Conceptual model and assessment of the Yangbajing geothermal field, Tibet, China

Abstract

The Yangbajing geothermal system in Tibet, China, is characterized by a deep and hot vertical upflow zone, which is feeding a shallow and horizontal outflow zone. The deep part has a measured temperature of 250-320?C at 1000-2000 m depth. One well there is already tapping the equivalent of 10 MWe from this volcanic and vertically fractured reservoir. Hot fluid ascends to shallow depths where it turns laterally towards the south-southeast, within the sedimentary, 150-170?C, shallow system, at 180-280 m depth. Hot springs and thermal lakes finally discharge the geothermal fluid to surface. More than 54 wells have been drilled into the Yangbajing system, most are tapping the shallow reservoir only. Electrical generation, which began in 1976, has led to pressure drawdown in the shallow system and a substantial land subsidence. Furthermore, the drawdown has initiated recharge from the system outer boundaries that in turn has cooled down some shallow wells to the south. On the other hand, some shallow wells in the north have become hotter due to increased fluid recharge from depth. The shallow reservoir may cover as much as 15 km2 while the deep system is considered less than 4 km2 in area extent. A simple 3-D numerical TOUGH2-EOS1 model has been developed for the Yangbajing system. At present it is only capable of simulating coarsely the initial reservoir temperature distribution, using 1176 rectangular elements in 7 horizontal layers and 6 rock types. Permeability in the vertical upflow zone was estimated near 20 mD and in the horizontal shallow system as high as 250 mD, while the outer model boundaries range from 0.1 to 5 mD. The model is heated with a deep, 330?C hot inflow of 25 kg/s.