The main purpose of this article is to provide an overview of the differences between p and T dependences of rock thermal properties. In the normal geothermal environment, p increases and l decreases. The two dependences are opposite, and the l/p relationship in rocks is not always the same. In the case of a crustal rock, p may increase by ten times.
Thermal conductivity is a fundamental property of rocks. It depends on the composition of the rock and its pore fluid. The greater the pore fluid, the lower the thermal conductivity of the rock. Higher temperatures result in lower thermal conductivity. And the higher the pressure, the lower the pore fluid. The volumetric ratios of the different phases affect the temperature and permeability of rock.
Thermal conductivity can be measured in laboratory or in situ. It increases with high pressure, and it is dependent on the density of water. High pressure also increases thermal conductivity and improves heat transport at grain-grain contacts. Depending on the chemical composition of the rock, high-pressure can increase the water content and close microcracks. This process can lead to increased porosity and reduce the density of the rock.
Typical thermal properties of rocks are measured by using a thermal analyzer. The average of the thermal properties of a given volume of rock is used. A proportional increase in T is common for crystalline rocks, and a decrease in l is observed in sheet silicates. Both l and p increase exponentially with increasing T. The temperature rises with depth, and this increases the heat capacity of a rock.
The thermal conductivity of rocks is calculated by calculating the density of water at a temperature above the Debye temperature. For cubic crystals, the pressure has a positive effect on thermal conductivity. However, the temperature decreases the heat transfer of rock. Thus, the difference between low- and high-conductivity rocks is a result of the pressure. In both cases, a decrease in the density of the rock will increase the heat capacity.
The thermal conductivity of rocks is the capacity of a rock to transfer heat in a thermal gradient. It is a major parameter controlling conductive heat flow density. A rock’s conductive heat flow density depends on its temperature. The higher the temperature, the greater the conductive heat flow density. Likewise, the lower the volume, the less a rock’s thermal conductivity.
The thermal conductivity of rocks is higher when they are saturated with water. In fact, the temperature derivative of carbonate rocks is higher than that of clastic rocks, despite their low-temperature sensitivity. In contrast, clastic rocks increase with temperature. In addition to their high thermal conductivity, they also have high water-permeability. They are often very difficult to drill through because of their porous structure.