Role of Geological Heterogeneity in Poroelastic Deformation of Geologic Medium due to Groundwater Extraction
Presented on Thursday, December 6, 2018
Ajit Joshi and Masaatsu Aichi, Department of Environment Systems, Graduate School of Frontier Sciences, The University of Tokyo, Tokyo, Japan
Land subsidence caused by groundwater extraction has been a major issue for developments in coastal cities where global climate change and sea level rise can exacerbate the situation. Data from satellite observations and leveling survey measurements from the Kujukuri region in Chiba Prefecture in Japan showed land subsidence have continued from past few decades until now because of extraction of methane dissolved in groundwater. For accurate prediction from numerical model, it must represent realistic subsurface conditions. Observations of outcrops from field site showed gradual fining upward/ coarsening upward cycles of vertical gradation with sediment size ranging from sandstone to siltstone/mudstone. This formation-scale sediment heterogeneity could affect hydraulic properties of geologic medium and affect its deformation.
Present research studied effect of vertical heterogeneity on poroelastic deformation of geologic medium and predicted land subsidence caused by groundwater extraction. One-dimensional numerical modeling of groundwater flow and sediment deformation was done by solving mass balance equations with Darcy’s flow mechanics for water-saturated poroelastic medium. Porosity and permeability of the medium were related using Kozeny-Carman equation. Simulations were conducted for an idealized 130 m thick geologic medium having a vertical sediment gradation ranging from coarse-grained sandstone at bottom to fine-grained siltstone (or mudstone) at top. A constant water production rate was assigned over a 50 m interval at the bottom. Simulations with heterogeneous sandstone/mudstone sequences were compared to homogenous sequences. The comparisons showed that deformation was higher for mudstone compared to sandstone due to less permeable and more compressible nature of mudstone. Another simulation was done using litho-stratigraphic section from the field site to calibrate model prediction with a field deformation data. Land subsidence prediction from the model showed a good match with deformation data from field measurements. Future research will be focused on studying deformation behavior of a poroelastic medium saturated with multiple fluids(water/methane).
Ajit Joshi
Department of Environment Systems, Graduate School of Frontier Sciences, The University of Tokyo, Tokyo, Japan
Ajit Joshi is a Project Researcher in the Department of Environment Systems at the University of Tokyo. He received his PhD in Geology from the University of Missouri in USA and BSc and MSc degrees from the Tribhuvan University in Nepal. His doctoral research was on the origin and hydrocarbon transport capability of porosity waves in sedimentary basins. Currently, he is working on numerical modeling of poroelastic deformation of geologic medium due to subsurface fluid production.
Masaatsu Aichi
Department of Environment Systems, Graduate School of Frontier Sciences, The University of Tokyo, Tokyo, Japan
Masaatsu Aichi is an Assistant Professor in the Department of Environment Systems at the University of Tokyo. His research interests are on land subsidence/uplift due to the changes in groundwater abstraction rate, strain measurement of rock masses by fiber bragg grating (FBG) sensor,thermo-poromechanical processes related to geothermal development and integration of numerical modeling and monitoring.
