Study on the NMR Response Mechanism of Micro-Fractured Tight Sandstones

Micro fractures serve as important storage spaces and migration channels for fluids in tight sandstone reservoirs. In order to fully understand the NMR (nuclear magnetic resonance) response mechanism of micro-fractured tight sandstones, we constructed several digital cores of tight sandstones with different micro fractures and simulated their NMR responses under oil-water saturation conditions by employing the random walk method. The simulation results showed that the aperture and length of micro fractures as well as the water saturation in fractures were the main controlling factors in the NMR response of reservoir fluids. In addition, the increase in the aperture, length, and water saturation could result in an improvement in relaxation time and signal amplitude of the water peak. However, the relaxation time of the water peak was not affected by above factors for micro fractures with length over 100 μm. Furthermore, the relaxation time of oil peak was independent of aperture, length, and water saturation, oil content was reflected from signal amplitude, and the inclination angle of micro fractures exerted no impact on the NMR response. The research results revealed the NMR response mechanism of micro-fractured tight sandstone reservoirs and provided a theoretical basis for the identification of favorable sections with micro-fracture developed.