Effect of saturated brine with high concentration on nuclear magnetic resonance (NMR) data in high-porosity sandstone reservoirs

Recent field applications of nuclear magnetic resonance (NMR) logging in wells that drilled with salt mud illustrate a lack of precise formation porosity and pore structure characterization because of the effect of mud filtrate concentration. In this study, 33 core samples were chosen for laboratory NMR experiments to demonstrate the effect of saturated brine’s concentration on NMR T2 responses. These core samples were separately recovered from tight, low-permeability sandstones and conventional reservoirs in two adjacent regions A and B. Each core sample was saturated with seven concentrations (0.0, 5.0, 20.0, 30.0, 50.0, 80.0, and 100.0 g/l) of brine. The experimental results illustrate that the NMR porosity and the shape of T2 distribution for core samples with porosity higher than 20.0% change significantly when the concentrations of saturated brine are higher than 80.0 g/l. An additional peak appears with the T2 relaxation time ranging from 11.0 to 40.0 ms, limiting the applicability of NMR logging in high-porosity sandstone reservoirs drilled with high-salinity mud. Based on the theoretical simulation, the relationship between the salinity and hydrogen index (HI) (defined as the ratio of the number of hydrogen atoms per unit volume to that of pure water at surface conditions) is established, and a method of correcting NMR porosity is proposed. Meanwhile, a model to correct the shape of the NMR T2 spectrum is also established. Based on the proposed method and model, the effect of saturated brine on NMR porosity and the shape of T2 spectrum in high-porosity reservoirs are eliminated. The accurate formation porosity, permeability, and NMR T2 spectra are acquired. Field applications of our target region B validate the reliability of the established method and model.