Issues in shale oil core porosity measurement

The primary methods for shale porosity measurement are the fluid invasion method and the electron/X-ray radiation methods. Among them, radiation methods, due to certain limitations, are currently not widely applied. Since shale pore structures have nanometer-scale characteristics, helium gas, with its small molecular size, stable chemical properties, and excellent permeability, has become the most commonly used fluid medium for shale porosity measurement. The penetration of helium gas into shale is influenced by core sample size and pore connectivity. Bulk samples require a long time for pressure equilibrium due to their complex pore tortuosity, while grain samples exhibit improved nanopore connectivity in shale with more pore space detected, making the measurement more accurate and reliable. This study aims to determine the sample with the optimal particle size and explore the impact of solvent extraction on porosity under the complex interaction mechanisms of soluble organic matter, organic solvent, and nanopores in shale oil cores. Recent progress in shale porosity measurement is systematically reviewed, focusing on sample size and solvent extraction, and experimental analyses of the shale oil core porosity in the Cretaceous Qingshankou Formation in Songliao Basin are conducted. The research indicates that the combination of bulk sample apparent density with grain helium porosity achieves the best measurement result for shale oil core porosity. It is recommended to use samples with particle sizes that cover 3 to 4 orders of magnitude larger than the main pore diameter to ensure both sample representativeness and experimental efficiency. It isn't recommended to perform solvent extraction on shale oil cores and use low-temperature vacuum drying to remove soluble organic matter in pores, thereby improving the accuracy of porosity measurements.