Enhanced gas recovery by nitrogen injection : the effects of injection velocity during natural gas displacement in consolidated rocks

Mohammed, NUHU; Abbas, AJ; Enyi, GC; Edem, DE; Suleiman, SM

Abstract

The choice of the flow velocity in EGR thus becomes important since higher injection rates could lead to premature
mixing of the fluids and lower injection rates generally provide longer resident times for the fluids in
contact and indirectly increases the mixing of the gases. Additionally, the medium peclet numbers mostly
indicate the best injection rates that translate to a smoother displacement with a lower dispersion coefficient
during the EGR process. Therefore, N2 Injection into natural gas reservoirs offers the potential to higher recovery
efficiency with less mixing compared to conventional CO2 injection. The atmospheric air contained 79% of N2,
making it readily available than CO2 with 400 ppm air composition. More so, N2 requires less compression ratio,
which is why a lower amount of it was required to initiate much pressure in the CH4 reservoir during
displacement. These made the use of N2 more economically feasible and friendly for the EGR process. A laboratory
core flooding experiment was carried out to simulate the effect of injection velocity on CH4 recovery and
dispersion coefficient. This was done at 40 ◦C, 1500 psig, and 0.2–1.0 ml/min injection rates. The results showed
that a medium peclet number could be used to predict the best injection rate that translates to a smoother
displacement with a lower dispersion coefficient during the EGR process. CH4 recovery and efficiency were
highest at lower injection velocities experienced in both core samples. This could be attributed to insignificance
nascent mixing observed as seen on their recorded low longitudinal dispersion coefficient results. Consequence,
the experimental runs at high injection rates (0.6–1.0 ml/min) present a different scenario with lower recovery
and efficiency due to their high interstitial velocities as the N2 plumes transverses into the core sample during
CH4 displacement. Overall, the least methane production and efficiency were noticed in the Bandera core sample
as a result of the heterogeneity effect due to the presence of higher clay contents in Bandera than Berea gray.
When the capillary forces within the narrower pores in Bandera core sample were overcome, the clay particles
occupied those pores thereby sealing some of the flow paths within the pore matrix. This reduces the flow
channels, significantly, through which the injected N2 will flow to displace the residual CH4.

• Journal Article Type: Article
• Acceptance Date: Jul 24, 2020
• Online Publication Date: Aug 6, 2020
• Publication Date: Nov 1, 2020
• Deposit Date: Aug 26, 2020
• Publicly Available Date: Aug 6, 2021
• Journal: Journal of Natural Gas Science and Engineering
• Print ISSN: 1875-5100
• Publisher: Elsevier
• Volume: 83
• Pages: 103513
• DOI: https://doi.org/10.1016/j.jngse.2020.103513
• Publisher URL: https://doi.org/10.1016/j.jngse.2020.103513
• Related Public URLs: http://www.elsevier.com/wps/product/cws_home/716470/description