Feasibility Study Of Cosmic Ray Muon Radiography In Monitoring Carbon Dioxide Geological Storage

The increasing amount of carbon dioxide (CO2) concentration in the atmosphere has proven to be the main cause for the growing greenhouse effect, which is posing great challenges to both environmental and ecological systems and rising to be a major concern in the economic development across the globe. How to reduce carbon dioxide emission into the atmosphere has become a pressing task in terms of relative theoretical studies and engineering implementation to alleviate the climate change. The energy source for human activities on a daily basis and industrial production still largely relies on the combustion of conventional fossil fuels, such as coal, oil, and natural gas. After years of both theoretical and experimental investigations, it has been widely accepted that carbon dioxide capture and storage (CCS) is the most promising option for reducing CO2 emission into the atmosphere on an industrial scale. A CCS process includes the separation of CO2 from the wastes produced in the carbon-based power plants, compressing it supercritical state, transporting it by pipelines to the destination, and finally injecting it into the proper underground porous formations, which is aimed to seal CO2 forever safely and efficiently. In some storage cases, CO2 may be injected with other impurities (mainly N2, H2S and SOX) incorporated to cut the total budget. Once injected, the injected fluids would be driven by buoyancy and pressure gradient and migrate both upward and laterally to the regions with less pressure.Due to the structural trapping by the upper seal formations, and various physical and geochemical trapping mechanisms, the migration of the injected fluids would be confined within the intended safety area in effective situations. However, the permanent seal would take a slow process in view of the characteristic time for geological behaviour, and in case of unsafe and ineffective situations for possible CCS projects to take place, a combination of monitoring techniques are essential to understanding storage processes and ensuring storage safety.Sites monitoring for a CCS project is required for decades and even hundreds of years into the future during the storage phase in view of the expected time scales for permanent storage underground. An effective and comprehensive monitoring system is designed for two separate goals, that is, deep and shallow monitoring. Deep monitoring of the injected fluids is to identify the location where they have extended for reasons of process control, storage safety and efficiency, and verification and modification of the numerical prediction models. Shallow monitoring is usually taken in leakage cases to determine the location, amount, and the influence on water quality, especially for the potable water formations. Effective monitoring system can not only provide information on the fluids migration behaviour but also help reduce the occurring rate of leakage by site-specific assessment together with relative remediation measures. Monitoring techniques that are under investigation or being tested in experiments and ongoing pilots conclude geochemical and geophysical measurements. These traditional monitoring methods usually borrowed from the experiences of exploiting fossil fuels are episodic, and thus the frequency and extent of monitoring are important problems to be settled in a practical storage phase. Continuous monitoring is an effective solution to address this problem. This paper aims to investigate on the feasibility of a novel radiographic method using cosmic ray muons as radiation sources to perform continuous monitoring for the storage phases in possible CCS projects.This non-destructive method uses naturally and continuously occurring cosmic ray muons to detect the structure or time-dependent interior change within a targeted object. Overcoming the confinements of the imaging scales by commonly employed X-ray and gamma rays, cosmic ray muons have been introduced in recent years as radiation sources for probing geological-scale objects by advantage of their high penetrating ability. The cosmic ray muons may have a wide range of energy spectrum depending on zenith angles. When cosmic muons pass through a targeted object, the interactions between muons and matter will cause the muons in lower-energy region to lose all their energy. Statistically, the number of the penetrating muon events is related with the muon path length, and the material type and density inside the target along the muon path. Should change take place within the scanning scope of this method, the attenuation of the cosmic ray muons and the resulting penetrating events may vary at a certain extent, and thus, by comparing the time-dependent measurements in different periods,change inside the target may be inferred with certain sensitivity.The feasibility of cosmic ray muon radiography still needs further investigation. The simulation studies involved in this paper aim to achieve three goals, that is, simulating with a higher accuracy, determining the spatial resolution that can be achieved by this method, and analysing the influence of different parameters on the performance of this technique to enhance the knowledge on its application scope. The specific contents are listed as follows:Firstly, a sea level muon generator was well developed, and the modelling results show a coincidence with the experimental data.Secondly, the muon propagation processes through the whole storage with different saturation situations by the injected fluids were simulated by using developed Monte Carlo programs, to investigate the performance of cosmic ray muon radiography in both deep and shallow monitoring. The simulation results show that, the attenuation of cosmic ray muons has a relative high sensitivity to the appearance of supercritical CO2 in the deep brine formations, and the intrinsic spatial resolution that can be achieved by this method can be confined within a scale of 10 m. As it goes shallower in the underground, the sensitivity of this method is more sensitive to the saturation of the injected fluids with a higher spatial resolution.Finally, simulations were conducted to probe into its sensitivity to the leakage of the stored gases in various scenarios, by virtue of which the influence of different parameters on its sensitivity was also qualitatively analysed, including the thickness of the permeation region by the leakage gases within the targeted monitoring scope of the technique, the constituents involved in the leakage gases, and the measurement period. The feasibility analysis in this study demonstrate that cosmic ray muon radiography could be applied as an effective complementary method in both shallow and deep monitoring in a possible CCS project. Since the radiation source in this radiographic technique is naturally and continuously occurring rather than episodically man-made, this method could serve as a continuous and effective way in practical applications. Once ineffective situations were indicated by such continuous monitoring within a relatively high spatial resolution, other episode methods could be further made to obtain more information.