When a production well depletes a reservoir with reduced reservoir pressure and no aquifer support, there still may be recoverable energy potential from a single well closed system solution;
After making the well safe, the well would have its original production tubing and any downhole packers removed;
The bottom of the well casing or liner would be plugged to isolate the reservoir section, then insulated production tubing would be installed into the well;
Heat transfer fluid would be circulated down the well annulus, getting heated from the surrounding rock through conduction and convection, and then produced back to the surface through the production tubing;
The amount of potential enthalpy is a function of thermal and hydraulic constraints of the well construction. For example, the annular space between the concentric production tubing and the applicable well casing or liner will have a material influence on potential circulation flow (i.e. smaller annulus means higher flow velocity which means less thermal heat transfer from the rock; and smaller annulus means hydraulic pressure losses so higher injection pressures on the surface would be required);
On the surface, this clean heated heat transfer fluid is circulated through the Evaporator of a Binary Organic Rankine Cycle (ORC) turbine expander power generation unit. The amount of electricity able to be generated is a function the product of the mass flow rate and the specific enthalpy difference between the produced and reinjected fluid and the efficiency of the ORC systems;
Due to the parasitic loads of the equipment, it is also possible in some locations to utilise a Standalone Power System with Solar PV and/or Wind Turbine to provide power to the pumps and fans so that the Gross Geothermal Electricity Power is able to be used.