Abstract
Water Alternating Gas (WAG) approach can improve the efficiency of gas flooding. However, the precipitation damage that is induced by the gas injection may be inevitable. The precipitation pressure point test of gas injection, and the WAG parallel double-tube long-core flooding experiment under different injection conditions were systematically performed to obtain the optimum injection parameters. The variations of petrophysical properties were caused by precipitation, and its morphology was also determined by centrifugal capillary force and environmental scanning electron microscope. The precipitation pressure rised with the increase of the amount of gas injection, generally 2.0 MPa ~ 3.0 MPa higher than the bubble point pressure (Pb), and it was confirmed by X-ray energy spectrum and scanning electron microscope that the precipitation was mainly asphaltene. The optimum injection parameters for WAG were Gas–Water Ratio (GWR) of 1:1 and slug size of 0.1 HydroCarbon Pore Volume (HCPV), which benefited the recovery of low-permeability and high-permeability pipe by additional recovery of 28.5% and 17.4% respectively, while WAG process enhanced the total oil recovery by 23.4%. The pore volume and median radius of capillary pressure of all cores were both reduced with more obvious effects on conglomerate. Combined with the results of sediment saturation, it also showed the poorer the physical properties of the cores, the severer the influence of the precipitation. Overall, the WAG could greatly improve the recovery but the influence of precipitation must be considered.
Highlights
Gas injection is an important and effective driving method to enhance oil recovery during the reservoir development
Due to the low viscosity of the gas, and the large density difference between the injected gas and the crude oil, poor macroscopic sweep efficiency is observed during the gas injection process, which leads to the oil bypassing at certain parts of the reservoir
The precipitation pressure rised with more gas injection, generally 2.0 MPa ~ 3.0 MPa higher than the bubble point pressure
Summary
Gas injection is an important and effective driving method to enhance oil recovery during the reservoir development. Methods for gas injection include continuous injection, Water Alternating Gas (WAG) injection, and cyclic injection. Due to the low viscosity of the gas, and the large density difference between the injected gas and the crude oil, poor macroscopic sweep efficiency is observed during the gas injection process, which leads to the oil bypassing at certain parts of the reservoir. The displacement front is unstable and an early breakthrough occurred in the reservoir [2]. This situation is especially exacerbated in reservoirs with strong heterogeneity. The WAG can reduce the adverse mobility ratio between the injected fluid and the crude oil to mitigate the channeling, to improve the macroscopic sweep efficiency and enable
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