Insight into the Pore Structure of Tight Sandstones Using NMR and HPMI Measurements

Laboratory measurements including porosity, permeability, high-pressure mercury intrusion (HPMI), nuclear magnetic resonance (NMR) measurements, and microscopic analysis of thin sections and scanning electron microscopy (SEM) were performed to provide insights into the microscopic pore structure of the Xujiahe Formation tight sandstones in Sichuan Basin. The relationships between microscopic pore structure parameters, such as pore geometry, pore size distribution, pore network, and macroscopic consequences, such as permeability, reservoir quality index, Swanson parameter, fractal dimension as well as NMR parameters, have been investigated. The results show that the pore systems are dominated by secondary dissolution porosity with minor amounts of primary porosity and microfracture. NMR T-2 pore size distributions are either uni- or multimodal. Long T-2 components are not frequently present due to the lack of the macropores, whereas as shorter T-2 components dominate the T-2 spectrum. T-2gm (the geometric mean of the T-2 distribution) shows good correlations with movable porosity and irreducible water saturation. The pore throat distributions from HPMI analysis show uni- or multimodal, and are narrower than the NMR T-2 pore size distribution. The r(apex), which is the apex of the hyperbola of Pittman (1992), is well correlated with the entry pressure, r(35) and r(50). The pore throats larger than r(apex), which only account for a small fraction of the pore volume, dominate the permeability of the reservoir rocks. Skin effect, high working pressure as well as oversimplification of cylinder pore shapes result in the high fractal dimension of the larger pores (>r(apex)). The smaller pores, which can be quantitatively characterized by the fractal dimension, control the microscopic heterogeneity of reservoir rocks. The reservoir quality index (RQI) shows good relationships with both the NMR parameters, such as T-2gm. and the HPMI parameters, such as r(apex) Integration of routine core analysis with HPMI test and integration of routine core analysis with NMR measurements show that the RQI, which links the pore-throat sizes resulted from HPMI tests with the pore-size distribution from NMR measurements, is a good indicator to reservoir heterogeneity in terms of macroscopic reservoir property and microscopic pore structure.