Experimental Evaluation Study on High Temperature Tolerance Foam for Profile Control of Steam Huff-n-Puff

In the process of steam huff and puff to develop heavy oil reservoirs, steam channeling is the main factor that restricts the development effect, which causes that the swept area of steam is limited and unable to heat heavy oil adequately. It is important to find an economical and efficient method to prevent steam channeling from happening at steam huff and puff production; thus, a DF-2 high temperature tolerance foam system is proposed to assist steam huff and puff production in this paper. Firstly, utilizing high-temperature aging tank and high-speed stirrer device, the foam quality of DF-2 foam and four kinds of other common industrial foams were evaluated and compared; this experiment result shows that the DF-2 foam has the best temperature tolerance. Whereafter, three groups of single-sandpack experiments were carried out to investigate the change of DF-2 foam resistance factor under different conditions, which included the influence of gas liquid ratio, injection pattern, and crude oil, and the blocking capabilities of DF-2 foam were studied. Through the dual-sandpack experiment, the profile control performance and diversion capacity of the DF-2 foaming agents were studied and evaluated systematically. Finally, the DF-2 high temperature tolerance foam agent was applied to on-site production. The sandpack experiment results indicated that the resistance factor of DF-2 foam agent decreased with gas-liquid ratio, and the optimal gas-liquid ratio was 1 : 1. Coinjection of gas and liquid with foam generator is better than slug injection. Crude oil in sandpack can reduce blocking capability to some extent. During the application of the DF-2 high temperature tolerance foaming agent in the steam huff-npuff process in M-12 Oilfield block, it had a good performance of profile control and foam blocking, solving the problem of steam channeling and improving the steam sweep area to increase the steam-oil ratio in the heavy oil reservoir, and the production performance of steam huff and puff was improved to a great extent.


Introduction
Heavy oil reservoir will play an important role in the future oil and gas resource development, which accounts for than 80% of annual heavy oil production in the world [1][2][3][4]. Thermal recovery by steam injection is one of the most important development technologies for heavy oil [5][6][7]. However, due to the low viscosity and high fluidity ratio of steam, the phenomenon of steam channeling generally exists in heavy oil production when utilizing this technology, which decreases the sweep area and the heat efficiency of steam, as shown in Figure 1. Meanwhile, the heterogeneity of the reservoir also can make the steam breakthrough worse, which causes that the steam injection performance will be seriously affected [8,9].
At present, the foam flooding is a profile control method which is widely used in the process of heavy oil thermal recovery, which has become a common measure to improve steam injection performance. Some researches show that steam foam can be formed by adding foaming agent to steam injection. For one thing, steam injection profile can be improved by foam [10][11][12][13][14][15][16]. For another thing, foam can block the large pores and drive the steam into the small pores, inhibiting steam channeling and improving the sweep area of steam [17][18][19][20][21][22][23], as shown in Figure 2. At the same time, foaming agent, as a surfactant, can effectively reduce the oil-water interfacial tension, improving the fluidity of heavy oil [24][25][26][27][28]. The foam property of most foam agents is stably only at low temperatures, and while their performance can be seriously influenced with the rise of pressure and temperature, the problem of steam channeling cannot be solved effectively.
In order to fully utilize the foam properties of the foaming agent to increase the steam swept area and enhance the ultimate oil recovery factor, this paper proposed the DF-2 high temperature tolerance foam system to assist steam huff and puff production. This work designed high-temperature singlesandpack and dual-sandpack experiments to evaluate the temperature tolerance and diversion capacity of the foaming agents. And the high temperature tolerance foaming was applied to the steam huff-n-puff process in M-12 Oilfield. Through the foam resistance and diversion ability, the problems of steam utilization imbalance and too small areal sweep in the thermal recovery of heavy oil in horizontal wells were solved.

