Study On The Deformation Failure Properties And Evolution Of CO₂ Permeability Of Sandstone In Deep Saline Aquifers

The increasing greenhouse gas of carbon dioxide?CO₂?leads to a series of ecological and environmental issues,e.g.,global warming.Deep saline aquifers have been identified as one of the most promising sites of geological storage of CO₂.When CO₂ injects to deep saline aquifers,physical and chemical reactions among brine,CO₂ and rock occur,which may induce the leakage of CO₂ and decrease the long-term safety and efficiency of sequestration.This study focuses on the mechanism of brine-CO₂-rock interaction and its influence on stability and permeability of rock in deep saline aquifers,which was supported by the National Natural Science Foundation of China?No.41272344?and Natural Science Foundation of Jiangsu Province for Distinguished Young Scholars?No.BK20150005?.The research object was sedimentary sandstone with high porosity.Laboratory experiment,numerical modelling and theoretical analysis were involved in this study.The following general results can be derived:?1?By using an adanced rock high temperature and high pressure triaxial testing system,real-time ultrasonic and acoustic emission?AE?monitoring systems,three-dimensional image capture and digital image correlation?DIC?analysis system,the mechanical properties of sandstone specimens with differernt salinities were analyzed,the real-time evolution laws of P-wave velocity and AE in sandstone under triaxial loading were obtained,the characteristics of full-field and local measured strain around the pre-existing flaws were investigated,and the influence of testing temperature on the failure mechanical behavior of saturated sandstone was studied.Furthermore,a damage model based on AE was proposed,which can simulate the experimental results of sandstone under high temperature and high pressure.These experimental results revealed the effects of brine salinity,buried depth,temperature and flaw geometry on the mechanical damage behavior of sandstone in deep saline aquifers before CO₂ injection.?2?A series of uniaxial compression,Brazilian splitting and fracture tests on brine-supper critical CO₂?scCO₂?co-saturated sandstone specimens were carried out by using the self-developed system of brine-scCO₂ co-saturation for rock and rock mechanics servo-controlled testing system.The influence of brine-scCO₂ on the uniaxial compressive strength,tensile strength and fracture toughness of sandstone specimens were investigated.High resolution X-ray computerized tomography?CT?and microscopy system were used to reveal the micro mechanism of brine-scCO₂-rock interaction.These experimental results increased the understanding of mechanical response of rock after CO₂ injection into deep saline aquifers.?3?To investigate the CO₂ permeabilty of sandstone under brine saturation,a system of gas and liquid two phases permeability measuring for rock was developed.Then,a series of seepage tests on saturated sandstone with different salinity levels were performed under different gas pressures,confining pressures,axial stresses and testing temperatures.Based on the results,the effects of brine salinity,gas pressure,effective confining pressure and temperature on the gasous and scCO₂ permeability of sandstone were revealed.These experimental results provide a reference for the evaluation of injectability and storage capacity of CO₂ in deep saline aquifers.?4?Based on the idea of mechanical equivalence and the mechanism of brine on the pore structure of sandstone,the laboratory experiments of brine salinity effect were verified by weakening the micro mechanical parameters in three-dimensional Particle Flow Code?PFC3D?.Open flaws were created by deleting particles in intact numerical model.The crack initiation,propagation,coalescence and failure process of sandstone specimen during triaxial loading were obtained.The experimental results of real-time high temperature and high pressure tests were testified by using linear thermal expansion method.The development of thermal crack at elevated high temperature was investigated.These numerical results deepen our knowledge of meso-mechanism of brine-rock interaction and thermal-mechanical coupling behavior.There are 196 figures,32 tables and 400 references in this dissertation.