Experimental study into the factors influencing rock thermal conductivity and their significance to geothermal resource assessment
ZHU Chuanqing, CHEN Chi, YANG Yabo, QIU Nansheng
1 College of Geoscience, China University of Petroleum-Beijing, Beijing 102249, China 2 State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum-Beijing, Beijing 102249, China 3 Guangzhou Marine Geological Survey, China Geological Survey, Guangzhou 510075, China
China’s ‘Double Carbon’ policy (carbon peaking and carbon neutrality goals), means geothermal energy, as a clean renewable energy, has gradually received attention in the energy domain. The thermal conductivity of rock plays an important role in studying the geothermal field and evaluating the amount of geothermal resources, as heat conduction is the main way that the Earth conveys heat to the outside. In order to quantitatively understand the thermal conductivity of rock and the factors influencing it, the thermal conductivity of 135 rock samples were measured by the transient plate heat source method (TPS) and the porosity, density and mineral composition of some selected samples were also measured to provide a dataset for the understanding on the influencing factors of rock thermal conductivity. The results show that the thermal conductivity of volcanic rocks is the lowest, the thermal conductivity of clastic rocks is close to that of intrusive rocks, and the average thermal conductivity of carbonate rocks is the highest, with the thermal conductivity of intrusive rocks, volcanic rocks, clastic rocks and carbonate rocks ranging from 1.62 to 4.00 W/m·K, 1.09 to 2.07 W/m·K, 1.52 to 5.23W/m·K, and 2.34 to 6.55W/m·K respectively. The average thermal conductivity of the four types of rock samples is 2.54±0.53 W/m·K, 1.50±0.24 W/m·K, 2.77±0.83 W/m·K and 4.21±1.28 W/m·K, respectively. There is a close relationship between thermal conductivity and rock fabric. Thermal conductivity of the rock-forming minerals is an important factor affecting rock thermal conductivity. Quartz, the typical mineral with a high thermal conductivity can obviously affect the thermal conductivity of intrusive rocks and clastic rocks, and dolomite content obviously affects the thermal conductivity of carbonate rocks. The thermal conductivity of clastic rocks is negatively correlated with porosity, it decreases with an increase of porosity in a larger porosity range, but the trend is not obvious when the porosity is relatively low. There is a positive correlation between rock thermal conductivity and density. The anisotropy of thermal conductivity is mainly caused by the stratified structure of rocks, that is, there is basically no anisotropy in the thermal conductivity of massive intrusive rocks. The anisotropy coefficients of stratified rocks range from 1.08 to 2.08. A low thermal conductivity cap plays an important role in the accumulation of geothermal resources, which makes the heat from depth accumulate at the bottom of the cap and the top of the basement, resulting in higher temperature distribution in the upper part of the geothermal reservoir. The results provide a reference for understanding the influence of rock fabric on thermal conductivity, the modeling of rock thermophysical parameters in a deep temperature field and geothermal resource evaluation.
