基于核磁共振实时成像技术的裂隙砂岩渗流特性研究

地下工程岩体内部存在着大量不规则、多尺度的孔(裂)隙,使得其渗流问题十分复杂,研究裂隙岩体的渗流特性及流场分布对岩体工程安全和深部资源开发利用具有重要的工程实际意义。利用核磁共振岩石渗流过程实时在线分析与成像系统对含不同裂隙性状的砂岩试样开展裂隙岩石渗流试验,对渗流过程中试样的体积含水率、T2谱曲线和渗透系数等参数的演变规律进行分析。结果表明:裂隙岩石试样的渗透特性与裂隙倾角和数量有关,岩石裂隙萌生和扩展受到渗透压力增加的影响,试样渗透系数呈现缓慢增加后平稳的趋势。当围压增大时,试样渗透系数减小,且当围压超过10MPa时渗透系数变化的敏感度降低。T2谱分布曲线的结果则表明流体首先在微孔隙中发生渗流扩散,然后在主裂隙中聚集,逐渐形成完整渗流通道。根据立方定律推导的渗透系数计算公式能较好的反映试验结果,为实际工程中低围压裂隙岩体渗透系数的计算提供更加安全可靠的方法。最后,通过对裂隙岩石试样的渗流过程进行核磁共振成像,直观获得了试样内部渗流场的分布规律,可准确描述流体在裂隙试样中的流动状态,为实际工程岩体裂隙渗流问题提供有意义的指导。

Investigations on the seepage characteristics of fractured sandstone based on NMR real-time imaging

The surrounding rock mass of underground engineering generally contains a large amount of irregular and multi-scale cracks or fissures,making seepage problems extremely complicated. Therefore,it is of great practical significance to study the seepage characteristics and flow distribution of fractured rock mass for the maintenance of engineering safety and the utilization of deep resources. In this paper,the online analysis and imaging system of NMR rock seepage process was used to carry out seepage tests on sandstone samples with different fracture characteristics,and the parameters such as volumetric moisture content,T2 spectrum and permeability coefficient in the seepage process were analyzed. The results show that the permeability of fractured samples is related to the angle and quantity of fractures,fracture initiation and propagation are affected by the seepage pressure,and the permeability coefficient of the samples presents a trend of slow growth and then stabilization. The permeability coefficient decreases with increasing the confining pressure,while the sensitivity of the permeability coefficient decreases when the confining pressure exceeds 10 MPa. The T2 spectrum indicates that the fluid diffuses through the micro pores and then accumulates in the main fractures,gradually forming a complete seepage channel. The formula for calculating the permeability coefficient derived from the cubic law can well reflect the test results,which provides a more safe and reliable method for calculating the permeability coefficient of fractured rock mass under low confining pressures in practical engineering. Finally,nuclear magnetic resonance imaging(MRI) was carried out for the seepage process to intuitively obtain the law of fluid distribution inside the fractured samples,which can accurately describe the flow state of fluids in the fractured rock samples and provide meaningful guidance for the seepage problems of fractured rock mass in practical engineering.