| Abstract |
To harvest enough heat from the subsurface and produce profitable energy at surface, traditional Enhanced Geothermal Systems (EGS) methods include a large number of proppant-propped fractures and desire that the circulating fluid have an ideal uniform flow distribution among all the fractures. An alternative method for heat extraction from hot dry rock is to maintain the surface and subsurface systems pressurized above fracture opening pressure, not connecting wells in the subsurface, and cycling (via Huff-n-Puff) only a fraction of the system fluid stored in the fracture system. In this alternative method, the number of fractures is lower by an order of magnitude compared to traditional EGS, making uniform split flow more manageable and not as critical. In each fracture, a large fluid volume always resides throughout the operations, absorbing significant amounts of heat. Then a smaller volume (relative to the residing hot fluid, but large enough for surface energy production) of cold fluid is injected into the fracture and the slightly larger volume is shut-in. During this process, fractures are maintained above fracture opening pressures but below the threshold that would initiate fracture propagation. After a pre-defined shut-in time, a volume (similar to that injected) of hot fluid is produced. A model has been developed that characterizes the thermal migration dynamics occurring within a single fracture maintained above fracture opening pressure during Huff-n-Puff operations. Specifically, pressure, temperature and flow behaviors in the pressurized fracture are determined by a thermal-hydro-mechanical (THM) solver for varying fluid volumes. Emphasis is placed on exploring the competing processes of balancing small-cold/large-hot fluid volumes in a pressurized fracture and its relation to thermal energy production. The results demonstrate that this alternative approach of operating fractures offers an efficient and sustainable alternative method for heat extraction in hot dry rock. |