Pakistan's First Alternate Energy Technology Company
Pakistan's First Alternate Energy Technology Company

Multiple Well Closed System (Doublet)

When a series of production wells deplete a reservoir with reduced reservoir pressure and no aquifer support, there still may be recoverable heat energy potential from a solution where two wells are connected at their toes to form a closed system called a Doublet.

The two wells have to be in close enough proximity that it is possible to sidetrack them until their toes intersect.  Each well would have their production tubing removed first, then casing/liner plugs inserted above the producing intervals downhole, then sidetracked out of the main well casing and geosteered towards the other well.  A wireline powered, tubing conveyed, magnetic field generating transmitter (solenoid) and a receiver (a specially adapted MWD survey system) would be used in the two wells to be able to intersect.  After intersection, a slotted liner would be used to mechanically connect the two wellbores with a stinger and then cemented into position.  Both wells would be circulated together and the final wellbore left clean and ready for geothermal fluids.

Heat transfer fluid can be circulated down the injection casing, getting heated from the surrounding rock through conduction and convection, and then produced back to the surface through the production well casing.  The combined length of the two wellbores, especially in the deeper rock formations, allows significant thermal conduction.  Without any hydraulic constraints from concentric production tubing, more heat transfer fluid can be circulated through the wells.

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.  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.