It’s been a while since I last posted anything here; I’ve been quite busy lately with my PhD and it has absorbed a lot of my time, but now I have a let up I can talk a bit more about what I’ve been up to.
Back in August last year I wrote a little about the background to my PhD. Essentially I look at whether offshore UK CO2 storage might require additional environmental regulations to limit the concentrations of a set of trace elements in overboarded produced water. I didn’t talk about exactly how I was going to achieve that, and so this post is a short overview of my research methods.
There are a number of general methods which I am applying to the research question: could offshore CO2 storage require additional regulations for disposal of produced water? They are:
- Experimental determination of trace element concentrations in produced water during CO2 storage in North Sea reservoir rocks
- Experimental investigation of sources of trace elements from North Sea reservoir rocks
- Investigation of a natural analogue in Utah for trace element mobilisation in CO2-charged waters expelled at the surface
- Compare the experimental and natural analogue data with a control dataset to assess requirement for additional regulations
The most significant part of my PhD is therefore the gathering, processing and interpretation of data generated from experiments carried out in the lab here in the School of Geosciences. To address point #1 on the list, I have carried out very simple batch experiments where I try and simulate – as best as the facilities I have allow – future UK North Sea CO2 reservoir conditions using rock samples, heated synthetic brines, and CO2. Carbon dioxide is bubbled through a flask containing warm/hot sodium chloride (salt) solution and reservoir rock collected from the British Geological Survey or held in our collection. The set-up from the experiment I did for my MSc is shown in the photo below (Figure 1):
This set up allows me to bubble carbon dioxide for several weeks and compare with a set of control flasks which have no bubbling CO2 (not shown in Figure 1). This allows a comparison to be made of the effect of CO2 on mobilising trace (and major) elements into solution. An example of one trace element, lead, is shown in Figure 2. While not necessarily true of all the trace elements I am looking at (viz arsenic, cadmium, chromium, copper, lead, mercury, nickel, zinc), the addition of CO2 does generally promote the mobilisation of elements into solution. These results follow those shown by other authors using batch and field experiments to determine element mobility, such as Lu et al. (2011), Little & Jackson (2010), Huq et al. (2012) and many more.
While the experiments of this nature can be carried out in the lab here at Edinburgh at reservoir temperature, unfortunately I do not have access to equipment which allows me to simulate reservoir pressure. This may be important since higher pressures allow more CO2 to dissolve, which reduces pH and can enhance mobilisation of elements. The concentrations therefore measured in my experiments may be an underestimate of what may occur in a reservoir. This becomes important when comparing to the control data set (point #4).
Point #2 is something I am working on at the moment. Unfortunately I can’t say much about it until I have published my results, but the aim is to determine – with experiments – where the trace elements are being mobilised from in the reservoir rock. If this goes to plan, I should then be able to build some sort of conceptual model which would allow a crude prediction of which elements could be mobilised in significant concentrations depending on the mineralogy of the rock. Ultimately this could be a tool for future CO2 storage operators to manage risk, depending on whether additional regulations are put in place to limit concentrations of trace elements in produced waters.
The investigation of the natural analogue, point #3, is outlined in this post, although I won’t talk about the results of that just yet. The aim here, though, is to compare concentrations of trace elements determined from field waters samples with samples collected from a batch reaction experiment (see above) using rock and water collected in the field. This will hopefully give some indication as to whether the batch results for the North Sea rocks are actually useful in simulating reservoir reactions or not – if the results from the Utah experiments match closely the field data then I can be confident that the experimental method is making suitable predictions.
Finally, the data collected from all the batch experiments will be compared to a control data set (point #4), supplied by DECC, which contains chemical data submitted for produced waters by UK North Sea oil and gas platforms. This data effectively shows what the UK government and various regional environment agencies are allowing in terms of concentrations of trace elements. By comparing the experimental data to this, I can make some assumptions about whether CO2 storage would be considered as safe/hazardous as the current oil and gas industry, or whether concentrations are significantly different (i.e. higher) and therefore perhaps require regulations to limit their impact to the environment.
So, plenty to do, and lots more to talk about at a later stage hopefully once there are publications on the way! Best get back to it…