Demystifying Fracking Fluid

This is what fracking looks like. Image: Day Donaldson

Hydraulic fracturing fluid has two functions - to convey pressure into the surrounding rocks, causing the eponymous fracturing, and to deliver the “proppants” into these gaps, to hold them open once the pressure is released. There are additional challenges such as bacterial growth, deposition of dissolved particles and absorption of water by surrounding clays, all of which are managed using their own classes of additives. Much of my information is from an excellent paper which you can find here (PDF).

Gelling agents - these are added to increase the viscosity of the water, allowing the proppant particles to stay in suspension. With water on its own, the particles would be inclined to sink to the bottom, which would make them rather difficult to manoeuvre into the fractures. Often these are derived from guar or cellulose based compounds, both of which are naturally derived and considered to be non-toxic and biodegradable, and a good thing too since they are used in concentrations of up to 1000mgL-1.

Foaming agents - they perform many of the same functions as gelling agents, with the advantage that they reduce the amount of water required. Carbon dioxide or dinitrogen gas are injected into the gelling liquids, causing them to expand. They are also less readily absorbed by clay, preventing clays in the well from swelling up and obstructing the operation.

Friction reducers - water has a habit of hydrogen bonding to things, so friction reducers are sometimes used to reduce the surface tension. This allows for a reduction in obstruction of fractures (something which is a possibility with gels and foams). Polyacrylamide is a commonly used friction reduces, and has a low toxicity. However reaction through heat or UV can cause it to degrade into acrylamide, which is rather carcinogenic. The risk of this happening during the fracturing process are very low, and evidence would suggest that aerobic biodegradation of polyacrylamide after use doesn’t produce acrylamide either.

Crosslinkers and breakers - two sides of the same process, crosslinkers cause polymer gels to bind into larger molecules, increasing viscosity, while breakers do the opposite, breaking them down again and reducing viscosity. High viscosity helps to suspend and deliver the proppant particles to their destination, then breakers can reduce the viscosity and allow recovery of the fluid from said cracks, preventing them from getting “gummed up”. Lowering the viscosity also facilitates the removal of the fluid from the well. Crosslinkers often function best at a particular pH, so pH adjusting chemicals are often added to optimise their performance.

Biocides - bacteria get around, and their dastardly activities can obstruct fracking. Sulfate reducing and acid forming bacteria contribute to the degradation of mine casings and other important equipment, so they are despatched using biocides. The range of substances used is vast, and while most degrade fairly quickly, Quaternary ammonium compounds have been observed to stick around in wastewater for some time. What’s more, their persistence inhibits many of the bacterially facilitated wastewater treatments which may otherwise have been employed.

Corrosion inhibitors - it’s not just the bacteria which are at it, various chemicals, either added to the fluid or dissolved from the surrounding rock, can corrode the metal parts of the well. The selection of corrosion inhibitor depends on the geological and environmental characteristics of the operation, and some such as acetaldehyde and thiourea are suspected to be carcinogenic.

Scale inhibitors - suspended ions like to form into crystals when they come into contact with solid surfaces. Scale inhibitors block the molecular attachment sites on growing crystals, inhibiting their growth, and preventing them from blocking every pore or pipe that they come into contact with.

Iron control - some geological formations contain a lot of soluble iron ions, which end up being deposited elsewhere in the works, and causing much the same problem as the previously mentioned crystals. Iron control chemicals bind with these ions to prevent them from being deposited.

Clay stabilisers - clay absorbs water and swells, causing obstruction of the well, or the pores and fractures from which the hydrocarbons are meant to escape. To prevent this, clay stabilisers are added, which inhibit the ability of clay particles to bond to water. Tetramethyl ammonium chloride has long been used, and is highly toxic. However the use of the far less toxic choline chloride is increasingly being adopted.

Surfactants - selectively used to perform any number of functions previously mentioned, surfactants can be used to control viscosity and surface tension, as a biocide and a clay stabiliser, depending on the formula used.

But will it kill me / my cat? As you can probably infer from that list, there are many chemical additives with varying toxicity, selectively used in different combination. The same paper analysed the data of 81 known chemical additives in fracking operations and could only obtain toxicity information for around two thirds of those. Three category 2 oral toxins were identified (propargyl alcohol, tetramethyl ammonium chloride and thiourea), meaning that mammalian toxic doses are between 50 and 500mg/kg body mass. Ten more were identified as category 3 (500 - 5,000mg/kg). Realistically, this means that the potential toxicity from contamination will depend on the concentrations of chemicals used, and the degree of contamination. It’s hard to say definitively, but you probably wouldn’t want to drink fracking fluid for a laugh. As mentioned in the previous post, contamination of groundwater from fracking, especially after disposal in injection wells, is not unheard of (PDF), so environmental monitoring and dilligent disposal practices are incredibly important so that we hopefully don’t ever have to learn first hand whether it’s a risk or not.

Stay tuned for:

• Earthquakes!
• The potential impacts of fracking on the local community and landscape

Something in the water

So, water. Useful stuff. We can perhaps conclude that faulty mining well casing is a source of methane in drinking water, but what about that somewhat mysterious “fracking fluid”? The evidence for this one is not so clear cut.

Image: ProgressOhio

I should probably explain why I keep on calling them mysterious, even though we largely know what sort of additives they are likely to contain. In the United States, disclosure of the ingredients in fracking fluids by drilling companies is largely voluntary, and many additives are not disclosed due to commercial sensitivity. The industry backed website FracFocus allows for the submission of additives by drilling companies, but regulation does not mandate their disclosure and it is up to the companies to decide what and whether to submit. It should be noted though that in the UK, regulations oblige mining companies to disclose all additives.

Hydraulic fracturing tends to be performed at great depth, an average of around 2500m below ground in the US, and the pressurisation of the fluid takes place in small “sections” of the horizontal well. This allows for greater control of the pressure, which can be optimised for the surrounding geology. Consequently, the chances of fracking fluid finding their way into local groundwater sources is fairly low, as it would have to migrate through hundreds of meters of rock. However, the depth at which fracking takes place is a function of the geology, rather than a measure to protect groundwater, and may range from over 5000m to as little as 30m. While fracking is not frequently performed at such shallow depths, this is well within the range of groundwater.

Even with a close vertical proximity to groundwater, our primary concern is whether any fracking fluid is allowed to migrate far enough from the mining well to pose a contamination hazard. It’s in the interest of mining companies to closely monitor the extent of the fractures which they induce, as too extensive a fracture could result in the escape of the gas, and too small would mean that they won’t make the best possible return on their investment.

Broadly speaking, most investigations have failed to find evidence of the chemicals associated with fracking fluid in drinking water. However, under some geological conditions, there is some evidence (pdf) of migration from fracture sites into groundwater reserves.

The vast quantity of water used during a hydraulic fracturing operation has to be disposed of after use. Often it is sealed underground in an “injection well”, and left there. This is associated with another issue, as the less constrained injection of pressurised water has the potential to unleash seismic energy stored in the surrounding geology. There is evidence that this has happened(pdf) (we’ll look at this more thoroughly in a future post).

So what can we take away from this? Fracking fluid may indeed find its way into groundwater supplies, but only under specific geological circumstances. It would seem that the biggest issue is with injection wells, where companies dispose of used water, rather than the actual mining site, since they’ll generally be under higher pressure and for a longer duration. Also, earthquakes.

Next time:

• A little bit about the stuff they add to fracking fluids. I’ve been promising it for ages but I keep running out of words.
• How much water? That’s a lot of water.
• Earthquakes? I’d rather the ground didn’t move, thank you.