A Simulation Study Of The Effect Of Hydrocarbon Injection Gas Below The Bubble Point Pressure Reservoir

Gas injection flooding gradually has become a mature technology to enhance oil recovery (EOR) after thermal recovery technology since the 21st century. It has become the main EOR technology for oilfield in the middle and later stage of development, which has been widely applied in every oilfield at home and abroad, and its effect is remarkable.Although we adopt various measures to improve or maintain reservoir pressure, reservoir pressure will reduce in the end along with the development, especially in low permeability reservoir, which pressure drop quickly and not easy to maintain. At present, the pressure of some reservoirs is less than or closes to the bubble point pressure of crude oil. It is universally acknowledged that there is some gas dissolved in oil under formation pressure. When the reservoir pressure is dropped to below the saturation pressure, dissolved gas began to emerge. The degassing degree is controlled by reservoir pressure. The more pressure is lower than the saturation pressure, the more severe the degassing degree is, and the more gas emerges. Due to the emergence of dissolved gas, the properties of crude oil changes. While if we injected gas in the reservoir, the nature of the injected gas will also change. It needs further research to study what kind of impact will do on the development effect of gas injection flooding.Fluid samples and core samples are picked out to study from reservoir which bubble point pressure is higher. For some constraints, it only carried out the slime-tube test and long core displacement experiment under the condition of higher than the saturation pressure. Then it uses Eclipse to establish a numerical simulation model after fitting the experimental results. Based on the idea of gas injection after depletion, it continues to carry out the gas displacement experiments under pressure which is below bubble point pressure, to studying the changes of oil displacement efficiency of gas injection on this occasion. Last, based on the known reservoir data, two kinds of mechanism simulation models (section model and areal model) are established to research the effect of gas flooding below the bubble point pressure in oil reservoir and study the topic of this article for further research on macro level. At the same time, the effect of reservoir angle, porosity, permeability, well pattern and rhythm on gas injection of reservoir which exists extracted gas is discussed.By the tubule experiment, the minimum miscible pressure (MMP) of hydrocarbon gas and crude oil is 51.36 MPa. Under the condition of formation pressure at present which is 49.7 MPa, it can achieve near-miscible flooding. By the long core displacement experiment, the ultimate oil displacement efficiency of hydrocarbon gas flooding is 86.48%. It illustrates a certain advantage in gas drive. Model predictions of indoor experiment below bubble point pressure, which is carried out below the saturation pressure, illustrate the impact of degassing degree on gas oil displacement efficiency, the more severe degassing degree is, the lower gas oil displacement efficiency is, but the rate of oil displacement efficiency decreased with pressure becomes slow, and the change trend of the curve gets upward relative to the situation of higher than saturation pressure. When pressure is less than 34%(plus or minus 1%) of the bubble point pressure, the pressure changes almost have no effect on oil displacement efficiency.Results of mechanism simulation models show that:the more severe the degassing degree is, the lower the recovery efficiency of gas drive is, but the recovery rate becomes slow along with the change of pressure. It shows that the influence of pressure on oil recovery is not becoming more and more obvious if we inject gas under lower reservoir pressure, when reservoir pressure is less than 34%(plus or minus 1%) of the bubble point pressure, oil recovery is hardly affected by the pressure. Therefore, if we want to enhance oil recovery by injecting gas, we should make sure that the reservoir pressure is kept above the bubble point pressure. Otherwise, gas recovery is lower, and the result is worse. Recovery efficiency of gas injection on reservoir which exists degassing gas is also under the influence of the angle, porosity, permeability, well pattern and rhythm of reservoir, which should be considered in the development of reservoir by gas injection.