盐水浸泡作用下石膏岩力学特性试验研究

EXPERIMENTAL STUDY OF MECHANICAL PROPERTIES OF GYPSUM SATURATED IN BRINE

为研究盐岩溶腔油气储库建造过程中,盐溶液对石膏夹层的侵蚀效应,在实验室对自然状态(干试件)、饱和与半饱和盐溶液中浸泡20d的石膏(湿试件)进行单调单轴压缩与小幅反复加卸载作用方式下的单轴压缩试验,初步揭示石膏在这一特殊条件下的力学特性。研究结果表明:反复加卸载使石膏单轴抗压强度由单调加载条件下的14.6MPa降至12.3MPa,降幅15.8%,而峰值强度所对应轴向应变也从0.39%降为0.19%,反复加卸载使得石膏的强度与变形均有所降低,但弹性模量基本不变。在反复加卸载作用方式下,与干试件强度相比,在饱和与半饱和盐水中浸泡20d之后的石膏强度并未降低,说明盐水对石膏侵蚀作用不明显。在盐溶液中浸泡之后试件变形能力增强,与干试件情形相比,增幅高达73%～147%,相应弹性模量也从干试件的6.6GPa分别降为4.5和2.8GPa。由于结构致密、孔隙率低,加之化学成分为难溶物质,在常温及酸性化学溶液作用下,石膏晶体在细观结构上未受到盐溶液的侵蚀损伤,强度并不随溶液浸泡作用而降低。但是,20d浸泡作用下有少量溶液由表及里的浸入,从而使石膏变形呈软化趋势,且随溶液浓度不同而不同。在层状盐岩矿床油气储库建造及运营过程中,对石膏夹层的这一力学特性变化应予以考虑。所获得的结果对揭示石膏力学特性及指导层状盐岩溶腔油气储库建造具有重要意义与价值。

In the laboratory,uniaxial compression experiments are carried out under monotone and cyclic loading states with gypsum specimens in dry,saturated and half-saturated states.The test results demonstrate that the peak strength of dry gypsum by cyclic loading is 12.3 MPa,strength decreasing of 15.8%at 14.6 MPa under monotone uniaxial compression.The strain of the specimens at peak strength also decreases from 0.39%under cyclic loading to 0.19%of monotone uniaxial compression.Both the peak strength and the corresponding strain of gypsum decrease when loaded in cyclic state.However,the elastic modulus keeps unchanged.Compared with that of dry specimens,the peak strength of the wet gypsum specimens,either saturated or half-saturated in brine,does not decrease.It demonstrates that brine corrosion effect on gypsum strength is minor.However,the elastic modulus decreases from 6.6 GPa of dry state to 4.5 and 2.8 GPa of saturated and half-saturated states,respectively.It means that the deformation ability of gypsum is enhanced greatly after saturated in brine.Due to tightness in structure and low porosity of gypsum,chemical solution can not easily immerge into it.In addition,the brine solution is acidicin chemical component in the test and no chemical reactions can happen with gypsum even less brine moving inside.Consequently,the strength of gypsum can keep undamaged when saturated in brine.However,the deformation ability will be enhanced and a softening trend will emerge because of long-term soakage in brine.It is suggested that this mechanical property should be taken into account in construction and operation of oil and gas storage in bedded salt deposit containing thick gypsum interlayers.