Environmental impacts of CO2-EOR: a joint industry report

Last year I researched and wrote a report titled “Environmental Impacts of CO2-EOR (The Offshore Context)” for a Scottish Joint Industry Project (JIP). The report has finally been published, so I thought I would provide an online summary of the key findings.

The report was commissioned and funded by the Scottish Carbon Capture and Storage CO2-Enhanced Oil Recovery Joint Industry Project (more snappily SCCS CO2-EOR JIP), which itself is supported by the Scottish Government, Scottish Enterprise, 2CO Energy and Nexen. I was the sole researcher and author of the report, although feedback was given by the JIP partners prior to publication. Any errors, omissions, or misinterpretations are of my own doing, and any feedback or corrections are appreciated via the comments! It is available through the following link:

SCCS JIP Report: Environmental Impacts of CO2-EOR (pdf, 5.5MB)

The following is a summary of the chapters of the report.

Report Framework

The aim of the report was to determine the possible incremental environmental impacts of future CO2-EOR projects in the UK North Sea, over and above those of the existing oil and gas (O&G) industry. Offshore hydrocarbon extraction already has some environmental impact, however the report did not aim to quantify this. Instead, current offshore O&G equipment and activities are listed at the start of the report, and used to identify what would be unique, or potentially a specific problem for CO2-EOR, which were determined to be:

  • CO2 leakage
  • CO2 stream impurities
  • Enhanced trace element concentrations from geological storage
  • Enhanced radioactive scale

Current Regulatory Environment

The report goes on to set out a general overview of the offshore regulatory environment in relation to emissions to the environment, such as oil, trace elements, and naturally occurring radioactive materials (NORM). Discharge of waste oil produced from CO2-EOR would certainly be regulated under the Offshore Petroleum Activities (Oil Pollution Prevention and Control) Regulations 2011 (pdf,  0.1MB), which requires operators to apply for permits to discharge up to 30 mg/L of oil-in-water. NORM, likewise, is covered by the Radioactive Substances Act 1993, which defines radioactive waste by the activity levels of its NORM constituents. However, as discussed later, NORM could be increased with CO2 injection, making it a specific environmental issue for CO2-EOR.

Of particular interest to me, given the research I do for my PhD, is how dissolved elements in produced waters are regulated. In fact, the UK does not specifically regulate for trace or major elements in produced waters beyond 3 nautical miles offshore, which covers the vast majority of offshore O&G activities (and hence CO2-EOR). The UK is, however, a member of OSPAR and therefore committed to meeting the recommendations set out by this trans-national agreement. OSPAR Recommendation 2012/5 (doc, 0.2MB) sets out a risk-based appraisal of produced water constituents, giving values for a number of elements against which offshore operators should compare their predicted concentrations in the environment. If the predicted values exceed the OSPAR recommended values, then the operators should take steps to lower concentrations of metals emitted to the environment. This is not legally binding, however, so far as I am aware.

O&G activities also currently exist within sensitive environment habitats, such as Shellfish Waters, Special Protection Areas and Special Areas of Conservation. That current offshore industry exists in these areas, and that CO2-EOR would probabaly not require much additional regulation and permitting, it seems unlikely that CO2-EOR activities would be prohibited in these areas. Figure 1 shows locations of these protected areas for England and Wales.

Figure 1.

Figure 1a. Special Protection Areas and Special Areas of Conservation in relation to UK North Sea hydrocarbon fields.

Ramsar and Shellfish Oct 2012

Figure 1b. Ramsar and Shellfish Waters in relation to UK North Sea hydrocarbon fields.

CO2 Leakage to the Marine Environment

CO2 is the main differentiator between CO2-EOR and traditional offshore O&G activities. CO2 may leak from transport pipelines, return up wells during injection, or escape geological storage. A review of the literature on the effects of increased dissolved CO2 in seawater was carried out, alongside literature (mostly from Jerry Blackford and colleagues at Plymouth Marine Labs) on modelling CO2 leaks in typical UK offshore environments.

When CO2 dissolves in water, it acidifies it and lowers the pH. Seawater pH varies naturally around the global average of pH 8.1 ± 0.3, with conditions (freshwater influx, biological CO2 demand, water column stratification, etc.) changing local pH values by up to 1 pH. Natural underwater CO2 seeps can also lower pH locally to values as low as pH 3.

Not only would a CO2 leak affect pH, but it increases the partial pressure of CO2 (pCO2). Both can lead to stress in marine organisms, including lowered body and skeleton calcification (formation of hard bodies with calcium carbonate), and hypercapnia (increased COin body tissue and fluids). The effects of these are changes in:

  • Mortality
  • Calcification
  • Metabolism
  • Growth and reproduction
  • Resilience
  • …and more

These more greatly affect organisms lower in the food chain, than those higher up, but there is obviously a knock-on effect if the quantity and quality of lower trophic levels species diminish. However, sensitivity depends on the species, including existing exposure to localised natural pH changes. Some species may therefore be more resilient to a CO2 leak, by virtue of its natural environment.

Modelling of leakage scenarios e.g. pipeline leak vs geological storage leak appear to indicate that the volumes of CO2 which interact with seawater are comparatively so small that dilution and mixing quickly occur. pH is therefore affected only to very minor degree (< 1 pH change) and/or is short lived before tidal and current mixing dilute any changes. That is not to say that effects are not possible, but any COleakage would be highly localised.

