核磁共振测井在火成岩地层应用的适应性分析

火成岩岩石具有高磁化率特征,与沉积岩差别明显,岩石的高磁化率会对核磁共振信号产生显著影响,开展核磁共振测井在火成岩地层应用的适应性分析研究,为广泛使用核磁共振测井用于火成岩储层评价提供参考.采用理论模拟、实验分析及实际测井资料相结合的方法,分析了高磁化率岩石的核磁共振响应特征.分析结果表明,火成岩岩石具有很高的磁化率,从酸性火成岩到基性火城岩,岩石的磁化率一般是逐渐增大的.高磁化率岩石孔隙内部会产生强梯度磁场,孔隙内部的磁场梯度越强,核磁共振的T2谱前移越明显,核磁信号衰减幅度也越大,核磁分析孔隙度与常规分析的孔隙度误差也越大;相同岩石磁化率情况下,岩石孔径越小,岩石孔隙内部磁场梯度越强,核磁信号衰减越大,核磁共振测井计算的孔隙度也越低.因此,核磁共振测井与岩性有关系,核磁共振测井会受到高磁化率岩石的显著影响.核磁共振测井在部分火成岩,尤其在中基性火成岩和小孔径火成岩储层中应用具有很大的局限性.

Analysis of adaptability of application of NMR logging in igneous rock reservoirs

Igneous rocks are characterized by high magnetic susceptibility.Significant difference in magnetic susceptibility was observed between igneous rocks and sedimentary rocks.The magnetic susceptibility of a great part of igneous rocks is higher than a great part of sedimentary rocks.NMR signal can be significant affected by the rock frame of igneous rocks because of the high magnetic susceptibility.A study on the analysis of adaptability of application of NMR logging in igneous rock reservoirs has been carried out and provided the reference for the evaluation of igneous rock reservoirs using Nuclear magnetic resonance logging.In the study,NMR response characteristics of the rocks with high magnetic susceptibility have been analyzed by using the synthetic means of theoretical simulation,experimental analysis and analysis of actual logging data.The results indicate that a part of igneous rocks have high magnetic susceptibility,and the magnetic susceptibility of igneous rocks increased gradually from basic to acid igneous rocks.The igneous rocks with high magnetic susceptibility can generate internal magnetic field gradient in porous media.The stronger internal magnetic field gradient in porous media is,the clearer characteristics of NMR T2 spectrum shifting forward is.The stronger internal magnetic field gradient in porous media is,the greater the magnetic signal attenuation is.The stronger internal magnetic field gradient in porous media is,the greater the error between conventional analysis porosity and NMR porosity is.Under the same circumstances of magnetic susceptibility of rocks,the smaller pore size is,the stronger internal magnetic field gradient in porous media is,and the greater the magnetic signal attenuation is,and the lower the NMR porosity is.Therefore,NMR logging is closely connected with lith,and NMR signal can be significant affected by the rocks with high magnetic susceptibility.It has obvious limitations on igneous reservoirs,especially in the basic igneous rocks and the igneous reservoirs with very small pore throat size.