页岩气储层总孔隙度与有效孔隙度测量及测井评价——以四川盆地龙马溪组页岩气储层为例

Measurement and logging evaluation of total porosity and effective porosity of shale gas reservoirs: A case from the Silurian Longmaxi Formation shale in the Sichuan Basin

在地层束缚水条件下,对页岩气储层有效孔隙度理解及测量方法尚无统一标准,由此造成同一样品的孔隙度测量结果显著差异,给储层评价带来困难。对来自四川盆地龙马溪组页岩气储层的平行岩心样品分别依据GRI和SY/T 5336—2006标准进行总孔隙度和有效孔隙度测量,并配套进行岩矿组分、粘土组分和有机碳含量(TOC)测量。通过实验数据分析认为,页岩气储层总孔隙度和有效孔隙度测量结果两者相差1~3倍,造成差异的根本原因是对与粘土有关的束缚水体积不同的处理方式;页岩气储层中充气孔隙度与TOC呈高度正相关关系,相关系数达到0.9以上,表明有机孔隙是页岩气储层中有效孔隙的主要贡献者;束缚水孔隙与粘土矿物含量呈现良好正相关关系,表明与粘土有关的微细孔隙被束缚水占据,为无效孔隙。基于上述认识,构建了有效孔隙定量关系。同时,基于干粘土体积模型建立了总孔隙度评价方法,利用测井资料确定了实际地层的总孔隙度和有效孔隙度。应用表明,测井评价结果与不同实验室测定的总孔隙度和有效孔隙度吻合较好。

There is no consensus on the understanding and measurement of effective porosity of shale gas reservoirs under irreducible water conditions in LAB. It gives rise to a great variance of core-measurement results,even on the same sample,and brings about difficulties and confusions in evaluation of shale gas reservoirs. To cope with the problem,both total and effective porosity are measured on paired-samples from the Longmaxi Formation in Sichuan Basin according to the criteria of GRI and SY/T 5336-2006,respectively. In addition,mineralogy,clay content and TOC are also measured.The results show that the total porosity and effective porosity of the paired-samples vary by a factor of 1 to 3. The large difference is mainly caused by the different treatment approaches of clay-related irreducible water volume in shale gas reservoirs. The gas-filled porosity（effective porosity） shows strong positive correlation with TOC. The coefficient of correlation is over 0. 9. It implies that the organic matter pore is the main contributor to effective porosity in shale gas reservoirs. The irreducible water-filled porosity shows significant positive correlation with clay content. It implies that the clayrelated micro-pores are occupied by irreducible water and are ineffective pores. A quantitative equation of effective porosity is established based on the above-mentioned understandings. Meanwhile,a logging evaluation method of total porosity is established based on the dry clay volume model. Both total porosity and effective porosity are calculated by using actual logging data,and the results coincide well with the core-measured results.