Chen Jinguo
(Hubei Provincial Geological and Environmental Station)
Abstract: This paper introduces the geothermal geological conditions and genesis types of the hot spring geothermal field in Xianning City, Hubei Province, and evaluates the geothermal resources, demonstrates the results of development and utilization in recent years, the basic experience, puts forward the problems in the process of development, and puts forward reasonable and feasible suggestions for the problems.
Xianning City is located in the southeast of Hubei Province, the south bank of the middle reaches of the Yangtze River, Hunan, Hubei and Jiangxi provinces junction, is the main corridor of the south to the north, known as the "southern gate of Hubei". The city has four counties, one city and one district and an economic and technological development zone, namely, Jiayu County, Tongshan County, Chongyang County, Tongcheng County, Chibi City, Xian'an District and Hot Spring Economic and Technological Development Zone.
Geothermal resources in Xianning City are rich and widely distributed, with hot springs exposed in one district, four counties and one city under its jurisdiction. With the extensive utilization of geothermal resources as a green energy source and the increasing attention paid to it, a detailed investigation of geothermal resources in the Hot Spring Geothermal Field of Xianning City, Hubei Province, an evaluation of the hot spring resources in Wuhong Mountain of Chibi City, Hubei Province, a census of geothermal resources reserves in the Geothermal Field of Wuhong Mountain of Chibi City, Hubei Province, a detailed investigation of geothermal resources in the Geothermal Field of Langkou in Chongyang County, Hubei Province, and a survey of geothermal resources in the Serpentine Mountain of Jiayu County have been carried out one after another since the beginning of the 21st century. Among them, the detailed investigation of geothermal resources in Hot Spring Geothermal Field of Xianning City, on the basis of the preliminary investigation, more y studied the nature and scale of the tectonic traces along Qianshan Mountain and its relationship with the distribution, runoff and discharge of geothermal fluids, and calculated that its extractable volume was 13320.1m3/d, and the maximum water temperature reached 55℃. The results of the survey, whether in the distribution range of geothermal fields, the amount of recoverable resources and the maximum water temperature, are a breakthrough compared to previous.
1 Regional Geothermal Geological Conditions
1.1 Geological Tectonic Profile
The geological tectonic position of Xianning is at the intersection of the Yangzi quasi-terrestrial Liangzi Lake depression and the Xianning terrestrial fold bundle, and belongs to the secondary tectonics of the northern wing of the Dumashan compound dorsal incline - the Gaogaoqiao dorsal inclination, the Sunqianpu reversed dorsal inclination, and the western extension of the Jajashan reversed dorsal inclination. The western extension of the inverted dorsal slope is composed of a series of compact folds, which form the basic tectonic skeleton of the area. The late New Huaxia tectonic system is strongly manifested in the area of Qianshan, in which the Dayishan type rupture is clearly reflected. These different tectonic systems interfere and compound each other, which not only control the combination of lithology, geomorphology and karst development in the area, but also strictly control the distribution of groundwater and its recharge, transportation and drainage. Qianshan dorsal slope is the main structural skeleton of geothermal field, which is characterized by comb, and inverted to the southeast, the largest scale of hot spring fractures in the area, and after many tectonic changes in the composite fracture, the first for the longitudinal tensile fracture and vertical movement is dominated by the later manifestation of the reverse composite, and by the counterclockwise stress and the compression and torsion characteristics of the deep cut fracture, but also control the distribution of thermal fluid in the area of the main fracture.
1.2 Hydrogeological conditions
The hot springs outcrop section is caused by the intersection of the North West West Fault and the North East Fault cutting the thermal reservoir, and the rocks at the intersection are strongly broken, with the development of fractured rocks and breccias, and the development of cleavage, and the porosity of tectonics rocks in the tectonic zone of the fracture is larger than that of the original rocks, especially in the part of the intersection of the faults, which provides a favorable space for the storage and transportation of geothermal fluids. Fractures in this area have experienced many phases of activities, due to its tectonic way, different strengths and weaknesses, the degree of rock fragmentation and cementation conditions are different, coupled with other fracture cross-cutting effect, resulting in the degree of fracture water enrichment, thermal conductivity of the unevenness of the fracture, in the rock fragmentation, cementation is poor, fissure development, geothermal fluid is relatively rich in a larger volume of water; in the rock is broken, the fissure connectivity of the poorer sections, because of the narrow space for storage, poor channel, so the water temperature is skewed, the water is not good. The channel is not smooth, so the water temperature is low, the water volume is small.
