Growing UK dependence on imported gas has increased the pressure to investigate the potential of non-conventional sources, including shale gas. Hydrocarbons in subsurface rocks make the rocks significantly more electrically resistive than when they are saturated with water. In cooperation with Petroleum Geo-Services and the Royal Academy of Engineering, the University of Edinburgh has been developing an electromagnetic exploration technology for application on the land surface, before drilling, to locate anomalous underground resistive bodies represented by hydrocarbons. The technology has the potential to reduce exploration drilling by 75%, reducing environmental impact and saving many £millions.
In parallel with the effort to harness renewable energy sources for the generation of electricity, it is recognized that the UK will be dependent on non-renewable energy sources for electricity, heating and transport for several decades. Hydrocarbon gas produces substantially lower greenhouse gas emissions than coal, per kilowatt of electricity generated; therefore greenhouse gas emissions can be reduced by switching from coal to gas for electricity generation. Gas is also the dominant energy source for heating.
For hydrocarbon exploration, the seismic reflection method is now very well developed and excellent for determining the geological structure of subsurface rocks. However, it is not so good at determining the nature of the fluids present in the rocks. This is where the electromagnetic method comes in: it can distinguish between salt water and hydrocarbons. If the fraction of rock pore space occupied by hydrocarbons rather than water is large, the resistivity can increase by an order of magnitude. Hydrocarbons contained in a rock volume of substantial thickness and area represent not only a source of energy, they also represent an anomalous resistive body in the Earth.
The Multi-channel Transient ElectroMagnetic (MTEM) exploration method invented at the University of Edinburgh is similar to the seismic reflection method, but complementary to it and with very little environmental impact. Its purpose is to identify the anomalous resistive bodies in the earth before drilling, thus minimising the number of required exploration wells. It has been used in the search for oil and gas in conventional reservoirs, both onshore and offshore, but never for shale gas.
Developers of this innovative electromagnetic exploration technology have much to learn from the well-established seismic exploration method. In one major respect, however, it is different: seismic waves propagate in the Earth essentially elastically, and the technology to image them is similar to that used in radar and ultrasound, except that there is a much greater range of velocities in seismic wave propagation. Electromagnetic propagation is diffusive, rather than wavelike and determination of resistivities from electromagnetic data is challenging. Edinburgh’s research focuses on this problem and on acquiring data to be able to identify targets down to 3000 m.