Objective Inland oil fields in China are progressively reaching the middle or late stages of exploitation, with the proportion of free and emulsified water in the surface gathering and transportation system constantly increasing. Under certain operating conditions, water accumulates at the bottom of gathering pipelines, creating a corrosive environment that heightens the risk of corrosion failure in the system.

Methods A numerical simulation study was conducted to analyze the formation process of water accumulation in oil-water two-phase flow for gathering pipelines with medium to low water content. The formation of bottom water accumulation was simulated under varying pipe diameters, flow velocities, water contents, and inclination angles. The volume fraction and location of water accumulation under different operating conditions were determined, and the influencing factors and formation law of water accumulation at the pipeline bottom were examined. Based on flow state statistics, a table of flow pattern and water accumulation state for oil-water two-phase flow was prepared, providing a reference for corrosion prevention and control in the production and operation of gathering pipelines with medium to low water content.

Results The formation of bottom water accumulation in gathering pipelines with medium to low water content occurred in four stages: flocculation, agglomeration, precipitation, and sedimentation. Under low flow velocities (below 0.5 m/s) and high water contents (above 30％), sedimentation gradually transitioned to stratified flow. Across all simulated conditions, the volume of bottom water accumulation was negatively correlated with flow velocity, with countercurrent sedimentation at elbows becoming more pronounced when the flow velocity was below 0.5 m/s. The volume of water accumulation was positively correlated with water content, with the greatest impact observed when water content ranged from 10％ to 20％. Changes in inclination angle and pipe diameter initially increased and then decreased water accumulation. Three flow states were identified: bottom water accumulation, homogeneous (dispersed) flow, and stratified flow. In simulations of 360 pipeline groups, bottom water accumulation and stratified flow occurred in 180 cases, representing a 50％ probability. These states created favorable conditions for pipeline corrosion, which, in severe cases, led to corrosion failure.

Conclusion The research findings offer a valuable reference for understanding the formation law of two-phase flow water accumulation in gathering pipelines with medium to low water content. Future mitigation of bottom water accumulation can be achieved by adjusting production and operational parameters of these pipelines.