Microscopic Displacement Characteristics And Influence Factors In Low-permeability Sandstone Reservoirs During CO₂ Injection Process

Low-permeability reservoirs play a more and more important role in oil and gas field development.Due to the narrow pore throats,there are two significant problems during the development of low-permeability oil reservoirs.First,water injection performs poorly and the oil is difficult to development.Secondly,pressure conduction is slow and formation energy depletion is fast.CO₂ is easier to injection into the reservoirs because of its strong fluidity,small interfacial tension with crude oil and good solubility in crude oil.In-depth understanding of the interaction between CO₂ and crude oil,microscopic displacement characteristics in low-permeability reservoirs and their influencing factors will not provide a theoretical basis for selecting feasible CO₂ development methods for low-permeability reservoirs,but also provide a technique support for improving microscopic sweeping efficiency of CO₂.Firstly,high-pressure swelling/extraction tests,interfacial tension tests and coreflood experiments were combined to understand the interaction between CO₂ and oil under the static and dynamic conditions.The CO₂-EOR mechanisms were illuminated from the perspective of phase behavior.Meanwhile,with considering the effect of high water content on the interaction between CO₂ and crude oil,a mathematical model was established to explain the mass transfer process of CO₂ in water film and time scale of CO₂ penetrating water film to diffuse into crude oil.On the basis of illuminating the CO₂-EOR mechanisms,the microscopic residual oil of the core samples after continuous CO₂ flooding,CO₂ flooding after water flooding and water alterning gas(WAG)injection was analyzed with NMR technique.The lower limit of pore radius and dominant pore range of different CO₂ injection methods was determined.The results show that when the microscopic heterogeneity is stronger and the proportion of small pores is larger,the lower limit of pore radius where oil coulde be recovered is higher.The lower limit of pore radius of CO₂ flooding is lowest,which could recovery oil in pores with different sizes and the residual oil distribution are relatively homogeneous.CO₂ flooding after water flooding could recover the oil in micro and small pores(pore radius less than 0.5?m),which couldn't be recovered by water flooding.WAG has better displacement efficiency in the pores which are larger than 0.2?m.Then,mixed wettability evaluation system on the basis of NMR technique for low-permeability sandstone reservoirs was established.The influence of wettability on CO₂flooding was analyzed by comparing the microscopic residual oil of core samples with different wettability after each CO₂ injection method.The results show that the continuity of oil distribution in pores is an important factor affecting the effect of CO₂ flooding.The better continuity of oil distribution in pores is beneficial for the CO₂ to dissolve into the oil and displace the oil.Therefore,each CO₂ injection method could get better displacement efficiency which were over 70%in oil-wet reservoirs.For water-wet reservoirs,CO₂ flooding after water flooding could have better displacement efficiency and injectivity.And for mixed wet reservoirs,WAG could have better displacement efficiency.Finally,the different CO₂ injection methods in low-permeability heterogeneous reservoirs and fractured reservoirs were respectively studied through physical simulation experiments.The effects of permeability contrast and fracture parameters on the different CO₂ injection methods were analyzed.Therefore,an optimization method for CO₂injection in low-permeability heterogeneous reservoirs was proposed.