Proceedings of the 7th International Conference on Gas Hydrates (ICGH 2011 ),
Edinburgh, Scotland, United Kingdom, July 17-21, 2011.
RISK ASSESSMENT OF ENHANCED GEOLOGICAL STORAGE
OF CO2 USING GAS HYDRATES
Takeshi Komai , Yasuhide Sakamoto and Atsuko Tanaka
Geo-resources and Environment, AIST-west
16-1 Onogawa, Tsukuba, 305-8569
JAPAN
ABSTRACT
T he framework of risk assessment system for CCS is proposed, especially for enhanced
recovery of gas, oil and gas hydrates. Based on the protocol proposed by European Union,
the framework of enhanced CCS has been modified to improve the functions of geologicalbased performance assessment. Some results on simulation of CO2 migration and the
estimation of the leakage of CO2 in pathways will be presented and discussed for the further
development. In addition, the promotion of methane gas hydrates would be one of the CO2
storage in geological structure of marine sediments. The original method of CO2 -CH4
substitution in the production of gas hydrates will be introduced in the presentation.
Keywords: gas hydrates, carbon storage, risk assessment, enhanced recovery
INTRODUCTION
Both aspects of risk and benefit as well as the
balance are very important in understanding the
feasibility of CO2 geological storage at specified
situation of enhanced recovery of gas hydrates.
Researchers in Europe and USA have considered
the framework for risk assessment of CCS,
especially for geological storage of CO2 and
enhanced recovery system of oil and gas
production. Various kinds of benefits of geological
carbon storage, compared with ocean and
atmospheric discharge, can be easily understood in
the scientific aspect of global environment and the
economical aspect of CDM. However, the
assessment of risks caused by CCS wo uld be
hardly undertaken, because of some difficulties in
determining the end point and parameters for
estimating ecological and human risks. In order to
achieve transparent risk governance for any
stakeholders who are involved in CCS project, it is
necessary to develop the general and/or common
framework, enabling to be fully communicated
among any party of concern.

In this paper, we propose the framework of risk
assessment system for CCS, especially for
enhanced recovery of gas, oil and gas hydrates.
Some results on simulation of CO2 migration and
the estimation of the leakage of CO2 in pathways
will be presented and discussed for the further
development. In addition, the enhanced recovery
of methane gas hydrates would be one of the CO2
geological storage in geological structure of
marine sediments. The original method of CO2 CH4 substitution in the production of gas hydrates
will be introduced in the presentation.
CO 2 GEOLOGICAL STORAGE
Capture and storage of CO2 in deep geological
structure is one of the mitigation measures of
carbon emission, regarding the issue of globa l
climate change. The possible target of geological
storage of CO2 has shifted from areas at onshore to
offshore. This change is considered to be the
reason that risks caused by the injection of CO2 at
offshore area might be smaller than at onshore
residential places. In terms of environmental risk
assessment for CCS, there are typical endpoints of
Corresponding author: Phone: +81 29 861 8294 Fax +81 29 861 8773 E-mail: [email protected]
geological storage of CO2 , human and animals,
ecological system, local and global environments.
The offshore storage of CO2 has a possibility to
reduce risks for some endpoints, except for marine
environment. The effects of retardation and
attenuation of CO2 emission would be expected
through various media of escape pathways of CO2
migration. IPCC report has stated the possibility of
CCS wo uld be in deeper geological structures.
Major institutions of geological survey in the
world have been investigating the possible places
of offshore geological storage of CO2 .
CO 2 GEOLOGICAL STORAGE USING GAS
HYDRATES
Preliminary study was carried out to clarify the
mechanism of CO2 gas hydrate formation in
marine sediments, which are targets to exploitation
of gas hydrates [1]. Regarding to actual CO2
reservoirs, the growth kinetics of CO2 hydrate
formation, the quantity of CO2 storage, and the
condition
change
in
reservoirs
were
experimentally investigated. Thus we clarified the
mechanism of gas hydrate formation in sand layers
representing marine sediments. Firstly, the Lw-HV ( water liquid, hydrate, and vapor) equilibrium
condition of the hydrate and microscopic
observation of the hydrate formation were
examined by using a small-scale pressure cell
filled with porous media in unsaturated condition
of water and gas. Then, behavior of the hydrate
formation front was observed by using a bulkscale pressure vessel filled with sand.
CO2 can be sequestrated as a form of solid
phase of gas hydrate in marine sediments. Because
the situation is in a condition of high pressure and
low temperature under the seabed, CO2 hydrate
can form easily and exists in a stable solid
condition in-situ by injecting CO2 or exhaust
combustion gas including nitrogen into submarine
sediments. Therefore, in order to examine
feasibility of this CO2 storage system, it is
important to investigate the mechanism of the
hydrate formation in sediments. This system
features not only the sequestration as CO2 hydrate
under the seabed, but also the combination with
methane hydrate production.
ENHANCED GAS HYDRATE PRODUCTION
USING CO 2
The enhanced recovery process of oil, natural gas
and gas hydrates by using CO2 is expected to be
one of the CO2 geological storage methods [2].
