四川盆地龙马溪组页岩吸水特征及3种页岩孔隙度分析方法对比

为研究分析页岩的吸水特征，有效测量页岩孔隙度，选取四川盆地黔浅1井龙马溪组5件页岩岩心样品开展吸水实验，分别从每件岩心样品中钻取一组不同直径（或不同长度）的页岩柱体，测量和计算每组各个小柱体的饱和绝对吸水量及骨架体积，建立二者的线性关系，由斜率K值表征各页岩样品单位体积的饱和吸水量，再据此计算出各页岩样品的有效孔隙度为3.51%~8.90%。为评价该方法所确定吸水孔隙度的准确性，又测定了样品的氮气吸附孔隙度和氦孔隙度，三者进行对比分析。结果显示：样品氮气吸附孔隙度为2.38%~7.04%，全部小于吸水孔隙度，二者相差0.54%~1.96%不等，这可能是由于氮气吸附实验无法探测泥页岩中孔径大于350 nm的宏孔，导致测定结果不包含这部分孔隙所贡献孔隙度而偏低。样品氦孔隙度为4.55%~8.09%，与吸水孔隙度相差仅为0.23%~0.81%，二者具有良好的一致性和可对比性，特殊地，QQ?45号样品的氦孔隙度比吸水孔隙度大2.65%，这是由于页岩柱体含微裂缝所导致的误差，而吸水实验可快速识别出含有微裂缝的柱体，能有效避免测量误差。由此可见，页岩柱体吸水实验法在有效保留页岩原生孔隙结构的前提下，通过统计分析多个页岩小柱体孔隙度测定结果而获得了页岩样品整体的吸水孔隙度，受页岩非均质性影响小，更接近页岩实际孔隙度。龙马溪组页岩孔隙度的变化与TOC含量具有良好的正相关性，与黏土及脆性矿物的相关程度不等，表明有机质是控制龙马溪组页岩孔隙度变化的主要因素。

The characteristics of water uptake and the comparative studies on three methods of determining porosity in organic-rich shale of Longmaxi Formation in Sichuan Basin

An experimental investigation of water uptake on five shale core plugs of Silurian Longmaxi Formation from Well Qianqian 1 in Sichuan Basin was conducted. The saturated water quantities on a series of shale cylinders with different diameters or lengths from one sample were determined, and a linear relationship between water uptake and the skeleton volumes of the shale cylinders was obtained. The slope of line, which is “K” value, represents the saturated water quantity per unit skeleton volume, and the water uptake porosity which is calculated by multiplying “K” value and the rock bulk density is from 3.51% to 8.90%. In order to evaluate the accuracy of the water uptake porosity determined from water uptake method, the nitrogen adsorption porosity and helium porosity of the same series of samples were comparatively measured. The results show that the nitrogen adsorption porosity ranges from 2.38% to 7.04%, which is less than the water uptake porosity, and the difference is about 0.54%-1.96%. The low-temperature nitrogen adsorption method failed to detect macropores with apertures larger than 350 nm in the shales, leading to the lower values of the nitrogen adsorption porosity without accounting the pore volumes of those macropores. GRI helium porosity ranges from 4.55% to 8.09%, and there are good consistency and comparability between the water uptake porosity and the helium porosity except for sample QQ-45, and the difference is only 0.23%-0.81%. For the sample QQ-45, the helium porosity is 2.65% larger than the water uptake porosity, and the difference is attributed to the microcracks in the shale cylinders. The water uptake experiment can discern the microcracks in the shale cylinders based on the rapid increase of water uptake curve at the early stage of water adsorption. The shale cylinders effectively retain the original pore structures and pore networks of the shales, and the water uptake porosity is a statistical result which was measured with varied sizes of shale cylinders and affected slightly by the shale heterogeneity. Therefore, water uptake porosity is more representative to the actual porosity of the shales. A good positive correlation exists between total organic carbon（TOC） and porosity values, but no direct correlation with clay minerals, suggesting that TOC is the one of key controls on the change of the Longmaxi shale porosity.