Analysis of Petrophysical Characteristics and Water Movability of Tight Sandstone Using Low-Field Nuclear Magnetic Resonance

Low-field nuclear magnetic resonance (NMR) coupled with gas–water centrifugation (GWC) was performed to investigate petrophysical characteristics and water movability of the Chang 7 tight sandstone. Microscopic observation, mineral composition analysis and grain size analysis were applied to determine lithology and pore characteristics. Then, the controls of petrophysical properties and geological factors on the water movability were discussed. The results indicate that the T2 distribution of the Chang 7 tight sandstone can be divided into three groups according to the shape, range and amplitude of T2 spectrum. The removable water volume increases with the centrifugal force, but its gradient presents a remarkable difference among the different reservoir types. The final removable water volume (Φmov) at centrifuge force of 400 psi (average value of 5.28%) is approximately half of the total pore volume (porosity). The T2 geometric mean (T2gm), fractal dimension (Df) and NMR permeability calculated from NMR data can be used to characterize the pore structure and petrophysical properties of tight sandstone. The reservoir quality index can also be estimated based on the NMR permeability models. Removable water volume under the different centrifugation conditions is positively related to porosity, permeability and T2gm and negatively related to Df. Furthermore, removable water volume under different centrifugation conditions is positively related to detrital mineral (quartz + feldspar) content and grain size but negatively related to clay content.