Research On Improving The Storage Performance Of CO₂ Injection In Deep Saline Water Layer By Hysteresis Effect

Global warming caused by a large number of greenhouse gases emission, which are mainly composed of carbon dioxide, has brought about a series of climate and environment problems. It is very important to take effective measures to mitigate climate change. The key is to reduce the emissions of CO2, which is of great significance to realize sustainable development. Carbon dioxide capture and storage(CCS) is a feasible means of alleviating the greenhouse effects. Deep saline aquifers are considered to have the most potential capacity because of its wide distribution around the world.In the process of storing CO2 in deep saline aquifers, due to the difference between multiphase fluid properties, relative permeability of the phases and capillary pressure at a point depend not only on the current local saturation of the phases but also on the saturation history the point has experienced. This means that the characteristic curves of relative permeability of the phases for the imbibition process will deviate from the curves for the drainage process. The same situation will also be seen in the capillary curves. This phenomenon is called hysteresis effects. Currently, using numerical simulation research for the hysteresis effect in the geological storage of carbon dioxide is relatively rare. In the intermittent injection mode, the hysteric relative permeability is not negligible effect in the movement of multiphase flow, and in turn it will affect the storage performance of CO2 in deep saline aquifers.This dissertation focuses on how to improve the storage performance of CO2 in saline aquifers by making use of the hysteresis effects through numerical simulations. The Darcy’s law of multiphase flow and hysteresis of relative permeability and capillary pressure in the movement of multiphase flow were reviewed and emphasized in the literature review. The effects of hysteresis on CO2 storage performance were numerically studied by running an ideal model and a real site model under several scenarios with the simulator of TOUGH2-ECO2 N. Compared with the simulation results without consideration of the hysteresis effects, results of the scenarios with hysteresis consideration indicated the findings as the following.Firstly, an intermittent injection procedure, where the hysteresis effects are significant in the switches from injection to injection pause or vice versa, can effectively reduce the pressure buildup in the storage aquifers and maintain the injection rate at a desired level. The scenarios with consideration of the hysteresis effects resulted in greater cumulative injection mass than the scenarios without consideration of the hysteresis effects. Under the same injection procedure, the mass fraction of CO2 in the aqueous phase was also much higher in the scenarios considering the hysteresis effects than in the scenarios without considering the hysteresis effects.Secondly, compared with nonhysteric scenarios, hysteric scenarios had more mobile gaseous CO2 and faster pressure propagation during the drainage process. Furthermore, more gaseous CO2 was trapped as the residual gas in the aquifers during the imbibition process, explaining why there was a higher mass fraction of CO2 in the aqueous phase in the hysteric scenarios.Thirdly, The calibrated permeabilities of the reservoir in the real site model with consideration of the hysteresis effects was one order of magnitude smaller than those resulted from the calibration without considering the hysteresis effects. The permeable characteristics obtained from the calibration with hysteric consideration were closer to the description about permeability revealed by the seismic survey.In conclusion, by making use of the associated hysteresis effects, a well-designed intermittent injection procedure could increase the injectivity of a reservoir with low permeability and low porosity, and reduce the risk of reservoir overpressure due to injection. This is of great importance in improving the reservoir performance for a project using low permeable saline aquifers as the storage place in the engineering practice.