| The greenhouse dominated by CO2 has caused a series of global climate change issues:polar ice melt, sea level rise, species extinction, all are shocking. It is an urgent responsibility to take active measures to deal with climate change. Currently, CO2 reduction has become a hot issue of common concern of mankind. Research has shown that carbon capture and storage technology (CCS) is one of the important measures which can effectively reduce CO2 emission to address climate change. Its principle is to transform CO2 into supercritical phase to storage in the deep geological media. There are mainly three geological media for CO2 storage:deep saline aquifers, depleted oil reservoirs and unminable coal beds. Among them, deep saline aquifers are considered to be the most promising and accessible geological container for CO2 storage due to its wide distribution, unexcessive penetration of upper aquiclude (while large numbers of traps will be destroyed by the oil exploration wells), relatively good sealing performance and large storage capacity. A lot of research and demonstration projects have been carried out on CO2 storage in deep saline aquifers. Most of these research forcused on the migration regularity of CO2, the trapping mechanism and the potential evaluation of CO2 storage in deep saline aquifer, etc., which have laid a theoretical foundation for further research and the implementation of CO2 geological storage project in deep saline aquifers. However, the geological conditions and physical and chemical characteristics are different from different sedimentary basins, or different deep saline aquifers, which makes the CO2-water-rock interaction varies from reservoir to reservoir, resulting in a complex trapping mechanism of CO2 in saline aquifers.The Ordos Basin, where the Shenhua CCS demonstration project located, covering an area of 250,000 km2, is the second largest sedimentary basin in China. Deep saline aquifers are widely distributed in the basin. There are multiple reservoir and caprock pairs suitable for CO2 geological storage, and the CO2 storage potential is estimated up to hundreds of million tons, which shows a broad prospect. However, due to the relatively high sandstone matrix content in the deep saline aquifers of basin, the mineral composition and structure maturity are generally low, poor physical, which is mainly a low porosity and low-permeable dense reservoir, making the CO2 injection capability and long-term storage complexity. This thesis based on the Shenhua CCS demonstration project in Ordos Basin, taking three main low porosity and low permeable reservoirs (Liujiagou Formation, Shiqianfeng Formation, Shihezi Formation) of the demonstration site as examples, exploring physical and chemical interactions of the CO2-water-rock and the influence on reservoir injection and long-term storage using numerical simulation method aiming at solving the actural problems. We analyzed the important influence factors on CO2-water-rock interactions, revealed the mechanism of injectivity enhancement of the low porosity and low permeable reservoir at the Shenhua CCS demonstration site in Ordos Basin, and clarified mineral transformation mechanisms during the long-term sequestration, and primarily forecast and evaluated the capability of CO2 injection and long-term storage in the deep saline aquifers, which can provide a scientific basis and technical support for the implementation of large scale CO2 storage project in deep saline aquifers.Injectivity is an important technical and economic issue for actual CO2 geological storage project, while porosity and permeability is a key index to determine whether the reservoir can be injected into easier or not. The enhancement in porosity can promote the injection, which is conducive to improve storage potential and implement actual project, otherwise, it is unconducived to CO2 injection. For the effect of physical and chemical reactions on the near-well injectivity caused by CO2 injection, we analysed the changes in porosity and permeability during CO2 injection by using a two-dimensional model, major conclusions are summarized as follows:(1) After CO2 injection, different amounts of salt precipitate for different reservoirs. The distribution area of the salt precipitation is:Liujiagou Formation-Shiqianfeng Formation>Shihezi Formation, but the amount of salt precipitation is in the following order of Liujiagou Formation>Shiqianfeng Formation>Shihezi Formation. Meanwhile, the reservoir permeability decreases with the increase of the salt precipitation.