Visual Physical Simulation Study Of N₂ Flooding In TH Fractured-vuggy Reservoir Well Group

As an effective means to enhance oil recovery after water flooding,gas flooding has been widely used in oil fields all over the world.As the most common gas,N2 has many ascendant characteristics,such as rich raw materials,low acquisition cost,inactive chemical property,high compression coefficient and good expansibility,so that the N2 flooding has a very good development prospect and potential.This paper relies on the National 13th Five-year Plan Demonstration Task Study on Gas Injection Mode and Technical Policy of Fractured-vuggy Reservoir(2016ZX05053-009).This paper mainly aims at many problems in the development of N2 flooding in TH fractured-vuggy reservoir,such as large difference of effect in field test,gas channeling in some well groups,low oil exchange rate of N2 flooding in well group,it is necessary to optimize the parameters of gas injection,and so on.There is an urgent need for in-depth study on N2 flooding of well groups.This paper mainly adopts the method of indoor physical simulation,based on establishing the high temperature and high-pressure visualization physical model of typical fractured-vuggy unit well group and matching physical simulation device,carries on the physical simulation experiment of injection N2 in well group,and considers the injection-production well pattern.The effect of gas injection mode and gas injection parameters on the mechanism of N2 flooding in fractured cave reservoir,the effect of oil displacement and the factors affecting recovery of N2 flooding are studied.The research results have certain guiding significance to the improvement of recovery rate by N2 flooding in TH fractured-vuggy reservoir group.The findings of this study are as follows:(1)The visual physical model of well group with high-pressure resistance is made by using laser-engraving technology,and the corresponding reaction kettle is improved to meet the requirements of high temperature and high-pressure(70MPa,150oC)visualization by selecting typical well group fractured-vuggy unit in the main area of TH fractured-vuggy reservoir.(2)The experiments of different injection-production well patterns in N2 flooding of well groups show that:high injection and low recovery well pattern flooding effect is the best,recovery rate is 85.24%,followed by high and low combined gas injection well pattern,equal high position gas injection well pattern,low injection high production pattern,low injection and high recovery pattern.The recovery rates were 82.31%,79.23%and 75.61%,respectively.(3)The experiments of different gas injection modes in N2 flooding of well groups show that the gas-water alternate:flooding effect is the best,the periodic gas injection is the second,and the continuous gas injection is the worse,and the oil recovery rate is 90.23%,87.16%and 85.24%respectively.(4)Through the experiment of gas injection parameters of well group N2 flooding,it shown that the best oil displacement effect cannot obtained by injecting gas prematurely or by injecting gas too late.The water cut of water drive can reach 90%and then transferred to gas injection,and the ultimate recovery rate is higher,reaching 85.24%.The gas injection velocity has the optimal value,and the gas injection velocity is suitable to choose 0.2mL/min,which makes the injection gas spread in a wide range and is not easy to occur gas channeling.Multi-cycle,moderate period gas flooding effect is the best,the effect of six-round of gas flooding is better,and the recovery of gas drive is 22.88%.With the increase of injection volume,the recovery degree increases.When the injection amount reaches a certain value,the oil increasing effect decreases obviously.Proper N2 injection can achieve good oil displacement effect,and the nitrogen utilization rate is higher.The larger the gas-water ratio is,the earlier the gas-water ratio is.The better the oil displacement effect is when the gas-water ratio is 1:1 in gas altermating water flooding,and the recovery of this mehod is 86.29%.(5)The distribution analysis of remaining oil in well group shows that many factors can affect the distribution of remaining oil.such as injection-production well pattern,gas injection mode,gas injection velocity,connecting mode and relative position of fracture cave unit.Optimizing the well pattern of injection and production.Gas injection mode and gas injection speed can improve the displacement effect and reduce the distribution range of remaining oil.The mechanism of flooding in well group is mainly due to gravity differentiation,expansion and viscosity reduction,supplementary formation energy,gas-drive leading edge,and water-cone suppression.