There has recently been a lot of attention to the concept of ‘deep closed-loop geothermal.’ The concept of closed-loop geothermal is to circulate fluid through a wellbore that is 1000s or even 10,000s of ft deep and then back up to the surface. No fluid ever leaves the well. Wells this deep would cost millions of dollars, and so they would need to produce a lot of energy to justify cost.

The design depends on heat conduction to bring energy into the wellbore. In contrast, typical geothermal and EGS designs use convection to produce energy – hot water or steam flowing through the rock and into the well.

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The problem is that heat conduction through rock is very slow. The thermal diffusivity of rock is around 1e-6 m^2/s. This is equivalent to the hydraulic diffusivity of a very low permeability shale – around 10 nd. Any design that relies solely on heat conduction to bring energy towards the well will result in extremely low energy production per ft of lateral drilled. Even with revolutionary reductions in the cost of drilling and high energy prices, these designs could not possibly come close to recouping the cost. It doesn’t matter whether you circulate water, or another more exotic fluid through the well.