Experiment Research Of Multiphase Seepage Mechanisms In Carbon Dioxide Flooding For Low Permeability Reservoirs

This paper focuses on the Yaoyingtai low permeability oil field, and has done experiment research of multiphase seepage mechanisms in carbon dioxide flooding for low permeability reservoirs. The research has figured out change rule of gas-liquid-solid interface tension and wettability, and determines the diffusion coefficient of CO2 in oil and porous media, and determines the phase state of fluid and multiphase relative permeability characteristics under high temperature and high pressure in CO2 flooding.With the increase of pressure, biggest drop occurs in the interfacial tension of oil/gas, then the second one is oil/water interfacial tension drop, gas/water interfacial tension drop is minimal. With the increase of pressure, reservoir wettability is changed to be hydrophilic step by step in CO2 flooding. Based on a large number of interfacial tension and wettability experiments, the mathematical models of oil/water, oil/gas, gas/water interfacial tension are established with pressure change. The research has established dynamic interfacial tension experiment technology of oil/gas/water and wettability test physical simulation experiment technology of rock-brine-CO2.The paper establishes two diffusion mathematical models for CO2/oil and CO2/oil in porous media. Based on two kinds of models, combined with the diffusion experiment under different pressure, diffusion coefficients for CO2/oil and CO2/oil in porous media are 10-8m2/s and 10-11m2/s under the condition of reservoir respectively. Nuclear magnetic resonance (NMR) test results show that by diffusion effect and solution expansion CO2 can displace the oil of the micropore of rock which can not be displaced by water flooding. Based on a large number of PVT experiments, physical properties of crude oil are measured under different CO2 injection rate in high pressure and high temperature. With the increase of pressure, the CO2 solubility in the crude oil and crude oil volume expansion coefficient increase, but viscosity of crude oil and density of gas oil ratio decrease, and gas oil viscosity ratio gradually improves. All combinations of these factors are the CO2 flooding mechanism. Based on the experiments, fluid parameters forecasting models of CO2 solubility-pressure, viscosity ratio-CO2 solubility, density ratio-CO2 solubility, volume expansion coefficient-CO2 solubility are established.Based on core displacement experiments, three displacement characteristics have been confirmed in the CO2 flooding for low permeability oil reservoirs. First, with the increase of injection pressure, the starting pressure gradient increases, but injection index decreases. Second, when CO2 is breakthrough, injection pore volume of fractured rock is less than that of no fractured rock. So effect of CO2 flooding less sharply. Finally, a function of double logarithmic of cumulative gas injection and the cumulative output is subsection linear relations. Considering the diffusion and dissolution effect, oil and gas PVT phase behavior, the interfacial interaction of oil and gas, the capillary number theory is updated. Then based on this theory, injection-production parameters of Yaoyingtai oil field are optimized. By means of displacement experiments in different production methods, water-alternating-gas, cycle gas injection, continuous gas injection, water flooding rank first to fifth respectively in terms of oil displacement efficiency.Based on unsteady experimental methods, considering CO2 diffusion and solution effect, oil/water and oil/gas two-phase relative permeability curves are determined under different pressure. With the increase of reservoir pressure, the interfacial tension of oil/gas decreases sharply, and then two phase seepage area gradually broaden, isoperm points gradually shift to the right, the endpoint values of gas phase relative permeability gradually rise. Based on STONE II updated model, three-phase relative permeability model is established, which contain the diffusion, dissolution and interfacial interaction. Oil isoperm curves results show that oil isoperm curves are concave to 100% oil saturation point, and oil isoperm curves area increases with the increase of reservoir pressure gradually, but the residual oil saturation decreases.