(2) According to the different influence on salt precipitation, the influence factors can be divided into shape factors and quality factors, which shape factors refer to the ability to change the spatial distribution of salt precipitation, mainly including the injection rate, the buried depth of strata and porosity; while quality factors refer to the ability to change the quantity of salt precipitation, such as salinity, permeability and irreducible water saturation. In addition, some factors influence both the shape and the quality meanwhile, such as permeability and residual water saturation.(3) During the injection, the reservoir porosity increases gradually over time caused by CO2-water-rock interactions, the scope of which is consistent with CO2 migration, but the distribution is non uniform. The porosity change of the Liujiagou Formation is the most gentle, while the increase in Shihezi Formation is the largest. The changes of porosity and permeability are directly related to the dissolution and precipitation of the initial and secondary minerals.(4) The reservoir injectivity of the Shenhua CCS demonstration project in Ordos Basin improves during the CO2 injection is because the dissolution of calcite and dolomite, among which dolomite is the most critical minerals that increases reservoir porosity. When calcite and dolomite coexist, the dissolution of dolomite would inhibit calcite dissolution, and promote calcite precipitation. The dissolution of oligoclase will improve the pore structure of the reservoir, which mainly locates in the two phase region of CO2-saline water, while the presence of oligoclase suppresses dolomite dissolution, is unfavorable for injectivity enhancement of the near-well region. High temperature and high pressure is conducive to the dissolution of dolomite and other minerals, and has a positive effect on reservoir injectivity.(5) Without considering the effect of CO2-water-rock interactions, the injection capability of the three reservoirs is in the order of Shiqianfeng Formation>Shihezi Formation>Liujiagou Formation, wherein injection capability of the Liujiagou Formation vary greatly over the previous two groups.For the geochemical processes of supercritical CO2- water- reservoir rock and long-term storage, based on the geological characteristics of the three main injection layers, we analyse single-well injection of CO2 with the actual perfusion parameters of the demonstration project. We take the geochemical processes in the reservoir as the main clue, characterize evolution of different trapping forms of CO2 (hydrodynamic trapping, solubility trapping, mineral trapping) with time, investigates the aquesous chemical changes and the rock mineral dissolution and precipitation process in the deep saline aquifers and the resulted porosity and permeability variation, determine the long-term CO2 trapping minerals, and compared with the natural analogies to assess its storage capability. The conclusions are shown as follows:(1) The controlled trapping phase of CO2 varies with time. At the end of the CO2 injection, the ratio of CO2 gas trapping is more than 80% in three reservoirs. CO2 is gradually dissolved in the formation water over time, CO2 gas trapping decreases, while the partial dissolution of CO2 generated carbonate minerals to form mineral trapping. Changes of trapping mechanisms in different reservoirs are quite different. The amount of CO2 mineral trapping of the Liujiagou Formation is similar to gas trapping after 500 years, followed by Shiqianfeng Formation, while the transformation of Shihezi Formation is the slowest, the amount of CO2 mineral trapping is mainly equal to gas trapping until nearly 2000 years. The amount of CO2 mineral trapping of Liujiagou and Shiqianfeng Formation are up to 28 kg/m3 medium after 10000 years.(2) The dissolved minerals are mainly oligoclase, chlorite, dolomite, illite and Ca-smectite. Initial mineral composition and content of the different reservoirs result in a difference in the dissolved minerals. The Liujiagou Formation is given priority to the dissolution of oligoclase and chlorite, while Shihezi Formation mainly dissolved minerals are oligoclase and dolomite, later mixed with a small amount of Ca-smectite; the main dissolved mineral in Shiqianfeng Formation is oligoclase, while illite early dissolves, and later stage begin to precipitate.(3) The carbon sequestration mineral assemblage in Liujiagou Formation is calcite+ dawsonite+siderite, Shiqianfeng Formation is calcite+dawsonite, and Shihezi Formation is calcite+dawsonite+magnesite. Through sequestration capacity calculation, the main sequestration mineral is calcite in three reservoirs, the amount of which is more than 50% of CO2 sequestration mineral in each reservoir after 10000 years, wherein Liujiagou Formation is of more than 80%.