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Thermal damage for deep geothermal engineering

1. Reservoir rock

The establishment of a fracture network is a key issue in deep geothermal exploitation. It is very important to study the thermal damage of geothermal reservoir rocks for deep geothermal exploitation. Therefore, we summarize the practices of deep geothermal engineering that have been built or are under construction in the world. According to the reservoir lithologies of each project, the demonstration projects and lithology distribution of deep geothermal exploitation in the world are given. Based on the frequentness of reservoir lithologies in the deep geothermal practices, igneous rock is the main rock in deep geothermal reservoirs (occupy 57.63%). The proportion of sedimentary rock is also large (occupy 23.73%). In igneous and sedimentary rocks, the deep geothermal reservoir is mainly composed of granite and sandstone. In addition, as deep igneous rock, granite is also the main lithology of deep strata, which can be seen in many geological settings, hence the study on granite has significant importance for other subjects. Therefore, granite and sandstone are selected as the test lithologies for thermal damage experiments.

Fig. 1 Lithology distribution and percentage of deep geothermal reservoir rocks. (a) Project distribution of deep geothermal exploitation, (b) rock types and their proportions, (c) proportions of lithologies.

2. Microstructure of rock after thermal damage

The thermal stress induced by heating or cooling is an important reason for the damage in the rock interior. During the processes of heating and cooling, the initiation and propagation of micro to macro cracks are the main expression of thermal damage. Through the thin section analysis of rock before and after high-temperature treatment, the increase of cracks caused by high temperature was observed. After temperature treatment, the surface roughness of the thin section becomes larger. The influence of rock damage on the microstructure is reflected in two aspects. The first is the increase of microcracks, such as the cracks in quartz and feldspar crystals in granite. The second is the roughness of the crystal surface. After high-temperature treatment, microcracks (in micro-scale) will appears inside the rock. These microcracks will easily lead to larger roughness during the polish process of the thin section. This phenomenon is easily observed in the specimens of G-2, S-1, and S-2, where the increase of slickensides in quartz and feldspar crystals is obvious, and the surface luster of the sandstone thin section is dim. Thermal stress causes the initiation and propagation of cracks inside the rock, leading to the propagation of stress waves, which provides the premise for the AE monitoring during rock damage.

Fig. 2 Changes of the microstructure of rocks before and after heat treatment (orthogonal polarized)

3. Simulations on thermal damage

Since the damage variable of granite after high-temperature treatment is quite different from that of sandstone, the post temperature damage model of granite after high-temperature treatment has been calculated by Rui and Zhao. The microstructure model simulates the thermal damage between two different minerals, and the microdefect model simulates the thermal damage in defective minerals. Both of them have practical physical significance, and they are not contradictory in the physical world. It is closer to the physical reality to combine the two models as a community to simulate the thermal damage process of rock. Considering the complexity of real rock structure, it is difficult to obtain the defects in rock, which brings great difficulties to the modeling of real rock structure (it needs to consider the combination of different minerals and various defects in minerals). Therefore, the simulation of the whole thermal damage process of rock is achieved by combining the results of the microstructure model and the microdefect model. Here we assume that a real rock material consists of defective materials (occupy w) and non-defective materials (occupy 1-w). By the piecewise fitting method based on the AE energy data of a single rock, the damage evolution model can be established. Results show that different rocks have their own damage evolution models. The theory model can well describe the damage evolution of rocks

Fig. 3 Damage variables of granites and sandstone obtained from numerical method and laboratory experiment.

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