Pore structure evaluation in ultra-deep tight sandstones using NMR measurements and fractal analysis

Abstract

The reservoir quality, pore spaces and pore size distributions of Bashijiqike sandstones in Kuqa depression were described by thin sections, SEM (scanning electron microscopy) analysis and nuclear magnetic resonance (NMR) tests. In order to quantitatively characterize the complexity of pore structure, NMR T2 (transverse relaxation time) spectrum was used for fractal analysis. This study unravels the relationships between petrophysical parameters, NMR parameters and fractal dimension of the ultra-deep tight sandstones. Primary intergranular pores are the main component of pore spaces, followed by secondary dissolution pores and micropores, and there contain microfractures. The NMR T2 spectra are either bi-modal or uni-modal distribution. The uni-modal T2 spectrum reflects uniform pore spaces. The coexistence of intragranular pores and intergranular pores leads to the bi-modal T2 spectrum. There is a positive correlation between fractal dimensions and T2gm (geometric mean of the T2 distribution). Consequently, four pore structure types are determined according to irreducible water content, T2gm, RQI (reservoir quality index), and the characteristics of individual pore structure are summarized. Because of the uniform pore space, Type I and Type IV have the lowest fractal dimensions. The structure of Type II and Type III are most heterogeneous due to their combination of intergranular and intragranular pores. The results help clarify the internal relationships between petrophysical parameters and microstructure, and have implications for pore structure evaluation of ultra-deep sandstones worldwide.