| The Shenhua CCS(Carbon dioxide Capture and Storage)demonstration project(100,000 tons/year)is the first pilot project of China to carry out geological carbon dioxide storage in aquifers,and is also the first full chain coal-based CCS pilot project from capture to storage.It provides good supports for the development of China’s carbon storage technology.From 2011 to 2015,a total of 303,000 tons of CO2 were stored,and the Shenhua multi-layered site(a total of 21 absorption layers)showed a dramatic increase in single-reservoir gas absorption.After compression,the cold CO2 was injected into the relatively high temperature deep aquifer.In this case,the fluid pressure and thermal stress in the first aquifer changes drastically,resulting in a large number of cracks,it reduces the total storage capacity of the system,and it may cause leakage risks.Taking the special geological characteristics in aquifer and storage pattern of the Shenhua CCS demonstration project as the research background,a cracking model of rock of CCUS(Carbon Capture,Utilization and Storage)engineering was established under the coupled processes of THM,which is based on the non-isothermal,multi-component(CO2-Water-NaCl),multi-field(THM)and multi-phase(CO2 gas-Liquid-Supercritical,H2O gas-Liquid-Vapor,NaCl solid crystallization-ion dissolution)coupled calculation program TOUGH-FLAC-ECO2N.we studied the basic laws of THM coupling in CO2 geological storage from a theoretical perspective,analyzed the factors of affecting the safety in CO2 geological storage site,simulated and studied the coupling effect and cracking of reservoir’s rock mass in the GCS engineering under the different injection schemes,and finally,studied the coupling effect of the reservoir under different-permeability.The main contents and conclusions are as follows:(1)Construction of a multi-layered hydraulic-thermodynamic coupling fracturing model for the Shenhua GCS site and development of THM coupling fracturing module.Basing on monitoring data of the Shenhua site,a multi-layered target rock mass hydraulic-thermodynamic coupled fracturing model in aquifer is proposed;the THM coupled cracking module TOUGH-FLAC-CRAMODULE is developed,which combines with the TOUGH-FLAC source program and the multi-layered hydraulic-thermodynamic coupled fracturing model;establishing a numerical model of the GCS site scale,and studying the basic laws of multi-field coupling in the GCS target aquifer.(2)Simulation analyses of the influence of reservoir properties and operating parameters on the processes of induced reservoir crack.Using numerical methods to study the influences of permeability,porosity,elastic modulus,Poisson’s ratio and different injection temperatures on the THM coupling effect and cracking effect of the reservoir,respectively,and the hydrodynamic,thermodynamic characteristics and multiphase seepage characteristics of the reservoir are analyzed.The analysis results show that formations under higher elastic modulus are more prone to fracture than formations under lower elastic modulus,and the growth rate of cracks in formations with a smaller Poisson ratio is slower,the Poisson’s ratio has a significant effect on pore pressure,while the impact on thermal stress is not significant.Porosity has little effect on the growth extent of cracks and the migration distance of CO2 plume,formations under higher porosity have a faster development of fractures.The accumulation of pore pressure around the well bore in lower permeability formations is more obvious.It is more easy to crack as the higher pore pressure and the higher temperature.It is not suitable for lower temperature gas injection which is far from the initial temperature of the formations,otherwise,it is easy to produce excessive thermal stress that causes thermal cracking.Therefore,the formation properties should be considered comprehensively,and reasonably injection operations should be selected to ensure the safety of the sequestration site.(3)Studies on reservoir coupling effect and cracking effect under different injection methods.Studying the changing laws of reservoir rock mass hydraulic characteristics,thermal characteristics and multiphase seepage characteristics under different injection schemes,and exploring the applicability and advantages and disadvantages of injection in formation.Numerical simulation’s results show that:in the early stage of injection,as the lower permeability rocks that are susceptible to change in permeability because of the stress changed,it is not advisable to use a higher rate of injection operation.In the case of economy and safety,a constant rate injection scheme with low rate can be used.In the constant rate scheme,the changes of temperature are small.Compared with the intermittent variable rate scheme,the temperature difference is 5℃,the resulting thermal stress is also the smallest,and the crack is the least likely to occur.Among the five injection schemes,the fracture development under the constant rate injection scheme is the least,only 2 units have ruptured in the simulation,and the cracked time is at the 10th day at the latest,which is most conducive to the safe storage of the sequestration site.(4)Studies on the coupling effect of formations with different permeability.Numerical simulation research on the coupled effect of carbon dioxide geological storage in depths is carried out for different formation permeability,and the simulation results are compared and analyzed.The research results show that:low permeability and ultra-low permeability formations tend to have characteristics that are susceptible to change in permeability because of the stress changed,and strong coupling effect,while the greater the permeability,the less the effect of coupling.The reservoir under 5mD permeability is at the critical point of strong and weak coupling effect,which is the permeability’s threshold that needs to be considered in coupling calculation for the geological storage sites.When the permeability is lower than this threshold,the influence of THM coupled effect should be considered when using the numerical simulation method to analyze the multiphase flow of the GCS reservoir at the site scale. |
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