3:30 PM - 3:45 PM
[MZZ40-01] Characteristics of apparent thermal conductivity by geology obtained from thermal conductivity profiling using heating cable
Keywords:shallow geothermal energy, apparent thermal conductivity, thermal conductivity profiling using heating cable, granitoid, sandstone, mudstone
1. Introduction
Apparent thermal conductivity is one of the key parameters for designing ground-source heat pump systems. apparent thermal conductivities are currently obtained only by thermal response test (TRT). The National Institute of Advanced Industrial Science and Technology (AIST) has conducted TRT using heating cable and borehole (Ishihara et al., 2023) in Fukushima, Saga, and Okinawa Prefectures since 2018 to obtain and evaluate apparent thermal conductivities under various hydrogeological environments. In this presentation, apparent thermal conductivities estimated by TRT using heating cable by depth are summarized and characterized for each geological facies.
2. TRT using heating cable
TRTs using heating cable and borehole were conducted at 47 sites in Fukushima Prefecture, 5 sites in Saga Prefecture, and 7 sites in Okinawa Prefecture based on the method of Ishihara et al. (2020, 2023), and apparent thermal conductivities were estimated at every 1 m depth at each site. Apparent thermal conductivities (2,187 in Fukushima, 204 in Saga, and 329 in Okinawa) were classified by the following geological facies: (sediments) e.g. gravel, sand, mud (rocks) sandstone, mudstone, tuff, and granitoid. Each classified data was summarized into frequency distribution at intervals of 0.1 W/m/K, while apparent thermal conductivities above 4.0 W/m/K were lumped together.
3. apparent thermal conductivities estimated from TRT using heating cable
This presentation reports statistical apparent thermal conductivities (maximum, minimum, mode, mean, and median) uof the geology described above for Fukushima Prefecture and Saga/Okinawa Prefectures, respectively. The data for Fukushima Prefecture are after Ishihara et al. (2023).
3.1. Apparent thermal conductivity of sediments [W/m/K]: maximum/minimum/mode/mean/median in this order
Gravel: 33.3/0.8/1.6/2.8/2.0, 443 data in Fukushima, no data in Saga/Okinawa.
Sand: 17.7/0.9/1.4/1.7/1.5, 190 data in Fukushima, 3.1/0.6/1.7/1.5/1.4, 30 data in Saga/Okinawa.
Mud: 5.3/0.7/1.2/1.5/1.3, 153 data in Fukushima, 1.8/0.3/1.2/1.2/1.3, 27 data in Saga/Okinawa.
Extremely large apparent thermal conductivities over 4 W/m/K indicate the effect heat advection by groundwater flow (Ishihara et al., 2023). The mean and median values are similar to literature values of effective thermal conductivity (Hokkaido University, 2020) and effective thermal conductivities measured in rock and core samples (Santa et al., 2020; Yoshioka et al., 2020). No regional differences were found between Fukushima Prefecture and Saga and Okinawa Prefectures with respect to apparent thermal conductivities of sand and mud layers despite the difference in the numbers of data.
3.2. Apparent thermal conductivity of rocks [W/m/K]: maximum/minimum/mode/mean/median in this order
Sandstone: 4.8/0.8/1.4/1.6/1.4, 236 data in Fukushima, 35.6/1.5/2.2, 2.3/2.9/2.3, 143 data in Saga/Okinawa.
Mudstone: 4.6/0.7/1.0/1.3/1.2, 544 data in Fukushima, 3.0/0.7/1.8/1.7/1.7, 134 data in Saga/Okinawa.
Tuff: 7.8/0.8/1.2/1.3/1.3, 283 data in Fukushima, no data in Saga/Okinawa.
Granitoid: 6.6/1.8/3.1/3.4/3.2, 190 data in Fukushima, 14.8/1.2/2.0/2.9/2.1, 65 data in Saga:/Okinawa.
Each statistical apparent thermal conductivity of sandstone, mudstone, and granitoid is different between Fukushima and Saga/Okinawa Prefectures. This suggests that there are regional differences in the trend of apparent thermal conductivities even for the same rock type.
In the future, the authors plan to analyze physical properties (e.g. effective thermal conductivity and porosity) of core samples of granitoids collected in Fukushima and Saga Prefectures to investigate the relationship between physical properties and effective thermal conductivity. If the trend of apparent thermal conductivities of the rock can be summarized considering the depositional age and formation environment, the data will be useful for the schematic design of the geothermal heat utilization system.