According to the analysis of geothermal fluid sampling in geothermal field, the gas composition of geothermal fluid in Xianning Hot Spring Geothermal Field is mainly dominated by N2, O2 and CO2, of which N2 accounts for 94.60% to 95.70% of the escaped gas and 66.30% to 68.80% of the dissolved gas, and the high abundance of N2 mainly exists in the atmosphere, and the source of N2 in geothermal fluid is brought into the underground by atmospheric precipitation, and the source of N2 in geothermal fluid is caused by atmospheric precipitation. The source of N2 in geothermal fluid is caused by atmospheric precipitation, and geothermal fluid is mainly recharged by infiltration of atmospheric precipitation.
The δD and δ18O of atmospheric precipitation have elevation effects, and according to the isotope results of the sampling and testing of geothermal fluids and atmospheric precipitation in this study, it is estimated that the recharge elevation of the hot spring geothermal field in Xianning City should be around 470m. According to the analysis of regional topography and geomorphology, in the southeast of the geothermal field along the Damaoshan Mountain is a low-middle mountainous area of 300-500m, and its exposed carbonate rocks, which receives the recharge from the atmospheric precipitation, is recharged to the Xianning Hot Spring Geothermal Field from a remote source through karst channels.
Additionally, according to the analysis of tritium content in the geothermal fluid, it can be seen that the geothermal fluid should be the geothermal fluid that infiltrated the atmospheric precipitation before 1954 and circulated to this area after a long time of runoff, i.e., the geothermal fluid in this area has experienced a runoff from the recharge to the discharge for 50 years or more, which represents the runoff time of the geothermal fluid in the geothermal area. In the thin surface cover and fissure development section, there is recent surface water and fissure water mixing.
1.3 Geothermal Temperature Field Characteristics
Based on the comprehensive analysis of the survey data, it is shown that the burial depth of the ambient layer in the geothermal field of Xianning Hot Springs is generally about 25m, and the temperature of the boreholes in the area is mostly in the trend of gently increasing temperature, i.e., as the depth of the boreholes increases, the temperature is subsequently increased. According to the characterization of the temperature change in the borehole, in the area, the borehole temperature increase is relatively slow, the temperature variation in the borehole is between 0.045～0.231℃/m, and due to the difference in the buried depth of the thermal conductivity and heat storage structure or the depth of some of the boreholes exposed is different, resulting in a difference in the temperature variation, in the thermal conductivity and heat storage structure buried depth of the different faults above the zone, the temperature increase is obvious, and in the fault zone, almost no temperature increase, and below the fault zone, the temperature increase is obvious. The temperature increase is obvious in the section above the fault zone with different burial depths of heat conductive and heat storage structures, and almost no temperature increase in the fault zone, and the temperature increase below the fault zone is not obvious, which shows that the convective heat transfer is dominated above the fault zone and in the fault zone, and conductive heat transfer is dominated below the fault zone. The size of the geothermal gradient value of geothermal field is directly related to the geological environment conditions of the geothermal field (location, lithology, geological structure, groundwater, etc.), and the geothermal gradient value of the rocks in each geothermal area has a certain degree of difference, and the value is mostly in the range of 1.15~26.74℃/100 m. According to the data of the follow-up drilling and temperature measurement, the average value of the geothermal gradient in the area is 6.57℃/100 m, and the thermal anomaly is obvious.
The long axis direction of the plane temperature contour is 290 ° north-west, which is consistent with the direction of the hot springs exposed on the surface, indicating that the distribution and exposure of geothermal fluids are mainly controlled by the north-west tectonic structure in this section.
