B&NES?Council has paid for a specialist report from the British Geological Society on the potential problems posed by fracking. The report concludes that any damage to the hot springs is unlikely and, if it did happen, it was likely to take several thousand years to manifest itself. However, any damage is likely to be irreversible. The hydraulic fracturing of coal seams is not considered so risky as shale gas fracking.

This assumes the work is properly managed and takes place several miles away from Bath.

The first hydraulic fracturing took place in the Hugoton Gasfield, in 1947 in Kansas. This was in a conventional, sandstone reservoir. Hydraulic fracturing has been applied to geothermal reservoirs (both sedimentary and igneous) and is known to generate microearthquakes. These quakes, which are not felt and have minus value magnitudes (ML), are monitored to assess how far the artificial fractures are developing. During the 1990s, hydraulic fracturing was applied to the Barnett Shale in the Fort Worth Basin of Texas and the subsequent improvement in well productivity sparked off the shale gas exploration, which is now worldwide. A larger volume of the shale reservoir was now accessible, which could therefore produce larger gas flows.

Hydraulic fracturing consists of pumping into the formation very large volumes of fresh water that generally has been treated with a friction reducer, biocides, scale inhibitor and surfactants and contains sand as the propping agent.

The treatment fluid maximises the horizontal length of the fracture, while minimising the vertical fracture height. The aim is to avoid fractures reaching other formations above and below the target, which may hold water. The fractures, which are held open by the sand, result in an increased surface area of the 'coal', which increases in the release of the gas and increases its mobility. The result is more efficient recovery of a larger volume of the gas-in-place.

Each chemical compound in the fluid serves a specifically engineered purpose, such as reducing viscosity or bacterial growth or bio-fouling reservoir surfaces. The make-up of fracturing fluid will vary. A fracture fluid used in the Fayetteville Shale, US, contained 99.5% water with less than 0.5% other compounds. Any toxicity, such as acid, is greatly reduced by dilution in the pumped fluid and by the reaction of the acid with the rock in the subsurface that converts any acid into salts.

Subsidence has not been shown to affect shale gas fields in US, which have been in production for more than ten years.

Large natural earthquakes, even those occurring on the other side of the world, can affect the productivity of water wells and are known to cause changes in hydrocarbon migration. The Lisbon earthquake of 1755 caused a reddening of the Hotwells water at Bristol (Hawkins & Kellaway, 1991). Also an earthquake in 1892 preceded, and perhaps caused, the appearance of oil in a water well at Ashwick, near Oakhill.

Enhanced geothermal systems, e.g. Basle in Switzerland, use hydraulic fracturing to increase permeability of rocks containing hot water, but the project there had to abandon operations after earthquakes up to 3.5 ML were created.

The B&NES and surrounding areas have very few water supply boreholes, but extensive (and old) coalfield shafts. It is vital to locate these prior to any drilling.

It seems unlikely that hydraulic fracturing activities in the centre of the Bristol and Bath Basin would have a significant effect on Bath's thermal water's temperature, quality or flow rate.

It is believed the main threat to the hot springs is in the immediate area of Bath city, where engineering boreholes and deeper wells drilled for various purposes have already encountered warm or hot water.

Coalbed methane exploration should not affect these migrating waters, because there is no evidence the hot waters have come into contact with the relevant strata. Properly conducted CBM exploration should not pose any problems to the hot springs, reports BGS.