基于核磁共振自由弛豫特征的含油性评价方法——以玛湖凹陷下乌尔禾组砾岩储层为例

Oil-bearing evaluation method based on NMR free relaxation characteristics:A case study of conglomerate reservoirs of lower Urho Formation in Mahu Sag,Junggar Basin

准噶尔盆地玛湖凹陷二叠系下乌尔禾组砾岩储层具有低孔低渗、孔隙结构复杂、强非均质性的特点,常规电法测井和核磁共振差谱法均难以反映储层的含油性特征。为了解决这一问题,开展了轻质原油核磁自由弛豫测量与下乌尔禾组储层岩心含不同流体的核磁共振对比实验,实验显示:①轻质原油核磁共振自由弛豫T2谱横向弛豫时间起始于100.00 ms;②饱和水岩心核磁共振T2谱基本收敛于100.00 ms;③饱和油岩心核磁共振T2谱在100.00 ms后出现了与原油核磁共振T2谱相似的自由弛豫特征。这说明下乌尔禾组岩心以水润湿为主,饱和油岩心核磁T2谱大于100.00 ms的信号由轻质油的自由弛豫引起。将100.00 ms作为核磁共振测井含油信号的标志,构建敏感参数,建立了基于核磁自由弛豫特征的流体性质识别图版。在考虑渗透率对含油性影响的情况下,建立了含油饱和度计算模型。利用该方法对研究区22口探井31个层位进行了含油性测井评价,结果显示测井解释符合率达93.5%,应用效果良好。该方法为核磁共振测井在含油性测井评价中的应用提供了新的思路。

The conglomerate reservoirs of Lower Urho Formation in Mahu Sag of Junggar Basin have the characteri-stics of low porosity,low permeability,complex pore structure and strong heterogeneity.Conventional electrical logging and nuclear magnetic resonance(NMR)difference spectrum method are difficult to reflect the oil bearing characteristics of the reservoirs.To clarify the oil-bearing characteristics,NMR free relaxation measurements of light crude oil and NMR experiments of reservoir cores containing different fluids of lower Urho Formation were carried out.The results show that:①The transverse relaxation time of NMR free relaxation T2 spectrum of light crude oil starts from 100.00 ms.②The T2 spectrum of saturated water core converges to 100.00 ms.③The NMR T2 spectrum of saturated oil core shows free relaxation characteristics similar to that of crude oil after 100.00 ms.The above phenomenon shows that the core of study area is mainly wetted by water,and the signal of NMR T2 spectrum of saturated oil core greater than 100.00 ms is caused by the free relaxation of light oil.Taking 100.00 ms as the oil-bearing signal of NMR logging,the sensitive parameters were constructed,and the identification chart of fluid properties based on NMR free relaxation features was established.Considering the influence of permeability on oil-bearing property,a calculation model of oil saturation was established.The above method was used to evaluate the oil-bearing property of 31 layers in 22 exploration wells in the study area,the coincidence rate of logging interpretation approached to 93.5%,indicating that the method was effective.This method provides a new idea for the application of NMR logging in oil-bearing evaluation.