How hydraulic-stimulation for geo-thermal energy is different from fracking for shale gas

David Townsend
David Townsend

By David Townsend

Concerns raised recently around the development of Scotland’s geothermal resources being hindered by the moratorium on fracking are valid but overstated.

Firstly, the moratorium on fracking is specific to shale gas exploration, and so does not apply to the development of a geothermal resource. Fracking is a term used exclusively in the oil and gas industry.

A similar process is sometimes applied in the development of a geothermal resource to enhance permeability, but the term most commonly used to refer to this process is hydraulic stimulation. However subtle distinctions of vocabulary are unlikely to differentiate the processes in the media and public eye.

Secondly, we must expand the debate to reflect the variety of geothermal resources available in Scotland: minewater, hot sedimentary aquifers (HSA), and ‘hot rocks’/granites.

The extensive network of abandoned coal mines in Scotland contain vast quantities of warm water, ready to be exploited by a simple geothermal doublet (a production and injection well system) or a single production well coupled with a passive treatment facility. Developing a minewater geothermal resource will never require hydraulic stimulation as the resource permeability is already high enough due to the mining process.

Scotland’s HSA geothermal resources are most commonly sandstone units which have been buried for example, in the Midland Valley sedimentary basin. To guarantee economic productivity hydraulic stimulation will be considered when planning the drilling programme, but will only be applied when absolutely necessary and after all environmental risks have been thoroughly evaluated and mitigated.

However, even in this unlikely scenario, ‘fracking’ an HSA poses far lower environmental risk than fracking for shale gas. This is primarily because a sandstone is ‘softer’ than a shale, and is usually ‘sandwiched’ between two shales of higher fracture resistance. Therefore when the sandstone is hydraulically stimulated lower pressures are required of the injected fluids than for shale gas production, and therefore fractures have a very low chance of propagating out of the sandstone.

Even if this were to occur, there would be hundreds if not thousands of meters of sediments for the fracture to bisect before it were to come close to any fresh water aquifers, and even in the very unlikely scenario that the fracture did reach a fresh water aquifer, the ‘contaminant’ would be brine, not methane.

As for ‘hot rocks’ which in Scotland are generally high heat potential (HHP) granites, hydraulic stimulation is most likely to be considered in the conventional Enhanced Geothermal System (EGS) approach, which has been applied safely in numerous R&D projects outside of the UK, Soultz-sous-Forêts in France and the Cooper Basin in Australia being the most famous.

For a ‘hot rock’ EGS project, natural fractures would be targeted by the wells, and enhanced only if permeability was insufficient to reach production targets as required by the project developer. In this case, a detailed survey of all freshwater aquifers would be completed, and the EGS process would only go ahead if it were deemed safe by the scrupulous environmental standards set by government and enforced by bodies such as SEPA.

My view is that the ban on fracking for shale gas does not pose a serious obstacle to the development of geothermal energy, but careful and consistent communication to the general public about the actual environmental risks and benefits of geothermal resource development must be practiced.

The main obstacle to the development of Scotland’s deep geothermal resources is lack of borehole data, so ironically the drilling of some exploratory wells for shale gas could benefit the geothermal community by providing new valuable data on resource presence and quality.

I propose that if any exploratory wells are drilled for onshore hydrocarbons in Scotland, the repurposing of the well as a geothermal system should be considered, as synergies exist whereby geothermal heat can be extracted simultaneously with hydrocarbons.

We need to heat our homes, and in most cases we will burn gas to do so for the foreseeable future.

There is an urgency to transition to renewable sources of heat, such as deep geothermal and heat pumps, whilst simultaneously reducing heat consumption through energy efficiency improvements and smarter system management. It is my view that we should leave shale gas in the ground, so as to avoid further CO2 or methane entering the atmosphere. Whilst it is widely recognised that at least half of the known fossil fuel reserves must be left in the ground as ‘unburnable stranded assets’ so as to avoid catastrophic climate change, developing new fossil fuel resources seems counterproductive (to say the least) to the wider challenge of decarbonising our energy supply.

Government support is required to better understand the deep geothermal resources of Scotland, through the careful financing and drilling of several deep geothermal exploratory boreholes.

Schemes such as dry-hole risk insurance – recently introduced in the Netherlands which is seeing enormous growth in its geothermal sector at very little cost to the Dutch government – will allow for the private sector to push forward with this exciting low carbon industry.

The opportunity to transition geoscience, drilling and engineering professionals and businesses from the oil industry into a new Scottish geothermal energy industry cannot be ignored.

David Townsend is founder and managing director of TownRock Energy


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