At a certain depth underground, when the internal heat of the earth and the solar radiation heat balance each other, this zone is the constant temperature zone, the solar radiation heat gradually weakens, the temperature remains relatively constant, and the ground temperature amplitude is close to zero. The temperature in the constant temperature zone varies greatly throughout the year, which is mainly related to latitude, altitude, surface water distribution, vegetation coverage and microclimate. The annual constant temperature zone temperature is generally higher than the local annual average temperature of 65438 0 ~ 3℃.
The temperature condition and temperature field in the warming zone are mainly controlled by the internal heat of the earth, and the ground temperature increases with the increase of depth. According to the temperature measurement data of geothermal wells in Henan Province, the normal geothermal heating rate in Henan Province is generally around 3℃/ 100m, and the local geothermal temperature is high due to geological structure, which belongs to the local thermal anomaly area. It can be seen that the change of shallow geothermal field is the result of the comprehensive action of internal heat and solar radiation heat. With the increase of depth, the former gradually plays a leading role, and the influence of the latter gradually decreases.
I. Main factors controlling and influencing shallow geothermal distribution
A large number of research results have confirmed that the ground temperature of a region is a comprehensive reflection of the geological structural conditions and geological history of the region. For the ground temperature within several thousand meters of the shallow crust, the influencing factors are generally the fluctuation and structural form of bedrock surface, groundwater activity and magmatic activity. A large number of practical data show that the ups and downs of bedrock surface and structural morphology have regional influence on the distribution of geothermal energy, and play a leading role in the shallow crust geothermal energy; Groundwater activity has an important influence on the ground temperature near the piedmont plain and some large-scale active fault zones, and the influence of magmatic activity may be limited to local areas where large-scale magmatic activity has occurred recently.
1. The undulation and structural form of bedrock
Most of Henan Province is located in the southwest of North China landmass, with Archean metamorphic rocks as the basement, on which a set of platform-type Mesoproterozoic, lower Paleozoic dominated by marine carbonate rocks, upper Paleozoic and Mesozoic interacting with land and sea are developed. Since Archean, this area has experienced many tectonic movements, and its early structural framework and caprock have been reformed to varying degrees. Among them, Yanshan movement is the decisive factor for the formation of the present tectonic framework in this area. In the early stage, the whole North China landmass began to disintegrate, and a series of NW-trending, nearly EW-trending and NNE-trending extensional faults developed, forming fault blocks and rift basins. Taihang Mountain, Songshan Uplift and North China Rift Basin subsidence form depression. Jurassic and Lower Cretaceous were deposited in the center of the depression, which laid the embryonic form of geological structure in this area. Since the Cenozoic era, the basin has been continuously declining to accept sedimentation, and a set of extremely thick relatively loose strata dominated by river and lake facies has been deposited. The deeper the depth, the stronger the consolidation diagenesis.
Except Luoyang, Nanyang and Sanmenxia, other cities in Henan Province are located in inland valley basins, and most of them are located in piedmont areas. The thickness of Cenozoic is increasing from piedmont to plain, and the thickness of urban area is generally more than 500m m. There are huge Mesozoic and Late Paleozoic clastic rock deposits below Cenozoic. A large number of practical data show that the fluctuation of bedrock surface has certain influence on the distribution of ground temperature. The Cenozoic caprock has a high geothermal temperature and a large geothermal gradient within a certain depth range. The negative tectonic geothermal is low and the geothermal gradient is small; The ground temperature distribution in the shallow crust is positively correlated with the fluctuation of bedrock surface. At the same time, under the condition of roughly the same structural form, because the strata with high or high thermal conductivity are covered by Cenozoic caprocks with different thickness and low thermal conductivity, the geothermal gradient in the shallow crust is also quite different. For example, the bedrock under the loose layer in Zhengzhou city is Triassic sandstone shale, and its thermal conductivity is higher than that of the overlying loose layer. Therefore, the warming rate of ground temperature and ground temperature at the same depth in Zhengzhou loose layer is higher in the western suburbs than in the eastern suburbs. For example, the geothermal heating rate of Huang Zhuang in the northern suburb of Rudong, which is more than 200m, is only1~ 2℃100m, and that in the western suburb is 2 ~ 3℃/ 100m.
