiting global warming 69 to less than 1.5 o C and 2 o C suggest that CO2 may be injected underground at rates of 10 Gt per year by 70 mid-century, and that >1000 Gt will need to have been stored by the end of the century 7 . This is a 71 similar scale to that of the fluids currently being handled by the hydrocarbon industry globally 8   identify a storage resource base between 10,000 -30,000 Gt available worldwide 17,18,19 . This is 83 potentially 3 -10 times more than the maximum global storage resources needed to support the 84 most aggressive CO2 storage scaleup trajectories identified by the IPCC, limiting global warming to less 85 than 2 o C 20 . The combined estimate of effective storage resources in Europe is 260 Gt, including 86 resources distributed among EU member states for both onshore and offshore (88 Gt in total), and 87 offshore UK (78 Gt) and Norway (94 Gt; Figure 1) 19,21,22,23,24,25 . However, concerns over onshore 88 storage of CO2 in the EU, e.g., by the European Commission 26 , could limit its use to offshore resources 89 alone (19 Gt, or 22% of the total, for EU member states 21, 27,28,29 ).

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At the same time, there are significant uncertainties in the scaleup of CCS to achieve climate 91 change mitigation targets. It has been recognised that current integrated assessment models (IAMs) 92 that identify scaleup trajectories for CO2 storage contain gaps in the representation of realistic 93 consumption of depletable natural resources 30, 31 . The constraints used in IAMS to determine 94 deployment projections of technologies are predominantly costs. For CCS, while some IAMs do 95 include an upper limit on available storage resources or a maximum injection rate, these are only 96 single-value constraints 5, 32 . As a comparison, the upscaling of other low-carbon technologies i.e., solar 97 and wind technologies in IAMs are constrained by a historical annual growth limit i.e., 10% yr -1 32 . geological features alone are inherently uncertain and can range in up to two orders of magnitude. As 100 a result, geological considerations alone are insufficient to describe actual development trajectories 101 of CCS. Rather, there is a range of factors that could potentially limit the growth of subsurface storage 102 sites, including the geophysical limit to the injectivity rate of CO2 as a result of the pressurisation of 103 the reservoir in repose to injection, latencies in project development, i.e., the discovery and appraisal 104 of suitable injection sites, and a combination of economic, social, and political constraints 105 33, 34,35,36,37,38,39,40,41 . The amount of CO2 stored underground in IAMs or regional or national energy 106 systems models does not reflect attributes from these potentially limiting processes 42 .

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In this work, we make use of the logistic growth model -a simple framework that has been 108 widely used in analogous industries, like the oil and gas industry, to investigate plausible growth

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The European Commission strategic long-term report, 'A Clean Planet for All', outlined the 168 decarbonization pathways for the EU to achieve net-zero commitments 9 . In this report, three CO2 169 storage targets stating that in 2050, injection rates of 80, 92, and 298 MtCO2 yr -1 will be necessary to MtCO2 yr -1 for 2050 13 . These four annual storage rate targets were determined based on the EU28 174 prior to the exit of the UK from the EU and are subsequently referred to as EU1-4 ( Fig. 1

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In this study, we will evaluate a group of scenarios we refer to as the 'UK Domestic Scenarios' 204 and identify growth rates and the storage resource requirements for UK storage targets. We also 205 evaluate the capability of the UK carbon storage resource to act as a regional CCS hub, servicing 206 additional storage needs from the EU in a group of scenarios we refer to as 'EU + UK Scenarios'. We 207 identify a range of annual growth rates and the necessary storage resource base required to achieve 208 these scenarios.

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In this context, the model can be used to approximate the relationship between the growth needed 240 to achieve near-term scaleup targets and the resource base that would be required to support that 241 growth which is key for understanding the deployment trajectory of CCS. Thus, another reason we do 242 not use linear or exponential models here is that they cannot capture the relationship between early 243 rates of growth and the available storage resource base.

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As aforementioned, a variety of logistic-like curve-fitting models exist, i.e., Gaussian, and 245 normal curves. The differences between these models are significant in their ability to fit existing data 246 or when used to predict future production and peak years 52,55 . However, our purpose here is to 247 explore a range of regional short-term growth trajectories of CCS that are dependent on fixed

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We numerically solve Equations 1 and 2 to meet climate change mitigation targets for a 282 region. This identifies rate and cumulative storage trajectories that meet proposed plans. Iterating 283 over a range of parameter space of storage resource requirement and initial (exponential) annual 284 growth rate allows us to identify the scenarios over which these plans may be achieved. From this, 285 minima in the initial growth rate that is supported by the maximum storage resource available can 286 also be identified.

