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.
Highlights
Introduction published maps and institutional affilIn Tahe oilfield (Tarim basin, China), there are a lot of fractured-vuggy carbonate reservoirs 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 different from the traditional sandstone reservoir and fractured porous carbonate reservoir [1]
The assumptions for building this model were the following: (1) the flow in fracturedvuggy reservoirs is laminar flow; (2) the porosity in the reservoir is isotropic; (3) the fluid medium is incompressible; (4) the reservoir temperature is stable; and (5) the physical properties of fluid in the reservoir are estimated by the given density and viscosity values
Summary
In Tahe oilfield (Tarim basin, China), there are a lot of fractured-vuggy carbonate reservoirs 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 different from the traditional sandstone reservoir and fractured porous carbonate reservoir [1]. In the early-stage development of these fractured-vuggy reservoirs, water drive was the main development method. 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 iations
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