Risk refers to the possibility of loss or damage to life and property. A more general and strict definition is as follows: Risk R is the product of accident probability p and environmental (or health) consequence c caused by the accident, namely:
Introduction of geological storage technology and method of carbon dioxide
For underground engineering, risk can be defined as the possibility that an activity or objective existence can lead to various direct or indirect losses in the process of action aimed at the normal construction of the project, then the project is said to have risks (Chi Xiuwen et al., 20 10). Risk assessment is a systematic process to describe and quantify the probability of bad results or accidents. In other words, risk assessment is a systematic process to evaluate the possibility and degree of damage to safety, health and ecology in a specific period.
(a) Assessment and rating
China's "Technical Guidelines for Environmental Risk Assessment of Construction Projects" defines the environmental risk assessment grades as Grade I and Grade II according to the material hazards, namely, highly toxic hazardous substances, general toxic substances, inflammable and explosive hazardous substances and functional units, namely, major hazards, non-major hazards and environmentally sensitive areas.
CO2 is the main substance for geological sequestration of CO2, but it is neither a toxic substance as defined in Technical Guidelines for Environmental Risk Assessment of Construction Projects, nor a combustible, inflammable or explosive substance, so its evaluation grade cannot be correctly determined. As one of the emission reduction technologies aimed at reducing CO2 emissions, it is suggested that the environmental risk assessment of CO2 geological storage should be determined as Grade I after comprehensive consideration. That is, it is required to quantitatively predict the impact of the accident, explain the scope and degree of the impact, and put forward prevention, mitigation and emergency measures.
Technical Guidelines for Environmental Risk Assessment of Construction Projects require that the scope of impact assessment should be determined according to the hazard threshold of chemicals and the locations of occupational exposure limits and sensitive areas of all harmful factors in industrial sites. It is suggested that the evaluation scope should be determined according to the maximum radius of CO2 migration.
In view of the limitation of toxicological research data, the calculation of CO2 leakage risk value has not included the chronic damage consequences such as acute death, non-acute death, injury, disability, teratogenesis and carcinogenesis.
(2) Risk identification
Risk identification includes production facility risk identification, material risk identification and risk category identification. For CO2 geological storage, according to the environmental impact mechanism and risk analysis, the facilities with risks include abandoned wells, integrity of injection wellhead and monitoring wellhead, wellhead equipment, pump room, valve room and other engineering units, and leakage channels of various geological factors. The only risky substance is carbon dioxide; ; Risk accident is leakage (Diao Yujie et al., 20 12).
According to the report of the International Risk Management Committee in 2008, generally speaking, the leakage risk will gradually increase with the beginning of CO2 injection and reach the maximum with the end of injection. After the site is closed, the risk of leakage will gradually decrease with the passage of time (Figure 1 1-22).
Figure 1 1-22 CO2 geological storage risk change diagram
(According to China Agenda Management Center 2 1 century, etc. , 20 12)
It is helpful to analyze the leakage risk to fully grasp the geological characteristics of the cover of CO2 geological storage site, and the focus of supervision work is the CO2 geological storage project with the highest risk (Benson, 2005). For different stakeholders, the risk situation is different, and the management of local risks (environment, human health and property rights) is completely different from the management of global risks.
The sudden leakage of carbon dioxide from borehole or fault poses the greatest risk to the health and safety of residents near the geological storage site of carbon dioxide. In addition, the risk situation is not static and can be effectively prevented and managed through engineering measures. Nevertheless, the lessons from the Fukushima 1 nuclear power plant accident in Japan show that the decision-making of risk countermeasures or risk scenarios that have not been tested by great disasters is fundamentally meaningful.
Source analysis
Source term analysis includes maximum credible accident probability analysis and calculation of harmful substance leakage. The most credible accident refers to the most serious accident that is harmful to the environment (or health) among all accidents with non-zero prediction probability. Qualitative and quantitative methods can be used to determine the probability of maximum credible accident and CO2 leakage.
1. Maximum credible accident screening
The most credible accident refers to the accident with a certain probability, a greater degree of harm and the greatest risk value. It is suggested that the analytic hierarchy process (AHP) be used to screen nine possible CO2 geological storage risk events, and the CO2 leakage risk accidents of water injection wells and monitoring wells are determined as the most credible accidents, as shown in table 1 1- 13.
Table11-13 Examples of screening the most credible accidents of CO2 geological storage.
2. Maximum credible accident analysis
After determining the maximum credible accident, through the analysis and comparison of various risk assessment methods in the first section of this chapter, the corresponding environmental risk assessment method is selected according to the actual situation of CO2 geological storage project.
(4) Consequence calculation
1. Source strength estimation
It is assumed that the typical CO2 leakage is the leakage at the connection of wellhead equipment transmission pipeline, and the leakage aperture is set to 10mm, and the leakage can be controlled within 3 minutes. When the gas velocity is in the sonic range (critical flow), the CO2 leakage velocity can be calculated according to the formula (1 1- 16).
Introduction of geological storage technology and method of carbon dioxide
Where: p is the pressure of the medium in the container. pa； P0 is the ambient pressure. pa； K is the adiabatic index (heat capacity ratio) of gas, which is 1.29.
Assuming that the gas is an ideal gas, the CO2 leakage rate can be calculated according to the formula (1 1- 17).
