background The circulation depth of geothermal water is one of the critical parameters in the study of geothermal systems as it determines the intensity of thermal energy and mineral compositions of geothermal water and influences the efficiency and sustainability of geothermal resource development and utilization.
Progress For low-medium temperature convective geothermal resources that being largely and widely utilized, methods for circulation depth evaluation and corresponding principles, applicable conditions and parameter acquisition approaches are systematically summarized. This paper summarizes the characteristics of typical low-to-moderate temperature convective geothermal systems from five aspects including "sources (heat source, fluid source), conduits, reservoirs, cap rocks" and "upwelling processes and their impacts". (1) Applicable methods for evaluating circulation depth, along with their fundamental principles, applicable conditions, and parameter acquisition approaches are systematically reviewed. Geothermal gradient method and numerical simulation method are proposed. The former requires representative geothermal gradient values and is more suitable for conductive or predominantly conductive geothermal systems. The latter is established based on the principles of mass and energy balance in heat exchange between the ascending hot water and surrounding rocks in convective geothermal systems. Depending on the two end-member models of the geometric shape of the upwelling conduits, it can be further divided into the conduit model and the fault-plane model, both of which assume that heat loss during the ascent of geothermal water primarily occurs through heat exchange with surrounding rocks. (2) Geophysical exploration and interpretation methods can provide spatial distributions of reservoirs and fracture networks, but due to limitations in accuracy and resolution, they often yield qualitative or semi-quantitative results. Given their high implementation costs, it is recommended that these methods be used in conjunction with borehole data for auxiliary support. (3) Based on the above analysis, this paper proposes a scientific and reasonable evaluation process for circulation depth. First, a conceptual model of the geothermal system should be established to check it is classified as either a conduit model or a fault-plane model based on the manner of hot water discharge. Next, two to three applicable geothermometer methods should be selected to calculate reliable reservoir temperatures. (4) Finally, the circulation depth is determined by combining parameters such as rock thermal conductivity and local annual average air temperature. Previous studies have shown that the Dengwu geothermal system in Fengshun, Guangdong Province, is a typical low-to-moderate temperature convective geothermal system along the southeast coast of China. Controlling by NE- and NW-trending faults, it exhibits characteristics of the conduit model. With a reservoir temperature of 143 ℃, the calculated circulation depth ranges from 6.0 to 7.0 km, which is consistent with the reservoir depth interpreted from MT exploration.
Perspectives Future efforts should focus on integrating geophysical, geochemical, and numerical methods to obtain reliable information on geothermal water circulation, and achieving quantitative evaluation of the contribution of convective heat to heat flux, thereby providing a scientific basis for formulating sustainable development and utilization plans for geothermal resources.
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