油润湿致密砂岩核磁共振弛豫机理与流体识别方法

核磁共振测井是一种能够进行流体识别的重要测量手段,岩石的润湿性对其具有重要影响。鄂尔多斯延长组长8段致密砂岩储层孔隙度低,渗透率差,为弱油润湿性,现有的核磁共振测井流体判别方法难以适用,解释符合率偏低。考虑润湿性因素的影响,研究了水湿、油湿以及混合润湿条件下的核磁共振驰豫机理,利用数值模拟计算了不同润湿性条件下核磁共振响应特征。分析发现,核磁共振差谱几何均值与有效孔隙度差对流体敏感,用这两个参数构建的交会图能有效区分油层和水层,利用核磁共振测井数据和构建的图版实现了储层流体类型的识别。应用实例表明,该方法能有效实现致密砂岩油润湿储层的流体识别,弥补了现有核磁共振差谱分析判识油润湿储层流体类型的不足。

Nuclear magnetic resonance relaxation mechanism and fluid identification in oil wet tight sandstone reservoirs

The nuclear magnetic resonance(NMR)logging is an important method for fluid identification.Rock wettability has an important effect on the NMR response.The tight sandstone reservoir of the Chang 8 member in the Yanchang Formation of the Ordos basin in China features low porosity,poor permeability,and oil wettability.Its NMR logging response is different from that of water-wettable rocks;thus,fluid identification is difficult.Considering the wettability effect,we investigated the NMR relaxation mechanism in porous media and derived its theoretical formulation.The NMR relaxation responses under different wetting conditions were simulated.The analysis of the results demonstrated that the T2 geometric mean of the NMR differential spectrum and the differences in effective porosity were sensitive to the presence of fluids.Moreover,it was shown that the cross-plot of the two parameters can be used to effectively distinguish the oil layer from the water layer.A fluid chart was constructed from the cross-plot and NMR logging data,so that an NMR logging interpretation method could be established.The application tests showed that the proposed method can effectively discriminate among the fluid types in oil-wettable tight sandstone reservoirs,thereby overcoming the limitations of the conventional NMR differential spectrum analysis technique.