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.

当前针对陆相页岩的氦孔隙度（helium porosity）测量方法，常因平衡时间不足导致测得的孔隙度偏低。基于氦孔隙度测定的实验原理，以陆相淡水湖盆页岩储层系统为研究对象，采用氦膨胀法（helium expansion method）对不同规格的页岩样品开展了长达60小时的超长时系统测试，测试覆盖不同注入压力（injection pressure）条件。据此提出了适用于陆相富有机质页岩的氦孔隙度测定方法。研究结果表明，影响氦孔隙度测量的关键因素包括孔隙结构（pore structure）、平衡时间（equilibration time）、温度与注入压力，这些因素显著制约氦气饱和度（helium saturation）的实现程度。延长平衡时间可有效提升氦孔隙度测量的精度。以松辽盆地白垩系青山口组一段（简称青一段）页岩样品为例，60小时测试时长下测得的孔隙度，相较25分钟测试时长提升了19.50%~37.64%；针对鄂尔多斯盆地三叠系延长组7油层组3亚油层组（简称长73亚油层组）的页岩样品，60小时测试下的孔隙度相较25分钟测试提升了20.44%~45.10%。粉碎样品可有效缩短测试的压力平衡时长，建议粉碎样品的粒度应为主导孔隙尺寸（dominant pore sizes）的3~4个数量级。通过延长压力平衡时间与提升注入压力，可增强氦气饱和度。此外，引入压缩因子（compression factor）、校正弱连接孔隙（weakly connected pores）与残余流体，可降低氦分子偏离理想气体状态带来的实验误差。不同沉积体系形成的页岩储层，其物理性质、孔隙类型（pore types）、孔径分布（pore size distribution）存在差异，且在成岩演化（diagenetic evolution）过程中受到的改造程度各不相同。因此，氦孔隙度测量的测试条件与实验参数应结合页岩储层的具体特征确定。建议长73亚油层组页岩的平衡时间不低于36小时，青山口组页岩的平衡时间不低于48小时。结合数值模拟（numerical simulation）构建了氦孔隙度-时间预测图，可有效降低测试成本并提升测试精度。