基于核磁共振的岩石孔隙扰动演化规律分析方法

Analysis Method of Rock Pore Disturbance Evolution Law by Nuclear Magnetic Resonance

岩石在外界各种扰动作用下内部孔隙结构将发生不同程度变化,轻则引起局部裂纹扩展而降低其力学性能,重则引起地下岩石结构或设施失稳破坏。采用核磁共振技术(NMR)分析岩样在离心试验前后T_(2)能谱分布规律,提出一种考察岩石孔隙演变量化分析方法,并以页岩孔隙结构的水化扰动演化为例进行分析。结果表明,通过核磁共振弛豫理论,分析了岩石横向弛豫时间T_(2)与孔径半径r之间的关系,并根据T_(2)能谱分布图制定一种孔隙分类方法,可分析岩石内部孔径分布规律。通过饱和岩样和离心后岩样T_(2)能谱图对比,建立了T_(2)能谱面积与孔隙体积之间的关系,并推演出基于核磁共振试验数据来求取孔隙比的计算方法。利用该方法考察页岩水化扰动作用下各类孔隙结构的演变量化规律及孔隙率和体积变化规律,揭示页岩水化扰动损伤的深层机理。该试验方法为考察其他扰动因素引起孔隙结构演化规律提供了一种新路径。

Under various external disturbances,the internal pore structure of the rock changes to varying degrees,which may cause local crack expansion and reduce its mechanical properties,or cause instability and failure of underground rock structures or facilities.Using nuclear magnetic resonance(NMR),a quantitative analysis method to investigate the evolution of rock pores was proposed,and the hydration disturbance evolution of shale pore structure was taken as an example to analyze.Research results show that the relationship between the rock transverse relaxation time T2 and the pore radius r is analyzed through the nuclear magnetic resonance relaxation theory,and a pore classification method is developed according to the T2 energy spectrum distribution map,which can analyze the distribution law of pore diameter inside the rock.The relationship between the T2 energy spectrum area and the pore volume was established by comparing the T2 energy map of the saturated rock sample and the rock sample after centrifugation,and a method for calculating the void ratio based on the NMR test data was deduced.The proposed method was used to investigate the evolution and quantification of various pore structures under the action of shale hydration disturbance,as well as the change laws of porosity and volume,and reveal the deep mechanism of shale hydration disturbance damage.This test method can provide a new way to investigate the evolution law of pore structure caused by other disturbance factors.