Hypergravity experimental study on immiscible fluid–fluid displacement in micromodels

Abstract

Hypergravity offers transformative potential for enhanced oil recovery (EOR) and CO₂ sequestration by mimicking subsurface geostress and pressure conditions, facilitating the study of large-scale physical phenomena like fluid migration and sediment compaction within reduced experimental timeframes and scales. In CO₂ sequestration, hypergravity shortens Ostwald ripening, facilitates bubble coalescence, and intensifies gas–solid mass transfer. While the dynamic process of two-phase flow under hypergravity remains insufficiently explored. Hence, a hypergravity microfluidic observation system (HMOS) was developed to investigate the aforementioned process. Seven sets of water–oil displacement experiments were conducted on two chips (channel depths of 160 μm and 30 μm) under 0 g, 1 g, and 50 g conditions, with capillary numbers (Ca) ranging from 9.55 × 10^-6 to 9.05 × 10^-5 (typical of the viscous fingering regime) and Bond numbers (Bo) ranging from −0.69 to 0. The results demonstrate that hypergravity (50 g) dragged down the bulk of the dense defending phase, reducing the local pressure gradient at the fluid–fluid interface, and thereby inhibited the upward advancement of the invading phase. In a wide flow channel (576 μm in Chip 1, Bo = -0.69), hypergravity overwhelmed viscous forces, accelerated the dense defending phase downward, even pinched off the invading phase (snap-off), and thus reduced displacement efficiency (S_nw) to 26.9 % (compared to 55.5 % at 1 g); while in a narrow flow channel (80 μm in Chip 2, Bo = -0.0133), the effects of hypergravity and viscous forces were comparable, resulting in enhanced lateral spreading of the invading phase, and thus drastically improved S_nw up to 60.9 % (compared to 29.6 % at 1 g). Meanwhile, hypergravity has a secondary influence on the displacement morphology, as evidenced by the fact that the slope of fluid–fluid interface length (l_nw) to invading phase saturation (S_nw) were constricted to narrow ranges (23.04 ∼ 29.12 for Chip 1, and 50.46 ∼ 64.96 for Chip 2). These findings shed lights on the immiscible fluid–fluid displacement efficiency and morphology under hypergravity, providing insights on applying hypergravity field on meter level models to simulate large-scale and long-duration physical phenomena encountered in deep-earth oil recovery.