Combining nuclear magnetic resonance and rate-controlled porosimetry to probe the pore-throat structure of tight sandstones

Rate-controlled porosimetry (RCP), nuclear magnetic resonance (NMR) and micro CT tests were conducted on five tight sandstone samples, the results were compared to reveal the limitations of RCP in determining the pore size distribution (PSD) and pore to throat ratio (PTR), and then an effective method to test the pore-throat structure of tight sandstone by combining NMR and RCP was proposed. The quasi-PSD derived by RCP was bimodal, the left peak of which corresponding to throats was in good agreement with that of NMR, while the right peak corresponding to pore bodies had similar volume content and different distribution range with that of NMR. RCP reflected an equivalent spherical radius of pore body, and the calculation was significantly larger than the maximum inscribed radius (MIR) of the actual pore body with the same volume; whereas in NMR, the ratio of pore volume to surface area was used to estimate pore radius, and the pore radius calculated was close to MIR. The full-range pore body size distribution was determined by subtracting the RCP-derived throat size distribution from NMR-derived PSD, and then the pore throat connectivity was evaluated comprehensively. The mean value of PTR calculated by RCP was larger than 100.0 due to the differences in the calculation method between pore body size and throat size in RCP, while the mean value of PTR calculated by combining NMR and RCP ranged from 7.5 to 64.0. It is concluded that the combination of RCP and NMR experiment is an effective way to comprehensively reveal the size distribution of pore body and throat in tight sandstone.