Principal Research Results

Field Investigation of CO2 Migration Behavior near the Ground Surface
for Environmental Impact Evaluation
BackgroundTo gain the public acceptance for CO2 geological storage, environment impact evaluation in groundwater and atmosphere isnecessary in addition to a feasibility demonstration of the geological storage technology. A possible leakage path of CO2 which was
not extracted in storage capability estimation of reservoir aquifer will allow CO2 to go upwards in the long term and to consequently
reach near the ground surface. Therefore, it is necessary to predict CO2 migration behavior from a lack of seal rock above the aquifer
to the surface and atmosphere, and to reflect it into the environmental impact evaluation of CO2 geological storage.

Objectives
This study aims to propose investigation methodology of CO2 migration behavior, geochemical influence of CO2 underground, and concentration distribution of the atmospheric diffusion of CO2 in cases of leakage of CO2 stored underground moving
upwards to the ground surface (Fig.1);

Principal Results
1. Investigation of gas migration undergroundAn uprising migration phenomenon of natural gas (mainly consisting of methane, CH4) from gas fields including dissolved
natural gas in groundwater was investigated by taking a natural analogue of CO2 migration in shallow layers. Geological survey ofoutcrops and boring cores, hydraulic measurement in boreholes, carbon isotope analysis of underground gas, and numerical analysis ofgas migration were conducted in a Pleistocene sedimentary rock region. As a result of the investigation, a gas migration model inwhich the gas moves upwards from deeper layers, is trapped by low permeability layers and goes up along the fault fracture zones wasproposed and was approximately validated by the numerical analysis (Fig. 2).
2. Investigation of CO2 geochemical influence on groundwater
Methods of quantitative analysis and dissolution rate evaluation were surveyed to clarify the elution phenomenon of traceelements (12 elements including B and As) from rock contacted with groundwater including dissolved CO2. A concentration analysis
technique of the trace element in rock sample directly using high precision X-ray fluorescence analysis (XRF) and a dissolution rateevaluation technique of trace element based on a laboratory or a field survey of carbonate spring were presented. A procedure for thegeochemical influence investigation of uprising CO2 including these techniques was proposed.
3. Investigation of CO2 atmospheric diffusion after leakage
A modeling method to predict the atmospheric diffusion phenomenon of CO2 leaked from the ground surface was investigated. Example analyses in which numerical simulation based on DEGADIS model ＊ 1 with attached functions of radiation source
calculation and weather data import was performed and yielded an efficiency for the prediction approach of CO2 atmospheric diffusion discharged from the ground surface (Fig.3).
Incidentally, this study was carried out as research funded from RITE (Research Institute of Innovative Technology for theEarth).

Future Developments
The proposed investigation methodology will be applied to the field including CO2 gas migration in a shallow undergroundlayer.
Main Researcher: Hiroshi Suenaga,
Research Geohydrologist, Geosphere Science Sector, Civil Engineering Research Laboratory
Reference
K. Nakagawa, et al., 2008, “Survey on understanding and estimation of CO2 migration behavior in a shallow layer around geologicalstorage site”, CRIEPI Report N990728 (in Japanese)＊1：One of the models that American environmental protection agency (U.S.EPA) allows to use for an atmospheric diffusion prediction of thehigh-density gas.

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2. Environment
CO2 diffusion investigation

WindLeakage
Soil

10m
fault ??

elution of traceelement

dissolution

mineral grain
uprising by
buoyancy force
CO2 dissolvedwater→pH≒4100m
CO2

CO2 migration
investigation

2geochemical influence
investigation
seal layer

in case of a lack in the layer

reservoir layer
1000m

Fig.1 Schematic diagram of this study

350m
100mCH 4 saturation

Fig.2 Results of the gas migration investigation (left: gas migration model, right: calculated resultof CH4 gas migration)
CO2
concentration（（ppm）
100

wind speed = 0.9m/s
at night
max 71.1ppm

10

leakage area
40×120m

10.10100（m100（ m））

Fig.3 Calculated result of the leakage CO2 diffusion in atmosphere55