Evaluation and development of geothermal resources in hot spring geothermal field in Xianning City, Hubei Province

(Hubei Geological Environment Station)

This paper introduces the thermal geological conditions and genetic types of the hot spring geothermal field in Xianning City, Hubei Province, evaluates the geothermal resources, shows the achievements and basic experiences of development and utilization in recent years, puts forward the problems existing in the development process, and puts forward reasonable and feasible suggestions.

Xianning City is located in the southeast of Hubei Province, on the south bank of the middle reaches of the Yangtze River, at the junction of Hunan, Hubei and Jiangxi provinces. It is the main passage from south to north and is called "the south gate of Hubei". The city has jurisdiction over four counties, one city, one district and one 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.

Xianning city is rich in geothermal resources, which are widely distributed, and there are hot springs exposed in one district, four counties and one city under its jurisdiction. With the wide use of geothermal resources as green energy, more and more attention has been paid to it. At the beginning of the 20th century, the detailed investigation of geothermal resources in Xianning City, Hubei Province, the evaluation of geothermal resources in Wuhongshan, chibi city City, Hubei Province, the general survey of geothermal resources in Wuhongshan, chibi city City, Hubei Province, the detailed investigation of geothermal resources in Langkou geothermal field in Chongyang County, Hubei Province, and the exploration of geothermal resources in Shewushan, Jiayu County were successively carried out. Among them, the detailed investigation of geothermal resources of hot spring geothermal field in Xianning City, based on the previous exploration, made a deep study on the nature and scale of structural characteristics in buried hill area and its relationship with geothermal fluid distribution, runoff and discharge. Through calculation, the recoverable amount is 13320. 1m3/d, and the highest water temperature is 55℃. In terms of distribution range and recoverable resources, geothermal field is the highest in exploration results.

1 regional geothermal geological conditions

1. 1 Brief introduction of geological structure

Xianning geological structure is located at the intersection of Liangzihu depression of Yangtze paraplatform and Xianning platform fold bundle, which belongs to the secondary structure of Gao Qiao syncline, Sunjianpu inversion syncline and Jiajiashan inversion syncline in the north wing of Damushan complex anticline, and consists of a series of dense folds, which constitutes the basic structural framework of this area. The tectonic system in the late Neocathaysia is strong in the buried hill area, and the Dayishan fault is obviously reflected. These different tectonic systems interfere and compound with each other, which not only controls the lithologic combination, landform and karst development in the area, but also strictly controls the distribution of groundwater and its recharge, migration and discharge. Buried hill anticline is the main structural framework of geothermal field, which is comb-shaped and reverses to the southeast. The scale of Wenquan fault in this area is the largest, and it is a compound fault with multiple structural changes. In the early stage, it is longitudinal fracture, mainly vertical movement, and in the later stage, it is reverse recombination, which shows compression and torsion under counterclockwise stress. It is a deep cutting fault and the main fault that controls the distribution of thermal fluid in the area.

1.2 hydrogeological conditions

The exposed part of hot spring is caused by the intersection of northwest fault and northeast fault cutting thermal reservoir. The rock at the intersection is strongly broken, with cataclastic rocks, breccia and cracks. The porosity of structural rocks in fault structural belt is greater than that of original rocks, especially at the intersection of faults, which provides favorable space for the storage and migration of geothermal fluids. The faults in this area have experienced multi-stage activities. Because of their different structural modes and strengths, the degree of rock fragmentation and cementation is different, and the interaction with other faults leads to the heterogeneity of fault water abundance and thermal conductivity. In areas with poor rock fracture cementation and developed fractures, geothermal fluid is relatively rich and water is large. In areas with broken rocks and poor connectivity of fractures, the water storage space is narrow, the access is not smooth, the water temperature is low and the water quantity is small.

According to the sampling analysis of geothermal fluid in geothermal field, the main gas components in geothermal fluid in Wenquan geothermal field in Xianning city are N2, O2 and CO2, in which N2 accounts for 94.60% ~ 95.70% of escaping gas and 66.30% ~ 68.80% of dissolved gas. High abundance of N2 mainly exists in the atmosphere, and the source of N2 in geothermal fluid is brought into the ground by atmospheric precipitation, and geothermal fluid is mainly replenished by atmospheric precipitation infiltration.

