Visualizing and quantifying the crossover from capillary fingering to viscous fingering in a rough fracture

Abstract

Immiscible fluid‐fluid displacement in permeable media is important in many subsurface processes, including enhanced oil recovery and geological CO2 sequestration. Controlled by capillary and viscous forces, displacement patterns of one fluid displacing another more viscous one exhibit capillary and viscous fingering, and crossover between the two. Although extensive studies investigated viscous and capillary fingering in porous media, a few studies focused on the crossover in rough fractures, and how viscous and capillary forces affect the crossover remains unclear. Using a transparent fracture‐visualization system, we studied how the two forces impact the crossover in a horizontal rough fracture. Drainage experiments of water displacing oil were conducted at seven flow rates (capillary number log10Ca ranging from −7.07 to −3.07) and four viscosity ratios (M=1/1000,1/500,1/100 and 1/50). We consistently observed lower invading fluid saturations in the crossover zone. We also proposed a phase diagram for the displacement patterns in a rough fracture that is consistent with similar studies in porous media. Based on real‐time imaging and statistical analysis of the invasion morphology, we showed that the competition between capillary and viscous forces is responsible for the saturation reduction in the crossover zone. In this zone, finger propagation toward the outlet (characteristic of viscous fingering) as well as void‐filling in the transverse/backward directions (characteristic of capillary fingering), are both suppressed. Therefore, the invading fluid tends to occupy larger apertures with higher characteristic front velocity, promoting void‐filling toward the outlet with thinner finger growth and resulting in a larger volume of defending fluid left behind.

High‐Speed Quantification of Pore‐Scale Multiphase Flow of Water and Supercritical CO2 in 2‐D Heterogeneous Porous Micromodels: Flow Regimes and Interface Dynamics

“…Furthermore, this trend implies that the leading edge of the CO 2 front advancing further downstream is not necessarily associated with the same finger but can instead alternate between fingers at different spatial locations as front advancement is more closely tied to local pore flow in the capillary fingering regime. Thus, these results reveal distinct pore‐scale invasion mechanisms for capillary and viscous fingering and are in agreement with previous experimental studies in rough fractures (Chen et al, ; Hu et al, ) and LBM simulations of flow in 3‐D porous rock media (Tsuji et al, ; Yamabe et al, ). For reference, x lead / L = 1 signifies the breakthrough of the CO 2 phase at the end of the porous section of the micromodel.…”

“…Wang et al () studied the crossover for supercritical CO 2 displacing water ( ) in a homogeneous micromodel over a Ca range of and noted a substantial decrease in CO 2 saturation at and −5.21, which is consistent with the findings of Lenormand et al (). Chen et al () investigated the crossover during water displacing oil ( ) in a hydrophobic rough fracture for and observed that the saturation of the invading fluid first decreased and then increased with increasing Ca , with the minimum value occurring at or , depending on M . The existence of a minimum value was attributed to the fact that both fingering propagation toward the outlet and void filling in the transverse/backward directions were suppressed during crossover.…”

Connecting Pressure‐Saturation and Relative Permeability Models to Fracture Properties: The Case of Capillary‐Dominated Flow of Supercritical CO2 and Brine

“…Consequently, the immiscible two‐phase flow regime gradually deviates from IP (Figure a) with increasing Ca and nonwetting phase saturation (Figures b and c). This transitional behavior of increasing S n with Ca was suggested in recent experimental observations (Chen et al, ) and earlier theoretical work (Lenormand et al, ) when a low‐viscosity fluid displaces a viscous fluid. It also agrees well with previous numerical results (Yang et al, ) showing that increasing Ca will yield a relatively higher S n during the primary drainage process when viscosities of the two phases are identical.…”

“…That is, with increasing capillary number, Ca = μu /γ , where u is mean velocity, viscous forces gradually become more important but still less significant than capillary forces. Since u = b 2 /(12 μ )·Δ P / L , and since in our simulations the maximum Δ P = ~1.4 × 10 3 Pa, L = 1 m, and < b > =5 × 10 −5 m, which therefore result in a maximum Ca = ~9.72 × 10 −6 , capillary forces fairly dominate over viscous forces (Chen et al, ). Consequently, the immiscible two‐phase flow regime gradually deviates from IP (Figure a) with increasing Ca and nonwetting phase saturation (Figures b and c).…”

“…Here the viscosity ratio = μ w / μ n = 0.048 ≪ 1. In principle, with a sufficient increase in pressure gradient, the flow regime would transition from capillary fingering, to the crossover, and end up with viscous fingering; this was observed in recent experiments (Chen et al, ).…”