The crust is divided into temperate zone, normal temperature zone and warming zone according to the thermal state from top to bottom. The ground temperature in temperate zone is controlled by temperature, which changes periodically day and night and every year. With the increase of depth, the change amplitude decreases rapidly. The depth where the temperature influence tends to zero is called the normal temperature zone, and the ground temperature in the normal temperature zone is generally slightly higher than the annual average temperature in this area 1 ~ 2℃, so the annual average temperature in this area can be used to represent the temperature in the normal temperature zone in rough calculation. The depth of normal temperature zone is 5 ~ 10~20m in low latitudes, 10~20m in mid latitudes, and it can reach about 30m in some areas. If the annual variation range of Nanjing 10m depth is less than 0. 1℃, it can be regarded as the annual normal temperature zone. The ground temperature below the normal temperature zone is mainly affected by the internal heat of the earth, and rises regularly with the increase of depth, which is called the warming zone. The depth (m) required for temperature rise 1℃ is called geothermal heating level, which generally rises 1℃ every 33m (expressed by 33m/ 1℃). However, due to the different thermal conductivity of rocks, crustal movement and hydrogeological conditions, the geothermal heating level varies greatly from place to place, with 33-43m/℃ in North China.

According to the temperature changes with time at different depths set by UFIDA Software Park in Yongfeng District, Beijing (Figure 5-3), when the formation temperature is above 10m, the temperature changes greatly and the curve fluctuates, while when it is below 10m, the temperature changes little and the curve basically fluctuates.

Figure 5-3 Temperature changes at different depths in Yongfeng area of Beijing during the year.

The curve of temperature variation with depth measured in Beijing Olympic Park also shows (Figure 5-4) that the formation temperature generally increases with the increase of depth. Due to the influence of atmospheric temperature, the surface temperature difference within the depth of 10m is large, and the temperature below 10m basically keeps a relatively stable upward trend.

Figure 5-4 Curve of Formation Temperature with Depth in Beijing Olympic Forest Park

It can be seen from the 70m geothermal isoline map (Figure 5-4) that most of the 70m geothermal values in the evaluation area are between 13 ~ 16℃, and there are some areas that are sporadically higher than 16℃, and relatively large areas are mainly distributed in Xueyuan Road-Wali, Shunyi, Xiaotangshan and other areas.

2. Geotechnical thermophysical properties test

1) Physical parameters according to lithology statistics

According to the classification of lithology and physical properties of rock and soil, the weighted average of the whole hole profile is carried out on the basis of mathematical statistics. See Table 5-2 ~ Table 5-6 for the physical parameters corresponding to different lithology of each sampling hole.

Table 5-2 List of Physical Parameters of Clay in Different Areas

Table 5-3 List of Physical Parameters of Silty Clay in Different Areas

Table 5-4 List of Physical Parameters of Heavy Silty Clay in Different Areas

Table 5-5 List of Physical Parameters of Clay Silt in Different Regions

Table 5-6 List of Physical Parameters of Sandy Silt in Different Areas

2) Comparative analysis of physical parameters of different lithology in the same place.

In order to visually show the difference of physical parameters between different lithology of the same sampling hole, the different lithology of each hole section is compared, as shown in Figure 5-5 ~ 5- 15.

By comparing the physical parameters of different lithology of these pores, it can be seen that the pore ratio, natural moisture content and specific gravity of clay, silty clay and heavy silty clay are slightly higher than those of clayey silt and sandy silt. The porosity ratio of the five lithology ranges from 0.5 1 ~ 1.09, and the natural water content ranges from17.6% to 36.88.

On the basis of the above changes, compared with the "empirical data about soil" in the engineering geology manual, it can be seen that the measured values are within the empirical parameters. By comparing the curves of thermal conductivity, it can be seen that the thermal conductivity of sandy silt in Figure 5-5Kj is small; Fig. 5-6Lm 1 porous sandy silt has a large thermal conductivity; Figure 5-8Xh 1 The thermal conductivity values of porous clay silt and sandy silt are small. In order to find out whether these abnormal values are within the scope of empirical parameters, the empirical values given in the chapter of geometric energy in HVAC application of ASHRAE manual in 2003 (Table 5-7) are consulted.

