Unrecoverable oil droplets represent a direct outcome of the snap-off phenomenon, and they play a pivotal role in shaping the microscopic distribution of the remaining oil within porous media in reservoirs
[5554][6463]. In the context of reservoir exploitation, waterflooding stands as a prevalent method. During waterflooding operations, the nonwetting phase, typically crude oil, undergoes displacement by the wetting phase, which is water. Within this process, snap-off events may transpire. To elaborate, when crude oil is displaced to the central region of pore throats and gradually dislodged from the pore walls by the advancing wetting phase, snap-off occurrences lead to the formation of oil droplets. These oil droplets become entrapped within the pores, rendering them immobile and resistant to further displacement, hence the designation unrecoverable oil droplets. The impact of these unrecoverable oil droplets on crude oil production is substantial
[6564].
4.2. Oil Bridging Effect
The oil bridging effect
[5554][6665] represents a significant potential outcome of snap-off phenomena, particularly noteworthy in heterogeneous reservoirs characterized by a diverse range of pore sizes and geometries. This effect arises from the intricate interplay between snap-off events and the inherent properties of such heterogeneous porous media. It results in the entrapment of nonwetting-phase droplets within pore throats, giving rise to bridging-like structures or obstructions
[66][67][68], rather than spherical shapes.
In these heterogeneous oil reservoirs, significant disparities exist in the dimensions of pores and throats, manifesting substantial differences in their interactions with fluids. Larger pores, characterized by their expansive cross-sectional areas and lower hydraulic resistance, tend to facilitate fluid flow, resulting in heightened fluid–pore interactions and consequential alterations in surface properties, particularly wettability. Conversely, smaller throats, characterized by their reduced cross-sectional areas and higher hydraulic resistance, impede fluid flow, maintaining their inherent wettability with limited fluid interactions. In instances where larger pores with significantly modified wettability are interconnected with smaller throats exhibiting unaltered wettability, forming integrated pore–throat systems, the formation of snap-off-induced droplets leads to distinct interfacial tension at interfaces near pore surfaces and those adjacent to throat surfaces. This variance in interfacial tensions results in droplet deformation, ultimately culminating in the formation of bridge-like structures.
4.3. Drainage–Imbibition Hysteresis
Snap-off serves as the fundamental cause of drainage–imbibition hysteresis
[5756][68][69][70], a phenomenon characterized by distinct variations in flow dynamics during drainage (the expulsion of liquid from pores) and imbibition (the infiltration of liquid into pores) processes within porous media.
This hysteresis can significantly influence fluid behaviors and flow mechanisms in such media, most notably evident in the nonalignment of relative permeability curves for the wetting phase during drainage and imbibition processes
[32][7170]. During drainage, the relative permeability of the wetting phase is higher, indicating relatively easier pore occupancy. However, in the imbibition process, particularly in porous media with nonuniform pore structure or microscale heterogeneity
[7271], the relative permeability of the wetting phase decreases, indicating the challenges for the wetting phase in completely occupying the pore space.
4.4. Strong Foam Generation
Foam generation is essentially synonymous with gas bubble generation
[7372]. Consequently, the production of foam within porous media is closely intertwined with snap-off phenomena. Foam generation denotes the occurrence wherein gas bubbles form within the porous medium during the multiphase flow, with the wetting phase and nonwetting phase within the porous medium assuming the roles of liquid and gas phases, respectively. Previous studies on the mechanism of gas bubble formation have identified snap-off as one primary principal mechanism responsible for the generation of strong foam
[73][74][75][76].
During multiphase fluid flow, foam generation can be classified into different categories, including strong foam and weak foam. Strong foam is the term used to describe relatively large and stable gas bubbles that are produced when the wetting phase rapidly pinches off the nonwetting phase. In contrast, weak foam consists of smaller and less stable bubbles. In porous media where gas and liquid coexist, gas, acting as the nonwetting phase, undergoes separation from the wetting liquid phase through snap-off phenomena, leading to the formation of larger gas bubbles.
4.5. Transient/Dynamic Effects
Apart from the aforementioned microscale consequences, snap-off can also exert an influence on specific macroscopic or continuum-scale parameters within the realm of porous media and multiphase flow. These effects are categorized as transient/dynamic effects, which are typically more pronounced in porous media characterized by coarser textures
[76][77][78].
Droplets and bubbles, generated through snap-off, have been proven to influence fluid redistribution and introduce macroscale inhomogeneities at transient state
[7776][7978]. When snap-off events occur, the entrapment of oil droplets or gas bubbles occupies a portion of the pore volume within the porous medium. Consequently, this augments the relative proportion of the nonwater phase in the reservoir, leading to a reduction in water saturation. It is essential to note that water saturation maintains a well-established constitutive relationship with relative permeability
[7776]. During multiphase flow, the decline in water saturation typically coincides with a decrease in the water relative permeability. This decrease implies greater challenges in displacing oil or gas by water, ultimately resulting in a reduced oil-recovery rate.
