Study On The Basic Physical Property Of CO₂-Alkanes Under Oil Reservoir Storage Conditions

At present, global warming caused by emissions of greenhouse gases, such as CO2, has a serious threat to human survival environment, which has a great effect to the world economic development. In order to reduce CO2 concentration in the environment, many scientists do all kinds of research to reduce emissions from all over the world. Carbon capture and storage (CCS) is considered to be one of the most economical and feasible method that could reduce greenhouse gas and slow down global warming in the future.In order to study the safety and feasibility of CO2 storage in oil, we must understand the physical and chemical property changes and the migration of oil after CO2 injected, and the density change of oil is the main factors of the CO2 diffusion and migration, thus, it is necessary to make a system research of density change of CO2 oil system. Based on magnetic suspension balance, this paper used undecane and dodecane as crude oil and studied density change of CO2-undecane and CO2-dodecane solution under different conditions of temperature, pressure and CO2 concentration. Based on the experimental data, combined with the thermodynamics characteristics, the CO2-undecane and CO2-dodecane solution density models were established, and the volume properties of CO2-undecane and C02-dodecane solution were studied.The result of C02-undecane indicates that in the temperature 313-353K,8-19MPa pressure range, the density of CO2-undecane solution decrease with increasing temperature and increase with increasing pressure. The densities of CO2-undecane mixtures increase at first with an increasing CO2 mole fraction, and then decrease at higher CO2 concentrations at high temperature. A crossover effect was found at high CO2 mole fraction, and the crossover pressure increases with increasing temperature. The result of CO2-dodecane indicates that in the temperature 313-353K,12-18MPa pressure range, the density change of CO2-dodecane has the same trend with CO2-undecane; the crossover temperature of CO2-dodecane is lower than CO2-undecane.Based on the experimental density data, BWRS model and PHSC model were established to predict the density of CO2-undecane and CO2-dodecane. The absolute average deviation of CO2-undecane and CO2-dodecane is 1.34% and 0.96% predicted by BWRS model. PHSC model was used to predict the density of CO2-undecane and CO2-dodecane and the absolute average deviation of is 3.38% and 3.63%, respectively. The value of s can be optimized for pure substances based on the experimental density data in this work, improved PHSC model predict the density of CO2-undecane and CO2-dodecane and the absolute average deviation is 1.25% and 0.92%, respectively. Compared with BWRS model, the accuracy of improved PHSC model is a little higher, interaction coefficient is constant, and more convenient to use.The excess molar volume and apparent molar volume of CO2-undecane and CO2-dodecane were calculated in this paper. The excess molar volume of CO2-undecane and CO2-dodecane were negative, the excess molar volume decrease at first with an increasing CO2 mole fraction and then increase at low temperature, decrease with an increasing CO2 concentration at high temperature. Under certain temperature and CO2 concentration, the excess molar volume increases with increasing pressure. Within the scope of the experimental temperature and pressure, the apparent molar volume of CO2-undecane and CO2-dodecane were positive. The apparent molar volume of CO2-undecane decrease with increasing CO2 concentration at low temperature, but increase at high CO2 concentration at high temperature. Under certain temperature and CO2 concentration, the apparent molar volume increases with increasing pressure for CO2-undecane. But for dodecane, the apparent molar volume increase with increasing CO2 concentration at high temperature, decrease at first and then increase with increasing CO2 concentration at low temperature.From above all, based on magnetic suspension balance, this paper has a systemic study of density of CO2-undecane and CO2-dodecane, high precision models were established, which could provide the database support and theoretical guidance for CO2 oil reservoir storage.