•Synclinal structure is favorable to formation of deep groundwater circulation.•Two deep groundwater circulation systems formed on the two wings of syncline.•Recharge area height and geothermal ...reservoir depth compose circulation depth.•Water-blocking/conducting faults are related to deep groundwater circulation.
Deep groundwater circulation is closely related to the earth surface processes of water, energy and matter, and imposes great influences on the earth critical zone. It is still unknown whether a deep groundwater circulation occurs in a syncline structure. Hydrogeochemical investigations of hot spring, cold spring and shallow groundwater were carried out in a syncline basin in Rucheng County, China. The water source, circulation depth, reaction temperatures, and mixing processes were evaluated by water chemistry and thermodynamic methods to reveal the pattern and controlling factors of the deep groundwater circulation. Results showed that two deep circulation groundwater systems with meteoric origin were developed in the syncline. The Tangkou system is related with the northwest wing of the syncline, the main fault F2, and the RS01 thermal spring. The Luoquan system is related with the southeast wing, the main fault F1, and the RS02 thermal spring. The main faults are water-blocking faults, separating the two systems. The mean elevations of recharge areas of the two systems were 1200m and 850m, respectively. The deep groundwater circulation depths of two systems reached 5.5 ± 0.5 and 3.6 ± 0.3 km, respectively. Within the geothermal reservoirs, deep circulating groundwater was heated up to 156°C for RS01 and 108°C for RS02. Before arriving at the ground, cold shallow groundwater mixed into the deep circulation systems. The mixing ratios of deep circulating groundwater were 14.8% at RS01 and 35.6% at RS02. The HCO3-Ca type deep circulating groundwater was slightly mineralized karst or karst-related water. Carbonate dissolution was the dominant water-rock interaction. The mixing obviously diluted the deep circulating groundwater, but didn’t alter its water type. In our case, the large-scale synclinal structure is favorable to deep groundwater circulation, which is closely related with the carbonate stratum and faults. The depth of the deep groundwater circulation can be determined by recharge area elevation and geothermal reservoir depth, which is a simple and effective method.
•A doublet system was selected to carry out reinjection tracer test with sodium naphthalenesulfonate as the tracer.•1D and 3D models were established to simulate and optimize the multiple ...exploitation-reinjection schemes.•A dominant fissure channel with a length of 513.7 m was shown between the exploitation well and reinjection well.
The imbalance between large-scale exploitation and reinjection year by year has led to the decline of the water level of the geothermal reservoirs in the North of China. The reinjection tracer test was carry out based on the doublet system in the Xianxian geothermal field in China with sodium naphthalene sulfonate as the tracer in this paper. The 1D and 3D models were modeled on the software COMSOL in this study to simulate the predictive analysis of multiple exploitation-reinjection schemes and optimize the design. The monitoring data of the tracer concentration in the water samples collected at the wellhead of the exploitation well showed that the tracer was firstly detected on 30th days after the tracer test started. Afterwards, the tracer concentration reached a peak on the 100th day and then gradually decreased until ∼ 140th day, with a tracer recovery rate of 0.0225%. And one dominant fissure channel between the exploitation well and reinjection well with a length of ∼ 513.7 m was shown in the test. The thermal breakthrough time of the exploitation well was defined as the time when the temperature decreases by 2 °C after 100 years of geothermal exploitation. Based on this, the optimal exploitation solution of the geothermal resources was proposed as the spacing between the exploitation well and reinjection well should be set as ∼ 375 m and the exploitation and reinjection volume is 75 m3/h.
