In addition to the heat source from the thermal reservoir on the surface of the Earth's crust which is normally generated by the geothermal gradient, waste heat from magma is commonly found in Yunnan and Tibet.
(1) Basic concepts
1) Earth's temperature field (T): The temperature of any point is a function of its spatial location and time: T=f ( x, y, z, t).
2) Geothermal gradient ( ): along the normal direction of the isothermal surface, pointing towards the center of the earth, the temperature increases per unit distance, the unit is ℃/100m.
3) Heat flux density (q): The heat flux dissipated from the unit of land surface per unit of time, in J/(cm2·s): 4) Thermal conductivity or thermal conductivity of rock (K): along the unit length (L) of the heat flow transfer direction, when the temperature changes by one degree, heat passes through the unit area per unit time, l 'unit is kJ/(m·℃·h): where: Q is heat; is the geothermal gradient; s is the thermal conduction zone; t is the heat transfer time.
(2) Commonly used parameters
1. Several geochemical methods for measuring the temperature of the thermal reservoir (body)
(1) Silica method
< p>Hot water type:t=32 .7 (SiO2) 0.311+1 (7-38)
Steam type:
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(2) Sodium method -potassium
Hot water type:
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Steam type:
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(3) Sodium-potassium-calcium method
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Where: Potassium (K), sodium (Na content), calcium (Ca) and silica (SiO2) are mass fractions, and the unit of measurement is mg/kg, the coefficient value β is if t>100℃, β is 1 /3, if t<100℃ and is a positive value, then β takes4/3.
In addition, there is also the standard sodium-lithium temperature method, which can also be measured directly in the borehole with the PSJ-2 digital logging tool.
2. Hot water and hot spring temperature classification
1) Cold water, <20 ℃
2) Low temperature water, 20 ~ 40. ℃;
3) Medium and low temperature water, 40~60℃;
4) Medium and low temperature water, 60℃ at local boiling point;
5) High temperature hot water, ≥ local boiling point.
3. Geothermal warming level and warming rate (geothermal gradient)
1) Geothermal gradient (warming rate): This is the area of normal surface temperature, each time it drops. 100 m, temperature increase [℃/100 m]. The Earth's average geothermal gradient is 3℃/100 m.
2) Level of geothermal heating: this is the depth that must change for each increasetation or decrease of 1 ℃ compared to the normal surface temperature zone [m/℃]. The average geothermal warming level of the Earth is 33 m/℃.
3) The solution formula for the geothermal gradient (geothermal warming rate q) is:
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In the formula: H is the depth of the temperature measurement point; h is the normal annual depth of the temperate zone; tcep is the normal annual temperature of the temperate zone T is the temperature of the depth point H;
For example, when calculating the geothermal gradient of a shallow buried thermal reservoir, the following formula can be used:
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Where: is the ground temperature gradient; t1 is the ground temperature at 1 m; t0.5 is the ground temperature at 0.5 m.
When calculating the geothermal gradient of deeply buried thermal reservoirs, the following formula can be usedused:
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In the formula: t0 is the temperature of the constant temperature layer or the annual average temperature, ℃; h is the depth of the thermostatic layer, m.
The conditions for calculating the geothermal gradient using equation (7-43b) are: the burial depth of the thermal reservoir is less than 1000 m, and its temperature must not be below 40°C.
4. Normal temperature zone
1) Daily temperate zone: The zone in which the heat from solar radiation affects the surface temperature and is not affected by the temperature difference between day and night is called the normal temperature zone. daily temperate zone; its depth is generally 0.7 to 1.5 m.
2) Annual normal temperature zone: an underground zone at a certain depth that is not affected by annual temperature differences. Its depth is gegenerally from 15 to 50 m and its temperature is equivalent to the local average annual temperature.
(3) Calculation of geothermal resources
Due to the different types of geothermal resources, the calculation methods of geothermal resources are also different. At present, the types of geothermal resources in China can be roughly divided into three categories: sedimentary basin type, fault (fissure) type, and recent magmatic activity type. Only common methods for calculating natural heat reserves and recoverable heat storage per unit area are presented.
1. Natural heat reserve calculation method - stored heat method
Commonly used formula:
QR=Ad［ρcCc（1-φ）+ρwCwφ]（ tr-tj) (7-44)
In the formula: QR is the geothermal resource (stored heat), kcal is the heat storage surface, m2 d is the storage thickness de heat, m; tr is the heat storage temperature, ℃; tj is the reference temperature, ρc, ρw are the densities of the rock and water, respectively, kg/m3, Cw are the specific heat capacities of the rock and water, kcal/( ; kg·℃ ); φ is the porosity of the rock (loose layer), %.