Introduction of geological storage technology and method of carbon dioxide
Where: QG is the gas leakage speed, kg/s; P is the container pressure, pa; Cd is the gas leakage coefficient, and the circular crack is taken as1.0; A is the fracture area, m2, and the leakage aperture is10mm; M is the molecular weight of the substance; R is the gas constant, 8.314j/(mol k); T0 is the gas temperature, 298Ky is the outflow coefficient, and the critical flow rate is 1.0.
On this basis, the leakage of carbon dioxide into the atmosphere can be calculated.
2. Leakage accident prediction
According to the complexity of CO2 leakage and its diffusion in the atmosphere, and according to the requirements of Technical Guidelines for Environmental Risk Assessment of Construction Projects (HJ/T 169—2004), the heavy gas diffusion model can be used for calculation.
(1) prediction mode
According to Technical Guidelines for Environmental Risk Assessment of Construction Projects (HJ/T 169-2004), it is suggested that the heavy gas diffusion model be used for calculation. According to the hourly sliding of meteorological data for one year or sampling according to meteorological specifications, the concentration values of each grid point and focus point are calculated, and then the concentration values are sorted from small to large, and the value with cumulative probability level of 95% is taken as the representative value of concentration of each grid point and focus point for evaluation.
The dense gas diffusion model of Cox and Carpenter is used to calculate the air mass diffusion at different times after stable continuous release and instantaneous release. The air mass diffusion under the action of gravity is calculated according to formula (11-kloc-0/8) to formula (1 1-20):
Introduction of geological storage technology and method of carbon dioxide
Diffusion under the action of air entrainment;
Introduction of geological storage technology and method of carbon dioxide
Among them: Qe is the diffusion under the action of entrainment around the flue gas; Ue is smoke sucked from the top by diffusion; R is the formation radius of instantaneous leakage smoke cloud; H is the height of the cylinder; γ is the edge entrainment coefficient, which is 0.6; α is the top entrainment coefficient, which is 0.1; μ 1 is the wind speed, m/s; K is the test value, generally1; R is Richardon number, which is obtained from the formula (1 1-2 1):
Introduction of geological storage technology and method of carbon dioxide
Where α is an empirical constant, taking 0.1; U 1 is the axial turbulent velocity; L is the length of turbulence.
(2) Forecast content
Under the assumed meteorological conditions, predict the impact of the accident on the focus and a certain range.
(3) Reference standard
The national standard "occupational exposure limits of Hazardous Factors in Workplace" (GBZ 2-2002) stipulates that the allowable concentration of CO2 in short-term contact is 18000mg/m 3, and the national standard "Indoor Air Quality Standard" (GB/T 18883-2002) stipulates that the daily average value of indoor CO2 is 0.1.
(V) Risk calculation and assessment
1. Risk calculation
According to the Technical Guidelines for Environmental Risk Assessment of Construction Projects, the environmental risk of CO2 geological storage projects is calculated by the formula (1 1- 15).
2. Risk assessment
Acceptable risk standard is a standard to measure system risk inferred from statistical data, and it is an important basis for planning and deciding the location of major hazard sources.
(1) Personal risk acceptance criteria
Personal risk acceptance criteria indicate whether the target population near the hazard source can be exposed above a certain risk level, and usually give an upper limit and a lower limit.
Even if the leakage rate of CO2 is within an acceptable range, the leakage amount or concentration of CO2 cannot meet the acceptable standards for human and animal health, agriculture, water resources and so on. Les believes that in the range of CO2 concentration ≤ 1%, it will not affect healthy human individuals, but may affect the health of infants and people. Health Canada suggested that the indoor carbon dioxide concentration should be ≤ 0.35%.
According to the health standards and human toxicity data, as well as the Data of Safety Technology of Hazardous Chemicals (GBZ2-2002) and Hygienic Standards for the Design of Original Industrial Enterprises (TJ36-79), the semi-lethal concentration (LC50), the allowable life health concentration of direct threat (IDLH) and the maximum concentration in the workshop (MAC) are selected as the evaluation standards (or effects) of various hazardous substances.
Table 1- 14 Evaluation Standard Table Unit: mg/m3
Table 1 1- 15 gives the personal risk standard of China on the basis of studying and summarizing the personal risk standard of the population formulated by foreign authorities.
Table 1l- 15 Classification of Objects Affected by Hazardous Sources and Acceptable Risk Criteria
(2) Acceptable standards of social risks
Acceptable standards of social risks are used to reduce the risk of accidents faced by the public. It is usually represented by two standard straight lines in the F-N curve, which represent the upper limit and the lower limit respectively. The F-N curve is usually divided into three regions: acceptable region, unacceptable region and ALARP region (as low as possible).
In the process of site selection of CO2 geological storage project, if the actual F-N curve of CO2 underground storage falls into an acceptable area, the site selection is acceptable. It belongs to an unacceptable area and the site selection is unacceptable, indicating that the density of affected people around CO2 underground storage is high; If it falls into ALARP area, measures should be taken step by step until the risk is acceptable. Figure 1 1-23 gives the recommended standards of social risks in the Netherlands, Britain, China and Hongkong.
Figure 1 1-23 Social risk standards for people formulated by authoritative departments in some countries and regions