Assessment and management of sedimentary geothermal resources

Abstract

Low-temperature (<150°C) sedimentary geothermal resources are widespread in the continental regions of the Earth’s crust. They are quite different in nature from the geothermal resources associated with volcanic systems or tectonically active regions of the crust. Their management during long-term utilization also requires somewhat different emphasis than that of the conventional geothermal resources. Good examples of sedimentary resources, which have been utilized heavily for direct applications, can be found in France, Hungary and China. More than 3500 geothermal wells had been drilled in China by 2010, most of them in sedimentary geothermal systems. Two Chinese sedimentary geothermal systems, the Xianyang system in Shaanxi and the Xiongxian system in Hebei Province have been assessed by lumped parameter pressure response modelling and volumetric calculations. They are of quite contrasting nature, the Xianyang system being a porous-type sandstone system and the Xiongxian system being a fissured-karst carbonate rock system. Xianyang lies in Wei River sedimentary basin in the centre of China. Today over 40 geothermal wells, ranging in depth from about 1500 to 4100 m, are distributed through the region. Well head temperature ranges from 55 to 120°C. Geothermal space heating is the main consumption in Xianyang with up to 3 million m2 approximately being heated, which is equivalent to reducing CO2 emission annually by 120,000 tons by replacing conventional coal boilers. The average permeability-thickness of the Xianyang reservoir is estimated to range from 2 to 36 Darcy-m through the lumped parameter modelling. Permeability inside the Xianyang reservoir increases northwards as well as indicating a significant difference between the two sides of the Wei River Northern Bank Fault.

The Xianyang sandstone reservoir appears to approach a closed reservoir in nature. The volume of geothermal fluid stored underground in the Xianyang territory is estimated to be 59•109 m3 and recoverable heat to be 13•1018 J. Xiongxian lies in the North China Basin at about 110 km and 100 km from Beijing and Tianjin, respectively. Over 56 geothermal wells (including 8 reinjection wells), which are mostly used for space heating, had been drilled by 2010. The Xiongxian reservoir is relatively shallow, with the deepest well being 1800 m, and the reservoir temperature ranging from 50°C to 95°C. Large scale reinjection started in Xiongxian in 2009. The internal permeability-thickness of the Jixian reservoir in Xiongxian is estimated to be up to 95 Darcy-m, and the external permeability over 14 Darcy-m, according to the lumped parameter modelling conducted. The potential of the Jixian reservoir is rather promising for future exploitation, partly because reinjection should be easy. The pressure response of the Jixian reservoir is comparable to that of an open reservoir. The volume of geothermal fluid stored in the Jixian reservoir, in the Xiongxian territory, is estimated to equal 9.6•109 m3 and the recoverable heat to be 2.5•1018 J. The lumped parameter models for the respective systems are used to calculate reservoir pressure predictions for different future production scenarios. Recommendations on the management of sedimentary geothermal resources in China, based on experience gathered in China and on mainland Europe, as well as on the results of the assessments of the work presented, are also provided. The most important ones relate to common management, enhanced monitoring and reinjection.