We report a study of the catalytic reactions of ethylene oligomerization over nickel impregnated in aluminum silicate using subcritical and supercritical media. We found the BET surface area decreases with increasing nickel loading, indicating the deposition of NiO particles on the catalyst surface, which we confirmed via XRD. We compared the performance of the NiBEA catalyst with the protonated form (HBEA) of the commercial support and obtained strong evidence that the protonated form promotes chain growth reactions. In addition, the introduction of nickel in the catalyst facilitates the desorption of the oligomers compared to the HBEA catalyst. Additionally, we used FTIR and GC-MS/FID to characterize the adsorbed and desorbed oligomers and developed reaction pathways for the ethylene oligomerization over the NiBEA catalyst. We found that both adsorbed and desorbed oligomers are aliphatic, and the non-desorbed products constitute the coke. Additionally, we found that pressure and temperature both increase the chain growth and desorption rates of adsorbed oligomers. Under supercritical conditions, the amount of coke formed on the catalyst and the desorbed products molecular weight both increase relative to subcritical conditions. However, the supercritical conditions promote the dissolution of the coke from both NiBEA and HBEA catalysts, which shows the standalone effect of the supercritical ethylene toward the dissolution of coke from the catalyst surface. We provided visual evidence of (1) the formation of coke on the catalyst surface, and (2) the coke dissolution phenomena under supercritical conditions. Finally, we characterized the coke molecules at supercritical conditions as macromolecules connected via C-C (sp3) bonds with the occurrence of cyclic species.