Investigation on pore structure and multifractal of tight sandstone reservoirs in coal bearing strata using LF-NMR measurements

Abstract

In this study, pore structures of tight sandstone reservoirs in coal bearing strata were evaluated by combining the low field nuclear magnetic resonance (LF-NMR) measurements with multifractal theory. The effects of petrophysical properties, pore structure parameters, and reservoir components on multifractal parameters in different probability measure areas were then discussed. The results show that tight sandstone reservoirs in coal bearing strata have complex pore structure, low porosity (2.43%–5.18%), low permeability (0.0005–0.0125 mD), and poor pore connectivity with significant multifractal characteristics. Multifractal characteristics are well correlated with petrophysical property, pore structure, and lithology, which is useful for evaluating pore structure heterogeneity of such tight sandstone reservoirs. Moreover, the multifractal parameters of different probability measure areas are strongly associated with petrophysical parameter porosity, whereas the multifractal parameters of low probability measure areas αmax, (Dmin-D0) are weakly related to the characteristics of pore structure parameters and reservoir components. This suggests that the multifractal parameters of the high probability measure areas are more sensitive than other multifractal parameters to the pore structure, and therefore can be chosen for effective pore structure characterization. Additionally, other parameters A, (D0-D1), and (D0-D2) can also reflect the property of pore structure to a certain extent.