Abstract:

In fault block G76 of the Jidong Oilfield, issues such as increased injection pressure and difficulty in water injection have arisen during the waterflooding development process. To analyze the variation in reservoir properties during water injection, high multiple waterflooding experiments were conducted on cores using two-dimensional nuclear magnetic resonance (NMR) technology. Laser particle size analysis was performed on the target reservoir cores to obtain particle size distribution, and X-ray diffraction (XRD) analysis was conducted to determine mineral content proportions. High multiple waterflooding experiments based on NMR technology were carried out to analyze reservoir property variations. The results showed that core 5-1 and core 6-1 consisted of medium sand-bearing silty fine sandstone and silt-bearing medium sandy fine sandstone, respectively, with high contents of fine sand, silt, and clay minerals. The relative permeability of the water phase and NMR porosity initially increased with cumulative water injection to a high value and then declined. In the NMR T₂ spectrum, the right endpoint values and the curves corresponding to medium and large pores shifted left as water injection increased. In the two-dimensional spectra, the free water signal intensity increased with cumulative water injection. As the injected water transitioned from bound water to a cumulative injection of 500 PV, the bound water signal continuously increased. When the cumulative injection is beyond 500 and up to 1 000 PV, the bound water signal of core 5-1 continued to strengthen, while that of core 6-1 weakened. The study suggests that, in the early stages of water injection, weak hydration of clay minerals occurs. In the later stages, due to water flushing, fine silt particles and clay minerals in the cement may detach and migrate to pore throats, causing blockage and damage to the pore throat structure, thereby reducing water phase permeability. The findings reveal the reasons for injection difficulty and increased pressure during waterflooding in medium-low permeability oil reservoirs and provide guidance for mitigating contamination and improving the effectiveness of waterflooding development.

Key words: medium-low permeability oil reservoir, water phase permeability, high multiple waterflooding, in-situ NMR, clay minerals