Shale gas in Europe: the chemical challenge
As the political debate continues, independent oilfield chemistry consultant Dr Henry Craddock looks at some of the chemical issues surrounding shale gas fracking in the UK.
Natural gas is the cleanest-burning fossil fuel available. Using hydraulic fracturing to extract this type of gas, the USA is currently exploiting its reserves of this fuel and there is talk of Europe following suit.
While natural gas resources trapped in shale rock formations are not a new discovery to those working in the oil and gas industry, the concept has only relatively recently become a topic of political and economic interest. This is because in the past decade energy companies have combined two established technologies – hydraulic fracturing and horizontal drilling – to tap into the resource. Looking to the example set by the USA, European countries are starting to explore shale gas.
Hydraulic fracturing involves pumping a viscous fluid into a well faster than it can escape into the rock formation. This causes the pressure to rise and the rock to break, creating artificial fractures and enlarging existing ones, which means the effective permeability of the reservoir remains unchanged, however the wellbore radius is increased. This leads to greater productivity, due to the larger surface area created between the well and the reservoir. The natural gas held in the rock is forced out of the well and can then be processed above ground.
This process was developed more than 50 years ago and is now the common method for extracting natural gas in shale formations throughout the USA. Furthermore, it is being actively considered in Europe, with some field tests already conducted. Shale gas bearing formations are extensive across the continent, particularly in France and Poland, where around 80% of the European reserves are located. The UK has several formations already identified and both the Cheshire and Wessex basins are currently being examined.
One of the critical differences between Europe’s exploitation of this resource and the direction travelled in the USA in particular is in the application of chemical treatments to the fluid used in the fracturing process. This will be especially true in terms of their technical requirements and their environmental control.
To produce gas from these shales by hydraulic fracturing, a certain number and proportion of chemicals are required to treat the fracturing fluids. This associated chemical use has come under public scrutiny, with particular focus on potential groundwater contamination. Such contamination has been reported in the USA, although differences in regulatory controls should ensure that groundwater contamination in the UK and Europe is highly unlikely, if not impossible.
Fracturing fluids provide the means to give the hydrostatic pressure required to create the hydraulic fracture. The pumped water has to be treated to increase its viscosity by the addition of viscosifiers or gelling agents. After fracturing, a proppant, mainly consisting of sand, is added to the pumping fluid. This slurry is used to prevent the newly formed fractures from closing when the pumping pressure is released. The transportability of this proppant depends on which viscosifiers have been added to the water or base fluid. Friction reducers and other chemicals are added to improve the efficiency of hydraulic fracturing.
The total amount of chemical additives used is less than 0.5% of the volume of fluid. These additives impart various properties that ensure the purity of the water, the rheological properties of the fracturing fluid and the prevention of degradation of the fluids so that productivity in flowing back the well is maintained. It should also be noted that the well design and construction ensure there is no possibility of fracturing fluids coming into contact with any aquifers. This is not only undesirable from an environmental perspective but also from an economic one, as such fluids are likely to be treated and reused.
While the amounts of chemical are relatively small, the amounts of fluid are large, with 7–14 million litres used in a typical job. Although a number of chemical additives can be employed, any single job would only use a few of these. Of the 12 common additives (see below), anything from three to all 12 could be used in a specific fracturing fluid.
Differences across the Atlantic
In Europe, there are a number of technical challenges that differ from the chemical applications so far used in the USA. These relate to the well depth and, therefore, the pressure and temperatures encountered by the fracturing fluid and the chemical additives. Also, the environmental regulations differ greatly from the controls placed on chemical use in the USA. In Europe, much more emphasis is placed on the chemicals’ ecotoxicological profile and available data to support their use. This strengthens the argument that groundwater contamination is less likely because regulators will not allow high-risk chemicals to be used. There is also a permitting system on the overall use and discharge of chemicals, which will include groundwater monitoring. This two-pronged approach to regulation is quite different from the permit-only approach generally employed in the USA. This means that the chemicals used in fracturing fluids will have to meet REACH chemical safety criteria and show that they have no adverse impact on the environment or cause any local disturbance or excessive noise.
The overall process of hydraulic fracturing and its environmental impact is a matter of some debate, which is likely to become even more heated, but in science and engineering circles the need for reasoned, factual argument is paramount – alongside openness and transparency. In respect of the latter, NOGEPA, a Dutch oil industry association, has developed a website that lists all the chemicals likely to be used in hydraulic fracturing in shale gas exploitation.
The USA has undoubtedly reaped great economic benefits from the development of unconventional hydrocarbon resources and the technical input into shale gas fracking is growing exponentially. Europe can gain vastly from these technical improvements, as there is little need for test bed developments – having been fully technically proven in the large shale deposits of midwest USA.
Common chemical additives
These three critical chemical additives will need to be adapted to meet technical and environmental requirements in Europe:
1. Viscosifiers and cross-linking agents
The majority of fracturing treatments conducted to date have used fluids that have been viscosified by guar gums or guar derivatives, such as hyropropylguar (HPG). Guar gum is a branched polysaccharide comprised of the sugars mannose and galactose in the ratio 2:1. Guar gums and derivatives are not self-gelling and require a cross-linking agent to be added in order to gel in water. In general these cross linkers (such as borates) are environmentally unsuitable. Also, they do not impart high enough temperature stability to the gel to provide rheological and fluid loss control and fracture conductivity properties in fluid temperatures higher than 105ºC. However, guar gum is very economical because it has almost eight times the water-thickening potency of similar materials and, therefore, only a very small quantity is needed for producing sufficient viscosity. Guar gum is a direct food additive and is registered for use and discharge in the north east Atlantic, including the North Sea. It is highly biodegradable and is recognised as posing no environmental or toxicological problems. As such, the critical challenge in the European application is to find and develop a cross linker that is both environmentally acceptable and offers a higher temperature stability of at least to 150°C. Work is ongoing in this area with magnesium salts and certain polyols.
Surfactants, such as cetyl ammonium bromide – a long chain quaternary ammonium salt – are used in fracturing fluids to generate viscoelasticity. They congregate into micelles, which interact to form a network imparting viscous and elastic properties to the formulated fluid. Surfactants are included in most aqueous fracturing fluids to improve compatibility with the hydrocarbon reservoir. It is also important that the formation rock is water-wet in order to achieve the maximum conductivity of hydrocarbon gas or fluids. The environmental fate of surfactants can be a complex issue and some careful consideration will be required as to which surfactant to use. This is an area where EU regulators will require considerable evidence on biodegradation and bioavailability before allowing use.
Where the fracturing fluid is composed of guar gum or other natural polymers, a small amount of biocide is included in the formulation, preventing any undesired degradation and changes in rheological properties. A number of biocides are used for this function. All are common in the oil and gas industry and are under strong regulatory control. By their nature, they are highly toxic to all sorts of aquatic organisms, however, their concentration in the chemical mix is very small and many are short lived in action and highly biodegradable.
For more information, email Henry Craddock at firstname.lastname@example.org