Fig. 5-5 Comparison of Physical Parameters of Different Lithology in Kilograms

Fig. 5-6 lm 1 Comparison of Different Lithological and Physical Parameters of Borehole

Fig. 5-7 Comparison of Physical Parameters of Different Lithology in LM2 Borehole

Fig. 5-8 xh 1 Comparison of different lithology and physical parameters of borehole

Fig. 5-9 Comparison of Physical Parameters of Different Lithology in XH2 Borehole

Fig. 5-Comparison of Different Lithological and Physical Parameters in10xh3 Borehole

Fig. 5-5- 1 1 Comparison of different lithology and physical parameters of borehole

Figure 5- 12lf 2 Comparison of Different Lithological and Physical Parameters of Boreholes

Table 5-7 Empirical values of physical parameters of several typical rock and soil bodies

Compared with the parameter range given in the table, the measured results are close to the empirical values in the table. Cohesive soil (including clay, silty clay and heavy silty clay) has high thermal conductivity [average1.75 w/(m k)] because of its large natural water content (average 27%) and heavy specific gravity (2.73). Silt (including sandy silt and clayey silt) has low thermal conductivity [average1.63 w/(m k)] due to its low natural water content (average 22.6%) and low specific gravity (2.69). It can be seen that the thermal conductivity of rock and soil is directly proportional to its natural water content and specific gravity.

3) Comparative analysis of physical parameters of the same lithology in different places.

In order to visually display the changes of physical parameters in different positions of the same lithology, the parameters in the same position of the same lithology are compared, and the results are shown in Figure 5- 13 ~ Figure 5- 16.

By comparing the physical parameters of different places in the same lithology, it can be seen that the natural water content, natural density, specific gravity, saturation and void ratio of cohesive soil and silt in different places have little change, but the thermal conductivity values are obviously different, among which Limaipai is the largest, Xinghuyuan is smaller and other areas are in the middle.

Figure 5- 13 Comparison of physical parameters of clay in different places

Fig. 5- 14 comparison of physical parameters of silty clay in different locations

Fig. 5- 15 comparison of physical parameters of heavy silty clay in different locations

Fig. 5- 16 Comparison of physical parameters of clayey silt in different locations

Fig. 5- 17 comparison of physical parameters of sandy silt in different locations

3. Obtain the relevant parameters of pumping and recharging test.

Static water level, dynamic water level and water yield are the measured values of pumping irrigation test, and other parameters are obtained by calculation or numerical simulation.

1) Calculation method of relevant parameters of pumping and recharging test

(1) Descending depth (m) = dynamic water level-static water level

(2) Unit water inflow (m3/d m) = water yield/drawdown.

(3) Permeability coefficient:

Where: k-permeability coefficient, m/d;

Q—— water output, m3/d;

S—— depth of water level decline, m;

M—— thickness of confined water aquifer, m;

R—— filter radius of pumping well, m;

R—— influence radius, m

(4) Numerical simulation of influence radius of pumping well.

Next, the influence radius of pumping well is simulated by Feflow software. Taking the groundwater heat pump system as an example, the simulation area is about 1km2, and the boundary condition is open boundary. The ground elevation is 40m, and the floor elevation is-60m. Two pumping wells W 1 and W2 (Figure 5- 18) are arranged in the area, with the well spacing of 100m, the pumping capacity of each well is 120m3/h, the initial water level elevation is 12m, and the simulation period is 3 days. After a series of processes such as simulation, fitting and parameter adjustment, the isoline map of groundwater level in the study area at the end of simulation is finally obtained (Figure 5- 19).

Fig. 5- 18 model schematic diagram

Figure 5- 19 Isogram of Groundwater Level in Demonstration Area

As can be seen from Figure 5- 19, the water level in the center of the borehole is about 7.28m, and the influence radius of borehole pumping is about 78 m..

In order to check whether the input geological and hydrogeological parameters are consistent with the local conditions and whether the simulation results are consistent with the actual situation, the actual pumping test results are compared with the software simulation results, as shown in Figure 5-20.