2. Buried fault structure
The underground temperature field is not only influenced by tectonic units, but also controlled by hidden fault structures. Main faults are generally beneficial to hydrothermal activity, especially active faults. The control cracks are thermal control cracks and thermal conduction cracks. The regional active faults and rock fractures in the deep crust reaching the mantle are well developed, which are good channels for the upwelling of deep heat energy. When the deep heat energy rises along the fault zone, it is absorbed by water and rocks, forming a certain width of underground hot water storage zone, so these faults belong to thermal control faults. Most of these faults are NW-trending and nearly EW-trending. The NE-trending and NEE-trending faults formed from late Yanshan to Himalayan are also active faults, which not only controlled the deposition of Cenozoic caprocks, but also dislocated caprocks in the later inherited activities. The activity of these new faults opens the channel of the caprock, which makes the underground hot water gush out along the NE-trending and NEE-trending fault zones, forming a low-temperature hot water reservoir zone with a certain width, which affects the distribution of shallow geothermal field. These faults belong to thermal conductivity faults. There are hidden faults in Luoyang basin and Nanyang basin, and there are many geothermal anomalies near the faults. The shallow groundwater temperature is higher than the normal water temperature 1 ~ 3℃.
3. Groundwater activities
Groundwater is widely distributed in the shallow crust, which is easy to flow and has a large heat capacity, and has an important impact on geothermal field. The degree of its influence depends on the scale of sedimentary basin, geomorphic units and characteristics, the distribution (thickness and continuity) of aquifer and its hydrodynamic characteristics. Usually, in the groundwater recharge area and strong runoff area replenished by cold water source, the activity of groundwater often leads to negative anomaly of ground temperature; However, in the hot water drainage area or near some deep active fault zones, positive anomalies are formed; In the hinterland of great basin, far away from the groundwater recharge area, groundwater runoff is slow or even stagnant, and this influence gradually weakens or even disappears.
The negative anomaly of ground temperature in groundwater recharge area and runoff area is due to the low-temperature groundwater entering from recharge area, exchanging heat with surrounding rock in the process of rapid flow, constantly taking away heat and cooling the surrounding rock, thus reducing ground temperature. Zhengzhou city is located in the front of the piedmont alluvial-diluvial inclined plain. Although the hydraulic gradient is large, the permeability coefficient of the aquifer is small, so there is basically no negative anomaly in the ground temperature, but the ground temperature gradient is slightly lower, especially in the area near the Yellow River, which is replenished by the Yellow River and has coarse aquifer particles, so the shallow ground temperature is low in winter.
Second, the regional geological background controls the distribution and utilization of urban shallow geothermal energy.
1. Influence of meteorology, hydrology and topography on shallow geothermal energy distribution
(1) Influence of Meteorology and Hydrology on Distribution of Shallow Geothermal Energy
Henan province is located in the transitional zone between warm temperate zone and subtropical climate, and the climate has obvious transitional characteristics. The climate in Henan is characterized by Leng Xia fever in winter. Annual average temperature 12.8 ~ 15.5℃. The average temperature in July is 27 ~ 28℃; In June 5438+10, the monthly average temperature was -2 ~ 2℃. The extreme minimum temperature in the whole province is mostly between-15 and-20℃. The extreme maximum temperature in the whole province is above 40℃ except in mountainous areas, and Luoyang Basin is the highest. Controlled by latitude and climatic conditions, the temperature in the constant temperature zone of cities in Henan is generally 15.5 ~ 17℃, which is lower in the north and higher in the south, so the distribution of shallow geothermal energy is also affected by climate.