In addition, the process of CO2 injection into
aquifers has also been studied as a C O2 storage
technique. Recently, based on the perspective of
gas hydrate exploitation under the seabed with
CO2 substitution reaction or stability and
environmental conservation of marine sediments,
there has been some basic researches for CO2
hydrate formation in marine sediments. This CO2
storage process using gas hydrates has the
possibility of satisfying simultaneously both
aspects resources development and environmental
conservation.
Large amounts of methane gas hydrates are
found in marine sediments and permafrost. To
extract methane gas from its reservoir in a
practical production way, it is necessary to obtain
fundamental information on the mechanism
underlying the formation and dissociation of gas
hydrates and their properties, including kinetics
and crystal growth. We have proposed an
advanced method of gas hydrate production, by
which methane gas is extracted from the reservoir
by replacing methane with CO2 at the molecular
level. Fig.1 illustrates a concept of enhanced
production of CH4 gas hydrates using CO2 [3].This
method would be feasible to achieve the
sequestration of CO2 into the sediments. The main
purposes of this research are to elucidate the
mechanism underlying hydrate reformation and
substitution and accumulate practical data for
completing the original concept of the proposed
method. The heat generation in CO2 hydrate
formation can be utilized to stimulate the
dissociation of methane gas hydrates.
Enhanced CO
CO2 storage system
Injection
well
well
MH Production system
N
N22+CO
+CO22
mixture
Production
Production
well
well
Water+N
Water+N22
Sea
Sea bottom
Formation kinetics
Formation rate
Formation
rate
Equilibrium
Hydrate saturation
Permeability change
Permeability
change
Equilibrium
Gas hydrate
bearing sediments
CO2 hydrate
N22+water drain
marine
marine
sediments
The nomenclature should follow the abstract as the
Fig.1 The concept of enhanced extraction system
of CH4 gas hydrate using CO2 sequestration [3].
FRAMEWORK OF CCS RISK ASSESSMENT
The methodology of risk assessment needed for
CCS operation is essential to achieve the
management of environmental risks and effective
operation. In order to implement transparent risk
governance for any stakeholders who are involved
in CCS project, it is necessary to develop the
general and/or common framework of risk
assessment, enabling to be fully communicated
within any party of concern [4].
Geosciences
（geo-structure）
Geological survey
Hydro-geology
Geochemistry
（mechanism）
Parameter fitting
Sequestration
Risk analysis
（assessment）
Geophysics
（monitoring）
Natural analog
Physical property
Fluid dynamics
（simulation）
Mass transport
Porous media flow
risk management
system of CCS
Social
aspects
Scientific
aspects
Company, Municipal
Fig.2 Synthesized approach for CCS investigation.
Fig.2 illustrates an integrated scenario of
various investigations for CO2 geological storage.
It is necessary for risk assessment of CCS to
synthesize some fundamental studies, such as
geosciences, geophysics, geochemistry, risk
analysis, and fluid dynamics. Both scientific and
social approaches are also needed to create more
realistic risk assessment scenario.
Integrated
Framework
for CCS
Risk
Assessment
Risk assessment should be undertaken for CO2
geological storage in three phases, like as in the
following contents. Different type of models have
be developed for process of Tier1 to Tier3
assessment.
Tier1: Risk source assessment, screening model
for selection of adequate site and its
identification,
Amplification
and
Fluorescence End Point. (FEP analysis)
Tier2: Exposure assessment, site specific
exposure model for environmental
impact. (Performance assessment)
Tier3: Effects assessment, detailed analytical
model for business planning, operation
and shut-down. (Risk management)
End Points of risk assessment
It is essential to clarify endpoints for quantitative
risk assessment.
The following are typical
envisioned endpoints of risk assessment for
geological storage. At each site endpoints and their
we ighting should be selected and identified.
1) Human or animals
Respiratory inhibition, earthquake damage,
exposure in operating.
2) Ecological environment
Population reduction of fishes, benthic
community, plants, and geo-microbes.
3) Regional environment
Reduction of environmental values and
services.
4) Global environment
Global climate change, change in ocean and
geo-environment.
Concerned Hazards due to CCS
Hazards caused by CCS operation would be
identified for each set of endpoint. The following
are the characterization of expected hazards.
1) Human effects
High CO2 exposure, trigger of geo-hazards,
elution of toxic components.
2) Ecological effects
Damage to fishery, reduction of ecological
diversity, effect to microbiology.
3) Effects to geo-structure
Ground slide, subsidence, induced earthquake,
effect to groundwater.
4) Effects to natural recourses
Pollution of aquifer, gas intrusion, effect to oil
reservoir, depletion of natural resources.
Whole System and Related Technologies
It is necessary to develop the whole system of
CCS and related technologies, regarding to various
kind of geological and environmental processes, as
listed in the following.
1) Geological performances
Fault, trap structure, reservoir, aquifer, and
seal performance.
