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.