Research On The Environmental Impact Of Carbon Dioxide Geological Sequestration On Groundwater

At present, the development of human economic activity has led to an increase in the concentration of CO2 in the atmosphere, this change led to an increase in global temperatures by about 0.6℃. Thus, it's urgent to take corresponding measures to mitigate climate change. Its core being is to reduce the emissions of CO2 and other greenhouse gases.One effective method is CO2 geological sequestration, which means depositing the CO2 produced by a fixed point source (mostly for power plant or similar industrial point source) into seas or some relatively closed geological structure to significantly reduce the anthropogenic emission of CO2 to the atmosphere. The main geological formations include depleted or depleting oil and gas reservoirs, un-mineable coal seams, and deep saline aquifers. Among these ways, the injecting CO2 into deep saline aquifers is one of the most promising geological CO2 sequestration options for the long term sequestration of the gas. Storage of CO2 in saline aquifers can be achieved through the following three trapping mechanisms:gas trapping, solution trapping and mineral trapping.However, due to the non-continuity of geological bodies, part of the injected CO2 will mitigate to the upper aquifer under the buoyancy through the faults, water channel and the exploitation wells. If the CO2 leak into the shallow aquifers, it will bring adverse effect to the water quality as well as to the environmental of the shallow groundwater, it's necessary to study the adverse effect for researching CO2 geological sequestration. In this paper, the simulation tool TOUGHREACT were used to establish the CO2 geological sequestration model of Jianghan Basin, the aim is to research the environmental impact of CO2 geological sequestration. The main contents are as follows:(1) Two-dimensional homogeneous model were setup basing on the relationship between caprock and reservoir, hydrogeological parameters, thermal kinetic parameters and mineral composition of Jianghan Basin. By analyzing the spatial and temporal variation of the reservoir pressure, porosity, permeability and chemical composition after CO2 injection, study the impact of CO2 geological sequestration on the formation pressure and crustal stability, regional groundwater dynamic field, hydraulic parameters of rock and chemical composition of groundwater.(2) However, due to the non-continuity of geological bodies, part of the injected CO2 will mitigate to the upper aquifer under the buoyancy through the faults, water channel and the exploitation wells and occupy the space of the aquifer, thus affecting the water quality and the environment. Two-dimensional CO2 leakage model were setup basing on the environmental impact of injection to study the impact of CO2 leakage on shallow groundwater, by analyzing the simulation results, study the spatial and temporal variation of gas saturation, liquid saturation, pressure, porosity, permeability, pH, the dissolution of primary minerals, precipitation of secondary minerals, heavy toxic metals and hardness of water, research the impact of CO2 leakage on shallow groundwater, and qualitative analyze the impact on near-surface ecosystems.By the above studies, we got some conclusions:During the process of CO2 geological sequestration and after the process, it will impact on the water chemistry and hydraulic parameters of reservoir, thus has an impact on the regional dynamic system. The main conclusions are as follows:1. The injected CO2 into the deep reservoir will produce remarkable influence on the chemical composition of reservoir water. The soluble CO2 in water leading to increased acidity of groundwater and solubility of parent rock minerals which changes the water-rock interaction. The primary minerals quartz, kaolinite, calcite and illite precipitated oligoclase, K-feldspar, hematite, and chlorite dissoluted, and generated the secondary minerals albite, siderite, iron, dolomite and dawsonite at the same time. Due to the precipitation and dissolution of minerals, the concentration of aqueous component has changed. The acidity has increased in a way but the dissolution of primary minerals and precipitation of secondary minerals and the changes of the concentration of aqueous component as before after injection.2. The pressure of reservoir has increased significantly as the injection, and it distributes uneven in the horizontal direction, thus destroyed the mechanical balance and triggered the shear failure and tensile failure of reservoir rock, may induce fracture and fault movement, leading to formation instability, particularly in the vertical profiles near the injection point, which may lead to formation uplift and cause an earthquake, the adverse effects only take effects during the injection period, it will renew to initial pressure gradually after the injection.3. The porosity and permeability of formation has changed and the gas saturation has changed significantly due to the dissolution of primary minerals and precipitation of secondary minerals, thus has an impact on hydraulic parameter of reservoir.4. The injection of supercritical CO2 to the reservoir leads to a significant change of the reservoir pressure and redistribution of water head, the redistribution may alter the fluid mitigation pattern. While the changes of porosity may induce the fracture and movement of fault, which changes the recharge, runoff and discharge of groundwater, especially the runoff condition and path changed significantly, thus significant changes of regional groundwater dynamic field took place as a result of above changes. By analysis of the environmental impact of CO2 leakage in Jianghan basin, the main conclusions are as follows:1. The leaked CO2 into the overlying shallow potable aquifer will reduce the pH value of groundwater significantly even the leakage rate is low. If the pH value is lower than the national drinking water standards, it will have adverse effect on residents'production and life who take the shallow groundwater as potable water source.2. The solubility of the parent rock minerals has changed due to the increased acidity, which leads to the dissolution of oligoclase, K-feldspar, chlorite, kaolinite, calcite and hematite, and the dissolution of quartz, illite, albite and smectite-Ca, which leads the concentration of aqueous component to change accordingly. Their spatial distribution pattern and spatial distribution of pH values are very similar, which illustrates that the pH value controls the mineral reaction and the changes of aqueous components concentration.3. The variations of galena containing toxic heavy metals were reanalyzed basing on the water chemistry of Jianghan basin. Results indicate that the dissolution of galena enhanced gradually as the CO2 leakage, but this change did not lead to significant changes of the concentration of Pb2+, because the increasing concentration of Pb2+ is mainly generated from desorption.4. The concentration of corresponding aqueous component increased significantly as the dissolution of primary minerals, of which the increasing of Ca2+ and Mg2+ concentration are particularly unfavorable, it will increase the water hardness directly, the groundwater of high hardness will have adverse effect on production and life of mankind.5. The hydraulic parameters changed as the CO2 leakage, which convert the mitigation pattern of fluid and water cycle, and because the elevated concentration of CO2, they will have effect on respiration and photosynthesis, make the germ species, life diversity and species complexity changed, these variation will have effect on the near-surface ecosystems.Through the research on the environmental impact of CO2 geological sequestration on groundwater, which provide theoretical principle for CO2 geological sequestration and the possible risk, the selection of candidate reservoirs, development and feasibility of CO2 storage projects, and help to the further research and development of geological storage of CO2 in China.