Lessons learned from hydrothermal to hot dry rock exploration and production

In this paper,we investigate geothermal exploration and production in 189 hydrothermal projects and 42 hot dry rock projects around the world.The hydrothermal fields for a working hydrothermal system to generate electricity should have the elements of heat source,water-saturated porous or fractured reservoir,caprock,heat transfer pathway,and good heat preservation condition and geothermal power energy intensity of 10-20 MW per km^(2)within at least 5 km^(2)area in tectonically active region.The hot water or steam flow rate in this hydrothermal system is normally larger than 40 L/s with temperature of 150℃or above.The power generated from enhanced geothermal system(EGS)in hot dry rock projects are generally less than 2 MW because the flow rate in most cases is much less than 40 L/s even with the hydraulic fractures using the modern stimulation technology learned from the oil and gas industry.The natural fracture in the subsurface is generally beneficial to the hydraulic fracturing and heat recovery in the hot dry rock.Moreover,the hydraulic fracture parameters,injection rate and well spacing,drilling strategy should be properly designed to avoid the short-circuit between injector and producer and low heat productivity.In the future,CO^(2)enhanced geothermal recovery associated with CO^(2)sequestration in the high temperature oil,gas,and geothermal fields maybe a good choice.On the other hand,both nearreal-time seismic monitoring to limit the pumping rate and the closed-loop of the Eavor-Loop style system without hydraulic fracture can contribute greatly to heat recovery of hot dry rocks and mitigate the risks of the hydraulic fracturing induced earthquake.Furthermore,the hybrid solar and geothermal system performs better than the stand-alone geothermal system.