Nitrogen Gas Flooding for Naturally Fractured Carbonate Reservoir: Visualisation Experiment and Numerical Simulation
Authors: Song Zhaojie , Hou Jirui , Liu Zhongchun , Zhao Fenglan , Huang Shijun , Wang Yong , Wu Jieheng , Bai Baojun 
Published: October 2016

Abstract
Tahe Naturally Fractured Carbonate Reservoir has implemented nitrogen gas flooding since 2013, with daily oil production of 7580 bbls and oil recovery increment of 0.83% by the end of 2015. However, the presence of fractures significantly affects gas swept volume and production performance, so large amount of oil reserves is poorly flooded due to gas channeling through fractures. The fluid flow mechanisms in fractured models were discussed in order to improve field gas flooding efficiency. Based on the geological constrains of Tahe Oilfield, fractured models with different apertures were fabricated using acrylic glass to model carbonate matrix wettability and for a better observation on fluid flow behavior. The models were placed vertically and horizontally to simulate the high-angle and low-angle fractures in the formation. Gas displacing oil experiments were performed at different injection velocities. The oil displacement characteristics were depicted and the production performance was recorded and discussed. Experimental data were history matched through numerical simulation, and thus a sensitivity study was conducted via design of experiments. For downward gas injection in the high-angle fractured model with a given aperture, a critical injection velocity was obtained below which piston-like displacement was observed. Channeling factor was defined to characterize the injected gas channeling features. It gradually increased and reached its maximum value with increasing injection velocities. The relationship between channeling factor and injection velocity was well fitted by Langmuir equation, and the mechanism behind it was elucidated. Based on their relationship, three gas/oil flow regions were illustrated including non-channeling, transitional channeling, and stable channeling. For all fractured models, the critical injection velocity increased and the maximum channeling factor declined with the increase of fracture aperture. A standard curve was plotted, which enables us to determine different flow regions according to the fracture aperture and injection velocity.
For oil displacement in the low-angle fractured models, the top part was flooded at a specific range of injection velocity. Gravity effect was weakened and the middle part was flooded at relatively high injection velocities.
Numerical fractured models were built and thus calibrated by history matching all the experimental data at different fracture apertures and injection velocities. A sensitivity study was conducted and the weighting of different variables was emphasized via DOE. Previous studies were mostly focused on gas flooding efficiency in naturally fractured carbonate reservoirs; however, this study visually depicted the gas-oil flow behavior through lab experiments, and demonstrated the weighting of different variables via numerical simulation using DOE. This paper could provide an insight into field gas injection projects and the development of the commercial numerical simulator that is specialized for naturally fractured carbonate reservoirs.