Apparent thermal conductivity is one of the key parameters for designing ground-source heat pump systems. apparent thermal conductivities are currently obtained only by thermal response test (TRT). The National Institute of Advanced Industrial Science and Technology (AIST) has conducted TRT using heating cable and borehole (Ishihara et al., 2023) in Fukushima, Saga, and Okinawa Prefectures since 2018 to obtain and evaluate apparent thermal conductivities under various hydrogeological environments. In this presentation, apparent thermal conductivities estimated by TRT using heating cable by depth are summarized and characterized for each geological facies.
2. TRT using heating cable
TRTs using heating cable and borehole were conducted at 47 sites in Fukushima Prefecture, 5 sites in Saga Prefecture, and 7 sites in Okinawa Prefecture based on the method of Ishihara et al. (2020, 2023), and apparent thermal conductivities were estimated at every 1 m depth at each site. Apparent thermal conductivities (2,187 in Fukushima, 204 in Saga, and 329 in Okinawa) were classified by the following geological facies: (sediments) e.g. gravel, sand, mud (rocks) sandstone, mudstone, tuff, and granitoid. Each classified data was summarized into frequency distribution at intervals of 0.1 W/m/K, while apparent thermal conductivities above 4.0 W/m/K were lumped together.
3. apparent thermal conductivities estimated from TRT using heating cable
This presentation reports statistical apparent thermal conductivities (maximum, minimum, mode, mean, and median) uof the geology described above for Fukushima Prefecture and Saga/Okinawa Prefectures, respectively. The data for Fukushima Prefecture are after Ishihara et al. (2023).
3.1. Apparent thermal conductivity of sediments [W/m/K]: maximum/minimum/mode/mean/median in this order
Gravel: 33.3/0.8/1.6/2.8/2.0, 443 data in Fukushima, no data in Saga/Okinawa.
Sand: 17.7/0.9/1.4/1.7/1.5, 190 data in Fukushima, 3.1/0.6/1.7/1.5/1.4, 30 data in Saga/Okinawa.
Mud: 5.3/0.7/1.2/1.5/1.3, 153 data in Fukushima, 1.8/0.3/1.2/1.2/1.3, 27 data in Saga/Okinawa.
Extremely large apparent thermal conductivities over 4 W/m/K indicate the effect heat advection by groundwater flow (Ishihara et al., 2023). The mean and median values are similar to literature values of effective thermal conductivity (Hokkaido University, 2020) and effective thermal conductivities measured in rock and core samples (Santa et al., 2020; Yoshioka et al., 2020). No regional differences were found between Fukushima Prefecture and Saga and Okinawa Prefectures with respect to apparent thermal conductivities of sand and mud layers despite the difference in the numbers of data.
3.2. Apparent thermal conductivity of rocks [W/m/K]: maximum/minimum/mode/mean/median in this order
Sandstone: 4.8/0.8/1.4/1.6/1.4, 236 data in Fukushima, 35.6/1.5/2.2, 2.3/2.9/2.3, 143 data in Saga/Okinawa.
Mudstone: 4.6/0.7/1.0/1.3/1.2, 544 data in Fukushima, 3.0/0.7/1.8/1.7/1.7, 134 data in Saga/Okinawa.
Tuff: 7.8/0.8/1.2/1.3/1.3, 283 data in Fukushima, no data in Saga/Okinawa.
Granitoid: 6.6/1.8/3.1/3.4/3.2, 190 data in Fukushima, 14.8/1.2/2.0/2.9/2.1, 65 data in Saga:/Okinawa.
Each statistical apparent thermal conductivity of sandstone, mudstone, and granitoid is different between Fukushima and Saga/Okinawa Prefectures. This suggests that there are regional differences in the trend of apparent thermal conductivities even for the same rock type.
In the future, the authors plan to analyze physical properties (e.g. effective thermal conductivity and porosity) of core samples of granitoids collected in Fukushima and Saga Prefectures to investigate the relationship between physical properties and effective thermal conductivity. If the trend of apparent thermal conductivities of the rock can be summarized considering the depositional age and formation environment, the data will be useful for the schematic design of the geothermal heat utilization system.