Because Henan is mostly a warm temperate continental monsoon climate, it is dry and cold in winter and hot and rainy in summer, so it needs to be warm in winter and cool in summer. The days of cooling and heating in this area are roughly the same, and the cooling and heating loads are basically the same. The utilization of shallow geothermal energy needs to extract underground thermal energy for heating in winter and store it underground in summer, which can keep the balance of underground temperature field and minimize the impact on the environment.
Henan province is located in the Yellow River and the Yangtze River. There are many rivers that branch radially from west to north and from east to south. The Yellow River and Huaihe River are the main rivers in Henan Province. In addition to the Yellow River flowing through Sanmenxia and Zhengzhou, the rivers that flow through the city with a large amount of water mainly include Luoyang Luohe, Nanyang Baihe, Xinyang Shihe, Pingdingshan Shahe, Luohe and Zhoukou. Other rivers flowing through the city are mainly urban sewage. Rivers have a certain influence on the distribution of shallow geothermal energy in cities. Not only does surface water contain some geothermal energy, but it also has a certain influence on the distribution, migration and occurrence of shallow geothermal energy in rocks and groundwater along the river. In recent years, the application of river water source heat pump system, lake water source heat pump system and sewage source heat pump system has made full use of shallow geothermal energy of surface water. Therefore, the shallow geothermal energy distributed in surface water can not be ignored, especially the use of treated urban sewage should be encouraged.
(2) The influence of topography on the distribution of shallow geothermal energy.
Henan province is located in the transition zone from the second step to the third step in China. Its topography is characterized by mountains, hills and terraces in the north, west and south, and vast and open plains in the east. 7.40× 104km2 in hilly area and 9.30× 104km2 in plain area. On the whole, its topography is high in the west and low in the east, and it gradually declines from west to east, and it transits from the middle and low mountains in the west to the hills, platforms and plains in the east.
Shallow geothermal energy mainly refers to the thermal energy with development and utilization value contained in rock and soil, groundwater and surface water within a certain depth below the surface. That is to say, the heat energy below 25℃ within 200 meters underground is utilized by the heat exchange technology of ground source heat pump. Generally speaking, all kinds of geomorphic units have shallow geothermal energy distribution, and the plain area is superior to the hilly area in terms of development and utilization conditions and value alone. Western Henan is mainly located in bedrock mountainous areas and loess hilly areas, and the conditions for the development and utilization of shallow geothermal energy are poor, while the plain area is vast and sparsely populated, with excellent geological, hydrogeological and shallow geothermal energy development and utilization conditions, which is the key area for the development and utilization of shallow geothermal energy. Henan Plain mainly includes Henan section of Huang-Huai-Hai Plain, piedmont inclined plain, Nanyang Basin, Luoyang Basin and Lingbao-Sanmenxia Basin. Of the 8 provincial cities in Henan Province/kloc-0, 3 are located in the above-mentioned Three great basin (valley plain), and the rest 15 are located in the piedmont alluvial plain and river alluvial plain. The distribution and utilization of shallow geothermal energy are controlled by its topographic position to some extent.
2. Analysis on the control of stratum and structure on the distribution and utilization of shallow geothermal energy.
All the cities under the jurisdiction of Henan Province 18 are located in the plain Quaternary distribution area, and the distribution and occurrence of shallow geothermal energy are mainly controlled by the lithologic structure of Quaternary and its underlying Tertiary.
(1) Formation lithology
Shallow geothermal energy in Henan plain mainly occurs in Quaternary and Upper Tertiary loose strata. Neogene is mainly distributed in basins and eastern plains, and its lithology is mainly alluvial-lacustrine sandstone (sand layer), siltstone, mudstone and marl. Quaternary is widely distributed in eastern plains, intermountain basins and piedmont hills. In the western intermountain basin and piedmont hilly area, the Lower Pleistocene (Qp 1) is alluvial sand, gravel and clay, and there are Wucheng loess on both sides of the Yellow River. The Middle Pleistocene (Qp2) is composed of alluvial-diluvial silty clay, sand and gravel, and there is Lishi loess in Lingbao-Zhengzhou. The western part of Upper Pleistocene (Qp3) is alluvial silt, silty clay, sand layer and gravel layer, and Lingbao-Zhengzhou has Malan loess. Holocene (Qh) is fluvial alluvium, with local aeolian deposits. See Table 2-2 for the specific stratum lithology of each city.
