The Experimental And Theoretical Research On Enhancing Oil Recovery Of Low Permeability Reservoir

Low permeable sandstone reservoir has enormous reserves ,but it is more difficult to be explored and developed than nonmal reservoir. Because the low permeable sandstone reservoir has some features as follows: low permeability, lacking of natural power in reservoir, elastic energy decreasing fast when developing with natural power, low oil recovery. For enhance oil recovery of low permeable reservoir, gas or water is injected to layer to make up power of reservoir.Pressure of waterflooding is higher than breakdown pressure of stone in some lower permeable reservoir, it is not only expensive but hard to carry out water flooding. And after injection of water, the permeability went down sharply in low permeable reservoir. Compare with water flooding, it was lower viscosity of gas than water, the viscosity of gas was far from oil, so phenomenon of bypass channel happen often.This paper make a research of a new displacement technique which is gas flooding with liquid(water or surfactant solution) droping ,and it will apply to low permeable reservoir. This flooding system can chance filtrational resistance at will by adjusting gas fluid ratio. The different filtrational resistance flooding system can apply to develop different permeability reservoir, and the system can improve problems, which were small swepting volume and finger advance and lower period oil recovery, of gas flooding; it can make up both higt pressure and difficult of water flooding as well. The research result of this paper is not only to make deeply understanding of low permeable reservoirs develop theory and filtration theory, but also to have a enormous meaning of both technicality and engineering with conducting oilfield developing and enhanve oil recovery in low permeable reservoirs.There are four parts in this dissertation, brief introduce as follows:1.The mechanism of gas flooding and why pressure gradient of gas flooding is low than water flooding.This part conduct a gas flooding mechanism research. The research is about recovery of CO₂,N₂,gaseous hydrocarbon flooding, analyse physical and chemic changing of some kind gas above injecting to reservoir, and the reason and effect in oil recovery of miscible phase happen or not happen between gas and oil. This part have introduced the feature and mechanism of WAG yet. It is shown that: the mechanisms of non-miscible phase gas flooding are that limited gas evaporating and extracting from oil, decreasing oil viscosity, oil expanding, gas extracting from oil after pressure low than bubble point pressure, decreasing interfacial tension force etc. the mechanisms of miscible phase gas flooding are that volume swelling, decreasing oil viscosity, gas extracting from oil after pressure low than bubble point pressure, decreasing interfacial tension force, improving relative permeability and imprving permealility near around well etc.The gas flooding pressure gradient is low than water flooding was analysed from gas viscosity, the IFT between gas and oil, two-phase permeability. It is shown that: the viscosity spectrum of N₂,CH₄,CO₂ is 100⁹00×10^-4mPS, N₂,CH₄,CO₂ gas flooding's mobility ratio is more than 1 when non-miscible phase happened, because of that, the sweep efficiency of gas flooding is less than water flooding, so at same reservior, the pressure gradient of gas flooding is more lower. Gas-phase's relative permeability in gas flooding is lower than water-phase's relative permeability in water flooding at same oil saturation, but gas is easy to go through porous channel than water at same oil saturation, because the viscosity of water is 10 times bigger than that of gas, and gas relative permeability is only 2 times smaller than that of water.2.The experiment and mechanism research on gas flooding with water/surfactant solution dropping changing filtrational resistanceThe finished experimental results, which was completed once, of non-oil saturated experiment research in room are that: N₂ gas flooding with water dropping system can increase filtrational resistance. The injecting system pressure gardient went up when increase frequency of dropping water. Use zero dimention pressure gradient to measure filtrational resistance, the least zero dimention pressure gradient is 0.103 when gas fluid ratio is 584:1, the biggest zero dimention pressure gradient is 0.607 when gas fluid ratio is 102:1. The least zero dimention pressure gradient increase by gas fluid ratio decreasing. The faster frequence of dropping, the bigger zero dimention pressure gradient increasing value. That mean more filtrational resistance were produced. It were same rules when gas flooding with dropping surfactant soultion. Moreover, it is more bigger zero dimention pressure gradient increasing value. At same dropping frequence, the zero dimention pressure gradient increasing values sorted form samll to big were that dropping water, dropping 0.05wt% surfactant solution, dropping 0.1wt% surfactant solution, dropping 0.2wt% surfactant solution. There is a rule that the samller gas fluid ratio, the bigger zero dimention pressure gradient increasing value.The results of oil saturated experiment research in room are that: experiments were taken on 15¹9×10^-3μm² artificial core. The filtrational resistance of water flooding is near same to gas flooding dropping 0.2wt% surfactant solution at gas fluid ratio between 38:1 to 32:1. The dropping system can chance filtrational resistance at will by control gas fluid ratio. There are different period oil recovery when different pressrue gradient flooding. The bigger pressure gradient, the bigger period oil recovery. The results of mechanism research of dropping system are that: at low permeable reservoir, gas flooding droping system have some feature as follows: there is a force p_I point to wall of pore,which caused by capillary force.The p_I and film of bubble can bring a friction force. Non-miscible phase gas meet water have bigger IFT, it can produce bigger resistance force. When the liquid is going to move, there is a new resistance force p_II pop up. When liquid dropping move to narrow throat, a new resistance force p_III was shown. It is same that when gas flooding with dropping surfactant solution, there are also p_II, p_III.3.Research on incresing filtrational resistance of Polymer floodingBase on flooding oil mechanism of HPAM, there are some flooding experiment on 10⁵0×10^-3μm² were carried on. It is shown that: coefficient of resistance and residual resistance factor increase by incresing polymer molecular weight or concentration. It did not block when inject 400×104 and 600×10⁴ polymer molecular weight at concentration of 600mg/L and 800mg/L to 10,30,50×10^-3μm² artificial core. So some low permeable reservoirs can flooding by HPAM.4.Research on filtrational resistance of different way flooding calculate corresponding to different permeablitity.Results of research on threshold pressure gradient of low permeability are that: Base on the biggest water-oil and gas-oil threshold pressure, breakdown pressure and distance between wells, it can be calculated that the least permeability fitting for gas or water flooding. Between gas and water flooding fitting permeability area, it fit for gas flooding with drop water or surfactant solution. The way of connect different dropping system to different permeability: To make it clear that the biggest threshold pressure gradient dropping system and oil, base on threshold pressure gradient, figure out this dropping system fit what permeability.To prove it is fitting or not that polymer flooding in different permeable reservoir: calculate the ratio of pore body radius and molecule circle round of polymer, make sure the ratio is rational, base on that to slect polymer; conduct flowing experiments, screen out non-block polymer system; after that, carry on different concentration threshold pressure gradient experiment, final figure out which polyer at how many concentration is suitable.