火成岩核磁共振数值模拟与影响因素分析

Analysis on influence factors based on NMR simulation in igneous rocks

核磁共振(NMR)在储层孔隙结构评价和孔隙度计算方面具有明显的优势,但是在某些火成岩地层核磁共振孔隙度比实际孔隙度偏低,这就限制了核磁共振测井在该类储层的应用.针对这一问题,首先开展了核磁共振的弛豫机理研究,从理论上分析了磁化率引起的附加内部磁场梯度是核磁共振孔隙度偏小的主要原因.然后,模拟了不同磁化率、含不同流体岩石以及不同回波间隔的核磁共振横向弛豫时间(T2)分布,分析了磁化率对核磁共振孔隙度的影响.模拟结果表明,对同一流体的孔隙模型来说,磁化率越大,其引起的扩散弛豫在总弛豫中的比重越大,T2分布前移也越明显,计算的孔隙度就越小.此外,岩心实验也证明磁化率核磁共振的影响较大.此项研究剖析了火成岩核磁共振影响因素,为该类储层的核磁共振测井解释提供了依据.

Nuclear magnetic resonance（NMR）logging can be used to evaluate reservoir pore structure and to calculate the porosity,which is more advantageous over other logging techniques. But in igneous rocks, NMR porosity is underestimated comparing to the actual porosity,which limits the application of NMR logging in such reservoirs.Aiming at this problem,a relaxation mechanism of MRI is firstly explored, and theoretically analyzed additional internal magnetic field gradient induced by magnetic susceptibility leads to NMR porosity decreasing.Then,numerical simulation is applied to study the effects of magnetic susceptibility and T2 distribution responses are calculated.The simulation results show that for the same porosity model,the greater the susceptibility,the larger the proportion of the diffusion relaxation in total relaxation,and T2 distributions shifts forward more obviously,and the porosity calculated is smaller.This study analyzes the influencing factors of NMR logging in igneous formation,and is helpful for NMR interpretation.