1.4 Geothermal fluid hydrochemical characteristics
According to the analysis of artificially exposed geothermal fluid well samples in the survey area, the hydrochemical type of geothermal fluid is sulfate-calcium water, in addition, according to the relationship between the content of sulfate ions and the temperature shows that: the concentration of sulfate ions in the geothermal fluid and the temperature of water in a certain range of the detection of a straight-line relationship between the temperature of water when the water temperature of 20 ~ 50 ℃, the sulfate ion concentration changes in the range of 17.50 ~ 50 ℃, the concentration of sulfate ions in the geothermal fluid in a straight-line relationship with the water temperature in a certain range of water temperature. When the water temperature is from 20 to 50℃, the concentration of sulfate ions varies from 17.50 to 1331.0mg/L. When the water temperature is above 48℃, the concentration of sulfate ions does not change much, and it can be assumed that 48℃ is the critical value for sulfate ions to become saturated in geothermal fluids. The hydrochemical type of surface water and ambient groundwater is calcium bicarbonate type, which is very different from the geothermal fluid. From the above data, it can be seen that the characteristic ions in the geothermal fluid in the survey area are potassium ion, calcium ion, magnesium ion, sulfate ion and fluoride ion, which are important hydrochemical markers of the geothermal field, and they are the main scalar elements.
2 Evaluation of geothermal resources in Warm Springs Geothermal Field
2.1 Establishment of thermal storage model
2.1.1 Thermal storage cover
The stratum between the east side of the Qianshan Dorsal Slope to the north and west flank of the Dashaoshan Dorsal Slope is the Silurian System, and the lithology is composed of rocks such as muddy siltstone, shale, and powdered and fine sandstone, with the total thickness of 1,762 to 2,856m, and the average thickness of 2,264m. The total thickness is 1762~2856m, with an average thickness of 2264m. Due to the high content of mud in the rocks, the permeability and water storage performance is poor, and the fractures and fissures developed in the local sections are filled with mud, so the heat transfer performance of the rocks is poor, and a "U"-shaped box structure is formed between the Dashuoshan and the submerged mountains, and a natural cover is formed in the upper part, which prevents the heat from being lost and plays an insulating role in the heat source of the heat flow field of the submerged mountains. The heat source of the lower heat flow field plays a role in insulation, is the geothermal area of the heat storage cover layer.
2.1.2 Heat-control and heat-conducting structures
In the area, the hot springs fracture is a representative trunk fracture, and its cutting depth and scale is also the largest, in the area of Qianshan Nandong, it controls the contact boundary between the Silurian and Ordovician strata, and the runoff of the geothermal fluid in the carbonate rocks is blocked near the hot springs fracture, and it is a heat-control fracture in the area. In addition, in the area of the Moon Bay, the north-west to the development of the tensile fracture for the heat and water fracture.
2.1.3 Thermal Reservoir
According to the analysis of the data obtained from drilling, the recharge, transportation and storage of geothermal fluids are carried out in the carbonate rock strata, and richer geothermal fluids have been obtained in carbonate rock strata with karst development. Therefore, the brecciated tuff, crystalline tuff, tortoise cracked tuff, bioclastic tuff, dolomitic tuff under the silicified rocks and Silurian strata, rock fissures, karst development, and the local section is the main storage place of geothermal fluids, and it is the thermal storage rock layer.
2.1.4 Formation of geothermal field
According to the analysis of the previous isotope data, the main source of geothermal fluid in the area is atmospheric precipitation, and it is recharged by alpine precipitation, and the recharge elevation effect of hot springs geothermal field is about 500m, and it enters the geothermal area after experiencing more than 50 years of runoff time.
The Aurignacian, Cambrian and Ordovician strata exposed in the north and west wings of the Damaoshan dorsal slope are mainly composed of carbonate rocks such as chert, dolomitic chert or gray dolomite and dolomite, with exposed rocks and karst development on the surface, which is favorable for infiltration and recharge of surface precipitation, and under the control of the tectonic structure, which is hydraulically connected with the karst system of geothermal area, and it also provides the channel of the deep transport of the groundwater, and the elevation of rock exposed in the north and west wings is about 500m, and the runoff time is 50 years before entering the geothermal area. In the north west wing, the elevation of rock outcrop is about 500m, which is about 20 times of the elevation of hot spring water outcrop in the geothermal area, so the natural head difference creates conditions for the deep circulation of groundwater.
