Foam Flow Behavior in Bentheimer Sandstone Cores

Foam injections in sandstone cores were performed by co-injecting CO2 gas and surfactant solution with a total injection rate of 50 cc/h. The core plugs were initially 100% saturated with brine, but two pore volumes of surfactant solution were injected prior to the co-injection. Foam was pre-generated in a foam generator located close to the core holder inlet. Measurements of water production, electrical resistance and inlet and outlet pressures during the co-injections have been evaluated.

Results from four foam injections are presented in Figures 49-52. One pore volume of surfactant solution injection is shown before the foam injection, to emphasize the variation in pressure gradient between pure gas injection and foam injection. Time foam entered the core plugs (foam start) was calculated based on inlet dead volume in the system, and injection rate. Water saturations calculated by water production measurements are not accurate relative to time, because the dead volume in the outlet tubing and the injected volume of surfactant solution have been subtracted from the total water production, assuming constant water production from the core plugs, and no further water production after gas breakthrough.

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Sw,res - Baseline S2i-11 Sw,prod - Baseline S2i-11 ∇P - Baseline S2i-11

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Uncertainties in the pressure gradients are calculated based on the uncertainties in the ESI pressure transducers, using formulas presented in Appendix B. A sharp and relatively large increase in pressure gradient, followed by fluctuations around a certain interval, is observed at the start of foam injection in core S2i-12, shown in Figure 51. The sudden increase in pressure gradient can indicate stable foam (Kam and Rossen, 2003). A smaller pressure increase is also observed in cores S2i-7, Figure 49, and S2i-9, Figure 50, before stabilizing around a certain interval. In core S2i-13, Figure 52, the pressure gradient increased more steadily during the entire foam injection, before it ceased at approximately 3.2 PV injected, when the resistivity stops decreasing. This indicates a slow displacement with late breakthrough.

It was not possible to define a time of foam breakthrough in the outlet tubing, since pure surfactant solution would be produced if either foam was not generated, or if the foam was broken down before reaching the end of the tubing. However, foam was observed as production at some point in all the experiments.

Figure 49 – Water saturation and pressure gradient during co-injection of CO2 gas and surfactant solution in sandstone core S2i-7. The core was initially 100% saturated with brine, but was flushed with two pore volumes of surfactant solution before foam injection. One pore volume of surfactant solution injection is included in the plot. Two pore volumes were injected during co-injection, with gas fraction 0.7 and injection rate 50 cc/h. Start of co-injection is indicated with the black, dashed vertical line. Water saturations (left axis) were calculated using resistivity and Archie’s second law, and water production measurements conducted during the injection, while pressure gradient (right axis) is based on measured differential pressures. Error bars in pressure gradients are calculated based on uncertainties in ESI pressure transducers.

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Sw,res - Foam S2i-7 Sw,prod - Foam S2i-7 Foam start ∇P - Foam S2i-7

Surfactant Co-injection of gas and surfactant

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Figure 50 – Water saturation and pressure gradient during co-injection of CO2 gas and surfactant solution in sandstone core S2i-9. The core was initially 100% saturated with brine, but was flushed with two pore volumes of surfactant solution before foam injection. One pore volume of surfactant solution injection is included in the plot. Two pore volumes were injected during co-injection, with gas fraction 0.7 and injection rate 50 cc/h. Start of co-injection is indicated with the black, dashed vertical line. Water saturations (left axis) were calculated using resistivity and Archie’s second law, and water production measurements conducted during the injection, while pressure gradient (right axis) is based on measured differential pressures. Error bars in pressure gradients are calculated based on uncertainties in ESI pressure transducers.

Figure 51 – Water saturation and pressure gradient during co-injection of CO2 gas and surfactant solution in sandstone core S2i-12. The core was initially 100% saturated with brine, but was flushed with two pore volumes of surfactant solution before foam injection. One pore volume of surfactant solution injection is included in the plot. Two pore volumes were injected during co-injection, with gas fraction 0.7 and injection rate 50 cc/h. Start of co-injection is indicated with the black, dashed vertical line. Water saturations (left axis) were calculated using resistivity and Archie’s second law, and water production measurements conducted during the injection, while pressure gradient (right axis) is based on measured differential pressures. Error bars in pressure gradients are calculated based on uncertainties in ESI pressure transducers.

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Foam start Sw,res - Foam S2i-9 Sw,prod - Foam S2i-9 ∇P - Foam S2i-9

Surfactant Co-injection of gas and surfactant

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Sw,res - Foam S2i-12 Sw,prod - Foam S2i-12 Foam start ∇P - Foam S2i-12

Surfactant Co-injection of gas and surfactant

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Figure 52 – Water saturation and pressure gradient during co-injection of CO2 gas and surfactant solution in sandstone core S2i-13. The core was initially 100% saturated with brine, but was flushed with two pore volumes of surfactant solution before foam injection. One pore volume of surfactant solution injection is included in the plot. Two pore volumes were injected during co-injection, with gas fraction 0.7 and injection rate 50 cc/h. Start of co-injection is indicated with the black, dashed vertical line. Water saturations (left axis) were calculated using resistivity and Archie’s second law, and water production measurements conducted during the injection, while pressure gradient (right axis) is based on measured differential pressures. Error bars in pressure gradients are calculated based on uncertainties in ESI pressure transducers.