The δD and δ 18O of atmospheric precipitation have altitude effect. According to the isotope results of geothermal fluid and atmospheric precipitation sampling, it is estimated that the recharge elevation of hot spring geothermal field in Xianning city should be about 470m m m. According to the analysis of regional topography and geomorphology, the exposed carbonate rocks are replenished by atmospheric precipitation in the low-Zhongshan area, which is 300 ~ 500 m southeast of geothermal field, and supplied to Xianning hot spring geothermal field through karst channels.

In addition, according to the analysis of tritium content in geothermal fluid, it is known that geothermal fluid should be a long-term runoff cycle of geothermal fluid infiltrated by atmospheric precipitation before 1954, that is, the runoff of geothermal fluid in this area has gone through 50 years or more from recharge to discharge, which represents the runoff time of geothermal fluid in geothermal area. In the area with thin surface caprock and developed fractures, there is a mixture of surface water and fissure water recently.

1.3 characteristics of geothermal field

According to the comprehensive analysis of exploration data, the buried depth of the normal temperature layer of the hot spring geothermal field in Xianning City is generally about 25m, and the borehole temperature in the area has a gentle rising trend, that is, with the increase of borehole depth, the temperature rises. According to the analysis of the characteristics of borehole temperature change, the borehole temperature in this area rises slowly, and the borehole temperature changes in the range of 0.045 ~ 0.23 1℃/m, and the temperature changes in different ranges due to the different buried depth of thermal conductivity and heat storage structures or the exposed depth of some boreholes. The temperature rise above the fault zone is obvious, but the temperature rise in the fault fracture zone is not obvious. The geothermal gradient value of geothermal field is directly related to geological environment conditions (location, lithology, geological structure, groundwater, etc.). ) geothermal field. The geothermal gradient values of rocks in the hot zone have some changes, and most of them are1.15 ~ 26.74℃/100m. According to the data of drilling temperature measurement, the average temperature gradient in the area is 6.57℃/ 100m, and the thermal anomaly is obvious.

The long axis direction of the plane temperature isoline is 290 northwest, which is roughly consistent with the direction of hot springs exposed on the surface, indicating that the distribution and exposure of geothermal fluid in this section are mainly controlled by NW-trending structures.

1.4 hydrochemical characteristics of geothermal fluid

According to the sampling analysis of geothermal fluid wells exposed artificially in the exploration area, the hydrochemical type of geothermal fluid is sulfuric acid-calcium water. In addition, according to the relationship between sulfate ion content and temperature, the detected sulfate ion concentration in geothermal fluid has a linear relationship with water temperature in a certain range. When the water temperature is 20 ~ 50℃, the variation range of sulfate ion concentration is 17.50 ~ 13 1. The hydrochemical type of surface water and normal temperature groundwater is bicarbonate, which is very different from geothermal fluid. It can be seen from the above data that the characteristic ions in geothermal fluid in the exploration area are potassium ions, calcium ions, magnesium ions, sulfate ions and fluoride ions, which are important hydrochemical indicators and main standard elements of geothermal fields.

2 Hot spring geothermal field geothermal resources evaluation

2. Establishment of1thermal storage model

2. 1. 1 thermal reservoir caprock

The stratum between the east side of the buried hill anticline and the northwest wing of Damoshan anticline is Silurian, and the lithology is argillaceous siltstone, shale, siltstone and other rocks, with a total thickness of1762 ~ 2,856 m and an average thickness of 2264m m. Due to the high content of argillaceous components in rocks and poor permeability and water storage performance, cracks and fissures developed in some areas are filled with argillaceous, and the poor heat transfer performance of rocks forms a "U" between Damoshan and the buried hill. It is a heat storage cover in geothermal area.

2. 1.2 Thermal control and heat conduction structure

In this area, Wenquan fault is a representative main fault with the largest cutting depth and scale. In the southeast of buried hill, it controls the contact boundary between Silurian strata and Ordovician strata, and the geothermal fluid flowing in carbonate rocks is blocked when it reaches the vicinity of thermal control fault and hot spring fault in this area. In addition, in the area of Moon Bay, the NW-trending extensional faults are thermal and water-conducting faults.

