The freeze-thaw process influences moisture and heat transport, which is of great importance for the runoff and groundwater infiltration processes. At present, the research on the freeze-thaw process is mainly conducted by means of field/laboratory experiments. The mechanism and details of the fluid and heat flow characteristics are still poorly understood. In this study, we developed the code to realize the elaborate description of each thermo-hydraulic phenomenon during the entire freeze-thaw process. Based on the framework of TOUGH2, we improved the EOS3 module to realize the function of phase transition between water and ice. The improved model can handle the system of water/air two components in gas/liquid/solid three phases. Moreover, the absolute and relative permeability change and thermal conductivity correction induced by the occurrence of the ice phase are all considered. The long-term field monitoring was conducted on two sites to observe the entire freeze-thaw process in the Changbai Mountain area. The monitoring data were used to verify the code and get a satisfactory result. It is found that the freeze-thaw process could be briefly divided into three stages, the gradually freezing stage, long-term stable stage, and a prompt thaw stage. The proposed method provides a solution for the in-depth investigation of the moisture and heat migration, and groundwater dynamics in seasonally frozen areas.

Clarifying the distribution and dynamics of soil moisture during the freeze-thaw process is crucial for surface ecology and is an objective requirement to investigate the mechanism of changes during the groundwater recharge process in a freeze-thaw zone. Based on the monitoring data of soil moisture and temperature in the Changbai Mountain area, the freeze-thaw process is classified into four periods. This study investigates the hydrothermal migration processes during different periods. The simultaneous heat and water model is used to simulate and analyze the infiltration of soil moisture into groundwater under five precipitation guarantee rates. The results are as follows: (1) The smaller the soil depth, the stronger is the correlation between soil temperature and air temperature during the freeze-thaw process. (2) The redistribution of soil moisture before and after freeze-thaw is significantly affected by the soil texture, and soil permeability affects the recharge of soil moisture from the upper region to the lower region during the thawing period. (3) Groundwater receives vertical infiltration recharge mainly during non-freezing and is supplied by freezing and snowmelt recharge during the stable thawing period. The percentage of soil water infiltration during the stable thawing period in the total annual infiltration increases gradually with the precipitation guarantee rate.

Super-thick shallow granites without a cap layer are widely distributed in the Wendeng geothermal field. To evaluate the field’s productivity potential for an enhanced geothermal system (EGS), we carried out field tests, laboratory tests and numerical simulations in succession. The geothermal characteristics and deep rock mechanical properties were identified based on real geological and core data from the borehole ZK1 in Wendeng geothermal field. Then, a numerical model of reservoir hydraulic fracturing based on a discrete fracture network was established. Thermal extraction simulations were then conducted to assess the long-term productivity of an EGS project based on the fracturing results. Possible well layout patterns and operational parameters were considered. Results indicated that, for naturally fractured formations, large well spacings should be used and reservoirs with overdeveloped natural fractures should not be selected. For the same reservoir, created by stimulation, the production performances of five-spot and triplet-well modes were different. The pressure indicator was more sensitive to the choice of well layout mode than the temperature indicator. The power generation of the five-spot well mode was slightly improved above that of the triplet-well mode. When selecting the target reservoir, the formations with high temperatures, moderate natural fractures, and high in-situ stress shielding are preferable. On this basis, a large volume of fracturing fluids should be injected to stimulate the reservoir, making the reservoir length and width as large as possible. If the desired large-scale reservoir is created, the five-point well mode should be selected.