In this calculator, the casing is 20 cm ID, 22cm OD (7.9 and 8.7 in). For the coaxial configuration, the inner pipe is 8 cm ID, 10 cm OD (3 and 4 in). Thermal conductivity of rock is 2.5 W/m °C, and conductivity of casing (and cement) is 15 W/m °C. The coaxial configuration has a fully insulated inner pipe, the U-tube has insulation (choose your length) on the producer only. (If you want to change the pipe sizes, feel free to download the html and adjust them.)

You can estimate the exit temperature and thermal energy output (divide by 8 to get approximate electrical output). For a commercial project using an Organic Rankine Cycle, you'd want a temperature of around 140 °C, and it needs to be available even after 10-30 years (3650-10,950 days).

This calculation is based on the Ramey equation, applied in three parts (downward vertical flow in the injector or annulus, horizontal flow along the lateral (through both inner and outer pipe, in the coaxial case), and upward vertical flow in the producer or inner pipe). The coaxial solution requires an iterative "shooting method" until the bottomhole temperatures of the inner pipe and annulus are equal -- be careful, sometimes it doesn't converge! (There's a minimum flow rate, so make sure the red and blue lines meet at the bottom -- increase the flow rate if they don't.)

Ramey, H.J.. "Wellbore Heat Transmission." J Pet Technol 14 (1962): 427-435. doi: https://doi.org/10.2118/96-PA

Horne, R.N., and K. Shinohara. "Wellbore Heat Loss in Production and Injection Wells." J Pet Technol 31 (1979): 116-118. doi: https://doi.org/10.2118/7153-PA

Horne, R. N. (1980). Design considerations of a down-hole coaxial geothermal heat exchanger. Geothermal Resources Council Transactions, 4, 569–572. GR Library