| The continuing growth in the concentration of greenhouse gases has leaded to global climate change which brings the side environmental effects such as extreme weather, melting polar ice, rising sea levels, ocean acidification and species extinction and has a serious impact on the ecosystems and human activities. Therefore, dealing with climate change and reducing the emission of greenhouse gas has become the survival problem global human must confront together and urgent need to be solved. CO2, as the major member of greenhouse gas, mainly comes from the burning of fossil fuels. There is a problem, however, that the energy structure in China is still dominated by coal and will have no change in the following40or50years. Therefore, it is unpractical to completely abandon the use of fossil fuels in a short time. The Chinese government is facing severe pressure on carbon emission in the face of large CO2emissions and continuing growth. Carbon capture, utilization and sequestration (CCUS) is a new emerging technology to be expected to achieve large-scale use of fossil fuels and meanwhile developing low-carbon economy in a short time. It is mostly likely to be the important strategic technology for the future development of China with the purpose of reducing CO2emissions and guaranteeing energy security at the same time.The vast majority of sedimentary basins in China belong to continental sedimentary basins, with the continental reservoir characterized by low porosity and low permeability, permeability in particular, because of the special nature of sedimentary and diagenetic processes. It will be of great significance for the promotion and application of CCUS in China to study the sequestration mechanism and injection capacity as well as feasible ways of resource utilization of CO2geological storage in the ubiquitous low-permeability reservoirs of continental sedimentary basins. In the present work, numerical simulation method was carried out to investigate the CO2injection, storage and utilization using TOUGH2and TOUGHREACT as simulation tools by choosing high-salinity and low-permeability aquifers and potassium-rich brine in Jiangling Depression of Jianghan Basin as the study area. Main research contents focus on CO2injectivity and sequestration mechanism in the high-salinity and low-permeability formations, methods to improve CO2injection rate in the high-salinity and low-permeability formations as well as potential advantages and process optimization of exhaustive brine production combined with complete CO2storage. The method and main conclusions are as follows:For the injection of CO2in high salinity formations, salt precipitation around the injection well has a significant impact on CO2injectivity. Numerical simulation was carried out to investigate the effect and mechanism of capillary pressure couple with salinity and injection rate on salt precipitation during supercritical CO2injection into the deep saline aquifer of Jiangling Depressin in Jianghan Basin. The results are shown as follows:(1) With low capillary pressure, salt precipitation is in a significant linear relationship with salinity. From the solid saturation profile of radial and vertical distance from the wellbore, we find that the imbibition of additional salt from capillary backflow is the main source of the increased salt precipitation.(2) With high capillary pressure, the initial salinity is no longer the dominant factor controlling the solid saturation. Even in the formation with low salinity, salt precipitation can continuously accumulate near the injection well until it completely blocks the pore throats which do not permit the fluid movement. After the subsection analysis of the liquid flow curve, we find that in spite of the enhanced salt precipitation under the retention, additional salt imbibition from the strong capillary backflow is fatal to severe injectivity impairment for high capillary case.(3) In addition, salt precipitation is highly controlled by injection rate. The higher injection rate, the lower salt precipitation will be. Even for the low capillary case, injection of CO2with a low rate can still lead to severe salt precipitation for the reason that low CO2flow prolongs the backflow of brine under the force of capillary pressure. Therefore, injection of CO2with high rate can effectively mitigate the effect of salt precipitation.Since mineral trapping is the most effective and safety sequestration mechanism of CO2, it is very meaningful to investigate and evaluate the CO2mineral trapping capacity of different rock types under specific and different geological conditions for the richness of CO2geological sequestering mechanisms and assessment method of the long term storage risk and safety of CO2geological sequestration. This paper takes the quartz-rich sandstone of Jiangling depression in Jianghan basin for example, through the batch geochemical simulation method, based on the cause and effect of the critical ions such as Na+, Al3+, Ca2+, Mg2+, Fe2+and so on, carries out the sensitivity analysis of critical minerals, explores the mineral capture mode of quartz sandstone, and evaluates the safety of long term storage risks, in the same time, sets up the different reservoir salinity to compare the salinity influence on mineral capturing capacity, the conclusions are as follows:(1) Mineral trapping of CO2for quartz sandstone which contains of calcite (and/or gypsum), chlorite, k-feldspar, albite, Fe2+, and Mg2+dissolved from chlorite can react with Ca2+that dissolved from calcite/gypsum to produce ankerite which can be stable for long time under the condition of rich Mg2+, otherwise dissolves into calcite and siderite and the providing Mg2+can help in the precipitation of illite. Magnesite can be generated under the condition of rich in Mg2+, but its precipitation amount is limited due to the reason that Mg2+is the necessary component for the formation of illite and smectite. The minerals order for the consumption of Mg2+in acid condition is illite>smectite>ankerite>magnesite. Ankerite and magnesite is the main form of carbonate mineral capture for the rocks rich in Mg2+and Fe2+, while it is ankerite and calcite for the rock typre rich in Fe2+but poor in Mg2+Although the dissolve of chlorite, k-feldspar and albite can provide with large amount of AlO2-, the majority of AlO2-will be consumed for the generation of clay minerals such as illite, smectite, kaolinite, even for the rock rich in Na+, the dawsonite precipitated earlier will also dissolve because of the short supply of AlO2-. The mineral order for the consumption of AlO2-in acid condition is illite>smectite>kaolinite>dawsonite. However, when clay minerals come into the equilibrium of dissolution and precipitation, the stable alternation of albite and dawsonite will be the main CO2mineral capture mechanism. K-feldspar totally transfers into illite under the environment with Mg2+stable supply and basically loses the potential of CO2mineral trapping. Moreover, the formation water rich in K+and Mg2+is also benefit for the production of illite which can consume large amounts of Mg2+and AlO2-and hinder the stable existing of CO2trapping minerals such as ankerite, magnesite and dawsonite.