Quantitative Evaluation Method of HPMI Pore-throat Distribution Based on NMR Calibration

Pore throat distribution is an important factor controlling the physical properties of low-permeability and tight sandstone reservoirs. The evaluation of pore throat distribution is based on the analytical methods of micro features of reservoirs, and it is necessary to synthesize multiple methods to quantify the pore throat distribution. Quantitative evaluation method of High Pressure Mercury Injection (HPMI) pore throat distribution was developed based on Nuclear Magnetic Resonance (NMR) calibration. Fluid states can be identified by the comparison of transverse relaxation time T₂ spectrum before and after centrifugation, which enables the redefining of T₂ boundaries (T₂₁ and T₂₂) of three pore components of NMR. Furthermore, pore throat is divided into irreducible fluid pore throat, transition fluid pore throat and movable fluid pore throat accordingly. T₂ boundaries are then converted to pore throat radius boundaries r₁ and r₂ by the correlativity between T₂ value and pore throat radius. Finally, pore throat distribution is evaluated quantitatively by statistics of the content of various types of pore throat (S₁, S₂ and S₃). This method combines the advantages of HPMI describing fluid states and NMR characterizing pore throat size. The method was applied to evaluate the pore throat distribution of low-permeability and tight sandstone reservoirs in the Huagang Formation of Xihu Depression. The average conversion coefficient C between T₂ value and pore throat radius is 0.0079, and pore throat radius boundaries r₁ and r₂ are 60 nm and 160 nm, respectively. Pore throat is classified into four types according to the content of various types of pore throat, which provides new parameters and thought for further reservoirs evaluation.