When the atmospheric precipitation under the action of gravity along the carbonate rock runoff system to the deep for circulation, coupled with the difference in topographic height, for the recharge of groundwater, transportation, storage provides a good geological conditions. In the upper part of the Ordovician system is Silurian siltstone with high content of muddy components, and a natural cover of geothermal heat is formed between the submerged mountains and the Damaoshan mountains. According to the regional data, the average thickness of the cover layer can reach 2,264m, which not only protects the earth's deep heat loss, but also reserves the energy for the absorption of heat by the deep circulating groundwater.
When the geothermal fluid runoff to the vicinity of the submerged mountain, due to the cutting effect of the hot springs fracture and a group of north-west-west oriented fracture, the geothermal fluid is exposed along the fracture zone, forming hot springs and geothermal fields.
2.2 Calculation of Geothermal Resource Reserve
Based on the characteristics of geothermal geological conditions (including cover, thermal storage, heat-controlling and heat-conducting tectonic structures, etc.) of the geothermal field in Xianning City, and combining with the "Geothermal Resource Evaluation Methods (DZ40-85)" and the research information about the evaluation methods of geothermal resources, it is proposed to evaluate the geothermal resources by using the thermal storage method.
After calculation, the resource of the whole geothermal field is 12.1145×1013kcal, or 507.2×1015J, which is equivalent to 17.3×106t of standard coal.The static storage capacity of geothermal fluid is 3875.5×104m3, and according to the recovery rate of 0.15, the amount of utilizable geothermal resource is 2074.40×104kcal/h, which converts to Power generation 2.44 × 104kW, equivalent to standard coal 10.7 × 104t / a, that is, the scale of the hot spring geothermal field in Xianning City for medium-sized geothermal field.
3 Development and utilization of geothermal resources
3.1 Overview
Hamming City has a long history of developing geothermal resources. Tianshun five years of the Ming Dynasty (1461) "Xianning County Records" described hot springs "water with sulfur gas", "sores and ulcers can be healed at any time by bathing". 1938 the end of the hot springs occupied by the Japanese invasion forces as a restricted area, the construction of "military hospitals". The hot spring was occupied by the Japanese invasion army at the end of 1938, and was designated as a restricted area for the construction of "military hospital". After the victory in the war, the Kuomintang army set up training camps in hot springs. 1949 September, the Chinese People's Liberation Army stationed in hot springs, hot springs in the old site of the People's Liberation Army set up 195 hospitals, from then on, the hot springs of Xianning geothermal began to benefit the people. Artificial exploitation and use of hot springs began in the 1960s, to the 80s to get more development, since the 90s gradually to the legalization, scientific direction.
Because the development and utilization of hot springs is increasingly concerned by all sectors of society, Xianning City Hot Springs Economic and Technological Development Zone has been the People's Liberation Army 195 Hospital, Quanshan Hotel, Zhengzhou Railway Bureau sanatorium, Han Shang Group Co., Ltd. Xianning Hot Springs Villa, Wuhan Changyin Group Hot Springs Hotel and other 20 enterprises and units to develop and utilize geothermal resources. Geothermal resources development and utilization is mainly concentrated in the hot springs economic and technological development zone around the original 28 geothermal wells, there are still 13 units **** 15 wells are still being exploited. Geothermal fluid is pumped by centrifugal pumps, vacuum pumps, submersible pumps and other equipment, and the extraction capacity is generally 30-100m3/h. In 2005, the average daily extraction of geothermal fluid totaled 4,470m3, and the peak can reach 5,020m3/d, with a minimum of 3,320m3/d.
Currently, the application of geothermal fluid is mainly used in the fields of service, medical care, aquaculture, residential life and scientific research, and has achieved good economic results. fields, and has achieved good economic, social and environmental benefits.
