Dependence of pressure drop on surfactant solution flow rate in foam coreflooding experiments through 50 mD limestone

The pressure drop (dP) and apparent viscosity characterize the ability of foam to control gas mobility in core flooding experiments. There is no clarity regarding which factor, total flow rate (qt), foam quality (fg), gas (qg) or liquid (ql) flow rate, determines the dP. The inconsistency in interpretation can be caused by the differences in conditions of experiments, which are carried out at one fixed and other varying parameter. In this study, dP and length of foam drops at the outlet are measured in three types of experiments at 400 and 500 psi for comparison: 1) scans at fixed total flow rate and varying fg; 2) tests at fixed liquid flow rate and varying gas flow rates; 3) tests at fixed gas flow rate and varying liquid flow rates. In foam scans 1, the dP measured at two qt decreased almost linearly due to low foamability starting at fg = 0.3, indicating a high quality regime. The length of foam drops and the bubble size increased with the increase in fg. In tests 2 and 3, there was no direct relationship between dP and qg at a constant ql, but dP increased with increasing ql at constant qg, which is in line with other publications. In addition, there was no direct dependence of the foam texture on the increase in qg and ql. The discrepancy in the dependence of dP on ql and qg between the three experiments was due to the fact that different combinations of ql and qg used in tests 2 and 3 resulted in the same fg and qt, which led to data overlap and data distribution in both high and low regimes. Meanwhile, dP measured in all three experiments was a linear function of the liquid flow rate with a line slope equal to the outlet pressure, regardless of the total flow rate. Due to the mathematical expression, the equation dP = P · ql can be called the equation of ideal foam and used as a test case for further comparisons. Thus, ql is the governing factor in high quality regime for a given surfactant and conditions. The slopes were also equal to the outlet pressures for the dP curves as a function of qt in tests 2 and 3. Irrespective the linear correlation between dP and ql, the deviations were observed at medium values of foam quality at 400 psi. These deviations were accompanied by the simultaneous deviations in the length of foam drops, showing that they were of nonrandom nature. In some deviating points, the foam texture at the outlet changed abruptly and disproportionately to fg changes. This could be the evidence of the intermediate state between the coarse-foam and strong-foam.