2. Calculation of recoverable heat storage per unit area
Commonly used formula:
Qr=K·hCr·(tr-t0) (7-44a)
In the formula: Qr is the amount of recoverable heat in the influence range of the geothermal production well, kJ/m2 is the geothermal extraction rate of the thermal reservoir (see table h); is the heat used by the hot water well. Reservoir thickness, m; Cr is the average heat capacity of the thermal reservoir, kJ/(m3·℃); tr is the average temperature of the thermal reservoir, ℃; of training, ℃.
3. Determination of heat storage area and thickness
1) The area ist bounded by the geothermal gradient anomaly and is determined according to equation (7-43a) and equation (7). -43b) in different situations Geothermal gradient;
2) Thickness geothermometer (PSJ-2).Digital well logging tool) is measured in the borehole and can also be measured at the using the geochemical temperature scale calculation method mentioned above.
4. Recovery rate
It is impossible to fully exploit the heat reserves calculated by the heat storage method, only part of them can be exploited. The relationship between the two is called. the recovery rate. It can be expressed by the following formula:
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In the formula: RE is the recovery rate; Qwh is the extracted heat, that is, the heat obtained from the wellhead; ; QR is the heat buried underground. The amount of stored geothermal resources.
It should be noted that this recovery rate is calculated on the basis of data obtained during the geothermal exploration period. As the production period extends, pumping from hot water wells causes the falling funnel to continue to expand, resulting in continued expansion of the falling funnel. affects the conditions of supply, runoff and discharge from the thermal reservoir; changes in these conditions will be reflected in the water flow from the extraction well or in the temperature of the pumped water. Practice has proven that in many hot field mining wells, as the production time passes, the water volume gradually decreases or the water temperature gradually decreases, and even the hot water is exhausted.
5. Estimation of the potential value of electricity production (E) and classification of geothermal field types
Division of geothermal field types: theLifespan of high temperature power generation is calculated as follows: more than 30 years and the lifespan of full use of medium temperature. The lifespan of low temperature thermal fields is calculated at more than 100 years. Geothermal fields are divided into three categories based on estimated power generation: large-scale power generation is greater than 5 × 104 kW; medium-sized fields are (1 ~ 5) × 104 kW; those on a small scale are less than 1 × 104 kW.
Estimated formula for electricity production:
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6. Methods of applying heat
The main current ways of applying heat to the floor. land are: power generation (such as commercial power generation in Yangbajing, Tibet); heating (Beijing, Tianjin, Zhengzhou, etc.); poultry farming (tropical fish); linehot springs) and extraction of natural mineral raw materials (sulfur), etc. Thermal reservoir temperature can also become the main source of thermal damage in mines.
(4) Examples
Example 1: Qiezishan Hot Spring is located in the Baoshan area. The thermal reservoir is a granite fracture zone. The temperature of the shallow part of the thermal reservoir was. measured using the geochemical temperature scale method. It is 84.95 ℃ and the deep part is 214.64 ℃, which indicates that the deep heat source is the residual heat of magma. Using equation (7-44a), the recoverable heat reserve within the influence range of the hot water well is 37.02 × 104 kJ/m2. The parameters used in the calculation are: the recovery rate K=0.20; the thickness of the heat reservoir h=41.01 m; average thermal capacity Cr=0.22×4186.8kJ/m2; the temperatureaverage ure of the thermal reservoir tr=74℃; the normal temperate temperature of the formation t0=25℃ (cited by Qi Wufu et al. “Hydrogeological investigation report of Supa Hot Spring of Qiezishan River in Baoshan, Yunnan”).
Example 2: The Tianlong Street geothermal well area is located in the Luxi Basin. The thermal reservoir consists of fine sandstone and Middle Jurassic limestone interbedded with brecciated limestone. geochemical temperature. The temperature of the layer is 51.64°C in the shallow part and 430.4°C in the deep part Using equation (7-44a), the heat reserves. recoverable in the influence range of the hot water well are calculated as follows.36.30×104kJ/m2 The parameters used in the calculation are: the extraction rate K=0.20; thermal h=75.80 m; average thermal capacity Cr=0.22×4186.8kJ/m2;the of the formation is t0 = 25°C (taken from “Geological Report on Cunpinfangjing Geothermal Well Exploration and Development and Geothermal Resource Evaluation, Luxi City, Yunnan”).