核磁共振研究致密砂岩孔隙结构的方法及应用

基于核磁共振的原理,推导T_2弛豫时间与孔隙半径的关系,由孔喉比将孔隙半径转换为喉道半径,结合压汞喉道半径分布,利用插值和最小二乘法,将岩心100%饱和水的核磁共振T_2谱转换为孔喉半径分布,并将核磁孔喉分布曲线应用到油田开发评价中。以鄂尔多斯盆地延长组致密储层为例,结合岩心驱替试验,利用转化的核磁孔喉分布对研究区块储层的孔隙结构、可动流体和可动油分布以及可动流体喉道半径下限进行研究。结果表明:研究区块孔隙结构复杂,发育微米级和纳米-亚微米级孔喉,孔喉半径均值在0.095~1.263μm,0.001~0.01μm的孔喉内束缚流体分布较多,可动流体主要分布在喉道半径大于0.01μm的孔隙内,水驱主要动用喉道半径大于0.1μm的孔隙内的油,研究区可动流体喉道半径截止值平均为0.013μm。

Characterization of pore structure of low permeability reservoirs using a nuclear magnetic resonance method

The flow conductivity of low permeability reservoirs is constrained by their pore structures. Nuclear magnetic resonance(NMR) T₂ spectra and mercury intrusion methods can be used to characterize the pore structures of different rocks. In this study, the correlations of NMR T₂ relaxation time with pore throat radius (in terms of pore-throat ratio) for fully water-bearing rocks were studied in combination with the mercury intrusion technique,in which the NMR T₂ distribution can be converted into a pore throat radius distribution using a method of interpolation and multiple regression. The pore structure, movable fluid distribution, movable oil distribution and the minimum movable fluid pore radius of a tight sand-rock from Ordos Basin were investigated using the inverted pore throat distribution curve. The results show that the complicated pore structure of the tight sand-rock studied is characterized by micro pores and nano-submicron pores with average pore throat radius in the range of 0.095-1.263 microns, and the fluid is mainly bounded in the pores of 0.001-0.01 microns. The movable fluid is mainly distributed in the pores with radius larger than 0.01 microns, and the movable oil that can be displaced by water is mostly in the pores larger than 0.1 microns. The average cutoff value of the pore radius for movable fluid is characterized as 0.013 microns.