Fluids flow behaviors of nitrogen and foam-assisted nitrogen floods in 2D visual fault-karst carbonate reservoir physical models

Abstract

Fault-karst fractured-vuggy carbonate reservoir is an important type of fractured-vuggy carbonate reservoirs in Tahe oilfield of China. The fluids flow behaviors in fault-karst fractured-vuggy carbonate reservoir are complex and unclear due to the complexity of the connection between fractures and vugs. In this work, the novel 2D visual fault-karst fractured-vuggy carbonate reservoir physical models (longitudinal model and horizontal model) were designed and fabricated to investigate the flow characteristics and phase behaviors of nitrogen gas and foam-assisted nitrogen gas floods. Experimental results showed that oil-water and gas-oil interfaces fluctuations had positive effect on improving oil recovery. Foam-assisted nitrogen gas can effectively inhibit the occurrence of gas channeling, improve swept volume and increase oil recovery. A four-phase dimensionless index (Nr’) was first proposed to describe the synergistic effects among the gas cap, foam cap and bottom water during foam-assisted nitrogen flooding process. Based on the analysis of oil recovery results, there was a turning point on oil recovery curve for foam-assisted nitrogen gas flooding. Moreover, the optimal production mode that was injection in high position of reservoir and recovery from low position of reservoir (HI-LR) in fault-karst carbonate reservoir was determined. This study is helpful to understand the flow characteristics of foam and nitrogen gas and provides a guide for increasing oil recovery in fault-karst fractured-vuggy carbonate reservoirs.