Mechanisms And Chemical Control Methods Of Gas Leakage And Channeling During CO₂ Flooding

The technology of CO₂ EOR and geological storage has become one of the research priorities worldwide in recent years,which has dual effects of enhancing oil recovery and reducing greenhouse gas emission.During CO₂ injection and storage process,existed or newly generated leakage channels are inevitable because of natural and man-made geologic activities.Hence,the storage safety in terms of CO₂ leakage has always been an eminent issue for concerns and questions.In the process of CO₂ flooding in oilfield,formation heterogeneity and the mobility difference between CO₂ and formation fluids can easily induce gas channeling,which will weaken the performance of enhancing oil recovery and CO₂ geological storage efficiency.Although the mechanisms and consequences of leakage and gas channeling are different,they are all associated with the high mobility of CO₂ and their control methods are interrelated.Therefore,how to identify and prevent gas leakage and channeling during CO₂ EOR and storage is of great importance and has become a key issue for research.In this dissertation,the influences of geology,technology and environment factors on leakage risk were investigated and the weak links during CO₂ flooding were identified using Fault Tree Analysis method,which can provide the references for proposing the corresponding strategy of leakage monitoring and control.Associated with the results from gas tracer test and micro-seismic monitoring,the mechanisms of gas channeling during CO₂flooding were analyzed through field case study on CO₂ flooding.According to the principles of chemical blocking and mobility control using foam flooding,the leakage blocking and mobility control methods based on CO₂-sensitive chemicals were proposed.The related features of chemicals on this category,which include CO₂ sensitivities,reaction performance,blocking capability,and feasibility of field application,were studied through experimental and numerical studies.Aiming at CO₂ foam systems for gas channeling blocking and mobility control,a mobility control method using CO₂-sensitive and self-enhanced foams was presented.Foam performances under different working conditions were evaluated by experimental works,and the effects of blocking and mobility control based on this new CO₂foam system were demonstrated.The results from the Fault Tree Analysis on CO₂ leakage during CO₂ flooding showed that channel type leakage（for example,the existence of leakage pathways in wells and the leakage through faults and fractures）was the primary cause for CO₂ leakage.The leakage through well was also demonstrated by the results from field monitoring in demonstration project of CO₂ flooding.Gas channeling during CO₂ flooding is mainly caused by the existence or formation of predominant pathway and mobility difference between CO₂ and formation fluids.Hence,blocking high permeability channels and CO₂ mobility control in reservoir are the main methods to control gas channelingFor blocking channel type leakage and large pore path,a kind of CO₂-sensitive gel system（modified polyacrylamide-methenamine-resorcinol system）is studied using experimental evaluation and its blocking performance is also analyzed.The experimental results showed that,with the presence of CO₂,increase of temperature can shorten the gelling time with advantageous viscosity-temperature characteristics.At the same time,pressure increase was beneficial for the gelation process.Compared with other kinds of CO₂ sensitive blocking systems,this system had much better blocking performance and scouring resistance with relatively robust temperature resistance and salts tolerance(when the temperature is90℃and the salinity is 2×10⁵mg·L^-1,the final permeability reduction rate of sand-pack model with permeability of 1698.5×10^-3μm² can be achieved as 90%using injection pore volume of 3PV).Numerical simulation results showed that gel can form and be absorbed more easily near the injection point which may limit the CO₂ leakage to a small range.Feasibility and effectiveness of this method for selectively blocking leakage using CO₂triggered gelation were also demonstrated in field-scale numerical experiment.Aim at gas channeling blocking and control during CO₂ flooding,CO₂-sensitve chemical or foam agent based on amine like compound with organic acid amide group is developed.The experimental results showed that this chemical can form CO₂ foams and viscoelastic wormlike micelles with the generation of entangled micelle network in a acid environment induced by CO₂,which made this system has much better blocking performance and mobility control capability than conventional surfactants with the presence of relatively harsh reservoir conditions（a good foam performance was still achieved even at 140℃with the comprehensive foam index of 9925.2min·mL）.In this category,improved long-chain polyamine（alkyl dipropylene triamine）,has the performance of CO₂-sensitive and self-enhanced foam agents,which can be converted into a Gemini surfactant like agent to generate CO₂ foams with the presence of CO₂.The generated foam based on this kind of foam agent has self-enhanced effect at some extent.Within a certain range of temperature and pressure,the blocking performance of the generated foam improved with increasing temperature and pressure.The experimental results indicated that this new foam system was sensitive to the CO₂ concentration in gas mixtures.Foam system also exhibited good foaming capability even at a low concentration of chemical with the presence of CO₂.The CO₂foaming capability of this new foam system can be enhanced at elevated temperatures,and good temperature resistance has been observed up to 160℃.The results which were achieved from sand-pack flooding experiments demonstrated the effectiveness of the capability of gas channeling blocking and mobility control based on this CO₂-sensitive and self-enhanced foam system with resistance factor up to 1000.