Fluid identification and tight oil layer classification for the southwestern Mahu Sag, Junggar Basin using NMR logging-based spectrum decomposition

Abstract

The intricate distribution of oil and water in tight rocks makes pinpointing oil layers challenging. While conventional identification methods offer potential solutions, their limited accuracy precludes them from being effective in their applications to unconventional reservoirs. This study employed nuclear magnetic resonance (NMR) spectrum decomposition to dissect the NMR T2 spectrum into multiple sub-spectra. Furthermore, it employed laboratory NMR experiments to ascertain the fluid properties of these sub-spectra, aiming to enhance identification accuracy. The findings indicate that fluids of distinct properties overlap in the T2 spectra, with bound water, movable water, bound oil, and movable oil appearing sequentially from the low-value zone to the high-value zone. Consequently, an oil layer classification scheme was proposed, which considers the physical properties of reservoirs, oil-bearing capacity, and the characteristics of both mobility and the oil-water two-phase flow. When applied to tight oil layer identification, the scheme's outcomes align closely with actual test results. A horizontal well, deployed based on these findings, has produced high-yield industrial oil flow, underscoring the precision and dependability of this new approach.