Blockage mechanisms of suspended particles in fractured ultra-low permeability reservoirs and optimization of water quality standards for post-fracturing injection

The ecological and environmental management department requires the treatment of oilfield produced water to comply with the "Technical Requirements and Analysis Methods for Water Quality Indicators of Clastic Rock Reservoir Water Injection" SY/T 5329 − 2022. The water quality indicators of oilfield injected water (mainly suspended solids content, median suspended solids particle size, petroleum content, etc.) are strictly required, and the above water quality indicators are mainly determined based on the actual permeability of the reservoir. Hydraulic fracturing creates complex "pore-fracture" dual-media systems in ultra-low permeability reservoirs, but the blockage mechanisms of suspended solids during post-fracturing water injection remain unclear—rendering conventional homogeneous-media water quality standards inadequate. Using core samples and Brazilian splitting to simulate fractures, this study reveals distinct blockage behaviors of particles and oil droplets in fractured media. A critical particle-to-pore-throat diameter ratio (PC = 1.059) is identified to quantitatively distinguish internal pore blockage from external filter cake formation, bridging classical bridging theory with practical diagnosis. Fractures exhibit a dual effect: they mitigate overall permeability damage as preferential flow pathways, yet enable deep invasion of solids and oil, complicating subsequent removal. Orthogonal experiments with proppants demonstrate that stringent Class I standards can be relaxed to Class IV (particle size≤5µm, concentration≤25mg/L, oil content≤30mg/L) while maintaining permeability loss below 25%. By clarifying fracture-induced blockage mechanisms and validating relaxed standards, this study provides theoretical insights and practical guidance for cost-effective water injection in fractured ultra-low permeability reservoirs.