不同初始损伤下大理岩卸荷的核磁共振试验研究

Experimental research into nuclear magnetic resonance in marble with different initial damages under unloading conditions

为研究不同初始损伤下大理岩的卸荷特性,开展相同初始围压、不同初始轴压的常规三轴加-卸载试验,并进行加-卸载后岩样的核磁共振(NMR)特性试验研究,得到岩样孔隙度、应变、核磁共振图像及T2谱分布等参量。结果表明:(1)随着卸荷的进行,岩样孔隙度增大,且当初始轴压为三轴抗压强度(TCS)的90%时,其卸荷过程比80%与70%时更加平稳;(2)初始损伤可以有效促进应变的增大,尤其对环向扩容效应影响显著,同时应变的增长能促进裂隙的发育;(3)初始损伤较小时,岩样经历弹性变形到塑性变形的转化,岩石的T2图谱中小孔隙谱峰先向左移,后向右移,大孔隙谱峰不断向右扩展,反映了卸荷初期产生新的小孔隙,卸荷后期主要是大、小裂隙的扩展、贯通。当初始损伤较大时,T2图谱只向右扩展,反映岩样以扩展大孔隙为主,岩样只经历塑性变形;(4)初始损伤越大,相同卸荷围压比的核磁共振图像白斑亮度和面积越大,岩样的孔隙度就越大。

To study the unloading characteristic of marble with different initial damages, conventional triaxial loading-unloading tests were conducted under the same initial confining pressure but different axial pressures. Moreover, the rock samples subjected to loading-unloading conditions were performed to investigate the nuclear magnetic resonance （NMR） behaviour. The rock porosity, strain, nuclear magnetic resonance image （NMRI） and crosswise relaxation time T2 distribution were obtained. The results showed that： （1） the rock porosity increased during the unloading process. When the initial axial pressure was at 90% of the triaxial compressive strength （TCS）, the unloading process was more steady than at either 70% or 80% TCS. （2） The initial damage advanced the increase of strain, in particular the hoop expansion, which in turn accelerated the growth of microcracks. （3） When the initial damage was low, the deformation of rock sample changed from elastic to plastic. Spectral peaks of the small microcracks moved from left to right, while spectral peaks of the large microcracks continuously extended to right, which indicated that small microcracks were generated at the initial stage of unloading, and both small and large microcracks propagated and coalesced at the last stage. When the initial damage was sufficiently high, T2 spectrtun curves just moved towards fight, which meant that the propagation of large microcracks played a vital role and the rock had undergone only the plastic deformation. （4） When the initial damage was increased, both the intensity and area of NMRI white spot and the rock porosity were increased at the same unloading confining pressure ratio.