常规及非常规储层岩石分形特征对渗透率的影响

Influences of fractal characteristics of reservoir rocks on permeability

为了刻画不同类型储层孔隙结构及其对渗透率的影响,以压汞、核磁共振及N2吸附实验为基础,基于分形理论对砂岩、煤岩及页岩的分形特征进行了分析,建立了砂岩、煤岩、页岩的等效毛管迂曲度模型,探讨了常规及非常规储层岩石分形特征参数对渗透率的影响。结果表明:砂岩、煤岩、页岩的分形维数主要为2.6~3.0,毛管平均迂曲度分形维数主要为1.1~2.3,分形维数大小与岩石渗透率具有负相关性;影响岩石渗透率的微观孔隙结构因素包括岩石的非均质性、孔喉分布、孔隙表面粗糙程度及毛管迂曲度等,其中毛管迂曲度对岩石渗透率的影响最大;煤岩的等效毛管迂曲度较小,砂岩中等,页岩较大。最后,利用分形理论模型对39组岩石样品渗透率进行预测,认为砂岩和煤岩的渗透率预测效果均较好,但对于具有强非均质性的页岩来说,其渗透率已经达到n D级,虽然预测结果具有一定的吻合度,但预测精度仍有待进一步提高。此次研究对深入探讨不同类型储层岩石微观渗流机理具有一定的参考价值。

In order to characterize the pore struc^tre of different types of reservoirs and cognize its influence on rock permeability, based on the mercury intrusion, nuclear magnetic resonance （NMR） and N2 adsorption experi- ments, the fractal characteristics of sandstone, coal and shale were compared and analyzed by using the fractal theory, the equivalent capillary tortuosity models were established, and the influences of fractal characteristic para- meters of conventional and unconventional reservoir rocks on permeability were discussed. The results show that the fractal dimension （Df） of the three kinds of rocks is in the range of 2.6-3.0, the capillary average tortuosity fractal dimension （Dr） is in the range of 1.1-2.3, and the fractal dimension is negatively correlated with rock permeability. The microscopic pore structure factors that influence the rock permeability include heterogeneity of the rock, pore throat size distribution, pore surface roughness and capillary tortuosity, among which the capillary tortuosity has the biggest influence on rock permeability. The equivalent capillary tortuosity of coal, sandstone and shale is relatively small, medium and big, respectively. Finally, the theory of fractal model was used to forecast the permeability of 39 sets of rock samples, the prediction results of sandstone and coal are good. But for shale with strong heterogeneity, its permeability reached nD level. Although the prediction results have a certain degree of coincidence, the prediction accuracy still needs to be further improved. The knowledge obtained in this paper has important reference value in in-depth knowing microscopic percolation mechanism of different kinds of reservoirs.