致密砂岩孔喉大小表征及对储层物性的控制——以鄂尔多斯盆地陇东地区延长组为例

Pore-throat size characterization of tight sandstone and its control on reservoir physical properties:a case study of Yanchang Formation,eastern Gansu,Ordos Basin

孔喉大小是评价储层质量的重要因素,而致密砂岩储层孔喉分布较强的非均质性使其表征难度较大。在分析目前孔喉表征方法的基础上,通过场发射扫描电镜、高压压汞和恒速压汞等实验,研究了鄂尔多斯盆地陇东地区延长组7段致密砂岩储层孔喉大小分布特征及其对储层物性的控制作用。结果表明:储层孔隙类型主要为剩余粒间孔、溶蚀孔、微孔和少量微裂缝;高压压汞表征孔喉的大小分布在0.014 8~40μm,大于1μm的孔喉分布较少;恒速压汞测试表明对于不同物性的样品其孔隙半径分布相对集中,主要分布在80~350μm;喉道半径则表现出较强的非均质性,分布在0.12~30μm;高压压汞和恒速压汞结合有效地表征了致密砂岩储层整个孔喉分布特征,孔径分布在0.0148~350μm。储层渗透率主要由比例较小的大孔喉(大于R_(50))所控制,对于渗透率大于0.1 mD的致密砂岩,其渗透率主要由微孔和中孔所控制,而小于0.1 mD的致密砂岩则主要由纳米孔和微孔所控制;此外,小孔喉的比例随着渗透率的减小而增加,虽然其对渗透率影响较小,但对储层储集性的影响却十分重要。

Pore-throat size is a key factor to evaluate reservoir quality,whereas the strong heterogeneity of pore-throat in tight reservoirs leads to a large difficulty in characterization.Based on analyzing the current pore-throat characterization methods,the experiments of field emission scanning electron microscope,high-pressure mercury injection（HPMI）and constant-rate mercury injection（CRMI）are performed to study the pore-throat size characteristics of tight reservoirs in 7 th Member of Yanchang Formation,eastern Gansu,Ordos Basin and their control on reservoir physical properties.The results show that reservoir pore types mainly include residual intergranular pores,dissolution pores,micropores and a few micro-fractures.Pore-throat sizes characterized by HPMI range from 0.014 8-40 μm,and the pore-throats with the radius1 μm are rare.The CRMI test reveals that for the samples with different physical properties,their pore radius distribution is relatively concentrated,ranging between 80 and 350 μm,whereas throat radius presents a strong heterogeneity,distributed between 0.12 and 30 μm.The overall pore-throat size distribution characteristics of tight reservoir are effectively characterized by integrating the HPMI and CRMI methods,and the pore sizes are found to range from 0.014 8 to 350 μm.Reservoir permeability is mainly controlled by large pore-throats（R_（50））with a small proportion.As for the tight sandstone with the permeability greater than 0.1 mD,the permeability is dominated by micropores and mesopores,while the tight sandstone with the permeability smaller than 0.1 mD is mainly controlled by nanopores and micropores.In addition,the proportion ofsmall pore-throats increases with the decrease of permeability.Although their impacts on permeability are small,they play an important role in contributing the reservoir porosity.