2. 1.3 heat storage

According to the analysis of drilling data, the supply, migration and storage of geothermal fluid are all carried out in carbonate strata, and abundant geothermal fluid is obtained in carbonate strata with karst development. Therefore, under silicified rocks and Silurian strata, breccia limestone, crystalline limestone, tortoise fissure limestone, bioclastic limestone and dolomite limestone, rock fissures and karst develop, and local areas are the main storage places for geothermal fluids and thermal reservoirs.

2. 1.4 Formation of geothermal field

According to the analysis of isotope data of predecessors, the main source of geothermal fluid in this area is atmospheric precipitation, supplemented by alpine precipitation. The elevation effect of hot spring geothermal field is about 500m, and it takes more than 50 years to enter geothermal area.

The Sinian, Cambrian and Ordovician strata exposed in the northwest wing of Damoshan anticline are mainly composed of limestone, dolomite limestone or limestone dolomite, dolomite and other carbonate rocks. Exposed rocks are beneficial to the infiltration and recharge of surface precipitation, controlled by structure, and have hydraulic connection with karst system in geothermal area, which provides a channel for the deep migration of groundwater. The exposed elevation of rock in the northwest wing is about 500m.

When atmospheric precipitation circulates along the deep part of carbonate runoff system under the action of gravity, and the difference of terrain height provides good geological conditions for groundwater recharge, migration and storage. Upper Ordovician is siltstone rich in Silurian argillaceous components, and geothermal natural caprock is formed between buried hill and Damoshan. According to regional data, the average thickness of overburden can reach 2264m, which not only protects the heat loss in the deep part of the earth, but also reserves energy for the heat absorption of deep circulating groundwater.

Local geothermal fluid flows to the vicinity of buried hill. Due to the cutting action of hot spring fault and a group of NW-trending faults, geothermal fluid emerges along the fault zone, forming hot springs and geothermal fields.

2.2 Calculation of geothermal resources reserves

According to the characteristics of geothermal geological conditions (including caprock, thermal storage, thermal control and thermal conductivity structure, etc.). ) geothermal field, combined with the research data of Geothermal Resources Evaluation Method (DZ40-85) and related geothermal resources evaluation methods, it is suggested that the thermal storage method be used to evaluate geothermal resources.

After calculation, the total geothermal field resources are12.1145×10/3 kcal, which is equivalent to standard coal 17.3× 65438. The static reserve of geothermal fluid is 3875.5× 104m3. According to the recovery rate of 0. 15, the available geothermal resources are 2074.40× 104kcal/h, which is equivalent to the power generation of 2.44× 104kW and 10.7× standard coal.

3 development and utilization of geothermal resources

3. 1 overview

Xianning city has a long history of developing geothermal resources. In the fifth year of Tianshun in Ming Dynasty (146 1), Xianning County Records recorded that hot springs "have sulfur in the water" and "all boils and carbuncles are healed by bathing". At the end of 1938, the hot springs were occupied by the Japanese invaders and designated as restricted areas, and a "military hospital" was built. After the victory of the Anti-Japanese War, Kuomintang troops set up training camps in hot springs. 1949 In September, the People's Liberation Army of China moved south to the hot springs, and established the PLA 195 Hospital in the former site of the hot springs. Since then, Xianning hot spring geothermal began to benefit the people. The artificial development and utilization of hot springs began in the 1960s and got great development in the 1980s. Since 1990s, it has gradually developed in the direction of legalization and science.

With the development and utilization of hot springs receiving increasing attention from all walks of life, Xianning Hot Spring Economic and Technological Development Zone has successively developed and utilized geothermal resources by 20 enterprises including China People's Liberation Army 195 Hospital, Quanshan Hotel, Zhengzhou Railway Bureau Sanatorium, Hanshang Group Co., Ltd. Xianning Hot Spring Villa and Wuhan Yin Chang Group Hot Spring Hotel. The development and utilization of geothermal resources are mainly concentrated in the hot spring economic and technological development zone. There are 28 geothermal wells, and there are still 13 *** 15 wells still being exploited. Geothermal fluid is pumped by centrifugal pump, vacuum pump, submersible pump and other equipment, and the water intake capacity is generally 30 ~100 m3/h ... In 2005, the daily average amount of geothermal fluid was 4470m3, the peak period was 5,020 m3/d, and the lowest was 3,320 m3/d. ..

