Extracting Energy from Hydraulically-Fractured Geothermal Reservoirs

摘要

The governing equations for heat and mass transfer were derived for hydraulically fractured geothermal reservoirs. When converted to nondimensional form it was shown that the equations can be considerably simplified. The resulting equations can be strongly influenced by the effects of buoyancy; the magnitude of the effect is measured by the ratio of the Grashof and Reynolds numbers, and the ratio of the actual permeability of the fracture and the square of the fracture gap width. Significant quantities of energy can be extracted from hydraulic fractures--even without thermal stress fracturing. The amount is limited by the size of the fracture and the low thermal conductivity of rock. The viscous pressure drop in open fractures is insignificant, and depending upon losses in piping and surface equipment, the entire system could be ''self-pumped'' due to buoyancy. Thermal contraction of the rock tends to increase the fracture gap width--thus decreasing viscous pressure drops and increasing the effects of buoyancy. This latter effect, which grows with time, eventually offsets the deleterious effects of ''short-circuiting'' in some cases. (ERA citation 02:000523)