CO2 geological sequestration Nagaoka Project


Survey of Storage Capacity in Japan


To show with confidence that Japan's aquifers have the potential for CO2 storage, the following efficacy evaluations were performed.

Evaluation of CO2 geological storage capacity based on the latest geological information and knowledge.
Evaluation of the possibility for CO2 geological storage near the emission sources, calculation of storage capacity, and improvement in calculation accuracy.


Classification of CO2 Geological Storage

The potential geological storage capacity was calculated based on the results of geological surveys and newly-obtained technical knowledge on CO
2 storage. Storage in aquifers was classified into Category A - "storage in an anticlinal structure" and Category B - "storage in a geological structure with a stratigraphic trapping, etc." Furthermore, considering that the evaluation accuracy of these categories varies according to the quality and quantity of geological data obtained by government-conducted basic surveys (basic drilling and basic geophysical exploration), Category A was classified into three sub-categories (A1, A2, A3) and Category B into two sub-categories (B1, B2).

Classification of CO2 storage in deep underground saline formation
Geological data Category A
(Storage in an anticlinal structure)
Category B
(Storage in a geological structure with a stratigraphic trapping, etc.)
Existing oil/gas field Well & seismic exploration data is abundant. A1 B1
(Water-soluble gas field)
Basic boring Well & seismic exploration data is available. A2
Basic seismic exploration Seismic exploration data is available, but no well data. A3 B2
(16 sea areas)
Major trap mechanism
Structural & stratigraphic trapping
Solubility trapping
Residual gas trapping
Mineral trapping
Concept of storage
(by category)


Calculation of Storage Capacity

Based on the knowledge obtained from the demonstration tests at the Iwanohara site and other actual aquifer storage, the geological storage capacity is calculated using the following equation.

CO2 storage capacity = Sf x A x h x p x Sg/BgCO2 x d
Sf : storage factor (Category A: 50%, Category B: 25%)
A : area (m2)
h : effective formation thickness
[formation thickness x sand-mud ratio (m)]
p : porosity
Sg : supercritical CO2 saturation (50%)
BgCO2 : CO2 volume factor (0.003)
d : CO2 density (0.001976 t/m3)

The storage factor, Sf, shows the volumetric ratio of supercritical CO
2 storage to the total void volume in the aquifer. In the current survey, considering the vertical migration of CO2 due to buoyancy, 50% was adopted for Category A. As for Category B, 25% was adopted because this category has a wider area than Category A and hence the reservoir is rather non-uniform in the horizontal direction.

For the supercritical CO
2 saturation (Sg), 50% was adopted in view of: the reference ratio of CO2 saturation derived from geophysical logging at the Iwanohara site was 40-50%, the Sg in a German case was 40-60% (May et al. 2004), and the result of a Canadian simulation was 30-90% (Keith et al. 2004).

Using the above equation, the CO
2 potential storage capacity in Japan was calculated. The storage capacity derived is 30.1 Gt for Category A and 116 Gt for Category B, which means the total geological storage capacity in Japan is 146.1 Gt. The reasons for the significant increase in capacity compared to the estimate of 1993 (91.4 Gt) are that the geological survey data after 1993 was added, and the ratio of supercritical CO2 storage was increased as a result of re-evaluating the 1993 assumption which considered that CO2 would be stored after dissolved into water.

Calculation results of CO2 geological storage capacity
FY2005 FY1993
Category Potential storage capacity (Mt) Category Potential storage capacity (Mt)
A1 3,492 1 1,987
A2 5,202 2 1,541
A3 21,393 4 72,042
B1 88,477
B2 27,532 3 15,847
Total 146,096 Total 91,417
*Inland basins and inner bays (Seto Inland Sea, Osaka Bay, Ise Bay, etc.) are excluded from the potential storage areas.
*The target is areas deeper than 800m but shallower than 4,000m.


Viability of CO2 Geological Storage near Emission Sources

Regarding the effectiveness of geological storage, matching an emission source with a storage site is very important in addition to the availability of storage capacity. This is because cost analysis reveals transport costs account for a relatively large percentage of the total storage costs. If we can show the potential for geological storage near the emission sources, it will significantly facilitate the practical application of this storage method.

From the existing data on potential reservoirs and locations in each category in Japan, it is known that deep ground data is not sufficient in areas like Tokyo Bay, Osaka Bay, Ise Bay, and the northern Kyushu area which are close to large-scale emission sources. To evaluate the storage potential in these areas, field surveys such as boring surveys and geophysical exploration by seismic reflection methods need to be performed.

Distribution of storable aquifers and related surveys

Click to enlarge

Based on the existing geological data, examination of geological structures, extraction and review of storage reservoirs and seal formations, the potential CO
2 storage capacity was estimated with a focus on such areas as Tokyo Bay, Ise Bay, Osaka Bay, and northern Kyushu which are close to large-scale emission sources. This estimation revealed for the first time that these coastal areas located near the large-scale emission sources have the potential for geological storage.



Aquifer storage was classified into Category A "storage in an anticlinal structure" and Category B "storage in a geological structure with a stratigraphic trapping, etc." Category A was further classified into three sub-categories and Category B into two sub-categories.
According to calculation of CO2 storage capacity in Japan, it was found that Category A has a storage capacity of 30.1 Gt and Category B a capacity of116 Gt, amounting to 146.1 Gt in total.
It was revealed that geological storage is feasible in the coastal areas located near the large-scale emission sources.


Future Challenges

It is necessary to improve the accuracy of estimating geological storage capacity in the coastal areas near the large-scale emission sources using existing data and the results of seismic exploration stratigraphic analysis, boring, etc.
It is necessary to formulate a field survey plan to elucidate the geological storage potential in areas close to mid-scale emission sources.

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