The Impact of Dynamic Wettability Alteration and Phase Behavior on the VES Flooding Recovery at the Pore-Scale

Wettability plays a crucial role in the recovery of hydrocarbons as it governs the adhesive forces between oil and rock samples, directly influencing the efficiency of the displacement process. Numerous research efforts have focused on modifying rock wettability to a more favorable state. Typically, the contact angle measurement with smoothed rock discs has been employed, but this experimental design has limitations in reflecting reality since the wettability effect occurs within the pores. The coverage area of a single drop is substantial, encompassing a wide range of pore and grain spaces, introducing uncertainties regarding the validity of the measurement. In this study, we delve into the pore-scale level to observe the actual process of contact angle alteration during flooding, utilizing microfluidic technology. Three different concentrations of Viscoelastic Surfactant (VES)—specifically, 0.5%, 0.75%, and 1.25% vol%, prepared using 57K ppm synthetic seawater—were employed. The microfluidic model initially underwent saturation with formation water and was subsequently displaced by oil to establish the initial oil saturation. The microfluidic setup encompassed a precise pump for flood control and a high-speed microscope to capture images, which would later be analyzed using image processing software to obtain the real contact angle. To ensure the reliability of our data, we divided the pore space into twenty divisions and measured the contact angle through image analysis. The contact angle was measured at various injection stages to observe the dynamic changes from the initial state to the final state and the resulting recovery from each fluid system. Additionally, we analyzed the in-situ generated emulsion to establish a link between phase behavior, wettability alteration, and recovery. The results demonstrated that using 0.5% VES altered the wettability from slightly oil-wet to slightly water-wet, resulting in a 55% recovery of the original oil in place (OOIP). Conversely, employing 1.25% VES did not significantly alter the wettability but yielded a recovery of 52% OOIP. The 0.75% VES altered the wettability from slightly oil-wet to extremely water-wet; however, this alteration did not translate into higher recovery. Instead, a water breakthrough was observed, which diminished the sweep efficiency, resulting in a recovery of 47%. This pore-scale investigation successfully demonstrated the effectiveness of utilizing VES solutions to modify rock wettability and enhance oil recovery.