渝东南地区下寒武统牛蹄塘组页岩孔径分布测试方法研究

低温氮气吸附法已普遍用于评价页岩的孔隙结构,但是文献中报道的脱气温度和脱气时间等测试条件不尽相同,此外测试结果中经常出现滞后环不闭合的情况,严重影响了孔径分布等结构参数测试的准确性。为了评价和探索有效的页岩孔径分布测试方法,本文选取渝科1井下寒武统牛蹄塘组黑色页岩,重点研究应用低温氮气吸附法分析其孔径分布的测试条件,系统探讨了样品质量、脱气温度、升温速率和脱气时间对测试结果的影响,通过进一步优化参数建立了可靠的孔径分布测试方法,并成功用于测定其他页岩样品。结果表明:吸附-脱附等温线呈反C型,属于Brunauer等提出的BDDT等温吸附曲线分类中的V型,滞后环完全闭合,属于IUPAC分类中的H4型,对应狭窄的狭缝型孔隙,说明此类页岩中除了含有峰值孔径主要集中在3.5~4.5 nm的中孔和一定数量的大孔外,还存在大量微孔。研究认为脱气温度是影响测试结果的主要因素。该研究是页岩孔径分布测试方法的一项补充,为我国页岩气的深入研究提供了关键参数支持。

Study on Analytical Method for Pore Size Distribution of the Lower Cambrian Niutitang Formation Shale in Southeastern Chongqing

BACKGROUND: The low-temperature nitrogen (N₂) adsorption-desorption method has been widely used to evaluate the pore structure of shale, however, the different analytical conditions such as degassing temperature and degassing time were reported in the literature. In addition, closure of the hysteresis loop often occurs in the test results, which seriously affects the accuracy of the structural parameters such as pore size distribution.BAOBJECTIVES:To evaluate and explore an effective method for measuring pore size distribution in shale, using the black shales from the Department of Chongqing 1 well Lower Cambrian Niutitang Formation.METHODS: Measurement conditions using low-temperature N₂ adsorption method to analyze the shale pore size distribution. The effects of sample weight, degassing temperature, heating rate, and degassing time on the analytical results were systematically discussed. A reliable measurement method of pore size distribution is established by further optimizing the parameters, which has been successfully used in the determination of other shale samples.RESULTS:The results show that the isothermal adsorption-desorption curves of the samples are inverse C type, which belong to the V type of BDDT isotherm adsorption curve proposed by Brunauer et al. The hysteresis loops belong to the H4 type in the IUPAC classification, which corresponds to the narrow slit-like pores, indicating that there are a large number of micropores in the shale, in addition to the mesopores with a peak pore size concentrating at 3.5-4.5 nm and a certain number of macropores.CONCLUSIONS:Degassing temperature is the main factor affecting the analytical results. This study complements the exploration on the testing method of pore size distribution in shale and provides the key parameters for further study of shale gas in China.