相对湿度对盐膏岩蠕变规律影响的机理研究

Mechanism of Effect of Relative Humidity on Creep Behavior of Gypsum Rock

为了弄清盐膏岩在井下环境下的蠕变规律,对盐膏岩蠕变规律影响机理进行了研究。采用调节盐溶液中含盐量的方法来控制岩样周围环境的相对湿度值,并在相对湿度为35%,65%,95%的条件下,进行围压分别为0,2,5MPa的三轴短期压缩试验和单轴应力为10MPa的蠕变试验。相对湿度从35%增加到95%,盐膏岩岩样强度由38.2MPa降至35.2MPa;弹性模量和泊松比变化量小。相对湿度为95%时,盐膏岩岩样产生负向的体积应变(膨胀);相对湿度为35%时,盐膏岩岩样产生正向体积应变(收缩)。研究结果表明,盐膏岩的强度对于相对湿度的变化不很敏感;结合Heard模型,利用蠕变激活能可以解释相对湿度对体应变速率的影响;晶体到毛细管空间的水分子相互运移的机械化学机制,可以解释相对湿度对体应变的影响。研究得到盐膏岩井下真实的蠕变规律,为盐膏层安全钻井提供了理论依据。

In order to find out the creep behavior of salt gypsum under the down hole conditions, the mechanism affecting salt-gypsum creep was studied. By adjusting the concentration of salt in the salt solu tion to control the relative humidity around the rock samples, the uni-axial compression test was carried out at 10 MPa and tri-axial compression tests were carried out at confining pressures of 0,2 and 5 MPa under relative humidity of 350//oo ,650//oo and 950//oo respectively. It was found that in a short-term test of salt-gypsum rock,the rock strength displays a downward trend from 38. 2 MPa to 35.2 MPa with the increase of rela- tive humidity from 300//oo to 90~//oo, the change in Young＇ s modulus and Poisson^s ratio is small. When the relative humidity is 95~/oo, the sample generates negative volumetric strain （expansion） and when the rela- tive humidity is 35~/oo, the sample generates positive volumetric strain （shrinkage）. The study shows the strength of salt-gypsum is not highly sensitive to the change of relative humidity; and combined with the Heard model, the impact of relative humidity on the volumetric strain rate can be explained by the theory of creep activation energy; and the influence of relative humidity on the volumetric strain can be explained by the mechanical-chemical mechanisms of mutual migration of water molecules between the crystal and capil- lary spaces. The creep behavior of salt-gypsum rock underground revealed by the study can provide theoret- ical basis for safe drilling in this kind of formations.