Gas and liquid flooding using carbon dioxide (CO2), nitrogen (N2), or brine solution have become one of the promising enhanced gas (EGR) and oil recovery (EOR) technologies for residual hydrocarbons (HCs) enhancement in conventional oil and gas reservoir respectively. However, the flow mechanism between the displacing and displaced fluids are not yet clear, especially for the novel gas alternating gas injection method adopted in this study. This experimental study investigates the flow mechanism of N2-CO2-CH4 through gas alternating gas injection techniques in consolidated rocks during EGR. The research presents a better flow behaviour characteristic using a novel N2 alternating CO2 during EGR. These values were used in determining the optimum injection rate with the minimum in situ mixing and high displacement front. An experimental laboratory core flooding, experiment was done to imitate a detailed process of an unsteady state N2-CO2-CH4 displacement in Bandera grey core sample at 35-40°C of temperature, 1500 psig of pressure, and at 0.2, 0.4, 0.6, 0.8 and 1.0 ml/min N2 alternating CO2 injection rates to evaluate the displacement flow characteristics, such as diffusion coefficient, dispersion coefficient, density and viscosity, mobility ratio, and dispersivity. The CO2 was injected after 4-5 cm3 of N2 injection throughout the runs at the experimental condition. The findings indicated that gas alternating gas injection technique presents a better flow behaviour characteristic compared to that of individual CO2 or N2 injection. Such prominent behaviour was observed at 0.4 ml/min injection, with higher displacement front and longer CO2 breakthrough time. The mobility ratio of N2-CO2-CH4 was lower compared to that of N2-CH4 and CO2-CH4. This was due to the inclusion of nitrogen which acts as a barrier between the CO2 and displaced CH4. The later contributed significantly for the delayed in CO2 breakthrough especially at lower injection rates (0.2-0.4 ml/min) during the gas alternating gas EGR process. The overall molecular diffusion coefficients were found to be 22.99, 18.48 and 17.33 ×10-8 m2/s for N2-CH4, CO2-CH4, and CO2-N2 binary interaction respectively at the test condition. The dispersion coefficient increases with an increase in the injection rate due to rise in the interstitial velocity as the CO2 plume traverses through the core sample during the EGR process.

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Ti t l e E n h a n c e d g a s r e c ov e ry b y ni t r o g e n inj e c tio n : t h e effe c t s of inj e c tio n v elo city d u ri n g n a t u r al g a s di s pl a c e m e n t in c o n s olid a t e d r o c k s

The promotion of enhanced gas recovery (EGR) and CO2 storage is still shrouded in contention and is not well accepted, due to the excessive in situ CO2 mixing with the nascent natural gas. This adulterates the recovered CH4 and thus results in a high sweetening process cost thereby making the technique impractical. This has not only limited the field application of EGR in actual projects to a few trails but renders it uneconomical. This study aims to present, experimentally, alternating N2 injection as a potential technique for EGR and CO2 storage in sandstone rock cores. A laboratory core flooding experiment was carried out to simulate a detailed process of unsteady-state methane (CH4) displacement using Bandera grey core plug. This was carried out at 40 °C, 1500 psig, and 0.4 ml/min injection rate by alternative injection of N2 and CO2 in succession designed to suit the application based on optimum operating conditions. The results show that both CO2 storage capacity and CH4 recovery improved significantly when gas alternating gas (GAG) injection was considered. The best results were observed at lower N2 cushion volumes (1 and 2 PV). Therefore, the GAG injection method with N2 as cushion gas can potentially increase both CO2 storage and CH4 recovery of the gas reservoir. This technique if employed will assert the current position and provide vital information for further researches aimed at promoting environmental sustainability and economic viability of the EGR and CO2 sequestration processes.

The use of carbon dioxide (CO2) for simultaneous methane recovery and CO2 storage is gaining recognition globally within the oil and gas industries. On the other hand, most of the residual natural gas recovered during the EGR process is highly contaminated with the injected CO2 due to their nascent miscibility nature, resulting in premature breakthrough. In this study, N2 gas was used as a buster to mitigate such early mixing between the CH4 and CO2. The experiment was administered at reservoir conditions of 40oC temperature, 1500 psig of pressure, the optimum injection rate of 0.4ml/min, and at varying N2 cushion volumes (8-36 cm3) using Bandera gray as the porous medium. Further experimental tests were administered to study the effect of this technique on connate water salinity with 5-20% water salinity been considered. The increase in buster gas volume was in direct proportion to delayed CO2 breakthrough, with the maximum at 36cm3 buster volume. This breakthrough occurred at 177 minutes which is 110min additional delayed than the conventional CO2 flooding with a breakthrough time of 67 minutes. This was due to the high shielding barrier inhibited by nitrogen, making it difficult for the CO2 to dispersed itself and mixed with the nascent natural gas resulting in delayed breakthrough as it plumes transverses into the CH4 during the displacement process. Furthermore, a poor performance was observed with the inclusion of the connate water salinity, especially at 20% wt. This was because the free pore spaces were already occupied by connate water molecules prior to the cushion gas injection which hinders its economic potential application.