GHGT-12 Conference Paper

I’ve been a little busy in the past few months with PhD thesis writing, hence the paucity of blog posting since the start of the year. However, I’ve just submitted an extremely large draft chapter to my supervisors and am taking a temporary break from writing to regain some sanity(!)

This post is a really just a quick advert for the conference paper I wrote for GHGT-12 last year. I’ve already mentioned my poster (here) and talked a little about how I found the conference (here), but without really touching on the science I presented.

The conference paper was published in the GHGT-12 conference proceedings, as the special volume (63) of Energy Procedia. These are not peer-reviewed papers, and effectively extended abstracts only of the work presented either as talks or posters, with mine being the latter. The work I presented in my paper (available here) was preliminary data and thoughts based on batch experiments and sequential leaching experiments, both undertaken on North Sea sandstones. I’ve previously described the rationale for this work, and the draft thesis chapter I’ve just finished is my final assessment of the data.

The paper essentially describes two types of experiments:

  • Batch experiments where North Sea sandstones were reacted with CO2 and synthetic brines at reservoir temperatures (but not pressures), to determine dissolved concentrations of elements
  • Sequential leaching experiments in which North Sea sandstones were subjected to a sequence of increasingly aggressive chemical leaching to determine where particular elements could be leached from (i.e. from mineral surfaces, carbonate minerals, etc.)

Sandstones collected from old cores drilled in UK North Sea oil fields, Figure 1, were used for the experiments since they represent possible geological formations in which CO2 may be stored for CCS projects.

Sample locations

Figure 1. Locations of Captain, Cormorant and Thistle fields from which sandstone samples were used for experiments. The Ross field is indicated as reference for Ross FPSO Bleo Holm location discussed further down the page.

Of particular interest to my work are the concentrations of 8 trace elements (As, Cd, Cr, Cu, Hg, Ni, Pb and Zn) since these are the 8 elements required by the UK Department of Energy and Climate Change (DECC) to be monitored by offshore oil and gas operators, and submitted to the Emissions to the Environment Monitoring System (EEMS) database.

From the batch experiments, concentrations of these elements appear to be low and therefore relatively immobile under the experiment conditions. The notable exceptions which are consistently elevated in concentrations when CO2 is added to the experiments (to simulate CO2 storage), are Cd, Ni and Zn. However, even elevated, their concentrations are much less than the total amounts of these elements in the samples.

In the paper I suggested that, since the experiments were undertaken at atmospheric (not reservoir) pressure, then pH was easily buffered by some mineral dissolution, and therefore was not low enough to more greatly attack minerals and increase mobilisation of these elements into solution from mineral dissolution.

The results of the sequential extraction indicated that the 8 elements of interest are largely bound to mineral phases which would not be dissolved by the presence of CO2, even under reservoir conditions.

Concentrations of these elements obtained from the batch experiments were also compared with concentrations submitted to EEMS for produced waters at the relevant offshore facilities: Cormorant, Thistle and Ross (closest facility to Captain), Figure 2.

Ross Thistle Cormorant histo

Figure 2. Histogram of concentrations of trace elements for Captain (Ross FPSO Bleo Holm), Cormorant and Thistle fields. Pinks are batch experiments, green is EEMS data. Dotted lines are analytical detection limits for batch experiment data.

Comparing batch experiment concentrations with data from the North Sea we can see that experimental concentrations lie within the usual range of produced water concentrations, although again Cd, Ni and Zn are at the upper end, or exceed the normal range (Hg is high because of analytical problems). However, while concentrations in the experiments exceed individual North Sea fields in some cases, overall concentrations fall within the range of all North Sea data submitted to EEMS. I would not expect, therefore, that CO2 storage is going to significantly mobilise these 8 trace elements.

The sequential extraction experiment overall provides more detailed information about element distribution throughout these rocks, and is therefore a better tool for predicting trace element release than other techniques such as thin section, XRD or XRF analysis. However, since element release in experiments doesn’t necessarily match up nicely with the sequential extraction determined mobility of these elements, it is not necessarily an accurate tool. As always, further work may be needed to improve our understanding of element mobility.

Reference:
Carruthers, K., Wilkinson, M., Butler, I. M. (2014). Metal mobility during UK North Sea geological CO2 storage. Energy Procedia, 63, 3149-3159.
http://dx.doi.org/10.1016/j.egypro.2014.11.340

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