致密砂岩气藏阈压梯度对采收率的影响

Influence of the Threshold Pressure Gradient on Tight Sandstone Gas Reservoir Recovery

致密砂岩气藏具有低孔、低渗、高含水等储层特征,致密储层中孔隙水的存在使得气体在渗流过程中产生了存在阈压梯度的非达西渗流,从而减小了单井控制储量降低了气藏采收率。通过对苏里格气田岩样采用气泡法与压差流量法相结合的实验方法,得出致密砂岩气藏具有储层渗透率越低、含水饱和度越高,阈压梯度越大,非达西渗流特征越显著的渗流规律,并根据实验结果建立了阈压梯度与渗透率、含水饱和度的关系式。所得关系式结合稳态产能方程计算表明阈压梯度与气藏采收率呈正线性相关关系,且储层渗透率是致密砂岩气藏采收率的最主要影响因素,当储层渗透率低于0.02×10-3μm2、阈压梯度大于0.1MPa/m时,会造成储层中的绝大多数流体无法被动用;含水饱和度也是致密砂岩气藏采收率的影响因素,当含水饱和度高于临界含水饱和度值时,采收率会随着含水饱和度的升高急剧下降,所得实验结果与数学模型能够正确地反映致密砂岩气藏的渗流机理和开发动态。

Low porosity and permeability and high saturation are the main characters of tight sandstone gas field and there are non-Darcy flow which exist threshold pressure gradient in the proceeding of the flowing of gas phase due to the water phase.It results in the reduction of the single well control range,thereby the reduction of gas recovery.Bubble method and differential pressure flow method were combined to get the conclusion that lower permeability,higher water saturation and threshold pressure gradient affected nondarcy flow more.The correlations between threshold pressure gradient,permeability and water saturation were built according to the result of the experiment.Threshold pressure gradient and gas recovery have positive linear correlation according to the empirical and capacity formula and permeability has greater impact on gas recovery.When the permeability is below 0.02×10^-3μm^2 and the threshold pressure gradient is above 0.1MPa/m,the fluid of reservoir would not be utilized.Water saturation is also a main factor on the gas recovery in tight gas reservoir.When the water saturation is above the critical value,the gas recovery will fall sharply with the rise of the water saturation.In conclusion,the result and mathematical model can reflect the seepage mechanism and exploitation dynamic correctly.