The medium-deep geothermal energy in China is abundant, with a great potential for development and utilization. To accelerate the exploitation of the medium-deep geothermal energy is important for promoting the rapid development of China's clean heating industry, with a diversified energy supply system and with the goals of the"carbon peaking and carbon neutrality" in the future. In the context of the efficient exploitation of the medium and deep geothermal energy, this paper reviews the researches of the heat extraction technology for exploiting the medium-deep geothermal energy, including various heat extraction technologies based on the doublet-well circulation, the closed-loop circulation in a single well, the circulation in a gravity heat pipe, the open-loop circulation in a single well, the circulation in a U-shaped well and the circulation in a multilateral radial well. The characteristics, the application conditions and the future researches of key heat extraction technologies are analyzed, as well as the development stage and the application scenarios of different heat extraction technologies. The suggestions for efficient exploitation of the medium-deep geothermal energy are proposed.

The reservoir modification technology is an effective means to improve the development capacity of deep thermal reservoirs. It increases the heat transfer capacity between the rock and the fluid by increasing the permeability of the thermal reservoir, maximizing the exploitation of the thermal energy in the rock. This paper reviews the development history and the production enhancement principle of the acidification technology in reservoir modification technologies, compares the relevant cases and researches of the acidification production enhancement carried out in the canonical geothermal fields abroad, as well as the commonly used acidic chemical stimulants and the production enhancement effects. By analyzing the experience and the lessons learned from the reservoir modification abroad, some guidance is provided for the modification of deep carbonate thermal reservoirs t in China.

Geothermal energy and petroleum are two kinds of resources coexisting in sedimentary basins. The exploitation and the utilization of the geothermal energy are highly integrated into the oil and gas related technologies. Oil fields in China contain rich geothermal resources, with a large number of abandoned drilling wells, and a huge geothermal utilization market space. This paper reviews the geothermal resource exploration, evaluation, development and utilization technologies in oil fields of China, including the geothermal geophysical exploration technology, the geothermal geological evaluation technology, the geothermal development technology and the geothermal utilization methods. However, the geothermal development and utilization in oil fields still poses some challenges, for example, the resource condition constraints the decision making, the insufficient technological innovation, and the imperfect incentive policies. Finally, some emerging technologies are proposed to accelerate the geothermal development and utilization in oil fields, such as the deep well direct heat transfer technology, the multi-branch radial horizontal well heat capture technology, the thermoelectric power generation technology and the nano technology.

The clear exploration direction and optimization prospecting targets can guide the exploration and improve the success rate of exploration wells for medium-high temperature geothermal resources. Based on the latest geothermal database, combined with the regional tectonic thermal background, this paper systematically analyzes the heat source, the channel, the reservoir, and the cap rock and fluid characteristics of the geothermal system. Four major exploration directions and 16 optimal target areas for the exploration of the geothermal resources of medium-high-temperature in China are delineated. Specifically, the inner zone of the Qinghai Tibet Plateau (the Yadong-Gulu rift zone, the Guna rift zone, the Shenzha rift zone, and the eastern and western syntaxis of the Himalayan), the outer zone of the Qinghai Tibet Plateau (the Gonghe Guide basin, the Songpan region in western Sichuan), the eastern area (the Songliao Basin, the Yitong Fault Zone, the Changbai Mountain Volcanic Area, the Bohai Bay Basin and its surrounding areas, the southeast coast area, and the Leizhou Peninsula-the northern Hainan Island) and the central rift valley area (the Erlian Basin, the Fenwei Graben, and the Shanxi Graben).

The development of the hot dry rock is one of the important ways to achieve the goal of "double carbon". In the context of monitoring the hot dry rock reservoir with the electromagnetic method and establishing a scientific and efficient hot dry rock reservoir evaluation system, this paper reviews the basic researches of the rock physical properties related to the electromagnetic hydraulic fracturing monitoring and evaluation, focusing on three aspects, namely, the electrical resistivity characteristics of the rock in the process of fracturing under pressure, the electrical resistivity characteristics of the fractured rock, and the relationship between the rock electrical resistivity and permeability. The application cases of the electromagnetic monitoring in the field of the hot dry rock hydraulic fracturing are introduced. Based on the basic researches of the rock electrical properties, it is concluded that:the pressure fracturing will be accompanied by significant electrical resistivity changes of the rock, and the characteristics of the electrical resistivity changes are closely related to the water saturation in the rock; the number, the width and the roughness of the fractures will affect the electrical resistivity of the rocks, and the influence trend is related to whether the fractures contain the water; the relationship between the electrical resistivity and permeability of specific rocks can be established by modifying the Archie's law and the Kozeny-Carman model. Finally, the tasks faced by the application of the electromagnetic method to the monitoring and the evaluation of the hot dry rock hydraulic fracturing are discussed, including:(1) the study of electrical resistivity properties of the fractured compact rocks under high temperature and high pressure; (2) the relationship model between the permeability and the electrical resistivity of the fractured compact rock under high temperature and high pressure; (3) new theories, new methods, new technologies and new equipment of electromagnetic method detection with higher accuracy, finer resolution, better anti-interference ability and longer detection distance.

In order to solve the problem that the simulation-based optimization method usually requires a large number of simulations, in this paper, after comparing the prediction performance of different machine learning methods, a surrogate model based on MLP was developed to reduce the computational cost, which was then combined with genetic algorithm to develop an optimization method of development parameters for geothermal doublets in heterogeneous geothermal reservoirs. Through the case study of a doublet system, the reasonability and efficiency of the developed optimization method of development parameters were demonstrated. The results show that when given a certain position of production well, the surrogate model-based optimization method of development parameters can accurately find the optimal placement of injection well, the rate of production and injection, and the temperature of recharge water with lower computational cost.