(4) At the beginning of the injection, the dissolution of carbonate minerals (dolomite, calcite) and feldspar (oligoclase) increases the porosity of Shiqianfeng Formation and Shihezi Formation. This will help reduce the pressure accumulation caused by CO2 injection, thus increase the injectivity. The porosity changes of Liujiagou Formation are not obvious in the initial simulation. As the CO2-water-rock interaction processes, the porosities of the three reservoirs begin to decrease from different time, among which the porosity decrease of the Liujiagou Formation appears at around 1000 years, Shiqianfeng Formation and Shihezi Formation appeares in 100 and 500 years respectively. The decline of reservoir porosity attributed to the injected CO2 added to the solid matrix as secondary carbonate minerals and make the amount of minerals precipitation greater than the dissolved ones. This restricts the migration of CO2, increases the contact reaction time among CO2, water and rocks, which is beneficial for CO2 mineral trapping.CO2 storage in saline aquifers is a complex hydrogeochemical process, increasing the acidity of the solution after CO2 is injected into saline aquifers, breaking the initial equilibrium, which results in a variety of geochemical changes in carbonate and aluminosilicate minerals. During the long-term process of CO2-water-rock interactions, CO2 can be permanently fixed by forming carbonate minerals contained Ca2+, Mg2+, Fe2+, Na+, Al3+(AlO2-).In order to understand influence factors and mechanisms for long-term CO2 storage, we analyzed the effect of different conditions (mineral composition, physical properties of the reservoir) on the long-term CO2-water-rock interaction processes, determined the main factors and critical minerals, also carried out sensitivity analysis. The main conclusions are as follows:(1) Temperature, porosity, reservoir thickness, permeability are the main physical parameters that influence the long-term CO2 storage in the deep saline aquifers. A condition of high capillary pressure makes CO2 plume distribution more uniform than low capillary pressure did, which is benefitial for CO2 mineral trapping. When horizontal permeability of the reservoir is equal, the smaller vertical permeability, the more conducive to CO2 solubility and mineral trapping.(2) We calculated the average sensitivities of each of the physical parameters influenced by the amount of mineral trapping. Besides, based on the average sensitivities, we made prediction and evaluation their actual physical conditions of the three reservoirs. It is concluded that the values were 6.5260,81.0883 and 88.0387, which shared the same sort of mineral trapping amount 6.12×108 kg,9.57×108 kg and 1.09×109 kg of the three reservoirs.(3) Overall changes of reactive surface area or kinetic rate constant can affect the geochemical reaction process of the whole system, bringing about the changes of CO2-water-rock interaction time scale, thus leading to the differences in final trapping minerals quantity.(4) Cations which were needed by the mineral trapping were derived from the corresponding minerals dissolution. Without corresponding minerals or their substitute, the corresponding carbonate minerals will not be generated. For example, chlorite is a key mineral in the long-term storage of CO2, whose role is to provide Fe2+ for CO2 trapping minerals such as ankerite and siderite. In addition, when both oligoclase and calcite exist in the initial minerals; oligoclase can inhibit the dissolution of calcite.(5) After CO2 injected into the deep saline aquifer, the initial equilibrium system was broken, which lead to dissolution and precipitation reactions of a series of carbonate and aluminosilicate minerals. In the process, CO2 can be fixed by precipitation of primary carbonate minerals such as calcite or secondary carbonate minerals like ankerite, magnesite, siderite, and dawsonite. Such basic chemical compositions of Ca2+, Mg2+, Fe2+ and Na+ of carbonate minerals were mainly provided by the dissolution of calcite, albite (oligoclase) and chlorite. Meanwhile, most Al+ (AlO2-) and SiO2(aq) from aluminosilicate were consumed by the precipitation of clay minerals such as illite and smectite. However, kaolinite is of a transition phase, which can be almost eventually replaced by illite after thousands of years of reaction. |
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