Because of the different lithologic conditions and causes, the distribution and utilization of urban shallow geothermal energy are also controlled by it. Generally speaking, the stratum lithology distributed in urban areas is mainly loose stratum with fine-grained sediments such as alluvial, alluvial and lacustrine deposits, including clayey soil, silty soil and fine-grained sandy soil in Quaternary and Neogene. , suitable for vertical buried heat exchange system. In urban areas, the lithology of strata is mainly loose strata, with alluvial and alluvial deposits, and the sediments are mainly coarse-grained pebbles, gravels, coarse sand and medium sand, or when the thickness is large, the influence of groundwater is considered, which is suitable for groundwater heat exchange.
(2) Geological structure
According to the characteristics of shallow geothermal fields in each city, the influence of geological structure is slightly different, but the influence is not great. The Cenozoic in the plain area is thick. When the bedrock basement is uplifted or uplifted, the geothermal gradient is large and the shallow geothermal is slightly higher. When the bedrock basement is depressed, the geothermal gradient is small and the shallow geothermal is slightly lower. The temperature difference between the two in the same area is generally not more than 3℃. The three great basin in western Henan Province are mainly controlled by concealed fault structures. Generally, near or at the intersection of active faults, the shallow ground temperature is slightly higher than other areas, and the temperature difference is generally less than 3℃ when the overburden is thick. It can be seen that the Cenozoic in the urban area is thick, and the geological structure has no obvious control on the distribution and utilization of shallow geothermal energy.
3. Hydrogeological conditions control the distribution of shallow geothermal energy.
The Huangzhunhai Plain in Henan Province, the sloping plain in front of Taihang Mountain, Nanyang, Luoyang, Lingbao-Sanmenxia Basin, Huaihe River and its tributaries, the shallow aquifers are mainly alluvial, alluvial sand, gravel and pebbles, with loose structure, good sorting, generally dual structure, shallow burial, large thickness, wide and stable distribution, strong permeability, quick recharge, good water storage conditions and strong water abundance. Especially, the alluvial fan in front of Taihang Mountain, alluvial fan of Yellow River, plain of Three great basin Valley and the main tributary valley in the upper reaches of Huaihe River have excellent hydrogeological conditions, with the water inflow of single well above 2,000m3/d, and the maximum water inflow can reach above 5,000m3/d, and its groundwater is mainly replenished by atmospheric precipitation infiltration, surface water infiltration and lateral infiltration, which is a good circulating water source for the development and utilization of shallow geothermal energy. Shallow groundwater is generally phreatic-slightly confined water, and locally confined water. The distribution, occurrence, migration and utilization of shallow geothermal energy, especially the way of using shallow geothermal energy in underground water source heat pump system, are controlled by hydrogeological conditions, even if the vertical buried pipe heat exchange method is adopted, it is also affected by groundwater.
The occurrence horizon of urban shallow geothermal energy in Henan Province is mainly all kinds of loose deposits in the upper part of Quaternary and Tertiary, and its groundwater is loose rock pore water with wide distribution, large thickness, abundant water, easy exploitation and strong recoverability. Especially in Luoyang, Nanyang, Sanmenxia and Anyang, Hebi, Xinxiang, Jiaozuo, Jiyuan, Zhengzhou, Xinyang, Pingdingshan and other cities or lots with superior hydrogeological conditions, when developing and utilizing shallow geothermal energy, groundwater heat exchange should be adopted.