Study About The Sweep Coefficient And Oil Displacement Efficiency Of Chemical Flooding

Since 1950s the chemical flooding after the water flooding or at the later period of oilfield-exploited has been studied in our country and abroad. In 1990s, as the main technology to improve the oil recovery in water-injected exploited oilfield, chemical flooding has been into the period of pilot test, the technology of polymer flooding has been applied in industrial scale and obtained good effect. However, the mechanism of chemical flooding is very complicated and many basic problems of it are still lack of enough understanding. So, in order to improve the mechanism and effect of chemical flooding, it is necessary to do deeper and systematical work about the physi-chemical seepage mechanism. This is the purpose of "the foundational research about improving the oil recovery greatly" (a national key project in "10th-five-years" period), "the study on oil displacement efficiency and sweep coefficient of chemical flooding" is part of the project.On the basis of the information analysis about the sweep coefficient and oil displacement efficiency of water flooding and chemical flooding, utilizing the plenty of the sealed coring inspection well data of Daqing oilfield, the dynamic characters of sweep coefficient and oil displacement efficiency of water flooding in Daqing oilfield were concluded, the sweep coefficient and oil displacement efficiency of water flooding in Daqing oilfield at present were given quantitatively. According to similarity theory and the sediment rule, the two-dimensional and three-dimensional heterogeneous physical models similar to the reservoir of Daqing oilfield were designed and produced. The seepage mechanism of chemical flooding, sweep coefficient, oil displacement efficiency were investigated by the core flooding tests with micro-model, two-dimensional planar homogeneous physical model and three-dimensional heterogeneous physical model. With these experiments the oil displacement efficiency, sweep coefficient and the last oil recovery of polymer flooding and ASP flooding were obtained, the contribution of oil displacement efficiency and sweep coefficient to enhanced oil recovery was also studied. The study confirms that polymer flooding can improve the oil displacement efficiency. The main conclusions are as follow:1. 5 kinds of physical model for experiment were designed and produced. According to the Archie's formula, the oil saturation in the model is obtained by the method to measure the electrical resistivity between the electrodes. A set of measurement, data acquisition system was established with the electrodes, data acquisition board and computer. The measurement difference of oil saturation is in the range of 1.0-1.5%. The distribution of oil saturation in the core at different period of chemical flooding and the position of chemical flooding front can be measured.2. Dynamic color image quantitative analysis system of micro-model was established successfully. This system has several functions: (1) It can take the photo of the dynamic process of oil displacement in micro-model continuously (1 photo/1.5s); (2) The dynamic image in the flooding process can be analyzed to obtain the degree of oil displacement and draw the curve of oil recovery; (3) The movement of the macroscopical target (such as the water front) can be displayed; (4) At the proper magnifying factor the movement of the target (such as the oil drop) in pore can be tracked and photoed, the characteristic parameter of the target can also be calculated, then the movement track of the target would be obtained; (5) The pore parameters of the micro-model can be analyzed to obtain the characters of the pore and throat.3. With the analysis of sealed coring well data and reservoir prediction, the sweep coefficient and oil displacement efficiency of water flooding in the various reservoirs of Daqing oilfield are given macroscopically. Now the sweep coefficient of water flooding in Daqing oilfield is in the range of 41.4%-67.7%, the oil displacement efficiency is in the range of 40.0%-65.0% and the mean value is about 53.2%.4. Both the micro-model experiment and core flooding test show that the main effect of polymer flooding is to increase the sweep coefficient, especially the sweep coefficient of middle and low-permeability layers. The mechanisms of polymer flooding are: (1) The decrease of mobility ratio improves the sweep coefficient of polymer flooding; (2) The shear stress of the polymer solution increases so it can displace the island and film remaining oil; (3) The visco-elasity of the polymer solution can reduce the amount of the "dead-ends" remaining oil.5. The core flooding test shows the polymer flooding can improve the oil displacement efficiency by about 8%. In a heterogeneous model with interlayer, the sweep coefficient can be increased about 21.9%, 10.1% and 7.1% at the low, middle and high-permeability layer respectively; while in a heterogeneous model without interlayer the sweep coefficient can be increased about 42.2% and 23.4% at the low and middle-permeability layer respectively. The sweep coefficient of the model without interlayer is a little lower than that of the model with interlayer, however, the increase of the sweep coefficient is inverse.6. The ASP flooding can not only increases the oil displacement efficiency, but also improves the sweep coefficient. The mechanisms of the ASP flooding are: (1) The ultra-low interfacial tension between oil and water can increase the capillary number and decrease the oil saturation of remaining oil, so it would improve the oil displacement efficiency; (2) The addition of the polymer can increase the viscosity of the system and decrease the mobility ratio, so it can improve the flow field, reduce the finger advace and increase the sweep coefficient; (3) The ASP system and crude oil can form the emulsion, the oil drop will aggregate continually and form the oil wall, then more and more residual oil will be emulsified and displaced; (4) The addition of surfactant and alkali changes the wettability of reservoir.7. The ASP flooding can increase the oil displacement efficiency by 32.18%. After the ASP flooding, in a heterogeneous model with interlayer, the sweep coefficient can be increased about 30.5%, 27.4% and 14.8% at the low, middle and high-permeability layer respectively. In contrast to the model with interlayer, the increase of sweep coefficient of middle-permeability layer in the model without interlayer is a little higher.8. As to the contribution of oil displacement efficiency to enhanced oil recovery, that of the polymer flooding is 11.85%, ASP flooding is 33.27%. The ASP system has significant synergic effect and it can enhance the oil recovery greatly. In order to make the best of the ASP system, firstly, the ASP system should be injected into the high-permeability layer and the polymer solution into the low and middle-permeability layer, then, the low and middle-permeability layer can be flooded with AS system. With this injection mode the oil recovery can be improved greatly at the same amount of chemicals.