Abstract
Crucial oil reservoirs are located in naturally fractured carbonate formations and are currently reaching a mature phase of production. Hence, a cost-effective enhanced oil recovery (EOR) method is needed to achieve a satisfactory recovery factor. The paper focuses on an experimental investigation of the efficiency of water alternating sour and high-nitrogen (~85% N2) natural gas injection (WAG) in mixed-wetted carbonates that are crucial reservoir rocks for Polish oil fields. The foam-assisted water alternating gas method (FAWAG) was also tested. Both were compared with continuous water injection (CWI) and continuous gas injection (CGI). A series of coreflooding experiments were conducted within reservoir conditions (T = 126 ℃, P = 270 bar) on composite cores, and each consisted of four reservoir dolomite core plugs and was saturated with the original reservoir fluids. In turn, some of the experiments were conducted on artificially fractured cores to evaluate the impact of fractures on recovery efficiency. The performance evaluation of the tested methods was carried out by comparing oil recoveries from non-fractured composite cores, as well as fractured. In the case of non-fractured cores, the WAG injection outperformed continuous gas injection (CGI) and continuous water injection (CWI). As expected, the presence of fractures significantly reduced performance of WAG, CGI and CWI injection modes. In contrast, with regard to FAWAG, deployment of foam flow in the presence of fractures remarkably enhanced oil recovery, which confirms the possibility of using the FAWAG method in situations of premature gas breakthrough. The positive results encourage us to continue the research of the potential uses of this high-nitrogen natural gas in EOR, especially in the view of the utilization of gas reservoirs with advantageous location, high reserves and reservoir energy.
Highlights
A key requirement of any enhanced oil recovery (EOR) process connected with fluid injection into the reservoir is to achieve a favorable mobility ratio between displacing and displaced fluid
Gas injection in EOR is associated with an unfavorable mobility factor, which leads to destabilization of the displacement front, gas fingering and premature breakthrough of the injected fluid into production wells
The review of 72 Water Alternating Gas injection (WAG) field cases indicates that the average incremental recovery was around 10%, and in some cases, up to 20% of the Original Oil in Place (OOIP) [3]
Summary
A key requirement of any enhanced oil recovery (EOR) process connected with fluid injection into the reservoir is to achieve a favorable mobility ratio between displacing and displaced fluid. Gas injection in EOR is associated with an unfavorable mobility factor (due to the significant difference in dynamic viscosity between the injected gas and crude oil to be displaced), which leads to destabilization of the displacement front, gas fingering and premature breakthrough of the injected fluid into production wells. Zones saturated with oil and not covered by the displacement front occur. This results in a relatively low recovery factor [1,2]. Water Alternating Gas injection (WAG) was initially introduced to enhance oil recovery in the continuous gas injection (CGI) process through improving macroscopic sweep efficiency. The technique, Energies 2020, 13, 2346; doi:10.3390/en13092346 www.mdpi.com/journal/energies
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