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The logistic modelling framework is ultimately a statistical model and comes with associated

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Second, the lack of data for storage resources, or deployment plans for CCS in some regions means 292 that there is a limit to the spatial resolution that can be achieved, e.g., we did not find it useful in

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MtCO 2 yr -1 ) and UK3 (175 MtCO 2 yr -1 ). The modelled scenarios identify a group of minimum growth 308 rates supported by the maximum storage resource available (88 Gt for the EU or 78 Gt for the UK) to meet the storage targets of the respective region. However, CO2 storage resource assessment is also 310 uncertain to over an order of magnitude 19 . Thus, an additional conservative group of higher growth 311 scenarios that depend on only 10% of the currently identified storage resources are also identified.

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The inflection year of each growth rate curve indicates the duration of exponential growth since 2030.

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In Figure 2, we use a solid line for the part of the trajectory where storage rate growth is close to 314 exponential. Beyond the inflection year, the trajectory is dashed to emphasise that these are not 315 predictive growth trajectories but rather are used to identify the resource base required to support 316 the early growth.

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The coloured point corresponds to a single trajectory, i.e., Fig.3 at 2030 on the cumulative graph in Figure 5). We show growth in annual injection rate from 2030 369 onwards at a range of rates from 9.5% -17.2% in Figure 5 and values are reported in Table 3. The  Table 3.

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The range of possible initial growth rate and storage resource base combinations needed to 386 achieve 2050 targets are shown with isocontours in Figure 6.

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with the more ambitious targets (EU3 and EU4) requiring over 14% average annual growth for at least 397 20 years. While these rates are frequently seen over short timescales, sustaining them for multiple decades is unusual for energy technologies 57 . If only offshore storage resource is available in the EU, 399 the growth rate required to meet EU1-4 is within a similar range of 10% ->15%. Additionally, Figure 6 400 shows that if <7 Gt of CO2 storage resources is identified, then EU3 and EU4 become significantly 401 difficult to achieve from a growth rate perspective -rates of growth that are >20% are ultimately 402 required. This is the case for EU1 and 2 if <2 Gt of storage resource is developed.

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The difference in the level of ambition between the targets for the range of growth rate and storage 407 resource requirement is illustrated; higher targets of 298

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In contrast, limiting the resource base to just 10% of the currently estimated 88 Gt results in 423 minimum growth rates increasing to 11.4%, 13.7% and 15.1% to meet UK1-3, respectively. Table 4 424 provides a summary of these values for the UK domestic scenarios.   to the colour lines in Fig.7 and dots in Fig.8.

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The tradeoff graph for the combined scenarios ( Figure 9) illustrates the minimum growth 445 rates bounded by the available storage resource in the UK (78 Gt), the Norwegian storage resource 446 (94 Gt), and the UK and Norway combined storage resource (172 Gt). The higher the storage rate 447 target, the higher the minimum growth rate necessary to achieve the target. Notably, the range of 448 minimum growth rates illustrated in Figure 9 is between 10.3%-14.8% depending on the size of the 449 supporting resource base (  Figure 9, the demands on the injection growth   Tables 2 for associated 475 targets). The black points correspond to minimal growth rates subject to various storage resource 476 constraints (See Table 5

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The storage resource of the UK and Norway, alone or combined, appear sufficiently abundant 490 to serve as a regional CO2 storage hub for the European continent ( Figure 9).

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In this study, we evaluate the scaleup of geological CO2 storage identified in European climate 510 change mitigation plans. We show that all storage targets require historically high rates of growth; 511 minimum average annual growth in injection capacity from 2030 through 2050 needs to achieve 10%-

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15% to meet European targets. In contrast, CO2 storage plans are not limited by current estimates of 513 the resource base available and can be accommodated by the offshore reservoirs of the UK or Norway 514 alone. Storage resource limitations will only occur if the resource base has been significantly 515 overestimated, i.e., around 10% or less of current best estimates. In such a case, higher rates of near-516 term growth of 11% -17% and the combined resources of the UK and Norway are ultimately required.

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Comparing these modelled growths with the production growth rates achieved by the petroleum