非饱和低渗砂岩突破压力试验研究——以柴达木盆地东部石炭系砂岩为例

摘要

Breakthrough pressure is a significant parameter in the processes of gas reservoir exploration and cap rock evaluation.This study investigates the breakthrough pressure of unsaturated low-permeability sandstones through a series of laboratory experiments.Three core samples of low-permeability sandstone were obtained from the Carboniferous sandstone formation in the eastern Qaidam Basin.The breakthrough pressure of each of the samples was tested under six different levels of water saturation.The mineral and clay compositions of the samples were analyzed using the X-Ray Diffraction and X-Ray Fluorescence method.Porosities and pore size distribution of the samples were measured with the He double chamber and mercury intrusion methods.CH4 was used in the seepage simulation experiments to measure the absolute permeability of the samples.The results indicate that porosityof the low permeability sandstones ranges from 9.02％ to 10.96％.The average pore radius ranges from 0.1082 μm to 0.3709 μm.The permeability ranges from 0.008 mD to 0.012 mD.The breakthrough pressure for the dry sandstone ranges from 0.05 MPa to 0.19 MPa.The breakthrough pressure for the fully saturated sandstone ranges from 1.51 MPa to 2.73 MPa.The pore structure will not be prominently changed by the swelling of the caly mineral and therefore the swelling has little effect on the breakthrough pressure.The pore structure and water saturation are the main factors controlling the breakthrough pressure.The results suggest that there is a negative correlation between the breakthrough pressure and the pore radius,i.e.,the increasing average pore radius or medium pore radius will lead to a decreasing breakthrough pressure.A quantitative relationship between the breakthrough pressure and water saturation is obtained.The breakthrough pressure increases exponentially with the water saturation.%突破压力是气藏开采和盖层评估中的重要参数.文章选取柴达木盆地东部石炭系低渗砂岩岩心3块,对每块岩心进行6个不同含水率下的突破压力试验.通过XRD、XRF分析方法对砂岩的矿物成分、化学成分进行定量测试;利用氦气双室法与压汞试验对砂岩的孔隙度和孔径分布特征进行分析;用CH4气体模拟渗流试验对砂岩的绝对渗透率进行分析.结果表明:研究区低渗砂岩孔隙度在9.02％ ～ 10.96％之间,平均孔隙半径在0.1082～0.3709 μm之间,绝对渗透率在0.008 ～0.012 mD之间,干岩石突破压力值在0.05 ～0.19 MPa之间,饱和岩石突破压力值在1.51 ～2.73 MPa之间.黏土矿物遇水膨胀对孔隙结构不会造成显著性改变,因此对突破压力没有明显影响.影响突破压力的主要因素是孔隙结构和含水率.试验结果表明突破压力与岩石孔隙大小之间呈现负相关的关系,随着平均孔径和中值半径的增大,突破压力随之降低.本研究得到了突破压力与含水率之间的定量函数关系:突破压力随含水率增加呈指数函数形式增加.