In-Situ Visualization of Imbibition Process Using a Fracture-Matrix Micromodel: Effect of Surfactant Formulations toward Nanoemulsion and Microemulsion

Abstract

Summary Spontaneous imbibition can help to improve the oil recovery of unconventional reservoirs owing to the significant capillarity. Although the dependence of imbibition dynamics of surfactants on wettability and interfacial tension (IFT) is understood, the mechanisms of nanoemulsion and microemulsion forming surfactants for higher imbibition recovery are not as clear. Herein, we conducted a series of imbibition experiments on a visual fracture-matrix micromodel, aiming to directly observe the imbibition processes of these surfactant formulations. Four surfactant-based fluids, including a common surfactant [fatty alcohol polyoxyethylene ether, sodium sulfate (AES)], a surfactant composition of nanoemulsion (nE-S), an ex-situ nanoemulsion (nE), and a situ microemulsion forming surfactant (mE-FS), were designed and used in this work for comparison with brine. The results suggested that AES, nE-S, nE, and mE-FS could substantially stimulate the imbibition invasion, and mE-FS generated the greatest imbibition depth and sweeping area followed by nE. The imbibition dynamics were governed by the interfacial interactions among oil, aqueous phase, and solid surface, leading to different imbibition patterns for these five fluids. AES and nE-S could reduce the oil-aqueous IFT to 10−1 mN/m and alter the wettability to a weak water-wet state as a result of surfactant adsorption, leading to a slightly higher imbibition invasion compared with brine. AES imbibition produced large oil droplets mainly because of the snap-off effect at the nozzle to the fracture, whereas nE-S produced smaller oil droplets due to the weak in-situ emulsification. nE as a formed nanoemulsion with an internal oil phase demonstrated a lower IFT of 10−2 mN/m and superior capacity in changing surface wettability mainly through the adsorption and spreading of nanosized oil droplets on the surface. The oil phase was heavily emulsified forming dense droplets on the oil-aqueous interface. mE-FS readily formed Winsor Type III microemulsion and produced an IFT of 10−3 mN/m magnitude. The wettability was changed mainly because of the peeling oil film and formation of microemulsion on the surface induced by solubilization. The dynamic increase of the oil-aqueous IFT at the imbibition front caused by the adsorption loss of surfactant to the surface and partitioning to the oil phase promoted capillary-driven imbibition for nE and mE-FS. We modified an imbibition model to incorporate the solubilization effect, leading to a much better fitting with the experimental data.

Nanomaterials in EOR: A Review and Future Perspectives in Unconventional Reservoirs

“…Spontaneous imbibition is a crucial mechanism in enhanced oil recovery (EOR) within unconventional reservoirs, especially in micropores (<0.1 mm) and mesopores (0.1–1 mm), as shown in Figure . It enhances oil recovery through several mechanisms, including interfacial tension (IFT) reduction, wettability alteration, structural disjoining pressure, oil solubilization, and emulsification …”

“…For instance, using nanoemulsion nE1 (0.4 wt %) improved oil recovery by 18.8% compared to brine, but it took longer to reach imbibition equilibrium due to complex interactions among oil, nE, and the rock surface. In the initial 50 h, IFT reduction and wettability alteration were the primary factors, while after 50 h, oil solubilization and emulsification, alongside IFT reduction and wettability alteration, played critical roles. , Jagar A. Ali employed a green nanomaterial, the Fe 3 O 4 /mineral earth nanocomposite, derived from natural plant extracts. This enhanced oil recovery in carbonate rock core experiments by 11.28%, facilitated by wettability alteration and spontaneous imbibition promotion …”

“…It enhances oil recovery through several mechanisms, including interfacial tension (IFT) reduction, wettability alteration, structural disjoining pressure, oil solubilization, and emulsification. 134 For instance, Liang et al crafted a nanoemulsion composed of a 10% hydrocarbon or olefin coat, 30−50% nonionic surfactant, and 20−40% alcohol. With a low concentration (0.1−0.15 wt %), this nanoemulsion effectively changed wettability, minimized IFT, and penetrated small pores, yielding a 45% recovery rate, triple that of brine.…”