RISK ASSESSMENT OF ENHANCED GEOLOGICAL STORAGE OF CO2 USING GAS HYDRATES

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1 Proceedings of the 7th International Conference on Gas Hydrates (ICGH 2011), Edinburgh, Scotland, United Kingdom, July 17-21, 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, JAPAN ABSTRACT The 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 CO 2 migration and the estimation of the leakage of CO 2 in pathways will be presented and discussed for the further development. In addition, the promotion of methane gas hydrates would be one of the CO 2 storage in geological structure of marine sediments. The original method of CO 2 -CH 4 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 CO 2 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 CO 2 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 CO 2 migration and the estimation of the leakage of CO 2 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 CO 2 geological storage in geological structure of marine sediments. The original method of CO 2 - CH 4 substitution in the production of gas hydrates will be introduced in the presentation. CO 2 GEOLOGICAL STORAGE Capture and storage of CO 2 in deep geological structure is one of the mitigation measures of carbon emission, regarding the issue of global climate change. The possible target of geological storage of CO 2 has shifted from areas at onshore to offshore. This change is considered to be the reason that risks caused by the injection of CO 2 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: Fax takeshi-komai@aist.go.jp

2 geological storage of CO 2, human and animals, ecological system, local and global environments. The offshore storage of CO 2 has a possibility to reduce risks for some endpoints, except for marine environment. The effects of retardation and attenuation of CO 2 emission would be expected through various media of escape pathways of CO 2 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 CO 2. CO 2 GEOLOGICAL STORAGE USING GAS HYDRATES Preliminary study was carried out to clarify the mechanism of CO 2 gas hydrate formation in marine sediments, which are targets to exploitation of gas hydrates [1]. Regarding to actual CO 2 reservoirs, the growth kinetics of CO 2 hydrate formation, the quantity of CO 2 st orage, 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 L w -H- V (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. CO 2 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, CO 2 hydrate can form easily and exists in a stable solid condition in-situ by injecting CO 2 or exhaust combustion gas including nitrogen into submarine sediments. Therefore, in order to examine feasibility of this CO 2 storage system, it is important to investigate the mechanism of the hydrate formation in sediments. This system features not only the sequestration as CO 2 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 CO 2 is expected to be one of the CO 2 geological storage methods [2]. In addition, the process of CO 2 injection into aquifers has also been studied as a CO 2 storage technique. Recently, based on the perspective of gas hydrate exploitation under the seabed with CO 2 substitution reaction or stability and environmental conservation of marine sediments, there has been some basic researches for CO 2 hydrate formation in marine sediments. This CO 2 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 CO 2 at the molecular level. Fig.1 illustrates a concept of enhanced production of CH 4 gas hydrates using CO 2 [3].This method would be feasible to achieve the sequestration of CO 2 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 CO 2 hydrate formation can be utilized to stimulate the dissociation of methane gas hydrates. Enhanced CO 2 storage system N 2+CO 2 mixture Injection well CO 2 hydrate MH Production system Sea bottom N 2 2 +water drain Production well Water+N 2 Formation kinetics Hydrate saturation Formation rate Equilibrium Permeability change Gas hydrate bearing sediments marine sediments Thenomenclature should follow the abstract as the Fig.1 The concept of enhanced extraction system of CH 4 gas hydrate using CO 2 sequestration [3].

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 Scientific aspects Risk analysis (assessment) risk management system of CCS Company, Municipal Geophysics (monitoring) Natural analog Physical property Fluid dynamics (simulation) Mass transport Porous media flow Social aspects Fig.2 Synthesized approach for CCS investigation. Fig.2 illustrates an integrated scenario of various investigations for CO 2 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 CO 2 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) En d 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 CO 2 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.

4 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 CO 2 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. CO 2 release might take place at surface and/or around borehole to reveal the emission into atmospheric environment. CO 2 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 CO 2 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 OF RISK ASSESSMENT SYSTEM FO R CCS An original risk assessment system for CCS and enhanced production of gas hydrates using CO 2 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 CO 2 geological storage, we have been developing risk assessment tool that consists of hazard impact estimation part, CO 2 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 CO 2 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 CO 2.

Target depth of CO 2 injection varies from deeper than -2000 [m] level of gas or oil reservoirs to shallower than -800 [m] level of unconsolidated seams or aquifers.

5 Target depth of CO 2 injection varies from deeper than [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 CO 2 will be smaller, compared to deeper formations as the matter of pressure balance between rock and injected CO 2. Even then, CO 2 storage in aquifers wo uld be possible. One of essential parts of risk assessment of CO 2 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 [m] level, a scheme has been proposed to evaluate sealing possibility [9]. As for faults shallower than [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 CO 2 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 CO 2 -CH 4 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 CO 2 geological storage at specified situation. The 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 CO 2 geological storage in aquifers and enhanced recovery of gas hydrates. The framework of enhanced CCS has been modified to improve the functions of geological-based performance assessment. Some results of a numerical simulation of CO 2 migration and the estimation of the leakage of CO 2 in principal pathways, such as faults and groundwater paths, are also discussed for the development of risk assessment system. REFERENCES [1] Inui, M., Komai, T., Behavior of Gas Hydrate Formation in Marine Se diments for CO 2 Se questration, Proc of 15 th ISOPE, 2006, Seoul, Korea. [2] Komai, T. Sakamoto, Y. and Kawamura, T. Carbon Capture and Storage System Using Gas Hydrates, 6 th Int Symp. Gas Hydrates, 2007, Vol.6, pp [3] Komai, T., Inui, M. and Sato, T. Behavior of Ga s Hydrate Formation in Marine Sediments for CO 2 Se questration, Int J Offshore and Polar Eng, ISOPE, 2005, Vol 14, No 1, pp [4] Corre, A., CO 2 storage risk assessment from site selection to project abandonment, AIST- Imperial College London annual seminar, 2008, Tsukuba Japan. [5] Pawar J. Rajesh, Development of a framework for long-term performance assessment of geologic CO 2 sequestration sites, Joint meeting with AIST and LANL, 2007, Los Alamos, USA.

6 [6] Okuyama, Y., Geo-chemical Effects on CO2 storage into Aquifers, J of Geological Science Japan, 107 (4), 2008, pp [7] Tanaka, A., Sakamoto, Y. and Komai, T., Framework of risk assessment of CO2 geological storage, proc. JGU, L , [8] Færseth, et.al, Methodology for risking fault seal capacity: Implications of fault zone architecture, AAPG Bulletin, 91(9), 2007, pp [9] Takahashi, M., Development of Evaluation Method for Sealing Abilities of Geo-Formations, RITE, 2008, pp [10] Ujihira, M., Geological study on Ga s Outbursts in Coal Mines, J of MMIJ, volume 90, 1989, 197.

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