Experiments
2.1. Materials. The viscosity of dehydrated crude oil used in this work was 12320 mPa·s, and the density was 946.4 kg/m 3 at 50°C. A high-temperature foam DF-2 (anionic) was used to perform experiments. It was provided in solution containing 42.0% active material. The foam agent was used at a concentration of 0.5 wt% without further treatment. Other four kinds of surfactant were selected from several surfactants commonly used in oilfield for EOR; NaCl (concentration of 17456 mg/L) and CaCl 2 (concentration of 477 mg/L) were applied in the experiments to simulate formation water. The     Figure 3. Before starting the experiments, the sandpack model was placed horizontally. A high-pressure piston pump (model 100DX, with a flow accuracy of <0.25 μL/min and pressure accuracy of <±0.5%) delivered water and foaming agent to the apparatus at a preset. Nitrogen flow was regulated with a gas mass flow controller (model 5850TR). Liquid and gas were mixed before they flowed into the sandpack through the foam generator. The pressure difference between the inlet and outlet of sandpack was measured using differential  Figure 3: The high-pressure and high-temperature displacement system.
3 Geofluids pressure transducers (model 3210PD, with measurement range of <50 MPa and accuracy of 0.1% full scale). Back pressure regulator (BPR) with open error less than 0.01 MPa was used to control the sandpack outlet pressure to 3 MPa. And the outlet fluid was measured by balance (model PL2002, with measurement range of <2100 g and accuracy of <0.1 g). There was a fixed-temperature chamber (accuracy of <1°C) outside of the sandpack, in order to heat the model to certain temperature. The foam properties were evaluated through foam evaluation instrument (foam value, with pressure range of <32 MPa and temperature range of <350°C).
(1) Single-Sandpack Experiments. In the single-sandpack displacement experiment, the sandpack model was filled with silica sand firstly and its parameters such as porosity and permeability were measured. The DF-2 foam was injected through single-sandpack under different situations to reveal the effects of injected gas-liquid ratio, injection mode, and crude oil on the resistance factor. Based on these experiments, the temperature tolerance range and resistance capacity of foaming agent were determined. The experimental scheme is shown in Figure 4(a).
(2) Dual-Sandpack Experiments. The foam profile control experiment which employed dual-sandpack model was carried out to select the foam with better diversion performance; the differential ratio in this experiment is the ratio of permeability between two sandpacks. According to the implemented experiments results, the DF-2 high temperature tolerance foaming agent was selected. The experimental scheme is shown in Figure 4(b).

Results and Discussions
3.1. Temperature Tolerance Experiment. Foam ability is directly decided by the foam volume and half-life. As shown in Figure 5(a), when the temperature increased, the most  4 Geofluids foaming agent foam volume was decreased from 400 mL to 0 mL. However, the reduction degree of the DF-2 foaming agent was lower than that of others. And when the temperature reached 300°C, the foam volume of DF-2, the DF-2 foaming agent still had high foaming capacity with volume reached more than 300 mL. Figure 5(b) illustrates that the half-life of DF-2 foaming agent decreased with increased of temperature, but the degree of reduction was lower than other foaming agents. It should be noted that the half-life of DF-2 foaming agent is still up to 350 s at 300°C. As well as known, foaming volume and half-life are two relatively independent parameters, one measures the foaming difficulty and the quantity of foam, and the other reflects the stability of foam, but neither of them is enough to reflect the performance of foam agent in porous media. Thus, foam comprehensive value, which is the product of foam volume and half-life, was proposed to accurately evaluate the performance of foaming agent in the process of profile control, as shown in Figure 5(c). It is obvious that the DF-2 comprehensive value was minimally affected by temperature; there-fore, the DF-2 foaming agent had superior temperature tolerance.

The Single-Sandpack Foam Resistance Experiment.
Blocking characteristic is the basic and important performance of foam. Furthermore, the resistance factor, which is the ratio of the inlet and outlet pressure difference during foam flooding to the pressure difference during pure water injection, is the key index to measure the foam blocking effect. Large number of experiments showed that the gasliquid ratio, injection method, and injection rate are the important factors affecting the foam resistance effect, and the sandpacks used in three experiments were Nos. 1~3, Nos. 4~6, and Nos. 7 and 8, respectively, and the sandpack parameters are shown in Table 1. The resistance factor of DF-2 high temperature tolerance foam agent under different conditions was tested; meanwhile, its blocking effects were synthetically studied in the laboratory experiment. As shown in Figure 6(a), the foam had the best resistance effect when the gas-liquid injection ratio was 1 : 1. With the increase of  5 Geofluids gas-liquid ratio, the blocking effect of DF-2 foam becomes worse [2], this is because the high gas-liquid ratio will cause the bubble lacking of stability to burst easily, and the foam cannot form effective blocking in the channel [13]. According to Figure 6(b), the foam resistance generated by simultaneous injection of mixed gas-liquid was great. It revealed that high-rate and high-dose foaming agent injection method was suggested in the oil displacement. It can be seen from Figure 6(c) that in the case of oil presence, the resistance factor of 6PV foam in the sandpack was much lower, and the resistance factor curve was unstable as the foam burst rate was accelerated by oil presence.

