3.1. Selection of the Foaming Agents
To optimize the foaming agent, the selected surfactants included anionic, cationic, and nonionic surfactants. The foaming properties of different CGA systems prepared by adding various modifiers were compared. SDBS, SDS, OP-10, Tween-20, CTAB, and SYHSY were all obtained from Sinopharm Chemical Reagent Co., Ltd. The experiments were conducted at 15 °C, with a stirring time of 4 min and speed of 8000 rpm, to study the foaming volume and half-life period of SDBS, SDS, OP-10, Tween-20, Tween-80, and SYHSY at a concentration of 5 g/L, and the results are shown in
Figure 2.
The results indicated that the addition of SDS solution attained the largest foam volume, almost 580 mL, followed by OP-10 at 455 mL, and the foam volume when using Tween-80 was the smallest at 290 mL. SDS also had the maximum half-life period, almost 9 min. The half-life period of the similar surfactant SYHSY was relatively shorter, close to 7 min, and that of OP-10 and SDBS was approximately 5 min. In this study, validation experiments were also conducted. CGA (610 mL) was obtained by adding 5 g of SDS foaming agent to 100 mL of water at 15 °C, stirring for 3 min at a speed of 8000 rpm. The bubble diameter range of the system was 0.5–20 μm, and the average diameter was 5.7 μm. The results were basically consistent with the literature [
9].
3.3. Selection of the Modifier Amount
To determine the proper modifier amount, experiments were conducted at 15 °C, with a stirring time of 4 min, a speed of 8000 rpm, and a foaming agent SDS concentration of 5 g/L; the results are shown in
Figure 4.
The results showed that the foam volume and half-life period of the SDS system firstly decreased and then tended to stabilize with increasing modifier concentration. When the concentration of GXJ-C reached 0.1 g/L, the foam volume and half-life period basically remained stable. The results showed that, when modifier GXJ-C was added to the SDS system, the surface energy of the microbubbles formed in the system slightly increased and the system stability decreased, resulting in decreases in the foam volume and half-life period of the foaming system; however, the decrease range was not large, and the overall effect on the foaming performance of SDS was not notable.
3.4. The Effects of Stirring Time, Speed, and Temperature on the Modified CGA Foam
At a modifier GXJ-C concentration of 0.1 g/L and a foaming agent SDS concentration of 5 g/L, the effects of the stirring time, speed, and temperature on the properties of the modified CGA foam were studied, and the results are shown in
Figure 5. It should be pointed out that, since the system generated very little foam at stirring speeds lower than 5000 rpm, the conditions at stirring speeds ranging from 5000–10,000 rpm were mainly studied.
The results showed that the stirring time, speed, and temperature had a large influence on the foaming properties of the modified CGAs, but the influence rules were different. When the stirring time was 3 min, the modified CGA foaming volume and half-life period basically reached the equilibrium state. Within the experimental investigation range, a higher stirring speed led to a larger foam volume and a longer half-life period. When the stirring speed was 9000 rpm, the modified CGA foaming volume basically no longer increased, and the half-life period was no longer extended. When the preparation temperature increased from 15 °C to 35 °C, the modified CGA foaming volume decreased by 40%. For the same half-life period, the modified CGA foaming volume was only 50% of that at a preparation temperature of 15 °C. When the temperature increased, the modified CGA foam volume tended to remain stable with the half-life period.
3.5. The Performance of the Modified CGA Foam Flooding
Water flooding, CGA foam flooding, and modified CGA foam flooding were carried out on three sandpack cylinders with similar permeability and oil saturation values (V = 9.5 mL/min). The injection pressure and recovery results are shown in
Figure 6.
The results showed that the injection pressure during water flooding increased over time. After 8 min, the pressure stabilized at approximately 3.1 MPa. Upon increasing oil phase displacement, the pressure gradually decreased; the pressure during CGA flooding and modified CGA flooding reached a peak at 11 and 12 min, respectively, and the oil phase started to be displaced. This phenomenon was related to the pressure accumulation process of foam in the oil zone. The recovery ratio results revealed that the water flooding oil yield was 40.3%, the CGA flooding oil yield was 54.7%, and the modified CGA flooding oil yield reached 63.6%. It is clear that the modified CGA system had a superior recovery effect.
In addition, a series of gradient displacement experiments with the same sandpack cylinder were carried out. That is, water flooding was firstly performed, then CGA flooding, and finally modified CGA flooding. The experimental results are summarized in
Table 2.
The data in
Table 2 indicate that, for the gradient displacement experiment with the same sandpack cylinder, the recovery ratio was 42.1% by water flooding, 10.7% by CGA flooding, and 16.4% by modified CGA flooding. It can be observed that the CGA flooding recovery was 10.7% on top of that of water flooding, and that the modified CGA flooding recovery was 16.4% relative to that of CGA flooding, indicating that the recovery efficiency of modified CGA flooding was notable.
In summary, the displacement experiments with different sandpack cylinders and the gradient displacement experiment with the same sandpack cylinder all demonstrated that the displacement effect of modified CGA flooding was better than that of water and CGA flooding.
To further clarify the mechanism, experiments were carried out from two aspects: the interfacial tension between the foam system and mixed crude oil, and the content of
n-heptane asphaltene in gradient flooding with the same sandpack cylinder. The measurement process of the interfacial tension was as follows: in a constant-temperature vessel, crude oil slowly formed droplets through the tip of the tube that fell into the CGA system or modified CGA system. The interfacial tension was calculated according to the equation, γ = f·V·ρ·g/R, where V is the measured crude oil volume, ρ is the crude oil density, g is the acceleration due to gravity, R is the emitter radius, and f is a correction factor [
15]. The asphaltene content was determined with
n-heptane as the solvent, and, according to the industry standard method, the relative error was within 3%.
The results (
Table 3) showed that the interfacial tension of the mixed crude oil and foam system was reduced from 3.29 to 2.68 mN∙m
−1 by adding 0.1 g/L modifier GXJ-C to the CGA system.
Figure 7 shows the asphaltene content data of the crude oil with different system gradient displacements. The figure reveals that the asphaltene content in the oil recovered by water flooding was 4.66%, and that the asphaltene content in the oil recovered by CGA flooding was slightly lower, at 4.44%. However, the asphaltene content in the oil recovered by modified CGA flooding was 5.48%. Compared with the asphaltene content in the mixed crude oil of 4.87%, the changes in asphaltene content in the oil recovered by water flooding, CGA flooding, and modified CGA flooding were −4.31%, −8.83%, and +12.53%, respectively. As the modifier is a crude oil fraction, which is miscible with crude oil, the modified CGA system formed by adding GXJ to CGAs as a modifier clearly reduced the interfacial tension and played a role in dissolving the heavy components in crude oil.
The experimental results showed that the modified CGA system had a good oil displacement effect. On the one hand, it reduced the oil-water interfacial tension, while, on the other hand, it had an elution effect on the asphaltenes deposited in the pores. That is, in the process of CGA flooding, small foam particles firstly entered the large pore channels, and the subsequent displacement fluid was forced to enter the small pore channels due to the foam resistance to displace the oil that could not be reached in water flooding, which enlarged the influenced volume. Compared with conventional CGAs, the modified CGAs reduced the interfacial tension between oil and foam and exerted a certain elution effect on the asphaltenes in crude oil. An emulsified oil zone was more readily formed with a notable oil displacement effect.