2) Hydro-geological properties
Groundwater transport, porosity, permeability,
temperature, dissolved matter.
3) Geochemical properties
Isolation,
transformation,
dissolution,
equilibrium and kinetic properties.
4) Mechanical properties
Physical strength, compaction, failure, run-off,
ground slide, earthquake.
5) Environmental media
Atmosphere, ocean, marine sediments, ground
surface, subsurface.
6) CCS technologies
Injection method, well system, injection rate,
pressure, flow rate, surface equipments.
7) Public acceptance
Risk scenario, risk trade-off, mitigation, risk
communication, risk and benefit analysis.
Site assessment and performance assessment
In the tier2 risk assessment, exposure assessment
or performance assessment, assuming site specific
conditions and geological structures, will be
introduced for environmental impacts assessment
of CCS. The scenario of site characterization and
performances on CO2 release, fluid flow and
geochemical reactions was proposed by Los
Alamos National Laboratory [5]. Different kind of
exposure models would be utilized in each
scenario, because that time and special dimension
are unlikely valuated from days to million years.
CO2 release might take place at surface and/or
around borehole to reveal the emission into
atmospheric environment. CO2 injected into deep
geological structure might migrate as fluid flow in
pore space of geological media and a part of that
could store as solid material in geochemical
reactions.
Although most of injected CO2 could isolate
inside the geological media, some portion might
flow and migrate through geological media for a
long period. The risk of leakage wo uld greatly
vary due to the geological and hydro-geological
structure at specific sites [6].
DEVELOPMENT O F RISK ASSESSMENT
SYSTEM FO R CCS
An original risk assessment system for CCS and
enhanced production of gas hydrates using CO2
has been developed, based on fundamental studies
of the framework of CCS risk assessment and the
numerical simulations [7].
To establish optimum risk assessment decision
basis for safety and risk management of CO2
geological storage, we have been developing risk
assessment tool that consists of hazard impact
estimation part, CO2 migration evaluation part and
risk evaluation part. Fig.3 illustrates the formation
of elements needed for risk assessment.
For hazard impact estimation, we introduced the
preliminary hazard analysis (PHA) methodology.
Using PHA format, the authors have been
collecting hazard elements, and gathering
information that will be able to be utilized for
semi-quantification of probability or frequency
and consequences, especially for hazards which
have potential impacts onto surface. The criteria of
environmental impacts due to the exposure of CO2
are listed in Fig.4.
Fig.3 Elements needed for risk assessment for
CCS.
Fig.4 Criteria data for impact assessment due to
exposure of CO2.
T arget depth of CO2 injection varies from
deeper than -2000 [m] level of gas or oil reservoirs
to shallower than -800 [m] level of unconsolidated
seams or aquifers. In Japan, aquifers are regarded
to be one of the promising targets for geological
storage. In shallower than -800 [m] aquifers,
storage capacity of CO2 will be smaller, compared
to deeper formations as the matter of pressure
balance between rock and injected CO2 . Even then,
CO2 storage in aquifers wo uld be possible.
One of essential parts of risk assessment of CO2
geological storage is the quantitative assessment of
fractures or faults [8]; whether they will act as
seals or paths for fluid? Regarding fractures or
faults deeper than -1500 [m] level, a scheme has
been proposed to evaluate sealing possibility [9].
As for faults shallower than -1000 [m] level, some
reports showed qualitative relationship between
depth of shallower faults and seal functions, as
illustrated in Fig.5, using gas outburst static data
in coal seams in Japan [10].
Fig.5 Fault distribution in Ishikari coalfield in
Hokkaido, Japan.
We have worked for designing functions
necessary for impact assessment of surface area,
including atmospheric and ecological impacts due
to little seepage of CO2 from ground surface. The
result of the calculation will be combined with
hazard consequences calculation using impact data
listed in Fig. 4. The developed risk assessment
system will provide stakeholders for transparent
risk communication.
CONCLUSIONS
The original method for enhanced extraction of
gas hydrates is proposed, based on the mechanism
of CO2 -CH4 substitution in the production of gas
hydrates. Various kinds of benefits of geological
carbon storage, compared with ocean and
atmospheric discharge, can be easily understood in
the scientific aspect of global environment and the
economical aspects. Both aspects of risk and
benefit as well as the balance are very important in
understanding the feasibility of CO2 geological
storage at specified situation.
T he assessment of risks caused by CCS would
be hardly undertaken, because of some difficulties
in determining the end point and parameters for
estimating ecological and human risks. In order to
achieve transparent risk governance for any
stakeholders, it is necessary to develop the general
and/or common framework, enable to be fully
communicated among any stakeholders.
We have proposed an integrated risk assessment
system for CCS, especially for the cases of CO2
geological storage in aquifers and enhanced
recovery of gas hydrates. T he framework of
enhanced CCS has been modified to improve the
functions of geological- based performance
assessment. Some results of a numerical
simulation of CO2 migration and the estimation of
the leakage of CO2 in principal pathways, such as
faults and groundwater paths, are also discussed
for the development of risk assessment system.
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