| With the development of industrialization and social economy, air pollution and greenhouse effect caused by combustion of fossil fuels have become serious threat to the earth’s environment for human survival. In order to avoid disasters that greenhouse effect may bring to human beings such as melting glaciers, rising sea levels, ocean acidification and so on, we must control emission of greenhouse gas CO2and CH4to the atmosphere, and reduce their excessive accumulation in the atmosphere. Among various mitigation techniques, in addition to improve energy efficiency, development of new energy and clean burning technology, more and more attentions are paid to carbon dioxide storage. Currently, the feasible storage ways mainly include the geological storage, ocean storage, mineral sequestration, forest-and land-ecological storage etc. At present, the relatively mature technology of saline-injection into groundwater layer is considered to be an important way to delay emissions of carbon dioxide into the atmosphere so as to slow down the greenhouse effect, whose storage capacity is large and the storage time can be up to several hundreds of thousands of years. According to the IPCC report, after the injection of carbon dioxide into the salt-solution layer, the capture in the early stage is physical capture, while by the scale of several thousands of years, with the increase of time, the capture is mainly (≥50%) by chemical trapping mechanism (dissolution trapping and mineral trapping). So, in the early period of the injection, carbon dioxide is mainly in dissolved state. In order to understand the migration of carbon dioxide after injection, we need to comprehend the diffusion mechanism of carbon dioxide in aqueous solutions after injection. In addition to geological storage, several other disposal methods such as mineral sequestration, ocean sequestration, injection of carbon dioxide into depleted oil and gas reservoirs, coal to promote the mining of oil and exploitation of natural gas, also involve diffusion process of CO2and CH4. In addition, the diffusion coefficients of CO2and CH4in the water and salty water are important parameters related to numerical simulation carried out in calculation for migration and distribution of carbon dioxide and the natural gas.At present, diffusion of gas in solution is experimentally determined by sampling for gas concentration test or indirectly by measuring changes of pressure and volume, so precision of concentration determination directly affects the accuracy of diffusion coefficient. Diffusion coefficients measured in previous experiments are mainly limited to low temperature and atmospheric pressure conditions, thus data of diffusion coefficients of geological storage conditions are in lack. Howerer, some calculation formulaes of the diffusion coefficient derived based on fitting the experimental data are extrapolated under high temperature and high pressure with uncertainty. Therefore, to obtain diffusion coefficients of CO2and CH4in salt solution at wide range of pressure and temperature to establish reasonable calculation model is very important.In this paper, experiments were carried out to determine the diffusion coefficients of carbon dioxide, methane in water and different concentrations of NaCl solutions at different temperature and pressure by transparent high-pressure capillary. Diffusion concentration was monitored with in situ Raman spectroscope to calculate diffusion coefficients under wide range of temperature and salinity, analyse the main factors influencing the diffusion coefficients, and establish a model to describe relationship between diffusion coefficients and temperature, pressure and salinity. The leakage of CO2through cover layer under storage conditions for a salt solution was simulated.Conclusions made in this paper are as follows:(1) According to the experimental results, the diffusion coefficients of CO2in water under different temperature and pressure conditions was fitted with respect to temperature. The relation could be described as Speed-Angell Power-law Function: D(CO2)=D0[(T/Ts)-1]γwhere D0=14.115×10-9m2/s,γ=1.673, Ts=228.9K. The results show that when the pressure increase from10MPa to20MPa and30MPa at393K, change of the diffusion coefficient is less than3%, while to45MPa, change of the diffusion coefficient is5.6%. In comparison to temperature, effect of pressure on the diffusion coefficient is rather small.(2) According to the experimental results, the function relation fitted by diffusion coefficient of CH4D(CH4) in water with respect to T under different pressure conditions was: D(CH4)=D0[(T/Ts)-1]γ where D0=16.318×10-9m2/s,γ=1.810, Ts=232.3K.Comparing the values that we measured at four pressures at298,353, and393K, the diffusion coefficients of methane in water increase with pressure increase below20MPa, but decrease with pressure increase above20MPa. However, at the studied temperature and pressure range, the pressure effect on the diffusion coefficients of methane in water is rather small. (3) Experimental data about diffusion of carbon dioxide in brine with different concentrations show that, at the same concentration, the diffusion coefficient increase obviously with the increase of temperature; with the same temperature and pressure conditions, the diffusion coefficient decreases with the increase of concentration Relationship between the diffusion coefficient and the concentration was fitted as: DS/DW=1-1.7138Cewhere Ds denotes CO2diffusion coefficient in NaCl solutions with different concentrations; Dw is the diffusion coefficient in pure water; Ce is the concentration of NaCl solution.(4) Diffusion experiment of CO2-CH4mixed gas in water showed that:the diffusion coefficient of carbon dioxide is larger than that of methane below433K, but rate of increase of the diffusion coefficient for methane with the increase of temperature was higher than that for carbon dioxide; while the diffusion of methane is slightly higher than that of carbon dioxide above433K.(5) Based on the diffusion coefficient of carbon dioxide in water, the effective diffusion coefficient of carbon dioxide in the aquifer was calculated; take the Texas Frio CO2Salt Solution Storage Project of the United States as an example, diffusion loss amount of CO2through low permeability layer was calculated according to the parameters of site. The calculation showed that the main factors affecting the diffusion amount of CO2in the rock cap are:the effective diffusion coefficient, thickness of and porosity of cover layer; there was less amount of CO2diffusion through the layer of rock cover with thick cover layer but small porosity. Therefore, in the future geological sequestration projects, cover layer with larger thickness and geological reservoir with smaller porosity should be selected to reduce the risk of carbon dioxide leakage. |
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