Innovative Applications of Nuclear Magnetic Resonance in Probing the Pore-Structure of Tight Sandstone Reservoirs

Abstract

Pore-structure plays an important role in controlling the storage and percolation of gas in tight sandstone reservoirs. Nuclear magnetic resonance (NMR) has been commonly used to qualitatively characterize pore size distribution (PSD) and connectivity and evaluating permeability, however, its application in tight sandstones faces great challenges due to the smaller PSD and poor pore connectivity, which must rely on the help of other experiments, such as N₂ adsorption (NA) and rate-controlled porosimetry (RCP). Thirty-five Lower Cretaceous tight sandstone samples, from the Songliao Basin in China, were measured by NA, RCP and NMR for broadening the effective application of NMR in revealing pore-structure of tight sandstones.

By comparing the experimental results of NA, RCP and NMR, the better correlation between NMR-derived T₂ spectra and RCP-derived throat size distribution (TSD) was found in tight sandstones with different petrophysical properties, which can be used to calibrate NMR T₂ distribution of tight sandstones; A new approach to characterize the full-range PSD by combining NMR and NA was established, showing that low clay and high clay tight sand samples contain the pores in main range of 0.2-20 μm and 10-300 nm, respectively.

The full-range PSD when compared with RCP-derived TSD can be used to investigating the pore-throat connectivity. Based on the fractal features of TSD, the pore bodies’ distribution can be distinguished from the full-range PSD by subtracting TSD. We found that throats are more common than pore bodies in pore spaces of lower permeability tight sands, the pore-throat ratio (PTR) controls the fluid mobility of pore bodies, and larger PTR are mainly produced by compaction and pore-bridging clay cementation.

Based on throat fractal inflection point and displacement pressure, pores were divided into mesopores, micropores and nanopores, which takes into account the differences in pore types and connectivities. The contribution of different scales of pores to percolation and storage can be investigating with NMR-derived PSD. We found that micropores dominate the permeability of tight sandstones, but the effects of nanopores must be considered with decreasing the permeability. Based on the contribution of nanopores and micropores, a new permeability model was established.

These above innovative applications of NMR can help to better understand the storage and percolation mechanism of tight gas sandstone reservoirs.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018