低渗砂岩气藏的孔隙结构与物性特征

根据毛细管束模型,在分形几何、表面与胶体化学和气体分子运动论的基础上研究了低渗砂岩气藏的孔隙结构与物性特征之间的定量关系。孔隙结构特征表现为孔喉半径及其高宽比小，分形维数,毛细管弯曲度和孔-喉径比大。物性特征表现为渗透率低并对应力敏感、毛细管压力和束缚水饱和度高，但原生水饱和度则可能很低,纵向气、水分布异常——气、水分界模糊和可能气、水倒置,自由分子流动与粘滞流动共存以及气高速非达西流动和水低速非达西流动显著。从已建立的理论关系式来看,各种结构参数对物性参数的影响不尽--致,如孔喉半径对渗透率的影响比对毛细管压力的影响大,而分形维数对毛细管压力比对渗透率的影响要大。水膜厚度不影响孔喉有效尺寸,仅通过改变润湿性影响气、水分布,从而影响气、水的相渗透率。在低渗气藏中因孔喉很小，分子自由流动明显,无论在室内或地下均需校正滑脱效应。

Characteristics of Pore Structure and Physical Property of Low Permeability Sandstone Reservoir

Quantitative relationship between pore structure and physical property characteristic of low-permeability sandstone gas reservoir is studied on the bases of capillary bundle model,fractal geometry,surface and colloid chemistry and gas molecule motion theory.The pore structure features small pore throat radius and low height-wide ratio,and much fractal dimension,big capillary tortuosity and high pore-throat radius ratio.The physical property features low permeability,sensitivity to the stress,high capillary pressure and irreducible water saturation,but low initial water saturation,vertically gas-water distribution abnormal,coexistence of free molecule flow and viscous flow as well as the high velocity gas flow and low velocity water non-Darcy flow.The given relationship in this paper shows that the effect of various pore structure parameters on some physical property data is not the same.The effect of pore throat radius on permeability is higher than that on capillary pressure,whereas the effect of fractal dimension on capillary pressure is higher than that on permeability.The thickness of water film has not affected the effective pore throat size,but affected gas-water distribution through altering wettability of rocks and then affected the permeability to gas and water.For very small pore throat in low permeability gas reservoir,the gas molecules assume free flowing,hence either in laboratory or in ield must consider the slippage effect corrections.