Figure 5-20 Comparison of Duration Curve of Pumping Depth Decline

By comparison, it can be seen that the simulated pumping depth is consistent with the actual situation, and the depth will reach stability in a short time, indicating that the Quaternary hydrogeological conditions in this area are good and the groundwater runoff speed is fast. Therefore, it can be judged that when the water yield of Quaternary single well reaches 1.20 m3/h, its influence radius is 78m.

(5) Influence radius of reinjection water temperature field.

Secondly, the influence radius of reinjection water temperature field is simulated by Flowheat software.

The simulated boundary is set as an open boundary, the grid size is 1m2/ grid, the groundwater flows from north to south, and the hydraulic gradient is 3‰. The setting of geological and hydrogeological parameters is consistent with the choice of Feflow model.

As shown in Figure 5-2 1, there are three wells W 1, P and W2 in the * * area, in which W 1 is a pumping well, with water output 120m3/h and initial temperature15℃; W2 is a recharge well, the recharge amount is 1 14m3/h, the recharge temperature is 20℃, 22℃ and 25℃ respectively, and the distance between W 1 and W2 well is 50m. P is an observation well, which is located between two wells.

Vertically, 90m stratum is divided into 18 stratum and 5m stratum. According to the histogram of Dada 2 # borehole, the strata in the work area are mainly clay, silty clay, sandy silt, fine sand and gravel. Among them, there are 4 gravel layers with a total thickness of about 35m. When the system runs continuously for 120h(5 days), the influence range of reinjection water with different temperature differences at 5℃ and 10℃ is shown in Figure 5-22 and Figure 5-23.

As can be seen from Figure 5-2 1, Figure 5-22 and Figure 5-23, under the geological and hydrogeological background of the University of Geosciences, after continuous reinjection at 5℃ and 10℃ 120h, the influence radius of reinjection temperature field is 42m and 46m respectively.

2) Analysis of pumping and recharging test results.

The main results of pumping and recharging tests are shown in Table 5-8. As can be seen from the table, when the water level drops within 5m, the water yield of a single well is between 102 ~ 172m3/h; When the water level rises within 3.2m, the single well recharge is between 80 ~ 1 14m3/h, and the water level is stable for more than 8 hours. According to the pumping test results, the permeability coefficient and unit water inflow are calculated.

Figure 5-2 1 model schematic diagram

Figure 5-22 Influence range of reinjection water when the temperature difference is 5℃

Figure 5-23 Influence range of reinjection water when the temperature difference is 10℃

Table 5-8 Summary of Test Results of Pumping and Recharge

By comparing the test results of pumping and recharging in four places, as well as the calculated unit water inflow and permeability coefficient, it can be found that the hydrogeological conditions of Haijian Building are the best, the Institute of Software of Chinese Academy of Sciences is the best, Sidaokou is the second, and China Geo University is the worst. Through the analysis of its stratigraphic structure and hydrogeological characteristics, it can be found that from west to east, the positions of the four projects are arranged in sequence from upstream to midstream on the alluvial fan of Yongding River, the thickness of Quaternary system is gradually increasing, the aquifer is gradually changing from single layer and single layer thickness to multi-layer, the thickness of single layer becomes smaller, and the lithologic particles become coarser and finer.

4. Determination of parameters related to field test of heat exchange capacity

1) technical requirements for field test of heat exchange capacity

(1) In general, the geotechnical field test shall be conducted at least 72h after the installation of the test buried pipe.

(2) In the field test, the unheated test should be conducted first to obtain the initial temperature of the formation. After the temperature is stable (the daily temperature change is less than 0.5℃), the test time shall not be less than 24h.

(3) During the field test, the number of heating power changes should be determined according to the test purpose, at least 2 times; During the test, the heating power and flow rate should be kept basically constant and the fluctuation range should be within 5%. Each heating power test, after the inlet and outlet temperature and temperature difference are stable, the test time shall not be less than 24h. After each heating test, the ground temperature recovery test should be carried out, and the test time should be not less than 8 hours after the heat exchange hole recovers to a stable temperature.

(4) According to the test data, numerical simulation software should be used to calculate the spacing of heat exchange holes, which provides a basis for the arrangement of heat exchange holes.

