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[MZZ48-08] Preliminary consideration about the permeability coefficients of confined groundwater in the Osaka area: For thermal utilization of groundwater
Keywords:geothermal utilization, open-loop system, permeability coefficient
Utilization of renewable thermal energies has been accelerating for the realization of carbon-free society. Geothermal utilization is one of the renewable energies expected to be further promoted in the future because the amount of heat supply can be stable regardless of the season. The closed-loop system has relatively small impacts on the surrounding environment. On the other hand, the open-loop system needs to be directly pumped-up groundwater for the thermal utilization, and it is difficult to evaluate the environmental impact. The total system design and initial cost tend to be oversized to consider about uncertain factors of groundwater pumping. Therefore, it is necessary to develop and standardize the method for estimating the appropriate amount of pumped groundwater. That depends not only on the capacity of the installed pumps but also on the permeability coefficients (hydraulic conductivities) of the aquifers.
Permeability coefficients can be determined by undertaking in-situ tests in boreholes. Especially for the underground construction, in-situ permeability tests have been conducted in order to safety managements and evaluation the impact on the surrounding environment. The shallow Pleistocene sandy gravel layers, so-called Dg1 and Dg2, are widely distributed in the Osaka Plain within the depth of 50-60 m from ground surface, which serve as confined aquifers with abundant groundwater. The in-situ permeability tests have been frequently carried out for these aquifers. Therefore, the permeability coefficient data were collected in this study to compile and input in the borehole database system, and the estimating methods of permeability coefficient from the particle size composition were preliminary considered.
The in-situ permeability test method involves variable head (rising or falling) or constant head procedures. The type of test undertaken depends on the soil type. The constant head test would be typically carried out in relatively permeable soils (more than 10-5 m/s), while the variable head test in less permeable materials (Shinshi and Matsuoka, 2020). Most of the permeability coefficient data compiled in this study were the results of variable head test. Comparing the results of the rising and falling groundwater head methods at the same point, the measured permeabilities by rising groundwater head method tended to be one to two orders of magnitude smaller than falling method, suggesting the effect of clogging due to water injection. Similar trends have been pointed out in previous studies (e.g., Tateishi et al, 2011). In addition, the variations of permeabilities measured by falling groundwater head method were within one order at most points.
Next, the permeability coefficients obtained from in-situ permeability tests were compared with the estimated value from the particle size composition performed at almost the same points. The method for estimating the permeability coefficients from the particle size composition were suggested by Creager et al (1945). As a results, the estimated values of the permeability coefficient by Creager tended to be larger than those obtained from the in-situ permeability test (falling method). This tendency suggests that it is necessary to consider the effect of consolidation (porosity) when estimating the hydraulic conductivity from the particle size composition. The relationships between permeability coefficients and various soil parameters should be compared in more detail to propose appropriately and simply permeability estimation methods in order to minimalize the initial costs of open-loop system.
Acknowledgement: This paper is based on the results obtained from a project, JPNP19006, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).
Permeability coefficients can be determined by undertaking in-situ tests in boreholes. Especially for the underground construction, in-situ permeability tests have been conducted in order to safety managements and evaluation the impact on the surrounding environment. The shallow Pleistocene sandy gravel layers, so-called Dg1 and Dg2, are widely distributed in the Osaka Plain within the depth of 50-60 m from ground surface, which serve as confined aquifers with abundant groundwater. The in-situ permeability tests have been frequently carried out for these aquifers. Therefore, the permeability coefficient data were collected in this study to compile and input in the borehole database system, and the estimating methods of permeability coefficient from the particle size composition were preliminary considered.
The in-situ permeability test method involves variable head (rising or falling) or constant head procedures. The type of test undertaken depends on the soil type. The constant head test would be typically carried out in relatively permeable soils (more than 10-5 m/s), while the variable head test in less permeable materials (Shinshi and Matsuoka, 2020). Most of the permeability coefficient data compiled in this study were the results of variable head test. Comparing the results of the rising and falling groundwater head methods at the same point, the measured permeabilities by rising groundwater head method tended to be one to two orders of magnitude smaller than falling method, suggesting the effect of clogging due to water injection. Similar trends have been pointed out in previous studies (e.g., Tateishi et al, 2011). In addition, the variations of permeabilities measured by falling groundwater head method were within one order at most points.
Next, the permeability coefficients obtained from in-situ permeability tests were compared with the estimated value from the particle size composition performed at almost the same points. The method for estimating the permeability coefficients from the particle size composition were suggested by Creager et al (1945). As a results, the estimated values of the permeability coefficient by Creager tended to be larger than those obtained from the in-situ permeability test (falling method). This tendency suggests that it is necessary to consider the effect of consolidation (porosity) when estimating the hydraulic conductivity from the particle size composition. The relationships between permeability coefficients and various soil parameters should be compared in more detail to propose appropriately and simply permeability estimation methods in order to minimalize the initial costs of open-loop system.
Acknowledgement: This paper is based on the results obtained from a project, JPNP19006, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).