Summary Foam flooding can minimize bypassing in gasfloods in fractured reservoirs. Finding a foam formulation effective in high-salinity brine is challenging, especially with divalent cations, e.g., American Petroleum Institute (API) brine (8% NaCl with 2% CaCl2). When formulating with nanoparticles, the colloidal dispersion stability is difficult due to the dramatic reduction in zeta potential and the Debye length at high salinity. The aim of this work was to develop a strong foam in API brine at the ambient temperature, using a nonionic surfactant and ethyl cellulose nanoparticles (ECNP), for gasflooding in fractured carbonate reservoirs. ECNPs was synthesized and dispersed in API brine using a nonionic surfactant (also denoted as SF). SF and SF/ECNP foams were generated, and their stability was studied at atmospheric pressure and 950 psi. Foam mobility was measured in a sandpack at high pressure. Foam flood experiments were conducted in oil-saturated fractured carbonate cores. The nonionic surfactant proved to be a good dispersion agent for ECNP in API brine. The SF/ECNP mixture stabilized foam in API brine, even in the presence of oil. Injecting a partially miscible gas (below its minimum miscibility pressure) as an SF foam into a fractured core more than doubles the oil recovery over injection of the gas alone. The injection of the strong foam (SF/ECNP) further improves the oil recovery over that of the SF foam, indicating the synergy between ECNP and surfactant. ECNP accumulates in the foam lamella and induces larger pressure gradients in the fracture to divert more gas into the matrix for oil displacement.

Carbon dioxide (CO2) flooding has a high microscopic displacement efficiency above the minimum miscibility pressure (MMP), but a low sweep efficiency due to gravity segregation and bypassing. In many fields, water-alternating-gas (WAG) is used to improve sweep efficiency, but early breakthrough and low sweep efficiency are common. The objective of this research is to evaluate the performance of polymer-alternating-gas (PAG) floods experimentally in the lab-scale. Continuous gas (CG), WAG and PAG flood experiments were performed in homogeneous and heterogeneous cores. Experiments conducted in homogeneous Berea cores showed that tertiary CG flooding and tertiary WAG flooding had similar incremental oil recovery and the tertiary PAG flooding had a lower incremental oil recovery. Heterogeneous core flood experiments showed that WAG flooding and PAG flooding had similar incremental oil recovery, which were slightly higher than that in the CG flooding. In linear cores, the gravity segregation effect is limited due to the small diameter of cores; so, the extent of bypassing is small. In heterogeneous cores, there is bypassing in CG floods due to permeability contrast and WAG / PAG floods help improve oil recovery. However, PAG oil recovery was no better than WAG oil recovery in these laboratory-scale core floods.

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