Unraveling the seepage and heat transfer coupled process between the working fluid and the fractured rocks in geothermal reservoir is of great significance to the exploitation and utilization of ...geothermal resources. In this study, based on the numerical modeling of fracture flow in geothermal reservoirs, the seepage and convective heat transfer behavior of fractured granite during geothermal extraction were investigated, and the effects of different fracture patterns, fluid injection temperature, and injection velocity on the temperature evolution of rock mass were comparatively analyzed. The results clearly revealed the influence of five different fracture patterns, i.e., single fracture, echelon fracture, parallel fracture, Y-shaped fracture, and crossfracture, on heat transfer capacity. Generally, the echelon fracture has the highest fluid outlet temperature, while the crossfracture has the largest total heat and shows the strongest heat transfer capacity. Besides the fracture shape, the fluid injection temperature and injection velocity also play significant roles in the heat transfer performance. The increase of fluid injection temperature would improve the total outlet heat of the crossfracture system and also benefit the system life. When the fluid injection temperature raises from 20°C to 35°C, the system life and the total outlet heat would be increased by 58.76% and 22.42%, respectively. However, higher fluid injection velocity would damage the system life of the geothermal reservoir, which obviously results in a decrease in total outlet heat. With the fluid injection velocity increasing from 0.004 m/s to 0.006 m/s, the system life could drop by 72.47%, and the total outlet heat was reduced by 55.72%. This work contributes to the preliminary understanding of the coupled seepage and heat transfer behavior in rock mass with various fracture patterns, and it could provide some practical implications for the rational exploitation of geothermal resources.
The surface morphology of rough fractures significantly affects the fluid flow and heat transfer characteristics in the fractures. A thermal-flow coupling model with specific geometric fractures was ...established to investigate the influence of surface morphology on the heat transfer characteristics of a single fracture. The effect of temperature on the physical properties of rocks and fluids was included in the study to reflect the actual situation more realistically. The research results show that the temperature of the fluid in the rough fracture is nonlinearly distributed along the flow direction and the higher the flow velocity, the higher the heat transfer efficiency. The fracture surface morphology has a significant impact on the heat transfer characteristics, and the surface fluctuation will greatly affect the flow velocity, causing the fluid temperature to change abruptly at the fracture surface. Under the same flow rate, with the increase of the fluctuation degree of the fracture surface and the fluctuation frequency, the larger the heat exchange area of the fracture surface, the stronger the heat exchange performance. The heat transfer efficiency of the fracture is directly related to the heat transfer area of the fracture, so even with the same permeability, the heat transfer performance of fractures with different surface topography is different.
•Micro-logging and small refraction were conducted to determine observation system and static correction models.•The post-stack wave equation for the PR splitting finite difference was used for ...migration imagery.•Imaging section was converted into geological model by seismic interpretation technologies.•The favorable areas of karst fissure heat storage and exploration holes were calibrated.
The seismic exploration was used to explore the potential geothermal resource and geological structure in Xian County, N China. Micro-logging was used to conduct surveys on the velocity structure near the surface, the static correction models and the observation system was determined by this method. By testing different borehole depths and different quantities of explosives, the best experimental parameters were obtained in a series of test locations and in two test sections. The surface differences were eliminated by the static correction, and an interference wave survey was conducted to improve the data signal to noise ratio (SNR). The post-stack wave equation for the PR splitting finite difference was used for migration imagery, and post-stack, underground imaging profiles were obtained. The corrected seismic signals were then interpreted, and converted into geological information. The results showed that the burial depths and morphology in each layer had been successfully obtained. The areas favorable for heat storage in karst fissures were mainly in the Jixian Formation and the Cambrian–Ordovician. Our investigation allowed us to predict and calibrate favorable locations for two geothermal energy exploration holes.
