Study On The Mechanism Of Oil Recovery By CO 2 Injection In Low Permeability Reservoirs Around Daqing

Due to the poor physical property of Da Qing marginal reservoir, wells barely show any natural productivity, in addition, a number of problens have encounted during the injection process such as high injection pressure, poor flooding effectiveness, as well as a sharp increased water cut ratio, which, in some respect, lead to a slow oil production rate and poor recovery efficiency. As a result, to achieve a sustainable development of Da Qing oil field, it is of great importance to exploit these marginal reservoirs in an economical and effective way.Taking into account of geological and development characteristics of these reservoirs, literature research shows that carbon dioxide flooding is an effect way to increase recovery ratio. However, limited preliminary tests and incufficient understanding of enhanced recovery via injected CO2 ramains potential weaknesses. Targeted at marginal reservoirs,a number of tests and numerical simulations have been carried out to deepen our understandings regards to the mechanism of CO2 flooding. Some of the findings are:(1) The property of reservoir fluids can be characterized as bellows:Oil riches in the content of heavy constituent but lacks in the content of intermediate hydrocarbon component. The saturation pressure and shrinkage factor of 10 wells are low, as well as the volume factor and average gas solubility coefficient. Density of crude oil is medium to large. Formation fluids ramain single phase under original reservoir pressure despite of its small expansion ability. So preliminary CO2 injection dwelling development should be considered.(2) A good compatibility between oil and CO2 has been witnessed during phase compatibility experiments and numerical simulations, which shows that injected CO2 can cause a great increase in volume expansion bur a reverse effect on density and viscosity. Thus creating an ideal fluidity of crude oil. What's more, according to simulation calculations of pseudoternary phase diagram and P-X phase diagram, it is considered that a relatively lightened oil can be acquired. But neither single contact miscible condition nor multiple contact miscible condition can be achieved under original formation pressure.(3) Experimental measuremenst and numerical simulations of one-dimensional long slim tube indicate that miscible flooding cannot be achieved due to the fact that the minimum miscibility pressures of 10 wells with CO2 injection are greater than original reservoir pressure, whereas, a lower minimum miscibility pressure is discovered in extended slim tube. Numerical simulations study found that once the injection pressure is settled, a better displacement effect can be achieved with enlarged injected gas, on the other hand, a higher recovery ratio is more likely to be appeared with higher experimental pressure under constant injecting rate.(4) Due to limitations and accuracy of the existing CO2-oil MMP prediction models, based on an overall analysis of identical fluids property and experiments from domestic and abroad,this thesis presents an optimized MMP prediction model using optimization algorithm. Besides, a good credibility of this model is displayed verifying the experimental datas afterwards.(5)Carried out under pressure acquired from slim experiments, long core experiments witnessed a increase in the recovery rate up to 29.15% more than that of original formation pressure. As a result, advanced gas injection can be used as a way of enhancing formation pressure to achieve higher displacement efficiency.