| Recently,the rapid economic and social development has excessively relied on traditional fossil energy,resulting in more and more serious problems such as environment pollution,energy crisis,climate change,etc.Faced with the current problems,developing novel clean energy and innovating energy development technology have been the long-term directions of China.Possessing considerably outstanding advantages,hot dry rock(HDR)geothermal resource is one type of the most promising novel clean energies to replace traditional fossil energy,while its exploitation is extremely difficult because the economy efficiency can be hardly achieved by the traditional development technology of geological resources.Supercritical carbon dioxide(SC-CO2)is a novel working fluid with superior performance to water-based working fluid in various aspects to develop geological resources,and it is one of the most potential technologies for HDR development and utilization.However,due to its chemical nature,continuous damage and deterioration can be caused to reservoir rocks when SC-CO2 is used to develop HDR.As two types of the most critical parameters in the processes of stimulation and operation,the pore structure and mechanical properties of reservoir rocks can continuously change.Since relevant studies have been lately started and a targeted research means is deficient,it is still difficult to accurately master the change laws of physical and mechanical properties of HDR reservoir rocks.Therefore,to break through the existing technical bottleneck of SC-CO2 exploitation and realize the efficient development and utilization of HDR,it is not only a critical prerequisite but also a key scientific problem to reveal the dynamic alteration mechanism of SC-CO2 on HDR reservoir rocks.In view of the above research status,it is aimed to reveal the dynamic alteration mechanism of SC-CO2 on HDR reservoir rocks in this study.First,a SC-CO2 dynamic alteration experimental system was developed to provide an apparatus basis for this study,and an accelerated test method was proposed for rock alteration reactions to provide a method basis for this study.Then,a chemical kinetics equation with the consideration of the resistance effect of SC-CO2 alteration resultants was established to fundamentally reveal the rock dynamic alteration mechanism from the mineral level.The mineral effect on pore evolution was clarified,the pore evolution calculation method was proposed based on the alteration kinetics equation,and the dynamic alteration mechanism on rock pore evolution was revealed.The mineral effect on alteration damage was clarified,the dynamic alteration damage variable based on the alteration kinetics equation was also constructed to reveal the rock dynamic alteration damage mechanism.The main research contents of this study are summarized as follows:(1)First,the common rock forming minerals were classified,and the dynamic alteration principle of SC-CO2 was clarified.To perform dynamic alteration test,the functional limitations of existing CO2 alteration apparatuses were investigated,and the key restrictions that hindered dynamic alteration test were found out.Through the optimization design of key structures,the technical limitations to relevant experimental functions were successfully broken through,and a SC-CO2 dynamic alteration experimental system was successfully developed,providing an apparatus basis for the study of dynamic alteration mechanism of SC-CO2 on rocks.(2)By analysing the prerequisites of accelerated test,performing accelerated alteration test was demonstrated from three aspects of the accelerated principle of alteration damage,the effect of high temperature on rock mechanical properties and on solution acidity,verifying the rationality and feasibility of increasing temperature as an acceleration method.By comparatively analysing the results of accelerated and unaccelerated tests,the accuracy of accelerated test method to study alteration laws was verified,and the universality was further verified.Finally,the accelerated test method of SC-CO2 on rock alteration was proposed.(3)Based on chemical kinetics method,the monomineral alteration kinetics equation was established suitable for SC-CO2 with the consideration of the resistance effect of resultants on alteration reactions.By analysing the multi-mineral alteration laws,the mutual weakening effect on different mineral alteration was clarified.Taking it as a bridge,the rock alteration kinetics equation was established according to the monomineral equation,and the dynamic alteration mechanism of SC-CO2 on rocks with different mineral compositions and contents was revealed.(4)Based on NMR technology,the rock pore structure evolution laws with reaction period were studied.The influence mechanism of different minerals on pore evolution was clarified,and the calculation method of pore evolution based on alteration kinetics equation was proposed.By analysing the pore evolution laws with temperature and pressure,the alteration mechanism of different minerals in different reservoir environments was revealed,and the pore evolution laws of rocks with different mineral compositions and contents were clarified.(5)The rock alteration damage laws caused by SC-CO2 were studied,the influence mechanism of different minerals on alteration damage process was clarified,and the alteration damage variable based on kinetics equation was constructed.By analysing the alteration damage laws in different reservoir environments,the alteration damage change laws with temperature and pressure were clarified.By establishing a mineral exfoliation model under dynamic alteration,the critical liquid flowing velocities that lead to different exfoliation types of mineral particles were calculated and analysed,and the influence laws of different liquid flowing velocities on mineral exfoliation were clarified,revealing the influence mechanism of the differential dynamic alteration damage caused by SC-CO2. |
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