Fracking – or "hydraulic fracturing" in full – involves high-pressure injection of fracking fluid into deep rock formations in order to create a fault system and increase the permeability of the rock.
The technique has been applied since the Second World War, and was initially used primarily for the conventional exploitation of oil and gas reserves. As a result of improved extraction technology and higher gas prices, fracking is now being used to exploit hard-to-reach shale gas deposits (unconventional gas reserves). The process of hydraulic fracking is used in the exploitation of conventional and unconventional oil and gas reserves, and involves increasing the pore pressure in existing or new, expandable fractures to such a degree that it exceeds the local rock pressure. This leads above all to tensile (Mode 1) fractures – and only to a lesser extent to shear (Mode 2) fractures – resulting from expansion of the rock without significant displacement, i.e. without an earthquake in the classic sense. Based on the physics of the process, the vibrations triggered by the opening and expansion of the fractures are smaller and less energetic and rarely lead to noticeable earthquakes (EGK "Risiken, Potenziale und Chancen von Hydraulic Fracturing (Fracking)"). For the most part the fractures would close again once pressure was removed, hence the use of “proppants”, materials such as small sand particles, which keep the fractures open. Earthquakes up to a magnitude of 4.8 (in Canada) have, however, also occurred in the context of fracking for unconventional oil and gas reserves. Earthquakes of this kind occur when the fracture grows into existing, prestressed faults.
A related technique known as hydroshearing is used to exploit geothermal resources, and involves drilling into natural, pre-existing fault systems in order to create shear fractures. Pore pressure is increased to a lesser extent than in hydraulic fracking, with the result that the local rock pressure is generally not exceeded (Conference Paper "Pros and Cons of Hydraulic Fracturing and Hydraulic Shearing for Deep Reservoir Stimulation") (acatech "Hydraulic Fracturing - Eine Technologie in der Diskussion"). Unlike the exploitation of conventional and unconventional gas reserves, however, deep geothermal energy does not necessarily depend on the use of proppants or chemical additives. In addition to this, deep geothermal energy projects involve less drilling than unconventional gas extraction, thereby reducing the amount of space and quantity of drilling fluid required.
Unconventional gas reserves have been exploited above all in the USA and Canada so far, and it is currently unclear as to whether this will ever be done in Switzerland. This is due to the unknown potential of shale gas reserves in Switzerland, the fact that the resources are not renewable, and the relatively large amount of space required, which could give rise to conflicts of use (SCNAT "Eine Technik im Fokus: Fracking").