Investigation of foam flow in a 3D printed porous medium in the presence

22 Foams demonstrate great potential for displacing fluids in porous media which is applicable 23 to a variety of subsurface operations such as the enhanced oil recovery and soil remediation. 24 The application of foam in these processes is down to its unique ability to reduce gas mobility 25 by increasing its effective viscosity and to divert gas to un-swept low permeability zones in 26 porous media. The presence of oil in porous media is detrimental to the stability of foams 27 which can influence its success as a displacing fluid. In the present work, we have conducted 28 a systematic series of experiment using a well-characterised porous medium manufactured by 29 3D printing technique to evaluate the influence of oil on the dynamics of foam displacement 30 under different boundary conditions. The effects of the type of oil, foam quality and foam 31 flow rate were investigated. Our results reveal that generation of stable foam is delayed in the 32 presence of light oil in the porous medium compared to the heavy oil. Additionally, it was 33 observed that the dynamics of oil entrapment was dictated by the stability of foam in the 34 presence of oil. Furthermore, foams with high gas fraction appeared to be less stable in the 35 presence of oil lowering its recovery efficiency. Pore-scale inspection of foam-oil dynamics 36 during displacement revealed formation of a more stable front as the foam quality decreased 37 which effectively improved the oil recovery. This study extends the physical understanding 38 of oil displacement by foam in porous media and provides new physical insights regarding 39 the parameters influencing this process. 40

Foam, a dispersion of gas in thin liquid films (named lamella), has been identified as a 69 remedy for these defects due to its unique properties [11][12][13][14]. Foam exhibits apparent 70 viscosity of a few orders of magnitude higher than its constituent gas and liquid in porous 71 media leading to low mobility [3,15,16]. This low mobility is caused not only by trapping of 72 bubbles within pores but also viscous dissipation associated with the moving bubbles through 73 pore throats [17]. The trapping of gas reduces the available pathways for gas flow thereby 74 reducing the gas relative permeability [18]. For foam to achieve its desired efficacy in oil 75 saturated porous media, it must remain stable. However, oil has proved detrimental to the 76 stability of foam [19] which could influence the success of foam in oil displacement 77 applications. 78 The effect of oil on foam stability has been studied by many investigators at bulk [19][20][21] and 79 bubble-scale [19, [22][23][24]. In many cases, the bulk foam test has been used as the 'litmus test' propagation velocities [28][29][30]. While the type of oil plays a significant role on foam 95 destabilisation in the bulk scale tests, Jensen and Friedmann [31] demonstrated that the oil 96 saturation was more influential to the stability of foams than the type of oil in porous media.

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This observation was also reported by Mannhardt and Svorstøl [28] in their study of the 98 effect of oil saturation on foam performance in porous media. Minssieux [25] studied the 99 influence of the foam quality on oil displacement in a sandstone core following the gas- and observations about how foam flow in a porous medium is influenced by oil. The rest of the paper is laid out as follows: In Section 2 we describe in detail the experimental setup, the 119 materials and the experimental procedure used in this study; Section 3 provides the results 120 and discussion and the final conclusions are presented in Section 4.

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The porosity was calculated by measuring the amount of water required to fully saturate the 135 model (i.e. the total pore volume). The pore volume was then divided by the bulk volume of 136 the model to obtain the porosity. This value was also verified by image analysis. The 137 permeability of the model was obtained in the following way: The model was first flooded by 138 CO 2 for 10 minutes at the flow rate of 100 mL/hr to displace the remaining air in the model.

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The model was subsequently flooded with water until 100% water saturation was established.    Table 1   In the next step, the grains were separated from foam using the picture of the empty model as          Consequently, foams with high gas fraction undergo catastrophic destruction in the presence 308 of oil.
309 Fig. 7b and 7c shows qualitatively that the displacement of oil by lower quality foam (i.e. low 310 gas fraction) is more effective compared to the higher quality foam (the latter exhibited 311 fingering phenomena even after one pore volume of foam was injected into the model).

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Bubble collapse and coalescence rate increases as the foam quality increases causing more 313 gas to escape the foam network consequently delaying the formation of stable foams.

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in lower recovery efficiency as more pore volumes of foam are used to recover a certain 316 fraction of oil. It must be mentioned however that, when the recovery efficiency is plotted 317 against time, the rate of oil recovery is higher for 80% compared to 85% foam quality. In our