More on hot-rock geothermal

When you drill a hole in the ground and flush water or CO2 through it, you cool the rock you've drilled through.  The amount of heat energy that comes up from that hole depends on how much rock you can cool.  And that depends on how conductive and permeable the rock is.

To get a feel for how much energy we're used to getting out of a hole in the ground, let's consider another industry that gets energy out of drilling holes in the ground: oil wells in the United States.  Half of US oil production comes from wells producing over 118 barrels of oil per day (ftp://www.eia.doe.gov/pub/oil_gas/petrosystem/us_table.html).  That's about 6.6 megawatts of chemical energy, which can be converted into about 4 megawatts of electricity.  These wells typically run for 20 years.

Geothermal wells will have to produce similar amounts of energy or the cost of drilling the well will make them uneconomic.

To get 4 MW of electricity from rocks that you cool from, say, 220 C to 170 C, you first need about 16 megawatts of heat, since your heat source is cooler than an oil flame and the heat engine it runs will have a lower Carnot efficiency.  Over 20 years, that flow will cool about 100,000,000 cubic meters of rock 50 degrees C.  If your well is 1 kilometer deep, you need to have cooled a region about 350 meters in diameter around the well.  Geothermal on a scale large enough to make a difference will need thousands of these wells separated by 500 meters or more, but connected by hot steam pipelines to bring otherwise diffuse steam to turbines large enough to be cost effective.

If we are to lose at most 10 degrees C of temperature delta to conduction, the cracks in the rock that carry the flow of fluid cooling it must be separated by less than 25 meters.  If there is significant flow restriction in those cracks, the separation must be closer still.  Rock 1 km down has been under 230 atmospheres of pressure for millions of years, which tends to crush the pores and small cracks the rock would otherwise have.

If the rock is heavily fractured, as it is in the Northern California Geysers field, then water flow through the rock can cool enormous expanses like this.  The Geysers geothermal field has been a great success over many decades, but it has unusual geology.

If the rock is not heavily fractured, then the production of heat from the well will look good at first but then fall off as the rock immediately surrounding the well cools off.  For the investment to pay off, heat production must stay high for two decades or so.

My guess is that using CO2 rather than water as a heat transport fluid might improve the permeability of the rock and make more rock viable for geothermal production.  This might make some areas that would not work with water-based heat extraction economically viable, but I don't think it's going to make hot-rock geothermal work across the country.