Online Nuclear Magnetic Resonance Analysis of the Effect of Stress Changes on the Porosity and Permeability of Shale Oil Reservoirs

In the process of volumetric fracturing of shale oil, physical property changes caused by stress changes will affect the recovery, so it is urgent to conduct an analysis and evaluation of physical property changes caused by stress changes. In this research, a physical simulation experiment with constant confining pressure and variable pore pressure combined with an online nuclear magnetic resonance (NMR) technique was constructed. The results show that the increased range of the macropore (IRM) and the total pore (IRT) of matrix samples is 11.70 similar to 13.24% and 10.54 similar to 14.27% with the increase of pore pressure, respectively. That of fractured samples are 7.94 similar to 8.48% and 9.21 similar to 10.51%. It indicates that increasing pore pressure is more effective for matrix reservoirs. The decreased range of the macropore (DRM) and the total pore (DRT) of matrix samples is 10.29 similar to 13.89% and 10.38 similar to 21.24% with the unloading of pore pressure, respectively. That of fractured samples are 8.49 similar to 9.66% and 10.91 similar to 11.10%. It indicates that unloading pressure has less influence on fractured reservoirs. The increased range of permeability of matrix rock samples is 99 similar to 120% with the increase of pore pressure. Greatly supplementing formation energy is an effective means to improve the permeability of matrix reservoirs. Compared with matrix reservoirs, unloading pore pressure is more damaging to fractured reservoirs.