The Dual-Sandpack Foam Profile Control Experiment.
Foam profile control was determined by resistance ability and diversion performance. Diversion means increasing the resistance of high permeability layer and diverting displaced fluid into low permeability layer; thus, the influence of dif-ferent permeability ratio on profile control capability was studied in this work. The results of DF-2 high temperature tolerance foam capacity to balance flow rate at each sandpack with different level permeability are shown in Figure 7, and the sandpack parameters are shown in Table 2.
In order to better demonstrate the profile control ability of DF-2 foam, DF-2 foam agent was injected in the range of 0~3PV, and the hot water was injected from 3PV. Hot water was injected into the experiments because the steam will condense into hot water because of the great heat loss through the wellbore in the process of steam huff and puff [14]. When the permeability difference was 3.3 (in Figure 7(a)), DF-2 foam preferentially entered the highpermeability sandpack to block the high-permeability sandpack quickly, resulted in that the outflow of fluid was gradually decreased, and the flow rate of low-permeability sandpack increased simultaneously. What is more, from 3PV to 5PV, the liquid flow rate of the dual-sandpack 6 Geofluids became closer during the foam profile control process, which indicated that the profile control performance of DF-2 foam was great. Began from 5PV, the difference of liquid flow rate of the dual-sandpack was enlarged again, which displayed that the profile control effect of DF-2 foam became weak gradually because the gas and foam were displaced out of the sandpack. When the permeability difference is up to 6.4 (in Figure 7(b)), the liquid production rate of the highpermeability sandpack was always greater than the diversion flow rate of the low-permeability sandpack, while the liquid production ratio difference was kept to 2.2 because of foam influence. With the further increase of permeability ratio (differential ratio was up to 9.1), the change of profile decreased, indicating that the duration of profile control was significantly shortened and the DF-2 foam was more easy to flow out of the high-permeability sandpack [14,25]; the duration time of foam profile control reduced obviously, as shown in Figure 7(c). Experiment results revealed that DF-2 high temperature tolerance foam agent had a greater profile control capacity to enhance liquid flow rate of low-permeability sandpack under the low permeability ratio. Meanwhile, the DF-2 foam still had an impressive profile control ability under the high differential ratio as well.

Application of DF-2 Foam in Steam Huff-n-Puff Process of M-12 Oilfield Block
Via several cycles of steam huff-n-puff process, most of the heavy oil field suffered the problems of low sweep efficiency and serious steam fingering. Similarly, a main problem for M-12 Oilfield block is that the high formation heterogeneity and the steam channeling are more serious in the process of steam huff and puff. As one method of profile control, foam can flow and block in the thief zone, increasing resistance and reducing steam mobility. Then, the steam cannot pass through the high-permeability cores and was diverted to 7 Geofluids the low-permeability area. The DF-2 foaming agent has a perfect performance at the high temperature above 200°C. Therefore, the DF-2 foaming agent was optimized for the profile control technology in steam huff-n-puff process of M-12 Oilfield.
Take the M-12 Oilfield block, which is a serious heterogeneous reservoir, as an application example of DF-2 foaming agent. Production performance statistics are shown in Figure 8, after two cycles of steam huff and puff production, oil production was gradually declining and the steam-oil ratio was relatively low. What is more, Figure 9 displays that the temperature difference of temperature horizontal well interval before injecting DF-2 foam agent was large, indicating that the phenomenon of uneven steam injection profile and steam channeling in this reservoir was serious and the steam thermal utilization factor was low. Then, the DF-2 foam agent was injected into this oilfield block to improve the production performance of steam huff and puff in the   8 Geofluids third cycle. Based on the previous experiments, the gasliquid injection ratio was 1 : 1 and injecting a large amount of foaming agent in a short time was adopted as the injection method. From the 3rd cycle to the 5th cycle, due to the preferable control profile performance of DF-2 foam, the oil production increased gradually and the oil production rate reached at 4 tons per day; meanwhile, the oil-steam ratio rose from 0.14 to about 0.25, indicating that the steam swept volume and steam thermal utilization rate were increased. Furthermore, the temperature difference of the horizontal well interval was decreased after injecting DF-2 foam, and the horizontal section reservoir was heated evenly. According to production situation on-site, the DF-2 foam can play an important role in profile control and foam blocking, which can improve the steam injectivity profile and inhibit the steam channeling to enhance heavy oil recovery effectively.

Summary and Conclusions
Through temperature tolerance test, the DF-2 foaming agent was selected with high comprehensive value above 200°C.
With the increased of temperature, the volume and halflife of foam reduced. In the single-sandpack displacement experiment, four factors on foam resistance property under high temperature were studied. The blocking capability increased as the injection velocity and decreased with oil presence. The maximum resistance factor was 84 when the gas-liquid injection ratio was 1 : 1. Coinjection of gas and liquid was better than slug injection. Via dual-sandpack experiments, the foam ability to balance flow rate at sandpacks with different permeability was tested. As the increased of permeability ratio, the liquid production ratio difference stayed around 2.0 because of the foam diversion effect, and the DF-2 high temperature tolerance foam agent had a greater profile control capacity under the low permeability ratio.
Finally, DF-2 foam had a good performance of profile control and foam blocking in the steam huff-n-puff process of M-12 Oilfield block, solving the problem of steam channeling and improving the steam sweep area to increase the steam-oil ratio in the heavy oil reservoir.

Data Availability
The data used to support the findings of this study are included within the article.

Conflicts of Interest
The authors declare that they have no conflicts of interest.