3.2 Existing major problems
With the wide application of geothermal fluids, the exploitation of geothermal fluids has also increased accordingly, and in the process of developing and utilizing the geothermal resources, some environmental and geological problems have arisen.
At present, within the geothermal area, the main problems arising are the mutual interference between well springs and boreholes, the decrease in temperature of geothermal fluid, the variation of water quality of geothermal fluid, and the decrease in regional water table.
3.3 Recommendations
In order to make better sustainable development of geothermal resources in Xianning Hot Spring Geothermal Field, the gradient comprehensive utilization, to avoid waste and cause environmental geological problems. Suggestions:
(1) Strict management of geothermal resources development and utilization, implementation of scientific planning, unified centralized mining, unified heat supply, and water withdrawal and distribution of full-time regulation and management. Strengthen the centralized heating engineering construction, construction of large-capacity insulation regulator, the use of advanced technology, improve resource utilization, improve the level of utilization, improve process technology, optimize the system parameters, the rational use of new materials, excellent material, make the best use of the material. Strengthen the management, put an end to the phenomenon of running, risking, dripping, leaking and wasting. Do a good job in the comprehensive utilization and extended development.
(2) formulate and promulgate the Measures for the Management of Geothermal Resources in Xianning City, and accelerate the legislation on geothermal management.
(3) Strict approval system for exploitation of geothermal resources, and control of the total amount, intensity and utilization conditions.
(4) Follow the procedure of geothermal resources exploration and exploitation, and strictly control the localized exploitation amount and intensity of geothermal resources.
(5) Improving the utilization rate of geothermal resources, adjusting the industrial structure, controlling the utilization of energy-consuming methods, and systematically eliminating the backward energy-consuming equipments, so that the equipments and systems can be developed in the direction of saving.
(6) Establishment of geothermal resource protection zones.
(7) Strengthen the monitoring of its dynamic change characteristics during the exploitation of geothermal resources, set up a systematic monitoring network to observe the changes in water quality, temperature, water quantity and water level of geothermal fluids under the conditions of exploitation, and at the same time, pay attention to the investigation and research of the environmental and geological problems.
(8) The geothermal mining area is subject to an annual geo-environmental assessment work.
(9) Strive for national funds to establish a national geothermal fluid monitoring point.
(10) Establish a dynamic monitoring network. Dynamic database is the most direct and intuitive trend response to the resources in management and development, therefore, a sound dynamic monitoring network should be established in the hot spring area to strengthen the dynamic monitoring of exploitation, which will provide a basis for the evaluation of the reliability of the resources and resource management to formulate the relevant policies.
(11) Construction of geothermal resources utilization demonstration project. Geothermal resources development and utilization involves many fields, each field and utilization must realize the unity of resources, environment and economic benefits. As many technologies and indicators are still in the exploratory stage, in the development of the role of demonstration projects is very much needed to drive the development of different types of utilization areas, training to establish the intensive use of geothermal demonstration projects, and in the management process of the demonstration project as a model to promote the application of demonstration engineering technology, and to play a point to lead the way to improve the role of the overall situation.
4 Conclusion
Geothermal resources is the advantage of Xianning City, as the geothermal itself is unable to circulate in the market in the form of commodities in the direct minerals, its value is through the development and utilization of thermal energy efficacy, geothermal resources industrialization and the development of the good economic, social and environmental benefits of a comprehensive embodiment of the benefits. Therefore, unified planning, scientific development, rational utilization, effective protection of geothermal resources is the city of Xianning economy, society, environment and coordinated development of a strong guarantee.
Geothermal development and utilization should be market-oriented, resource-based, industry-based, give full play to the advantages of geothermal resources, make full use of geographic and transportation advantages, combined with the local natural landscape, humanities and historical background and other tourism resources, to open up a distinctive style, outstanding personality of the construction of the facilities, constituting the development and utilization of geothermal resources for the development and utilization of the new pattern for the city of Xianning, making greater contributions to the economic construction and social progress.