The quantitative evaluation method of low permeable sandstone pore structure based on nuclear magnetic resonance (NMR) logging
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摘要: 低渗透砂岩油气藏已成为油气增储生产的重要勘探开发目标,但孔隙结构复杂使得储层及其有效性难以准确识别.笔者利用物性、压汞、核磁等资料,对东营凹陷南坡沙四段(Es4)低渗透砂岩孔隙结构进行分析,划分出了3种类型.核磁T2谱与毛管压力曲线都在一定程度上反映孔喉分布,但常规方法利用T2谱重构伪毛管压力曲线所得到的孔隙半径与压汞孔喉半径有较大误差,而岩石孔隙自由流体T2与压汞孔喉分布对应关系更好,以此建立了不同孔隙结构类型二者之间不同孔喉尺度对应的关系式(大尺度:线性;小尺度:分段幂函数),可在井筒剖面上通过识别孔隙结构类型,进而利用核磁共振测井(NML)定量反演孔径分布,省去了构建伪毛管曲线环节,为低渗透砂岩储层有效性评价提供了直接依据,也是测井用于定量反演储层微观孔隙结构信息的有益探索.Abstract: The low permeability sandstone reservoir, which has been an important target of exploration and development for oil and gas increase in reserves and production, is difficult to identify accurately because of the complex pore structure. The pore structure classification of complex low permeability sandstone and the investigation of the petrol-physical diversity of rock samples in different types are helpful to determine the reservoir type and the fluid properties of low permeability sandstone reservoir.#br#After the analysis of the pore structure based on physical properties data, mercury injection, nuclear magnetic resonance data and the considering of the properties division level standard of clastic reservoir (SY/T 6285-2011), the low permeability sandstone of Es4 in the southern slope of the Dongying sag is divided into three types of pore structures. And the diversity of every type in mercury injection, nuclear magnetic resonance (saturated T2 spectrum, centrifuged T2 spectrum and free-fluid T2 spectrum), the pore throat distribution, the porosity and permeability are discussed. Many results indicate that the T2 spectrum and capillary pressure curves could reflect the pore throat distribution in some degree. Meanwhile the T2 spectrum could be used to evaluate the pseudo capillary pressure curve to get the pore throat radius. However, it's large of the deviation between the pore throat distribution from this way and mercury injection because of the membrane bound-water affect. In fact, the free-fluid T2 spectrum and the pore throat distribution of mercury injection correspond much better. Based on this feature, after the comparing of the free-fluid T2 spectrum and the pore throat distribution of mercury injection in different pore structure types, the conversion relationship is established between the relaxation time and the pore throat distribution of mercury injection in different pore structure types and the pore size scale (large scale-linear relationship; small scale-piecewise exponential function). Meanwhile, the electrical standards and identification methods of every pore structure are also established based on the cross-plot analysis of logging response.#br# Therefore, the pore structure could be identified along the well hole, and then the pore throat distribution of different pore structure can be quantitatively calculated by using nuclear magnetic resonance logging data. Not only the segment of pseudo-capillary curve's building is avoided, but also the identification of pore structure could be more effective. Overall, this research provides a direct evidence to recognize the low permeability reservoir and sheds a new light on quantitative reconstruction of microscopic pore structure with well logging.
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Key words:
- Nuclear magnetic resonance /
- Low permeable sandstone /
- Mercury injection /
- Free fluid /
- Pore structure
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