Determination of movable fluid percentage and movable fluid porosity in ultra-low permeability sandstone using nuclear magnetic resonance (NMR) technique

• Ultra-low permeability cores show five patterns of T2 distribution.
• Bimodal distribution is the dominant T2 distribution.
• Movable fluid percentage correlates poorly with core porosity and permeability.
• Movable fluid porosity shows some correlation with core porosity and permeability.

In ultra-low permeability reservoirs, the movable fluid parameters (including movable fluid percentage and movable fluid porosity) are very important measures in that they can provide realistic assessment of the amount of fluid that can flow in the porous media. In this study, the movable fluid percentage and movable fluid porosity of 264 core samples obtained from the Yanchang group of Ordos basin (China) are determined using the nuclear magnetic resonance (NMR) technique. The test results show that the T2 distributions present five typical patterns: bimodal distribution with higher left peak and lower right peak (occupying 26.52%), bimodal distribution with lower left peak but higher right peak (occupying 37.50%), bimodal distribution with similar amplitudes of the two peaks (occupying 35.23%), unimodel distribution (occupying 0.38%), and triple-peak distribution (occupying 0.38%). The pore throat radius exists mainly over the ranges of 1–10 μm and 10–100 μm, while the T2 cutoff value is found to mainly distribute between 5.34 and 20.00 ms. The average movable fluid percentage and average movable fluid porosity of the 264 core samples are 48.35% and 5.43%, respectively. The movable fluid percentage shows almost no correlation with porosity; it varies dramatically when the permeability is low and tends to converge to a higher value with an increase in permeability. The movable fluid porosity exhibits a relatively good correlation with core porosity as well as core permeability. There is obvious difference between the movable fluid parameters and physical property in different reservoirs because of the difference in wettability, microcrack and pore structure. The movable fluid parameters are comprehensive reflection of micro-characteristics of ultra-low permeability reservoir.

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