Huge UK shale potential must be exploited responsibly

By Kasia Patel
Published: Wednesday, 09 July 2014

Though shale in the UK extends to around 30% of England and Wales, according to BGS geologist Rachel Bell steps must be taken to prevent pressure on groundwater supply and contamination, while a secure proppant supply is also essential.

Although there is widespread potential for unconventional gas exploration in the UK, according to the British Geological Survey (BGS) there are a number of steps that need to be taken first to protect groundwater resources.

Shale plays in the UK extend to around 30%
of England and Wales
Source: Jared Rodriguez
BGS geologist Rachel Bell told delegates at the Extractive Industry Geology (EIG) Conference 2014 last month that before further development of shale gas occurs, groundwater resource demand and potential pollutant pathways with regards to exploration need to be considered.

“Extraction of shale gas will use and mobilise extraction of chemicals that are potential pollutants, so these risks will need to be fully assessed and managed effectively from evaluation stage to post abandonment as well,” Bell said.

Shale plays in the UK extend to around 30% of England and Wales, with the USGS identifying two specific areas with potential for shale gas development.

Shale gas development to date has been limited to the north west by Cuadrilla Resources, but proposals have already been put forward for further developments in South Wales, North Yorkshires, West Sussex, Kent and Northern Ireland, which have led to concerns regarding additional water resource pressure.

In the UK, groundwater provides around 30% of the water supply, with some areas in the south east and East Anglia sourcing up to 80% of their public water supply from groundwater.

According to Bell, in order to carry out shale gas exploration in the UK, a lot of water would need to be produced in a small amount of time.

Existing data from the US estimate that a maximum of 4,000m3 water is needed to drill, and around 23,000 m3 water is need for fracking, though figures vary depending on geology, drilling complexity and well length.

However, based on the assumption that 100 wells are drilled and fracked in one year, annual water usage would not be unreasonably high – the challenge lies rather with managing overlap between exploration and areas with limited groundwater.

“The water that shale gas will potentially use is actually a very small percentage – 0.02% for shale gas extractions,” Bell said.

“The issue is more because there are challenges in areas where there are very limited water resources, in areas of the south east and also large chunks of the midlands as there isn’t much resource available,” she added.

Contamination sources

Aside from additional water pressure, contamination of groundwater is also a concern.

One potential contamination source to groundwater from shale gas exploration is the fracturing (fracking) fluid used to prop open the fissures.

Principle acquifers in England and Wales
Source: BGS
The fluid contains additives such as sand and acids, although these make up only around 0.1-0.2% of the total volume, and disposal of any of these chemicals requires prime authorisation.

Another potential contaminant is the flowback water used in fracking as 20-80% of water used in the process is returned to the surface.

“That can contain things like heavy metals and also naturally occurring radioactive materials so it’s really important that we look into the safe handling, storage and disposal of this kind of water,” Bell cautioned.

She added that the last source of potential contamination is the shale gas itself, which is composed of methane and other light hydrocarbons.

According to Bell, the presence of higher hydrocarbons enables a distinction to be made between thermogenic methane, formed in deep shales, from biogenic methane, formed in much shallower environments such as marshes, bogs and landfills.

One study, which was carried out in the US, used the distinction between thermogenic and biogenic methane to determine methane concentration based on distance to the nearest gas well and found much higher methane concentrations close to the active extraction sites.

However, Bell noted that monitoring was important before any activity even begins: “Because they didn’t know how much methane was there before they started extracting the shale, they weren’t sure about the migration pathways, and how exactly it was getting into the groundwater.”

Groundwater contamination pathways

With contaminant sources such as chemicals and oils, produced water and shale gas, Bell said it was important to note how contaminants can get into groundwater and the likely sources.

“We can then examine where this is going to be a problem in the UK,” Bell added.

Shale clay units in England and Wales
SourceL BGS

Pathways can include factors such as surface spills, well integrity failure – such as leaks or poor construction – the fracking process itself and natural geology faults.

To pinpoint problem areas in the UK, the BGS has carried out work on a national 3D geological model, which has identified areas where aquifers overlie source rocks as a potential issue.

“Now that we know where shale gas is overlying principle aquifers, we’re going out and collecting baseline methane samples in public water drinking supplies and in any private drinking supplies near shale gas rock,” Bell said.

All the data collected so far has been collated and the BGS plans to extend its research into northern areas, eventually looking to publish all the data on the BGS website.

The results collected so far have shown that biogenic methane is present in all UK aquifers but at low levels.

“In spite of this, methane isn’t actually a contaminant of concern – in terms of human health it has no impact really – but it could be an early warning indicator of other contaminants that might come through from shale gas operations,” Bell added.

The future of UK shale

According to Bell, though the UK has significant shale gas potential, this is not yet a proven resource.

Additionally, before any exploration takes place, local needs must first be considered, and relevant regulation and monitoring be put into place.

“We do have an existing extraction licencing system which is well used to dealing with competing water demands,” Bell said.

“We also have groundwater contamination regulations which are well developed, and baseline and groundwater monitoring is essential and needs to be implemented throughout the whole lifecycle of the shale gas process,” she said.

Frac sand sufficient

In order to sufficiently exploit the UK’s shale resource, however, a secure proppant supply is also needed.

According to BGS industrial mineral specialist, Clive Mitchell, the UK is nearly self-sufficient in silica sand, with 40 sand quarries producing 4m tonnes of silica sand.

While all frac sand is made from silica sand, not all silica sand is suitable for use as frac sand, with some grades far more efficient than others.

According to Mitchell, the closest equivalent to the frac sand suggested by the British Standards for proppants used in Europe is foundry sand. This has high-quartz content and features round grains, with good sphericity and a similar size range to frac sand grains (100-500 microns).

“There are 20 quarries producing foundry sand in the UK. The main sources are the Sandringham Sand Formation (Norfolk), Woburn Sand Formation (Bedfordshire), Folkestone Formation (Surrey & Kent), Chelford & Congleton Sands (Cheshire) and Wind Blown sand (North Lincolnshire),” Mitchell said.

There is of course also potential for ceramic proppants – composed of sintered bauxite or kaolin –within Europe, however these are more expensive to produce, and until exploration gets underway it will be difficult to tell if the shale within Europe requires them.