The Impacts Of Leaked Carbon Dioxide From Underground On The Thermal Parameters Of Unsaturated Soil

There is no doubt that global warming is a true fact. Accordingly, greenhouse effect has attracted global attention. CO₂ capture and storage（CCS） in geological reservoirs may be part of a strategy to reduce global CO₂ emissions of the anthropogenic behavior. However, CCS is not absolutely safe because of a risk of the leakage. It is note that the CO₂ escaping from cap rock would bring about changes in the soil thermal properties of the unsaturated zone. Soil thermal parameters of the unsaturated zone directly control heat exchange and water movement between atmosphere and soil, and then impact on the ecological environment of soil.In order to study the effects of the soil’s thermal parameters in unsaturated zone, caused by the escape of the high concentration of CO₂, the experimentwas designed in this study to artificially simulate the leakage of CO₂ in the aeration zone.The experiment consists of two parts: the outdoor section and indoor section. The formal usesthe CO₂ concentration of5′104ppm, 10′104 ppm, 15′104ppm to be injected from the bottom of the testing columns（h= 330 cm, d = 100cm） containing different configurative loess.After one year operation,different layers of the loess were sampled from the testing columns and the then the soil thermal conductivity, volumetric heat capacity, and thermal diffusion rate are monitored. In the indoor section experiment, a purity of 99.99% of the CO₂ with a flow rate of 50 m L/min was introduced into the bottom of the test tube（h = 175 cm, d = 60cm）. The impacts of CO₂ on soil thermal properties through on-line monitoring moisture content, negative pressure and other negative factors of the soil thermal properties were investigated. The main experimental results areshowed as follows.（1） With the increase of concentration of CO₂ setting from 5′104 to 15′ 104 ppm in the outdoor section trial, average thermal conductivity of the unsaturated zonewas reducedfrom0.9 W·m-1·K-1 to 0.5 W·m-1·K-1, while volumetric heat capacity was reduced from 1.5MJ·m-3·K-1 to 1.2 MJ·m-3·K-1.In addition, the thermal diffusion rate of the unsaturated zone was also decreased.（2） From the upperlayer to the lower layer of the testing columns of the outdoor sectiontrial, the thermal conductivity has a decreasing trend, from the highest point of 1.0 W·m-1·K-1to the lower region of 0.4 W·m-1·K-1in the case of CO₂ injection concentration of 10 ppm. The impact of this regional CO₂ concentration on soil thermal conductivity is more obvious,indicating a sensitive area of the thermal conductivity.（3） CO₂ affects the linear relationship between the volumetric heat capacity and moisture content.Low density regions have the best linear relationship, and the linear regression correlation coefficient R2 is 0.7419.The linear correlation coefficient R2 in the middle region and the low concentration region are less than 0.5. This shows that the high concentrations of CO₂ alter the composition ratio of the three phases of soil.（4） Through the on-line dynamic monitoring system, four probes of moisture content in the indoor section trial showed decline compared to the control sample. The 4th experimental sampleshowed the highest drop in moisture content, reduction range from 0.315m3·m-3 to0.295 m3·m-3.（5） Due to the CO₂ injection, the temperature order of the experimental group was alteredin the area of laboratory trials. The first point, which was initially thelowest temperature point among the four monitoring points, becomes the highest temperature point after the trials.