Droplet fragmentation: 3D imaging of a previously unidentified pore-scale process during multiphase flow in porous media

Using X-ray computed microtomography, we have visualized and quantified the in situ structure of a trapped nonwetting phase (oil) in a highly heterogeneous carbonate rock after injecting a wetting phase (brine) at low and high capillary numbers. We imaged the process of capillary desaturation in 3D and demonstrated its impacts on the trapped nonwetting phase cluster size distribution. We have identified a previously unidentified pore-scale event during capillary desaturation. This pore-scale event, described as droplet fragmentation of the nonwetting phase, occurs in larger pores. It increases volumetric production of the nonwetting phase after capillary trapping and enlarges the fluid−fluid interface, which can enhance mass transfer between the phases. Droplet fragmentation therefore has implications for a range of multiphase flow processes in natural and engineered porous media with complex heterogeneous pore spaces.Multiphase fluid displacement processes in porous media are important for a broad range of natural and engineering applications such as transport of nonaqueous phase liquid contaminants in aquifers, oil and gas production from hydrocarbon reservoirs, subsurface CO₂ storage, or gas transport in fuel cells. Herein, capillary trapping is a fundamental mechanism that causes immobilization of a portion of the resident nonwetting phase when it is displaced by an invading wetting phase. As a result, production of the nonwetting phase is always less than 100%.The pore-scale physics of capillary trapping are broadly understood, as the underlying mechanisms such as piston-like displacement, snap-off and film development have been observed in physical micromodel experiments and quantitative theories have been established for them (1–4). The conventional view considers such pore-scale processes to occur between multiple pores, i.e., they are interpore processes and the pores are defined as volumes connected by narrower pore throats. By contrast, intrapore processes, as presented in this paper, are not well established in the literature. During drainage (i.e., where a nonwetting phase displaces the wetting phase), the wetting phase can establish films in the corners of the pores, which results in its continuous production and hence low residual saturations of the wetting phase. During imbibition (i.e., where the wetting phase displaces a nonwetting phase), swelling of the corner wetting films causes snap-off of the nonwetting phase, which results in capillary trapping of the nonwetting phase. The trapped nonwetting phase exists as disconnected ganglia within the pore network. Numerical pore network models have been developed to include these pore-level mechanisms with the aim of predicting the macroscopic flow properties of porous materials such as the structure of the phase distributions, residual saturation, relative permeability functions, and capillary pressure curves. Some of these models, referred to as quasi-static models, assume that fluid flow is only governed by capillary forces (5–8), and hence are limited in capturing the dynamics of fluid displacements that occur under the action of both capillary and viscous forces. In another class of pore network models, referred to as dynamic models (9–11), capillary and viscous forces are considered simultaneously. Such models are more applicable in modeling the dynamics of pore-scale events controlled by both capillary and viscous forces.The saturation distribution of two immiscible fluid phases in a porous medium is influenced by the wettability of the system, i.e., the distribution of surfaces that are preferentially water wet or preferentially wetting to a nonaqueous phase such as oil (12). It is known that a trapped nonwetting phase can be remobilized and recovered when the wetting phase is injected at capillary numbers N_c that exceed a critical level. N_c is a dimensionless ratio quantifying the relative importance of viscous to capillary forces, i.e., N_c= vµ/σ where v is the apparent velocity, µ is the viscosity of the invading phase, and σ is the interfacial tension (13). For homogeneous sandstones, remobilization typically occurs at N_c of the order of 10⁻⁵, an effect known as capillary desaturation (14).Recent advances in X-ray computed microtomography (µCT) methods have enabled the visualization and quantitative analysis of the static distribution of fluid phases, fluid rock interactions, and the structure of wetting and nonwetting phases in porous materials (8, 15). A particular focus has been on capillary trapping (16–20). Using synchrotron X-ray μCT facilities, it has also become possible to visualize dynamic pore-scale mechanisms, including snap-off and Haines jumps (21). Most of these imaging studies have focused on relatively homogeneous pore systems such as bead packs (22), sand packs (22–26), and sandstones (8, 18, 21, 23), but less attention has been paid to carbonate rocks. However, more than 50% of the world’s remaining oil reserves are located in carbonate reservoirs (27), and carbonate aquifers supply water wholly or partially to one quarter of the global population (28). Carbonates rocks can have complex multiscale pore structures, which render the application of X-ray µCT more challenging because of the need to select a representative sample that is small enough to achieve high resolutions on µCT images but that also captures the essential heterogeneities of the pore structure (29, 30).In this contribution, we use X-ray µCT to quantify the structure and distribution of a nonwetting phase (oil) after drainage and after its displacement by a wetting phase (brine) at low and high capillary numbers in a heterogeneous carbonate with multiple pore scales. Using image analysis, we demonstrate the effect of capillary desaturation on the cluster size distribution of the trapped oil phase. We identify a previously unidentified pore-scale event, which we refer to as droplet fragmentation. Droplet fragmentation is responsible for further production of the oil phase beyond capillary trapping. This fragmentation process occurs mainly in larger pores. It results in the production of additional oil from these large pores, contributes to a change in the structure of residual oil, and increases the oil−brine surface area. As a consequence, the trapped phase may subsequently be more difficult to mobilize after droplet fragmentation has occurred but mass transfer between the phases can increase.