(5) On-site test instruments and equipment should be checked and calibrated regularly.

(6) When analyzing the field test results, we should pay attention to the influence of test conditions such as temperature on the test, and use mathematical statistics to exclude abnormal data.

(7) When conditions permit, observation wells should be arranged around the exploration holes.

2) Determination of average thermal conductivity

(1) calculation method:

The following assumptions are introduced into the simplified analysis model for determining the average thermal conductivity: ① the periphery of the borehole is uniform (simulation requires average parameters); ② The heat exchange between the buried pipe and the surrounding rock and soil can be regarded as the linear heat source of the heat exchange between the drilling center and the surrounding rock and soil, ignoring the heat transfer along the length direction; ③ The heat exchange intensity between the buried pipe and surrounding rock and soil remains unchanged (which can be achieved by controlling the heating power).

According to the above assumptions, the relationship between the average temperature of the fluid in the pipe and the initial temperature of the deep rock mass can be determined by the heat exchange equation between the heat exchanger and the surrounding rock mass, which can be expressed as:

Shallow geothermal energy resources in Beijing

Where: is exponential integral; Db is the borehole diameter, m; Cs is the specific heat capacity of rock and soil, j/(kg k); Ks is the thermal conductivity of surrounding rock and soil, w/(m℃); Ql is the heat flux density of linear heat source per unit length, w/m; R0 is the total thermal resistance per unit length of borehole,℃/w; Tf is the average temperature of the fluid in the buried pipe,℃; Tff is the temperature of rock and soil at infinity,℃; ρs is the density of rock and soil, kg/m3; It's time, S.

There are three unknown parameters ks, R0 and ρsCs in the simplified model. Among them, ρsCs can be obtained by analyzing and testing soil samples and selecting empirical data for weighted average calculation. Ks and R0 can be determined simultaneously by solving the inverse heat transfer problem with optimization method. According to the field test of heat exchange, the water temperature in the loop and its corresponding time are measured, and the thermal conductivity ks of rock and soil around the borehole and the thermal resistance R0 in the borehole are deduced from the known data. The average temperature of fluid obtained by heat transfer model is compared with the actual measurement results. By adjusting the thermal conductivity of surrounding rock and soil and the thermal resistance in the borehole in the heat transfer model, when the error between the calculated results and the measured results is the smallest, the corresponding thermal conductivity value is the obtained result. The sum of variance (f) is:

Shallow geothermal energy resources in Beijing

Where Tcal, i is the average temperature of the fluid in the buried pipe calculated by the heat transfer model at the first moment,℃; Texp, i is the average temperature of the fluid in the buried pipe measured at the first moment (approximately the average temperature of the inlet and outlet fluid),℃; N is the number of groups of experimental measurement data.

The minimum value of variance sum (f) can be obtained by optimization technique.

(2) Calculation results:

Taking the heat exchange field test of Xinghuyuan as an example, the borehole is 150mm, the original temperature of underground rock and soil is 13.5℃, and the heating power is 60 W/m. The average thermal conductivity of rock and soil measured by the above method is KP = 2.45 W/(m℃). Forward and inverse simulation and curve drawing are shown in Figure 5-24. Compared with the measured results, the two results are in good agreement, which shows that the simplified heat transfer model is feasible to measure the thermal conductivity of deep rock and soil in situ.

Figure 5-24 Positive and Negative Fitting Results of Xinghu Garden

Then, according to the field test results of heat exchange in Forest Park, Beijing Institute of Geological Exploration Technology, Air China Flight Simulation Training Base and UFIDA Software Park, the average thermal conductivity of rock and soil in the corresponding drilling depth range is calculated. Forward and backward fitting results are shown in Figure 5-25, Figure 5-26, Figure 5-27 and Figure 5-28.

Through the above forward and backward modeling calculation, the average thermal conductivity of rock and soil at five field test points of heat exchange capacity is shown in Table 5-9.

