One of the most promising technical solutions is Multiple Well Open Systems. Assuming good rock formation porosity, permeability, conductivity, and thermal gradients, it may be possible to circulate fluid through the systems to capture geothermal heat for use in surface power generation facilities;
For existing oil & gas fields and wells – if a hydrocarbon reservoir has been produced until end of life and there is substantial water production (e.g., aquifer support), it is possible that reservoir fluids (mainly produced water) could be circulated through the reservoir to obtain heat through conduction and convection. Some of the existing production wells could be repurposed to be used as injection wells. Existing production tubing and packers would need to be removed to minimise hydraulic constraints in the well bores. Water would be injected down the existing casing / liners to the existing reservoir completion (perforations or open hole) where the fluid would enter the reservoir rock formation. Reservoir simulations would be required to ensure sufficient heat transfer, flow rates, and distance between injection and production wells (to ensure fluid temperatures are recovered) to ensure the hydrothermal reservoir is able to economically produce the desired energy with the specific numbers and characteristics of wells;
For new drilled wells – in some instances there may be areas with substantial heated aquifers (called hydrothermal reservoirs) discovered during exploration and/or development drilling. Sometimes these aquifers were located beneath conventional hydrocarbon reservoirs (likely due to the migration of both hydrocarbon and water fluids from source rocks below). If new drilled wells are selected as the development option, it is likely that they would be specifically designed for geothermal hydraulic flow considerations – large diameter casing at depth (9-5/8”, 11-3/4”, 13‑3/8”, and sometimes even larger) would be used to accommodate fluid flow up to an order of magnitude more than many conventional oil & gas wells. Another good design option is to use horizontal wells to facilitate flow into and out of the reservoir rock formations. Good recent geothermal projects used as benchmarks have used horizontal wells 2-2.5km in length spaced 1-1.5km apart. Using new drilled wells can significantly increase the potential Geothermal Energy able to be extracted from some hydrothermal reservoirs.