The Mechanism of Reservoir Damage by Water Injection in Ultra-Low-Permeability Reservoirs and Optimization of Water Quality Index

Injecting liquid into the formation has an impact on the microstructure of the reservoir and formation fluids, and negative effects often lead to the failure of oil well stimulation measures to achieve the expected results. It is crucial to clarify the reasons for the decrease in the injection capability of low-permeability reservoirs in China and the mechanisms of the impact of on-site injection water quality. This study first conducted injection experiments with different water qualities. To study the micro factors that cause damage, clay mineral X-ray diffraction (XRD) analysis, high-pressure mercury injection experiments before and after damage, nuclear magnetic resonance (NMR) during the damage process, scanning electron microscopy (SEM) after damage, and energy dispersive spectroscopy elemental spectrum analysis (EDS) of incompatible sediment were carried out on the experimental core. In injection experiments with different water qualities, the permeability decreased by up to 65.35% when the injection volume reached 60 PV. The main causes of the decrease in injection capability are poor reservoir porosity and permeability and formation particle blockage. The particles mainly come from suspended particles, emulsified oil, migration of formation particles, and sediment formed by the injected water. This paper also proposes a reference for water quality index optimization in similar reservoirs. The new water quality index reduced permeability damage by at least 3.22%.