Conventional hydraulic fracturing techniques are often found problematic for extracting geothermal energy in hot dry rock (HDR). As an alternative, employing the less viscous gas to replace water as ...the fracturing fluid showed great potential for more effective fracturing of HDR. In this work, the failure behavior and mechanism of granite during gas fracturing under different confining pressures and gas injection rates are comprehensively examined. It is shown that the breakdown pressure increases with the increase of confining pressure, whereas higher gas injection rate can result in evident decrease of the breakdown pressure. As the confining pressure grows, the acoustic emission (AE) event increases rapidly, with much higher AE counts observed at high gas injection rates than at low injection rates. Comparatively, the AE energy decreases under high confining pressure, due probably to granite transitioning from brittle to ductile. It is interesting that the
b
-value of AE varies dramatically as the gas injection rate becomes higher with significant fluctuations, indicating the ratio of large fracture and small fracture changes drastically during gas fracturing. In addition, the length of the induced fractures decreases with the increase of confining pressure during gas fracturing, and the length and width of vertical fractures are evidently larger when at high gas injection rate. Last, a novel theoretical predictive model is proposed for estimating breakdown pressure during gas fracturing based on the average tensile stress criteria, which is featured by considering the effect of confining pressure and gas flow behaviors. The theoretical prediction agrees with the experimental results. The present study can provide valuable results for theoretical analysis and engineering applications of gas fracturing in stimulating the HDR reservoirs.
Article Highlights
Gas fracturing on granite under different confining pressures and gas injection rates are conducted.
Higher gas injection rate can result in evident decrease of the breakdown pressure.
AE event increases rapidly while AE energy decreases with increasing confining pressure.
A novel theoretical model is proposed for estimating breakdown pressure considering the features of gas fracturing.
Geothermal energy is a kind of renewable energy with the characteristics of stability, zero carbon emissions, less land occupation, and localization. It has enormous development potential in the ...transition from traditional fossil energy to renewable energy, especially in Guangdong, geothermal resources are also abundant. However, the existing resource potential evaluation is relatively broad, and the uncertainty of the distribution of geothermal resources is not considered. Therefore, accurately and scientifically assessing the potential of geothermal resources is currently a research focus, Monte Carlo simulation is an ideal solution to quantitatively measure the distribution of geothermal resources through probability distributions. In this study, based on the volume method, considering the uncertainty of geothermal resource distribution parameters, Monte Carlo simulation was introduced, and the triangular distribution and uniform distribution model were used to simulate the input parameters of geothermal fields, and the potential of uplifted mountain geothermal resources in this area was evaluated. The results show that the fracture-banded reservoir geothermal resources are 5.648–5.867 × 10
16
kJ (mean 5.743 × 10
16
kJ), the karst-layered reservoir geothermal resources are 5.089–5.536 × 10
15
kJ (mean 5.328 × 10
15
kJ), finally the uplifted mountain geothermal resources potential of Guangdong are 6.176–6.399 × 10
16
kJ (mean 6.275 × 10
16
kJ). It quantitatively shows that the existing uplifted mountainous geothermal resources potential in Guangdong Province is enormous, the total amount of uplifted mountainous geothermal resources is equal to 2.11–2.18 × 10
5
Ten thousand tons of standard coal (mean 2.14 × 10
5
Ten thousand tons of standard coal).
Hot dry rock (HDR) has great development potential because of its advantages of clean, environmental protection and renewable. The study of physical and mechanical properties of HDR is one of the ...important links in the process of geothermal energy development. Previous studies have mainly focused on the granite thermal reservoir but research on the sandstone thermal reservoir, especially the physical properties of sandstone after multiple thermal shocks, is scarce. Therefore, in this study, cyclic thermal shock experiments of sandstone at different temperatures are carried out, and the variation law of P-wave velocity of sandstone after heat treatment is revealed. It is found that the P-wave velocity of red sandstone decreases with the increase of temperature and cycle times, especially when the temperature is higher than 400 °C, the maximum change rate of wave velocity reaches 52.6%. It is particularly noteworthy that the P- wave velocity of sandstone increases abnormally at about 600 °C. And this article puts forward three hypotheses to explain the wave velocity anomaly. In addition, the nonlinear relationship model among P-wave velocity, temperature and number of cycles is established for the first time, and the correlation coefficient
R
2
is more than 0.9. This study serves as a reference for the development and utilization of the sandstone geothermal reservoir.
Article highlights
The effects of temperature and cycle times on wave velocity of red sandstone are discussed.
The threshold temperature of wave velocity change after cold heat cycle is 400 °C.
The nonlinear fitting relationship among temperature, thermal shock times and wave velocity is established.
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