Key issues and reflections on the helium porosity determination of continental organic-rich shales

The current methods of helium porosity measurement for continental shales tends to yield underestimated porosity due to insufficient equilibration time. Based on the experimental principle of helium porosity determination, we conduct an ultra-long-term (60 h) systematic test using the helium expansion method on shale samples of varying specifications under different injection pressures, based onthe continental freshwater lacustrine basin shale reservoir system. Accordingly, a helium porosity determination method for continental organic-rich shales is proposed. The results indicate that the key factors influencing helium porosity measurement include pore structure, equilibration time, temperature, and injection pressure, which significantly restrict the degree of helium saturation. A prolonged equilibration time can effectively enhance the accuracy of helium porosity measurement. For shale samples from the 1st member of the Cretaceous Qingshankou Formation (also referred to as the Qing 1 Member) in the Songliao Basin, the porosity measured under a test duration of 60 h increased by 19.50%~37.64% compared to that measured under a test duration of 25 min. Meanwhile, for the shale samples from the 3rd oil sub-group of the 7th oil group of the Triassic Yanchang Formation (also referred to as the Chang 73 oil sub-group) in the Ordos Basin, the porosity measured under a test duration of 60 h increased by 20.44%~45.10% compared to that obtained under a 25-min test. Crushed samples can effectively shorten the time for pressure equilibration for tests. It is recommended that the grain sizes of crushed samples should be 3-4 orders of magnitude of the dominant pore sizes. The helium saturation can be enhanced by extending the pressure equilibrium time and increasing the injection pressure. Furthermore, the experimental errors caused by the deviation of helium molecules from their ideal state can be reduced by introducing the compression factor, correction of weakly connected pores, and residual fluid correction. Shale reservoirs formed under different sedimentary systems exhibit different physical properties, pore types, and pore size distribution, as well as varying degrees of modification during the diagenetic evolution process. Therefore, the test conditions and experimental parameters for the helium porosity measurement should be determined based on the specific characteristics of shale reservoirs. It is recommended that the equilibration time should be set at equal to or longer than 36 h for shales from the Chang 73 oil sub-group and equal to or longer than 48 h for those from the Qingshankou Formation. A helium porosity-time prediction chart is developed in combination with numerical simulation, which can help reduce test costs and improve test accuracy.