松辽盆地沙河子组页岩孔隙结构表征——基于低场核磁共振技术

Pore structure characterization of Shahezi Formation shale in Songliao Basin:Based on low-field nuclear magnetic resonance technology

为了表征松辽盆地沙河子组页岩微观孔隙结构,选取沙河子组9个页岩样品开展了饱和与干燥状态下的核磁共振T2测试,同步进行了岩石物性测试和场发射扫描电镜观察,系统分析了核磁共振T2弛豫特征,孔隙类型与分布特征,对比了核磁孔隙度与气测法孔隙度的差异。结果表明:沙河子组页岩样品的核磁共振T2谱以单峰型为主,弛豫时间较短,孔隙直径主要分布在10~1000 nm;孔隙类型以纳米级无机孔为主,有机孔和微裂缝相对不发育;扣除基底信号后计算的核磁共振孔隙度分布在0.68%~3.66%,与He(氦)孔隙度具有较好的匹配关系;孔隙度较小的样品更容易受岩石基质背景信号影响而造成测试结果的相对误差。总体认为,核磁共振技术能够准确分析低孔、低渗页岩样品的孔隙度和孔径分布,但需要注意页岩中有机质和黏土矿物束缚水产生的核磁信号干扰。

In order to characterize the micro pore structure of Shahezi Formation shale in Songliao Basin,nine shale samples from the lower part of the Shahezi Formation in the Songliao Basin were selected to carry out NMR T2tests in saturated and dry conditions,and simultaneous petrophysical testing with field emission scanning electron microscope observation.The T2relaxation characteristics,pore types and distribution characteristics were systematically analyzed.And the difference of porosity measured by the NMR method and the gas method was compared.The results show that the NMR T2spectrum of the shale samples from the Shahezi Formation is dominated by a single peak type with shorter relaxation time,and the pore diameter is mainly distributed in the range of 10~1000 nm.The pore types are mainly nano-scale inorganic pores,and the organic pores and microfracture generally underdeveloped.The NMR porosity calculated after deducting the base signal is 0.68%~3.66%,which has a good matching relationship with Helium porosity.The samples with smaller porosity are more likely to be affected by the background signal of rock matrix,resulting in the relative error of test results.In generally,the nuclear magnetic resonance technology can accurately analyze the porosity and pore size distribution of low porosity and low permeability shale samples,but attention should be paid to the NMR signal interference caused by organic matter and clay mineral bound water.