Insight into porosity of shale oil reservoirs: Comparison of helium, low-temperature nitrogen adsorption−desorption, and nuclear magnetic resonance methods

Porosity is a fundamental parameter in characterizing the pore structure of shale oil reservoirs, as it directly affects the accuracy of shale oil reserve estimations. Despite the availability of various measurement techniques, accurately quantifying porosity in such reservoirs remains a significant challenge. In an effort to identify the most effective porosity testing method, this study collected samples from four shale oil reservoir intervals across five sags in three different basins. Five porosity testing methods were employed to detect shale porosity, including helium porosity, low-temperature nitrogen adsorption−desorption (LTNA/D), oil-saturated wetting, and nuclear magnetic resonance (NMR) T₂ and T₁−T₂. NMR T₂ porosity acted as a touchstone against which the other methods were compared. The pros and cons of each evaluation technique were explored to select the optimal analysis method for shale oil reservoirs. Results indicate that LTNA/D porosity, derived from powdered samples, commonly fails to reflect shale porosity effectively. Helium porosity, widely used for detecting nanoscale pores, is constrained by extended equilibration times and the retention of residual pore fluids after oil washing and drying, leading to systematic underestimation. In contrast, oil-saturation wetting and NMR T₂ exhibit strong agreement, both reflecting pore fluid content. However, residual fluid distribution can also impact the accuracy of NMR T₂ measurements. NMR T₁–T₂ is an innovative technique for quantitatively evaluating shale oil reservoirs. NMR T₁–T₂ spectrum at the water and oil restoration state can provide accurate shale porosity. NMR T₁–T₂ porosity estimates generally align with those obtained from T₂ porosity. When residual pore fluids are not entirely removable, the NMR T₁–T₂ method offers a more realistic porosity assessment. The NMR technique is recommended for evaluating the porosity of shale oil reservoirs, and the combination of T₂ and T₁–T₂ can accurately determine the effective and total porosity. This research serves as a valuable reference for accurately determining porosity in shale oil reservoirs.