energies Article Numerical Simulation via CFD Methods of Nitrogen Flooding in Carbonate Fractured-Vuggy Reservoirs Kexing Li 1,2,3, *, Bowen Chen 2 , Wanfen Pu 1,2,3 , Jianhai Wang 4 , Yongliang Liu 5 , Mikhail Varfolomeev 1,3, * and Chengdong Yuan 1,3, *   Citation: Li, K.; Chen, B.; Pu, W.; Wang, J.; Liu, Y.; Varfolomeev, M.; Yuan, C. Numerical Simulation via CFD Methods of Nitrogen Flooding in Carbonate Fractured-Vuggy Reservoirs. Energies 2021, 14, 7554. https://doi.org/10.3390/en14227554 Academic Editor: Hossein Hamidi Received: 7 October 2021 Accepted: 9 November 2021 Published: 12 November 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (SWPU), Chengdu 610500, China; pwf58@163.com 2 School of Petroleum and Natural Gas Engineering, Southwest Petroleum University, Chengdu 610500, China; 201911000134@stu.swpu.edu.cn 3 Department of Petroleum Engineering, Kazan Federal University, 420008 Kazan, Russia 4 Northwest Oilfield Company, Sinopec, Urumqi 830011, China; wjianh.xbsj@sinopec.com 5 Northeast Sichuan Gasfield, PetroChina Southwest Oil and Gas Field Company, Dazhou 635000, China; liuyongl@petrochina.com.cn * Correspondence: likx@swpu.edu.cn (K.L.); mikhail.varfolomeev@kpfu.ru (M.V.); jchengdong@kpfu.ru (C.Y.) Abstract: A reservoir-scale numerical conceptual model was established according to the actual geological characteristics of a carbonate fractured-vuggy reservoir. Considering the difference in density and viscosity of fluids under reservoir conditions, CFD (computational fluid dynamic) porous medium model was applied to simulate the process of nitrogen displacement in a fractured-vuggy reservoir after water flooding. The effects of gas injection rate, injection mode, and injector–producer location relation were studied. The results show that nitrogen flooding can yield additional oil recovery of 7–15% after water flooding. Low-speed nitrogen injection is beneficial in obtaining higher oil recovery. High speed injection can expand the sweep area, but gas channeling occurs more easily. In gas–water mixed injection mode, there is fluid disturbance in the reservoir. The gas channeling is faster in low injector–high producer mode, while the high injector–low producer mode is beneficial for increasing the gas sweep range. Nevertheless, the increment of recovery is closely related to well pattern. After nitrogen flooding, there are still a lot of remaining oil distributed in the trap area of gas cap and bottom water in the reservoir that water and gas injection can’t sweep. The establishment of the numerical conceptual model compensates for the deficiency of physical simulation research, stating that only limited parameters can be simulated during experiments, and provides theoretical bases for nitrogen flooding in fractured-vuggy reservoir. Keywords: fractured-vuggy reservoir; nitrogen flooding; numerical simulation; computational fluid dynamic; enhanced oil recovery 1. Introduction In Tahe oilfield (Tarim basin, China), there are a lot of fractured-vuggy carbonate reser- voirs with depths of 5320–6200 m. This type of reservoir is mainly composed of paleokarst cave, dissolution cave, and fracture with different sizes and shapes, which is essentially dif- ferent from the traditional sandstone reservoir and fractured porous carbonate reservoir [1]. The reservoir is highly heterogeneous [2] and fluid flow mode is mainly pipe flow [3]. In the early-stage development of these fractured-vuggy reservoirs, water drive was the main development method. The injected water and natural bottom water channeling occur fast along high-angle fractures. For water drive development in fractured-vuggy reservoirs, formation energy depletion is obvious. Gas injection can effectively supplement formation energy and enhance oil recovery. The macroscopic mechanism of nitrogen injection (such as nitrogen huff and puff and nitrogen flooding) in fractured-vuggy reservoir is gravity Energies 2021, 14, 7554. https://doi.org/10.3390/en14227554 https://www.mdpi.com/journal/energies