Numerical Investigations of NMR T1–T2 Map in Two-Phase Fluid-Bearing Tight Sandstone

Аннотация

Study of two-dimensional nuclear magnetic resonance (NMR) longitudinal and transverse relaxation time (T₁–T₂) responses of tight reservoirs and optimization of T₁–T₂ acquisition parameter are of significance for exploration and development of tight oil and gas reservoirs. In this paper, a tight sandstone digital model was generated using a micro-CT scanning technique, and the random-walk method was modified to simulate the T₁–T₂ measurements in a two-phase fluid-bearing tight sandstone. The echo data of the saturation-recovery Carr–Purcell–Meiboom–Gill pulse sequence were acquired and processed to obtain a T₁–T₂ map. We compared oil–water-bearing tight sandstone T₁–T₂ maps with gas–water-bearing tight sandstone T₁–T₂ maps, and analyzed how fluid types, various degrees of fluid saturation, acquisition parameters, and signal-to-noise ratio (SNR) of NMR data affected T₁–T₂ maps. The research indicated that NMR T₁–T₂ logging is more suitable for identifying gas-bearing reservoirs, rather than oil-bearing reservoirs. NMR-logging acquisition parameters have different influences on various fluid-bearing tight sandstone T₁–T₂ maps. Increasing either the magnetic field gradient or echo spacing leads to a leftward shift of the gas signal, but does not affect the water signal and the oil signal in a T₁–T₂ map; the T₁–T₂ map with a mixed group of wait time is better than that with a long or short group of wait time in providing fluid relaxation information. The NMR data with lower SNR result in stronger divergence of fluid signal in T₁–T₂ map, rendering it more difficult to identify the fluid according to the map, and a data SNR more than 21 is recommended.