My PhD looks to address whether there might be a regulatory requirement to limit produced water discharge from North Sea CO2 storage facilities. Specifically, produced water contains concentrations of trace elements, such as arsenic, lead and mercury (so-called ‘heavy metals’) which can become detrimental to the marine environment if the concentrations are significant enough. This post is a quick overview of the background to my PhD, explaining why there might be a concern about trace element concentrations in produced water, and why we are ‘producing’ water in the first place while storing CO2 offshore.
So firstly, a clarification, for those not entirely familiar with CO2 storage: when we talk about storing carbon dioxide offshore, we are not storing it on the sea bed. CO2 is, or will be, stored in porous rock deep under the seabed. In order for CO2 to be stored as efficiently as possible, it must be compressed and heated to turn it from its gas phase (the bubbles in your Irn-Bru) to its dense phase, where it becomes a supercritical fluid. In order for the CO2 to remain in this dense state, it must be stored above a certain pressure (7.39 megapascals) and temperature (304.25 Kelvin), which means a minimum storage depth of around 800m below the surface, and ideally from around 1km.
The porous rocks into which CO2 would be injected offshore of the UK are sandstone formations, all of which bear saline waters. Some may also bear oil and gas, which has been great news for the UK since the 1960’s and 70’s, but these hydrocarbon formations are small in volume compared to the total saline sandstone formations. So, with the sandstones already containing fluids, injecting CO2 into them can lead to pressure building up as the CO2 and saline fluids are squeezed into a finite amount of porespace.
Figure 1 shows a typical North Sea natural pressure profile with depth. ‘Hydrostatic gradient’ on the graph, taken from Moss et al. (2003) is the pressure increase due to the weight of fluids, and can vary considerably from this graph, depending on the formation or hydrocarbon field of interest. The ‘Lithostatic gradient’ is due to the weight of the overlying rock. So, hydrostatic pressure is less than lithostatic pressure, but as CO2 in injected, the hydrostatic pressure increases for the injection depth and approaches the lithostatic pressure. The purple squares are data points from the North Sea which represent the pressure at which the rock in a particular formation will fracture and fluids will ‘leak off’. Given that we are trying to store CO2, and don’t want it leaking, then we want to avoid the leak off pressures, therefore there is a limit to the hydrostatic pressure increase that is allowed before the formation fractures and is compromised.
So pressure limits the amount of CO2 which can be stored in a given formation. Taking this into account, the Department of Energy and Climate Change (DECC) estimate that the UK has around 100 years’ worth of storage for UK CO2 output. However, there is a ‘simple’ solution: relieve pressure by allowing water to escape from the formation. This would be achieved by producing water at the offshore injection platform. In a study carried out by Scottish Carbon Capture and Storage (SCCS, 2011), the storage capacity of Scottish formation alone was uprated to 200 years’ worth of CO2. Other papers (e.g. Lindeberg et al (2009) and Neal et al (2011)) also confirm that pressure relief in this way would increase storage capacity and improve the economics of storage.
In fact, producing water is already an everyday oil and gas industry activity, as oil and gas reservoirs not only contain hydrocarbons, but saline water too. In the North Sea, these produced waters are routinely overboarded (dumped) into the sea, Figure 2. Insofar as environmental regulations for this activity are concerned, provided that the produced water contains no more than 30 milligrams per litre of oil, then it can be overboarded without treatment.
Produced water does not just contain oil, however. Dissolved elements such as calcium, potassium, sodium, chlorine and sulphur are also present in various amounts, with sodium and chlorine being the dominant elements. These are all present due to the chemical interaction of the formation rock and the fluids they contain. These interactions also bring into solution elements which have the potential to be harmful to marine life, such as arsenic, chromium, lead, and zinc, in trace concentrations. So far, the concentrations of these potentially harmful elements are not significant enough in produced waters to have prompted tight environmental regulation. However, this may change with CO2 storage.
As you will know already, when CO2 is dissolved in water, it acidifies it which is why the dentist tells you off for drinking lots of fizzy drinks as the slight acidity attacks your teeth. The same thing happens when CO2 is injected to sandstone formations: the CO2 dissolves into the saline fluids, acidifying them, which can affect the minerals which make up the rock. Experimental work, both at laboratory and field scale, has shown that CO2 increases trace element concentrations in the fluids. This may mean, therefore, that by injecting CO2 into North Sea formations and producing fluids back to the surface, higher concentrations of trace elements are being released in to the marine environment. If this is the case, and depending on the concentrations of those elements, additional environmental regulations may be required. This is what my PhD seeks to determine, and I’ll post more about that in the future, explaining the sort of work involved!
 Moss, B., Barson, D., Rakhit, K., Dennis, H., & Swarbrick, R. (2003). Formation pore pressures and formation waters. In D. Evans, C. Graham, A. Armour, & P. Bathurst (Eds.), The Millennium Atlas: Petroleum Geology of the Central and Northern North Sea. The Geological Society of London.
 Scottish Carbon Capture and Storage, 2011. Progressing Scotland’s CO2 Storage Opportunities.
 Lindeberg, E., Vuillaume, J.-F., & Ghaderi, A. (2009). Determination of the CO2 storage capacity of the Utsira formation. Energy Procedia, 1(1), 2777–2784. doi:10.1016/j.egypro.2009.02.049
 Neal, P. R., Cinar, Y., & Allinson, W. G. (2011). The economics of pressure-relief with CO2 injection. Energy Procedia, 4, 4215–4220. doi:10.1016/j.egypro.2011.02.369