If I read right “Future of Enhanced Geothermal Systems” MIT School of Engineering did for federal government 2007 (http://geothermal.inel.gov), CO2 is better than H2O as hydraulic/heat-transfer fluid for Enhanced Geothermal Systems. Also, 70 years output from 500 MW coal-fired plant would fill 1 of 15 hot-rock reservoirs needed to replace 100MW coal-fired plant sustainably, assuming 100MW EGS would need 0.5km3 pore space approx. 0.1 of 5 km3 total volume of hot rock reservoir. Thus 500MW/100MW x 70 years x 15 reservoirs = 5250 CO2 output could be used in Enhanced Geothermal System to replace 100MW coal-fired electric generating capacity, which is over 26 times as much as CO2 produced over remaining 200 years of coal reserves for 100MW plant’s share of coal. Chapter 9 discusses economics with section 10 describing a sample EGS well field with 1 injector well and 3 producer wells for each stack of 3 reservoirs and a total of 30-40 wells implying 10 stacks each producing 5 MW for a total of 50 MW for field with need to tap a new reservoir in each stack every 6 years over a 90 year cycle. We also ought to replace petroleum and natural gas with sustainable energy before we run out. We will need as electricity about 2/3 of the same total energy as now. Even allowing for sequestering CO2 from other fossil fuels and even if we are able to use artificial trees that look like naked billboards to pull enough CO2 from air to reduce present almost 400 ppm to 300 ppm of CO2 normal for warm spells between ice ages, EGSs can still make good use of more CO2 than will be available. Thus, there is a technological fix for global warming if we will use it.
They estimate a cost of about $1 billion (2004 prices) for the R&D involved in developing the technology over five years and losing close to $1 billion over another five years getting from pilot project to competitive assuming a price on CO2 emissions of $35/ton (2004 prices) to make fossil fuel more expensive and thus sustainable energy more competitive. Considering how expensive failing to mitigate global warming and thus having to rely entirely on adaptation is likely to be, Enhanced Geothermal Systems are likely to be more valuable as a way to sequester CO2 from burning fossil fuel than as competition to coal in producing base load electric power. It might help to have the administration and the 100 largest firms incorporated in the United States exchange itemized lists of cost of present non-tariff regulatory burden on pollution including greenhouse gases, of a system of user fees and rebates to control pollution, and of business as usual. Property and casualty insurance firms already fear global warming and might be willing to chip in something towards mitigating it, even if only by investing part of their reserves in federal government bonds dedicated to controlling greenhouse gases. Large commercial farms are likely to be hurt by heat waves and drought brought by global warming and should chip in towards its mitigation as part of insurance against crop failure they already buy. Fossil fuel firms should pay for much of it because it is largely their mess to be cleaned up. Coal mine owners should pay for Enhanced Geothermal Systems as eventual replacement for coal. If everyone agrees on user fees to cover the cost of pollution with rebates for money spent on controlling pollution and/or developing replacements for products causing pollution and/or federal green bonds to help finance the needed investments, Exxon Mobil could get rebates for R&D on bio-diesel from algae, Peabody Energy Systems could pay its user fees towards Enhanced Geothermal Systems what the coal they replace is worth, natural gas firms could invest in either solar power to replace gas or methane from organic wastes. Maybe it would be possible to manage with contracts between the federal government and the various firms involved with the threat of the Commerce Department giving a hard time to any outsider trying to butt in without paying the pollution user fees.