At present, the application of geothermal fluid is mainly in the fields of service, medical treatment, aquaculture, residents' life, scientific research and so on, and has achieved good economic, social and environmental benefits.

3.2 Main problems

With the wide application of geothermal fluid, the exploitation of geothermal fluid has also increased correspondingly, and some environmental geological problems have also occurred in the process of developing and utilizing geothermal resources.

At present, in geothermal area, the main problems are the mutual interference between Jing Quan and borehole, the decrease of geothermal fluid temperature, the change of geothermal fluid quality and the decrease of regional groundwater level.

3.3 Suggestions

In order to make the geothermal resources of Xianning hot spring geothermal field get better sustainable development and gradient comprehensive utilization and avoid waste and environmental geological problems. Suggestion:

(1) Strictly manage the development and utilization of geothermal resources, implement scientific planning, unify centralized mining and heating, and conduct full-time regulation and management of water intake and distribution. Strengthen the construction of central heating project, build large-capacity heat preservation regulating pool, adopt advanced technology, improve resource utilization rate, improve utilization level, improve process technology, optimize system parameters, rationally use new materials and make the best use of them. Strengthen management and put an end to running, running, dripping, leakage and waste. Do a good job in comprehensive utilization and delayed development.

(2) Formulate and promulgate the Measures for the Management of Geothermal Resources in Xianning City, and speed up the legislation of geothermal management.

(3) Strictly examine and approve the exploitation of geothermal resources, and control the total amount, intensity and utilization conditions.

(4) Follow the exploration and development procedures of geothermal resources and strictly control the exploitation amount and intensity of local geothermal resources.

(5) Improve the utilization rate of geothermal resources, adjust the industrial structure, control the utilization mode of high energy consumption, and gradually phase out the backward equipment with high energy consumption, so as to make the equipment and system develop in an economical direction.

(6) Establish geothermal protection zones.

(7) Strengthen the monitoring of the dynamic change characteristics of geothermal resources in the process of exploitation, establish a systematic monitoring network, observe the changes of water quality, temperature, water quantity and water level of geothermal fluid under the exploitation conditions, and attach importance to the investigation and study of environmental geological problems.

(8) Geothermal mining area geological environment assessment once a year.

(9) Strive for national funds and establish national geothermal fluid monitoring points.

(10) Establish a dynamic monitoring network. Dynamic database is the most direct and intuitive trend response to resources in management and development. Therefore, the dynamic monitoring network should be established and improved in the hot spring area, and the dynamic monitoring of mining should be strengthened to provide a basis for resource reliability evaluation and resource management to formulate relevant policies.

(1 1) demonstration project construction of geothermal resources utilization. The development and utilization of geothermal resources involves many fields, and each field and utilization mode must realize the unity of resources, environment and economic benefits. Because many technologies and indicators are still in the exploratory stage, it is very necessary to promote development with the role of demonstration projects, select different types of utilization fields, cultivate and establish geothermal demonstration projects with intensive utilization methods, and take the demonstration projects as a model in the management process to promote the application of demonstration engineering technologies and play an overall role in upgrading.

4 conclusion

Geothermal resources are the dominant minerals in Xianning city. Because geothermal itself is a direct mineral product that cannot be circulated in the market as a commodity, its value is comprehensively reflected by the good economic, social and environmental benefits obtained through the development and utilization of thermal energy and the industrialization of geothermal resources. Therefore, unified planning, scientific development, rational utilization and effective protection of geothermal resources are a powerful guarantee for the coordinated development of economy, society and environment in Xianning City.

The development and utilization of geothermal resources should be market-oriented, resource-based, industry-based, give full play to the advantages of geothermal resources, make full use of geographical and transportation advantages, and develop facilities with distinctive styles and outstanding personalities in combination with local natural landscapes, cultural and historical backgrounds, thus forming a new pattern of geothermal resources development and utilization and making greater contributions to Xianning's economic construction and social progress.