Real-time NMR investigation of water infiltration mechanisms and pore structure evolution in fractured sandstone near-wellbore regions
Sheng-Feng Wu, Yongfa Zhang, Yu Zhao, Chao-Lin Wang, Jing Bi, Anfa Long, Xiaobin Li
Abstract
During the exploitation of sandstone gas reservoirs, natural fractures near the wellbore affect water infiltration and pore evolution; however, the impact mechanisms of these fractures remain unclear. This study utilized real-time nuclear magnetic resonance (RT-NMR) technology to investigate the influence of near-wellbore fracture angle ( α ) at 0°, 15°, 30°, and 45°on water infiltration, migration patterns, and pore evolution mechanisms during water injection. Throughout the experiments, T 2 curves and magnetic resonance imaging (MRI) were monitored in real time during the water injection process. The pore evolution and water infiltration were translated by the evolution of T 2 curves and MRI. The results show that increasing injection pressure ( P inj ) transforms adsorption pores into seepage pores, leading to enhanced pore damage. Pore damage predominantly occurs during the rapid pressurization stage and is concentrated around the near-wellbore fracture. The maximum infiltration area and rate were observed at α = 0°, while the minimum values occurred at α = 45°, which can be attributed to the significant influence of α on water infiltration and migration pathways. The increasing inclination of the infiltration front with α is attributed to the fact that the rate of water infiltration along the fracture wall is always higher than that at the fracture tip. In field fracturing, it is recommended to adjust the perforation direction to align with the natural fracture orientation and optimize pressurization strategies by reducing the slow pressurization duration while extending the rapid pressurization stage. These findings can provide important guidance for setting fracturing sections and optimizing injection parameters in sandstone gas reservoir exploitation.