Blackford et al 2012

Figure 2. Modelled pH changes due to three CO2 leakage scenarios over approx. 33km x 20km area: a) low flux point source; b) high flux point source; c) pipeline leak. Large panel indicates maximum pH change. Small panels top left-right are pH change at 6, 12, 36 hours, bottom panels left-right 72, 120, 240 hours. From Blackford et al., (2013)

Impurities in the CO2 stream, as remnants from the capture process, may include various acid gasses (NOx, SOx), trace elements, hydrocarbons and capture chemicals. However, despite some strong environmental effects of a number of these, their concentrations are anticipated to be so small as to be considered a low risk to the environment if CO2 leakage occurred.

Trace Elements in Produced Water

As already mentioned in previous blogs, COdissolves in water to acidify it, promoting the mobilisation of elements into solution from sandstones targeted for CO2 storage. A number of elements have been identified as priorities for monitoring in offshore discharges by the Department of Energy and Climate Change (DECC). They are:

  • Arsenic (As)
  • Cadmium (Cd)
  • Chromium (Cr)
  • Copper (Cu)
  • Lead (Pb)
  • Mercury (Hg)
  • Nickel (Ni)
  • Zinc (Zn)

While some of these elements (e.g. copper and zinc) are essential nutrients, in large enough concentrations they all exhibit toxicity to marine life, in similar fashion to CO2 hypercapnia. Unlike CO2, however, these elements accumulate in organisms and are magnified in concentration up the food chain, hence humans can also be affected if eating contaminated fish and shellfish (see: mercury poisoning at Minimata).

The O&G industry already produces discharges with these elements, which are not regulated (other than the OSPAR recommendations, see above). These elements could present a problem with CO2-EOR only if concentrations from these projects are elevated above what the current O&G industry produce. This was estimated using batch experiments, where sandstone samples (8518 & 8579) from a North Sea oil field was reacted with COand saline water, at reservoir temperature. The experiments were ran for 3 weeks, with control flasks run alongside which had no COadded.  The setup is shown in Figure 3.

Batch experiment setup

Figure 3. Batch experiment setup. L-R are flasks F1-4. F1 and F3 had CO2 bubbled through, F2 and F4 acted as controls without bubbled CO2.

Concentrations of the eight elements of interest, listed above, generally did not deviate greatly between the added CO2 and the control experiments, hence CO2 does not seem to have a significant effect on concentrations of these elements. The exception is Ni, where concentrations were consistently above the controls, Figure 4.

Nickel concentrations

Figure 4. Concentrations of nickel, for two sandstone samples 8518 and 8579, comparing control and bubbled CO2 experiments.

Major elements, such as iron and calcium, show stronger concentration trends with the addition of CO2, indicating that minerals are being dissolved and releasing elements. However, comparing these concentrations with oil field concentration data from which the experiment sandstone samples were collected (field name subject to non-disclosure agreement!), they are much lower overall than the field concentrations. And say we were to compare the experimental concentrations of Ni to DECC supplied data for Ni concentrations in produced waters, we see that the experiments lie within the range for existing O&G activities, Figure 5.

Nickel DECC compare

Figure 5. Histograms of nickel concentrations (log10 scale), comparing experimental data (Atmos CO2 = control) with DECC supplied North Sea data (EEMS).

So, while CO2 did enhance concentrations of elements, they lie within the range of normal activities, therefore it would not be expected that – comparing with the North Sea as a whole – element release from CO2-EOR will be any different to current activities.

Scale and Radioactive Scale

Scale is the formation of suphate and carbonate minerals within O&G reservoirs, wells and production equipment in response to changes in fluid chemistry. These scales can incorporate radioactive elements, such as uranium and radium, which are dissolved from minerals in the reservoir. Scales are waste products, and if they contain enough radioactive activity, must be disposed of in the correct manner. Therefore, with the issue of enhanced dissolution of minerals with CO2, there is the potential for more scaling and hence more radioactive waste generated. However, this is unlikely to be much of an issue as scaling inhibitors are already in use in the O&G industry, and although CO2 can in some cases reduce the effect of the inhibitors, scaling is likely to be well controlled.

The Conceptual Risk Model

The results of the analysis of the potential risks associated with CO2 leaks, CO2 impurities, trace element mobilisation, and radioactive scaling were assessed using a conceptual risk model.

The potential environmental impact of each issue was plotted against the likelihood of that impact occurring. This gives us the potential risk of each case. The report goes into more detail of the rationale behind each ranking; here is presented only the final risk model, Figure 6, for an ‘anchor’ project (i.e. local risks).

Risk Model Anchor

Figure 6. Conceptual risk model for a CO2-EOR anchor project.

From the figure, we can see that CO2 leaks from a well or trace element release from produced waters are the most ‘risky’, but are still low environmental risk.

Various mitigation options could be deployed to further reduce risk, if required, such as contingencies for stopping CO2 leaks, or the treatment of produced waters to reduce trace element concentrations. Overall, however, CO2-EOR appears not to pose significantly more environmental risk than current O&G activities.


Blackford, J. C., Torres, R., Cazanave, P., & Artioli, Y. (2013). Modelling dispersion of CO2 plumes in sea water as an aid to monitoring and understanding ecological impact. Energy Procedia 37, 3379-3386


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