Abstract
Geothermal resources as clean and renewable energy can be utilized for agriculture, tourism, and industry. The assessment of geothermal potential and the study of genetic mechanism of the geothermal system is an essential part of geothermal resource development. In this study, 16 steady-state temperature logs are obtained in the mountainous area on the northern margin of North China. Thermal conductivity and heat production rates are tested or collected from more than 200 rock samples of these wells and outcrops around the study area. Based on these data, for the first time, the detailed delineations of temperature distributions, genetic mechanisms of geothermal systems, and resource potential of Hot Dry Rock in the study area are achieved. The heat flow map indicates a low heat flow state with an average value of 53.1 mW/m2 in the study area, which is lower than the average value of 62.5 mW/m2 in mainland China. The distribution of hot springs in the area is mainly controlled by fault systems. Heat flow only exhibits a minor effect on the temperature of hot springs and geothermal wells. On this basis, the deep temperature distribution within 3–10 km depths of the study area is calculated using the one-dimensional steady-state heat conduction equation. With it, the reservoir depths of hot springs are estimated to be 3–5 km with temperatures ranging from 70°C to 110°C. Furthermore, a conceptual model for the geothermal system in the study area is derived. According to the results, Northeastern Chengde and northern Beijing exhibit the highest temperatures at all depths. Similar patterns are observed in the temperature distribution maps and the heat flow map, which suggest that the deep temperature distribution is mainly controlled by regional heat flow. With the depth increases, the temperature shows larger variation at each depth level, which is possibly caused by the heterogeneity of crustal composition. According to our resource assessment by volumetric method, the exploitable potential of Hot Dry Rock within the depth of 7–10 km of the study area is equivalent to about 3.1 × 1011 tons of standard coal, but the barrier is still existing for development under the current technical and economic conditions.
Highlights
Geothermal resources as renewable energy with enormous potential have received significant attention worldwide in recent years, especially under the goal of diminishing carbon dioxide emissions (Gao et al, 2021; Jacko et al, 2021)
Previous studies on geothermal resources in North China mainly focused on the resources in the North China Plain (Wang et al, 2018; Zhang et al, 2019), the mountainous area on the northern margin is scarcely studied based on individual boreholes or springs (Shen, 2017)
The average heat flow of the mountainous area of northern margin of North China (NMNC) is 51.3 mW/m2, which is less than the global continental average of 65 mW/m2 (Pollack et al, 1993) and the average value of 62.5 mW/m2 in continental China (Hu and Huang, 2015; Jiang et al, 2016b), indicating that the NMNC has a low heat flow state (Figure 4)
Summary
Geothermal resources as renewable energy with enormous potential have received significant attention worldwide in recent years, especially under the goal of diminishing carbon dioxide emissions (Gao et al, 2021; Jacko et al, 2021). In our study, based on the investigation of geothermal manifestations, borehole temperature, thermal conductivity, and heat production rate of rock samples, the terrestrial heat flow, and deep temperature distribution are analyzed. These deep boreholes penetrate older formations, among which most of them exhibit linear conductive temperature curves in a large range of depths (Figure 2) Based on these temperature data from deeper boreholes, the impact on geothermal gradients from the convection at shallow depths can be reduced, and the quality of the heat flow in the area could be improved. Based on the computed result of temperature distribution at depth, the volumetric method could be applied to assess the total amount of the Hot Dry Rock (HDR) resources within the deep strata with low permeability and porosity in the study area, via the equation of. Where ρ is the rock density (kg/m3); Cp is the heat capacity of rock (J/(kg·K)); V is the rock volume (m3); T is the rock temperature at a specific depth (°C), generally greater than 150°C (Olasolo et al., 2016; Rybach et al, 1978; Tc is the average surface temperature or reference temperature (°C), set as the temperature of constant temperature layer
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