(2) The influence of salinity on mineral CO2trapping capacity mainly depends on the initial mineral content of albite and k-feldspar. Higher concentration of Na+in aqueous solution will inhibit the early dissolution of albite and evolve into the lower pH buffer environment. However, acid environment is not helpful for the generation of smectite, but has no impact on illite. If the rock is rich in k-feldspar, the low pH can promote the dissolution of k-feldspar and provide with much more K+to support the precipitation of illite. When illite precipitation is no longer limited to K+, CO2mineral trapping capacity will decrease with salinity. If the rock is poor in k-feldspar, illite precipitation is restricted and the transformation of dawsonite from albite will be prompted. In this case, CO2mineral capturing capacity will increase with salinity.For the specific of the sand reservoir of Jiangling Depression in Jianghan basin which has high salinity, low permeability and is rich in carbonate rocks, we conduct a series of modeling studies to examine the salt precipitation and CO2injectivity in different cases such as pre-injection of fresh water, salt water, CO2saturation solution and diluted hydrochloric acid as well as hydraulic fracturing. Findings are as follows.(1) The injection of fresh water and salt water with different salinities can mitigate the salt precipitation which occurs near the CO2injection wellbore in different degrees. Larger injection of fresh water and salt water can certainly obtain greater improvement. But the economic cost has to be considered, therefore, finding a best injection rate of fresh water and salt water is a must.(2) The injection of CO2saturated solution or diluted hydrochloric acid solution not only mitigrate the salt precipitation, but also can dissolve the calcite and gypsum near-wellbore and increase the permeability which enhances CO2injectivity. However, due to the low permeability of the reservoir, the movement of the acid solution is limited. It is difficult to achieve impressive promotion for CO2injection rate within a short time. (3) Hydraulic fracturing can enhance CO2injectivity effectively, mainly depending on the length and width of the fracture. The data shows obvious exponential growth in the length of the fracture with the injection rate. Improving fracturing permeabilitycan also increases the injection rate, but the improvement room is not so obvious. Giving more fractures near-wellbore can also improve CO2injectivity, but the increase is less than that of a single fracture with the same length. It is maybe caused by the eclipsing effect of the effective contact area between the sidewall of the injection well and the reservoirs.(4) For singe vertical well, it is possible to inject hundreds of1000t of CO2into low permeability reservoirs in one year through reservoir fracturing and several sandstone layers.For the investigation on CO2injectivity and containment in the sandstone system consisting of multiple interbedded sandstone and mudstone, we built5different models such as (A [(S100+M100)×1], B [(S50+M50)x2], C [(S20+M20)x5], D [(S10+M10)×10], E [(Es-6real drilling)x7]). The results show that the injectivity and containment of CO2is obviously affected by interbedded sandstone and mudstone, and decrease with the increase of the interbedded sandstone and mudstone. The thin interbedded mudstone cannot be able to act as independent caprock. In addition, the reasonable design in the opening of injection well and location can obtain the maximum benefit on ecology investment and beneficial output.As a salt lake rift basin, Jiangling depression is rich in large hyperthermal potassium-bearing brine in Shashi Formation and Xin Gouzui Formation of Paleocene period. This is valuable liquid mineral resource for Jinzhou city in Hubei province. Taking the potassium-bearing brine formation of the Jiangling depression in Jianghan basin as research objective, we build a3-D homogeneous model with multiple wells to study the efficiency of the only industrial-scale brine production and the environmental geological problems which may occur during the production. Then simultaneous brine extraction and CO2storage is proposed researched and optimized. Results that have been obtained are as follows.(1) For only industrial-scale production of brine, the production rate of single well declines significantly with time, and multiple wells of brine production with higher economic cost are also unable to obtain higher efficiency. In addition, the tremendous pressure difference not only breaks the balance of the fluid and the medium which may not only seduce land subsidence but also cause the leakage between the target reservoir and the overlying formation, mitigating the leakage risk regardless of the permeability of the confining bed.(2) Simultaneous brine extraction and CO2storage can not only effectively regulate the region pressure balance of the storage formation, significantly enhance the brine production capacity and CO2injectivity as well as the storage capacity, but also can effectively avoid the leakage between the target reservoir and the overlying formation, mitigating the leakage risk regardless of the permeability of the confining bed. The constant pressure injection scheme is much superior to the constant rate injection scheme. The simultaneous brine production of nine wells and CO2injection of four wells under the constant pressure injection scheme act best in all respects of pressure regulation, brine production efficiency, CO2injectivity and storage capacity as well as leakage risk mitigation. (3) Several ways to further optimize the combined strategy are investigated and the results show that increasing the injection pressure and adopting fully penetrating production wells can further significantly enhance the combined efficiency within the pressure near-wellbore under control. Meanwhile there is no obvious promoting effect by shortening the well spacing and using the triangular pattern of well placement, but on the basis of the specific site terrain and project objectives, a reasonable layout of wells can also be obtained. On the contrary, the reduction of the brine production wells decreases the brine production capacity and CO2injectivity, due to that the wells group composes a relatively independent unit so that every single production well is important for the whole system’s efficiency. |
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