关键词: 压缩系数, 体积模量, 单轴冷压, 蒙脱石, 石英

Abstract: As clay minerals, montmorillonite is usually enriched in fault rocks, mudstones and clastic rocks. It is important to measure its elastic properties accurately for seismic sounding and interpretation of sonic logging. Many researches on the elastic properties of clay minerals have been carried out by predecessors, including the theoretical modelling of the elastic modulus of clay, the experimental determination of the elastic modulus of aggregates of clay minerals by cold pressing and the calculation of clay elastic modulus based on the corresponding elastic modulus of shale and two-phase bodies composed of mixture of clay-epoxy, and so forth. The comparison results of these modulus obtained by different methods show a huge difference. Previous investigations display a massive gap on the bulk modulus of montmorillonite ranging from 6GPa to 63GPa. Here, we present the new results of bulk modulus of montmorillonite based on uniaxial compression experiments on the two-phase bodies composed of mixture of montmorillonite and LR-White glue(hereinafter referred to as LRWG)and of quartz-LRWG performed at the pressure of 0.1~600MPa and at room temperature. On the base of the changes of sample length and density with pressure, the uniaxial compressibility of LRWG, quartz and montmorillonite are obtained. The internal porosity in these starting materials(two-phase bodies)were checked by scanning electron microscope to ensure the porosity within the range of allowable error. Observations show that the porosity in the starting materials is extremely low. Although very small amount of cracks inevitably exist at the boundaries between the montmorillonite aggregate and LRWG, attributed to drying shrinkage during sample preparation(heating and solidification), the pores disappear at the pressure increasing to 200MPa approximately. Hence, the samples could be regarded as pure two-phase bodies when the pressure is higher than 200MPa. After the uniaxial compression, the mineral particles in the specimen exhibit uniform distribution and no anisotropies are observed. In the initial stage of pressurization, the non-linear shortening of the montmorillonite-LRWG sample is explained to rapid reduction of the pores which is expected to be between the starting materials and sample chamber and between montmorillonite aggregate and LRWG. During the pressure range from 250MPa to 600MPa, the length of specimen linearly decreases with increasing pressure. The calculated bulk modulus of quartz is 37.49GPa which is consistent with previous report of 37GPa. The bulk modulus of montmorillonite of 10.44GPa, 6.36GPa, 8.34GPa and 7.86GPa with the average of 8.25GPa are obtained. The variation range of the bulk modulus of montmorillonite is about ±2GPa, which is significantly better than the previous experimental results. The experiments provide both reliable data and an effective technique for investigating the elastic properties of clay reservoir.

Key words: quartz, montmorillonite, compressibility, bulk modulus, uniaxial compression experiment