assessment of geothermal resources in xi'an, china - Orkustofnun

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Yin Lihe 428Report 20of the production water.The above predictions arebased on a singleproduction-injection pair;if, on the other hand, thereare several wells aroundthe injection well, thetemperature drop will belower than that predictedin Figure 14.Although potential coolingis a disadvantage ofreinjection, it is beneficialfrom the view of long-termmanagement of the Xi’angeothermal system. If thereinjection rate is 5 kg/slasting 3 months duringthe wintertime, theincrease in annual energyproduction amounts to 3.8GWh, assuming aproduction increase equalto the reinjection.Temperature (°C)80757065605550Reinjection rate=5kg/sReinjection rate=10kg/s0 10 20 30 40 50 60 70 80 90 100Time (years)FIGURE 14: Calculated temperature changes in the produced waterwhen the production well is 1000 m away from reinjection well6. SUMMARY AND RECOMMENDATIONSThe main conclusions of this report are summarized as follows:1. The Xi’an geothermal reservoir is a low-temperature sandstone reservoir, with a conductiondominatedheat flow. It is bounded on all sides by four faults, considered to be flow-barriers. Thearea of the Xi’an field is about 376 km 2 . The Lantian-Bahe formation, at a depth between 1000 and1900 m, is widely exploited and has an average temperature of 86°C.2. Hot water production for bathing and space heating has increased dramatically from year to year since1994, from 250,000 tons per year up to 3.3 million tons in the year 2000. This results in an increasinglong-term water level drawdown of 10 m per year.3. A simple lumped parameter model, using the software LUMPFIT, was used to simulate water levelchanges in well XA-1 from 1994 to 2000. According to the model, the volume of the Xi’an reservoiris estimated at 37 km 2 and it is connected to another hydrological system with a much larger volume.The estimated permeability of the system is 15 mD assuming radial flow, but by assuming differentmodel geometry, higher permeability is obtained.4. The lumped parameter model was used to predict water level changes due to three long termproduction scenarios in order to assess the production potential of the Xi’an geothermal system.According to the model, the system is capable of sustaining constant 158 l/s production, or 5 milliontons annually, until the year 2010, with water level drawdown less than 150 m.
Report 20429Yin Lihe5. The effect of reinjection in the Xi’an reservoir was estimated with the lumped parameter model. If10 l/s are injected into two reinjection wells each, the water level will increase about 5 m on averageuntil 2010, but the increase will be about 8 m on average if four reinjection wells are used.Some recommendations are put forward:1. A coordinated action is needed to improve geothermal management in general in Xi’an. Monitoringis an essential part of geothermal management. Long-term monitoring of the Xi’an geothermalsystem must be continued and improved. The main parameters to monitor include production rate,water level and water temperature for each production well. Collection of water samples for chemicalanalysis is also recommended to provide information on chemical changes, which may indicaterecharge change or temperature change. It is important to update existing models as the productionhistory proceeds.2. The Xi’an geothermal system should be considered a single reservoir and rapid production increaseshould be avoided.3. Results of the modelling in this report indicate that reinjection will increase the production potentialof the reservoir. It is, therefore, quite important to carry out tracer tests before a reinjection projectis launched. In that way, the potential of premature cooling in the production wells can be estimated.4. Numerical modelling is recommended in order to simulate both the whole system and individualwells, when considerable data has been collected.5. Due to the existence of a poisonous element, fluorine, in the geothermal water, monitoring ofwastewater is required to prevent soil and groundwater pollution.ACKNOWLEDGEMENTSI would like to take this opportunity to thank the director, Dr. Ingvar B. Fridleifsson, and the deputydirector, Mr. Lúdvík S. Georgsson for giving me the opportunity to participate in the UNU GeothermalTraining Programme, and to Mrs. Gudrún Bjarnadóttir for her kind help. I am deeply indebted to mysupervisors, Dr. Gudni Axelsson and Mr. Arnar Hjartarson, for giving me excellent guidance and helpwith the English. I am grateful to staff members at Orkustofnun for their valuable lectures and I amdeeply grateful to Mr. Wang Deqian and Mr. Liu Zuzhi for providing the data used in my report.REFERENCESAxelsson, G., 1989: Simulation of pressure response data from geothermal reservoir by lumped parametermodels. Proceedings of the 16th Workshop on Geothermal Reservoir Engineering, Stanford University,California, 257-163.Axelsson, G., Björnsson, G., Flóvenz, Ó.G., Kristmannsdóttir, H., and Sverrisdóttir, G., 1995: Injectionexperiments in low-temperature geothermal areas in Iceland. Proceedings of the World GeothermalCongress 1995, Florence, Italy, 3, 1991-1996.Axelsson, G., Flóvenz, Ó.G., Hauksdóttir, S., Hjartarson, A., and Liu J., 2001: Analysis of tracer testdata, and injection-induced cooling in the Laugaland geothermal field, N-Iceland. Geothermics, 30, 697-725.
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