Table 5-9 List of Average Thermal Conductivity

Figure 5-25 Positive and Negative Fitting Results of Forest Park

Figure 5-26 Positive and Negative Fitting Results of Exploration Technology Research Institute

(3) Analysis of results:

Comparing the average thermal conductivity of five different places, it can be found that the value of forest park is the largest, followed by the Institute of Exploration Technology, and the values of Xinghuyuan, UFIDA and Air China are the smallest. By comparing the geological and hydrogeological conditions of the five regions, it can be known that the forest park is relatively rich in water, coarse in lithology and fast in groundwater runoff, and it is the region with the best geothermal geological conditions among the five regions, so the average thermal conductivity is the largest and the heat exchange effect is the best. Dongxiaokou area, where the Institute of Exploration Technology is located, is located at the junction of Yongding River alluvial fan and Nankou alluvial fan. It is a region with good geothermal geological conditions in the five regions, with large average thermal conductivity and good heat exchange effect. Although the Houshayu area where Air China is located, the Houhai area where UFIDA is located and the Taihu area where Xinghuyuan is located are in different hydrogeological units, they are all located in the lower part of the alluvial fan, with fine lithologic particles and slow groundwater runoff, so the average thermal conductivity of these three places is small and the heat transfer effect is relatively worst.

Figure 5-27 Positive and Negative Fitting Results of Air China

Figure 5-28 UFIDA Forward and Backward Fitting Result Diagram

5) Determination of influence range of buried pipe temperature field

The influence range of underground pipeline temperature field is simulated by Fluent software, and the rationality of 5m spacing between heat exchange holes in the demonstration area is tested.

Based on the geological, hydrogeological and thermodynamic parameters of the work area, the temperature field changes of single hole, three holes and five holes in a cooling season are simulated by Fluent software. Simulate the temperature field change of five holes in a heating season; The boundary condition of Fluent model is set to constant wall temperature, the initial temperature of rock and soil is set to 14.2℃, the average thermal conductivity of rock and soil is 1.90 w/(m℃), and the heat is removed by 62W per linear meter, 43W per linear meter, and the hole spacing is 5m. The software doesn't consider the influence of factors such as geothermal flow in the earth, so the actual engineering operation effect is better than the simulation result, and the influence radius of temperature field determined by the model is larger than that under actual operation.

The simulation of temperature field changes of five holes (hole depth 120m, distance between adjacent holes 5m) in a natural year is as follows: first, the cooling period (8h every day, operation120th day), then the recovery period of 60 days, then the heating period (8h every day, operation120th day), and finally the recovery period of 60 days.

As can be seen from Figure 5-29, after a natural year, the temperature from the center of the central hole 1.0 ~ 2.4m has returned to the original temperature of the rock and soil. It shows that when the heat exchange hole discharges 62W and takes away 43W heat per linear meter, the rock and soil can recover to the original temperature after one natural year. In Figure 5-30, the maximum variation range of the average temperature field of rock and soil in the simulation area in a hydrological year is about 65438 0℃, and it basically returns to the original temperature at the end of the year.

In order to visually display the temperature change and their mutual influence at different distances between two adjacent heat exchange holes during the heat removal process, the results of heat removal 120 days under the above simulated working conditions are displayed by using Tecplot post-processing software, as shown in Figure 5-32.

Fig. 5-29 Geotechnical temperature field after five holes pass through a hydrological year.

Fig. 5-30 Temperature change with time at a distance of 0. 1 ~ 4.5m from the center of the central hole.

Fig. 5-3 1 average temperature field curve of rock and soil in five-hole hydrological year

Fig. 5-32 Temperature changes with time at different distances from heat exchange holes

As can be seen from Figure 5-3 1, when two heat exchange holes with a distance of 5m continue to emit heat, the temperature of rock and soil around the heat exchange holes gradually increases with the passage of time. At the position between the two heat exchange holes, the temperature basically does not increase after 30 days of heat removal; After 60 days of heat removal, the temperature rises by about 0.3℃; After 90 days of heat removal, the temperature rises by about 0.7℃; After 120 days of heat removal, the temperature increased by about 1. 1℃. This shows that in the whole cooling season (under the condition of continuous heat removal), the temperature field will be superimposed on the heat exchange holes with a distance of 5m, but the temperature difference of 1. 1℃ will not obviously affect the heat exchange capacity of a single hole, and the distance between heat exchange holes with a distance of 5m is basically reasonable.