4.6. Interconnections between Effects
The various effects induced by snap-off, as previously elucidated, are not isolated but rather interconnected. The most conspicuous and common effect of snap-off is the formation of unrecoverable oil droplets. When these unrecoverable oil droplets occur within heterogeneous oil reservoirs characterized by a wide range of pore sizes and geometries, factors such as disparities in the interfacial tension, local pressure, and saturation, resulting from significant variations in pore and throat dimensions, lead to the deformation of trapped oil droplets, transforming them from spheres into bridge-like shapes and giving rise to the oil bridging effect.
Whether in the form of spherical trapped oil droplets or deformed oil bridges, these entities, serving as obstacles within the porous medium, can alter fluid distribution at the microscopic level. Consequently, this alteration initiates drainage–imbibition hysteresis and transient/dynamic effects, leading to a reduction in key macroscopic parameters, such as water saturation within the porous medium and water relative permeability in the context of two-phase flow. This reduction has a significant adverse impact on oil- or gas-recovery processes.
5. Prevention and Utilization of Snap-Off
5.1. Prevention of Snap-Off in Waterflooding for Oil Production
During the crude oil production through waterflooding, the occurrence of snap-off typically exerts a detrimental influence on crude oil-recovery rates. This adverse impact arises from the interaction of oil and water phases during the waterflooding process, where the oil phase frequently undergoes snap-off events
[149][5066][67]. Consequently, this leads to the formation of unrecoverable oil droplets, some of which may become entrapped in the narrow constrictions of the reservoir’s porous media. These trapped oil droplets pose significant challenges to effective displacement, rendering them irrecoverable residual oil droplets. Furthermore, the subsequent oil bridging effect, induced by the presence of these residual oil droplets resulting from snap-off, further obstructs the unimpeded flow of the displacing phase within the porous media. This additional hindrance substantially diminishes crude oil-recovery rates.
Essentially, during the waterflooding process, the formation of unrecoverable oil droplets, the oil bridging effect, and drainage–imbibition hysteresis caused by the snap-off phenomenon collectively contribute to an increased volume of trapped oil within subterranean reservoirs. This trapped oil becomes inaccessible, ultimately leading to lower oil recovery. Therefore, extensive research and technological advancements within the petroleum industry are directed towards mitigating or preventing the snap-off phenomenon to enhance recovery rates
[79][80][81].
5.2. Utilization of Snap-Off in CO2-EOR
CO
2-EOR, also referred to as CO
2 flooding, is a reservoir engineering technique employed to enhance oil recovery using CO
2 [81][82][83][84]. Typically, it finds application in reservoirs where conventional waterflooding has been conducted, yet a substantial quantity of crude oil remains within the porous media of reservoirs. The fundamental principle underlying CO
2-EOR involves the injection of CO
2 into the reservoir, primarily to reduce the viscosity of crude oil by blending with it within the interstitial spaces of the porous medium. This process plays a pivotal role in enhancing the flowability of crude oil, consequently elevating the sweep efficiency, and thereby increasing both the oil-production efficiency and oil-recovery rate. Furthermore, during the implementation of CO
2-EOR, the introduction of appropriate foaming agents alongside CO
2 can lead to interactions with reservoir fluids that induce the occurrence of snap-off phenomena
[8584]. This phenomenon results in the formation of gas bubbles within the oil phase, which are effectively stabilized by the foaming agents.
5.3. Utilization of Snap-Off in CO2 Storage
During the process of CO
2 geological storage, the occurrence of the snap-off phenomenon significantly enhances storage efficiency
[8281]. The characteristics of CO
2 storage in saline aquifers share similarities with oil production through waterflooding in oil reservoirs, both involving immiscible two-phase fluids within porous media. Typically, brine or water serves as the wetting phase, while CO
2 gas or oil function as the nonwetting phase. Therefore, findings related to snap-off in oil reservoirs can contribute to a deeper understanding of the theoretical aspects and mechanisms behind CO
2 storage in saline aquifers.
In summary, CO
2 geological storage in saline aquifers leverages the strong foam generation effect of snap-off to promote the formation of large, stable CO
2 bubbles. Subsequently, transient/dynamic effects induced by snap-off lead to a reduction in water saturation. As a consequence, water relative permeability decreases, impeding water flow within the porous medium, thereby facilitating the primary goal of stably storing CO
2 bubbles within saline aquifer porous media. To optimize the efficient application of snap-off for CO
2 geological storage, several technical measures should be considered. It is necessary to evaluate the storage capacity and sealing properties of the selected saline aquifers. The CO
2 injection flow rate and pressure should be controlled within manageable ranges. Additionally, the implementation of numerical simulations before CO
2 injection, real-time monitoring during injection, and long-term poststorage monitoring are essential components of this process.