低渗透岩石结构体风化初期的水气分子扩散

低渗透岩石具有孔隙狭窄、连通性差、流动阻力大等特点,这致使地下水渗流速度极低。本文以二长岩及玄武岩结构体的核心及紧靠核心的外环区域为对象对低渗透岩石结构体的初期风化机理进行研究。二长岩及玄武岩核心区的渗透率分别为0.0047×10-3μm2和0.0081×10-3μm2,孔隙度分别为1.58%和0.24%。对两种岩石的核心及外环进行化学全分析表明两个区域的主要元素含量无明显变化,外环烧失量有所增加,Fe2+与Fe3+变化敏感。薄片鉴定发现二长岩核心及外环的斜长石及钾长石均发生了强烈风化,外环中的辉石变为绿泥石。玄武岩的核心区矿物基本无变化,外环的长石及辉石发生了皂石化,部分基质绿泥石化。微观结构上,两种岩石核心区均致密坚硬,外环不同程度地出现了长石及辉石的晶内溶孔。风化初期,水以气态形式扩散进入低渗透岩石内部。水气的扩散方式为介于菲克型扩散与Knudsen扩散之间的过渡型扩散。计算得二长岩和玄武岩中的水气扩散系数分别为48.09×10-8m2/s和3.74×10-8m2/s。

WATER VAPOR DIFFUSION IN LOW-PERMEABILITY ROCKS AT THE EARLY WEATHERING STAGE

Low-permeability rocks have the characteristics of narrow pore,bad connectivity,big flow resistance,leading to low seepage velocity.In this paper monzonite and basalt cores and rinds were chosen as objects to study rock weathering mechanism during the early stage.Intrinsic permeability and porosity of the monzonite and basalt are 0.0047×10-3 μm2,0.0081×10-3 μm2 and 1.58% and 0.24% respectively.Chemical analysis of core and rind of the two rocks demonstrated no obvious difference in major element content between the two regions,while ignition loss slightly increased in rind,and changes in Fe2+and Fe3+are sensitive.Plagioclase and potassic feldspar had experienced serious alteration in both core and rind in monzonite,and pyroxene had changed to chlorite in rind with thin section analysis.Basalt core was unchanged,and feldspar and pyroxene partly steatited in its rind.Core microstructures of the two rocks are compact and intragranular pores in feldspar and pyroxene were observed in the rind.Water diffused into the rocks in the form of gas in the early weathering stage.Hydrosphere diffusion mode is transitional between Fick and Knudsen diffusion.The diffusion coefficients of moisture in monzonite and basalt are 48.09×10-8 m2/s and 3.74×10-8 m2/s,respectively.