Foaming Agent-surfactant Principle of Action

May 10, 2021

The surface activity of the foaming agent The foaming agent is a surfactant, and its surface and interface properties are important indicators. The surface tension and interfacial tension of the foaming agent should be measured first. Since the injected formation is a gas-liquid two-phase, it is necessary to measure the surface tension and interfacial tension of the mixed solution injected into the formation and the vapor-condensed aqueous solution. The measurement method may be carried out by the hanger method or other surface tension measurement methods.

The test results show that different foaming agents have lower surface tensions, and the surface tensions of condensate and solution are not exactly the same. This difference is due to the different distribution of surfactants in solution and steam. At the same time, it can be seen that the pH value also has a certain influence on the distribution of surfactants. The interfacial tension is the result of the interaction between the aqueous surfactant solution and the crude oil, and directly reflects the magnitude of the interaction between the two. The selected foaming agent should be able to greatly reduce the interfacial tension between oil and water. There are currently two main methods for interfacial tension testing, one is the hanging drop method, and the other is the rotating drop method.

The former is the discharge of one phase from the capillary to the other phase. The geometric size of the droplet at the moment it leaves the capillary is a function of the interfacial tension. The latter is the high-speed rotation of low-density droplets in a high-density liquid, and there is a functional relationship between its rotation speed, density difference, temperature, etc. and the interfacial tension. Both methods have specialized instruments. Temperature and pressure have a certain influence on the interfacial tension, but the determination of interfacial tension is usually to find out the change rule of interfacial tension with other factors and to screen the formula. Therefore, most of the tests are carried out under normal pressure. Since temperature has a relatively large influence on the viscosity of crude oil, it is easier to prepare samples when the temperature is high. However, due to the limitation of the temperature range of the instrument, the general test temperature is 50～90℃. Table 2 shows the interfacial tension between several surfactants used as foaming agents and corresponding crude oils.
2 Thermal stability of foaming agent Steam foaming agent is used at high temperature, therefore, its thermal stability is also an important technical indicator. To evaluate thermal stability, use a dedicated thermal stability evaluation system-high-pressure evaluation tank, roller furnace, etc., or use high-pressure reactors and other equipment capable of high-temperature and high-pressure experiments. If a high-pressure container is used, fill the evaluation solution to 2/3 or less of the container, seal it under nitrogen purge, and place it in a roller furnace or corresponding oscillating constant temperature device after inspection. If an autoclave is used for evaluation, it can be replaced with nitrogen after filling the evaluation solution. After loading the sample, it can be continuously stirred under controlled temperature. Generally, the evaluation temperature is above 250'12 and the time is above 10d. The evaluation temperature and evaluation time can also be determined according to the specific conditions of the oil field. After the sample has been kept at a constant temperature for a long time, it can be taken out and tested according to the required indicators. Generally, the temperature is around 10d, and the structure and performance should not change too much.
3 Foaming performance
The foaming performance of foaming agent-based surfactants is evaluated based on the amount of foam generated and foam stability. Stable foams are generally relatively small and are not prone to coalescence. During evaluation; put a certain amount of drag foaming agent solution in the foamer, and determine the specific requirements of the foamer by measuring. Generally, the foamer with a dosage of 15ml can be selected. Then inject a certain amount of air, and record the volume of foam and the change of foam volume over time. The high temperature foaming performance is similar to the normal temperature foaming performance; but the whole experiment process needs to be performed at high temperature. Table 3 shows the foaming properties of different foaming agents at room temperature and 190°C.

4 Adsorption loss Almost all surfactants will adsorb on the surface of rock minerals in the reservoir, and foaming agents are no exception. The adsorption that occurs on the surface of rock and mineral is an ineffective loss of foaming agent, and the amount of adsorption should be as small as possible. There are two main ways to solve this problem, the use of surfactants with small adsorption capacity and the use of sacrificial agents. Studying the adsorption loss of foaming agent is the same as studying the adsorption loss of other surfactants. Not only dynamic, but also static should be considered. Dynamic adsorption is to extract the natural rock sample, then saturate the formation water, do a flow experiment with a foaming agent, detect the change in outlet concentration, and calculate the adsorption loss after passing through the core. Static adsorption is to take a certain amount of cleaned oil sand, add a certain amount of foaming agent solution, shake, and then test the concentration of the foaming agent, and calculate the amount of adsorption based on the concentration difference. In practical applications, the foaming agent exists in the form of a liquid film, and the adsorption loss is smaller than the result measured in the aforementioned experiment. But as an economical and practical system, the smaller the adsorption capacity, the better. The adsorption capacity of common foaming agents on natural cores is about 0.3mg/g. 5 Steam foam plugging performance and displacement effect Like other methods of enhancing oil recovery, the plugging performance of steam foam can also be tested by macroscopic and micro (micro) seepage simulation experiments. The experimental equipment is mainly composed of ink: steam generating device; model holder; metering and other auxiliary devices. As the high-pressure steam is used, the entire system should be able to withstand high pressure and high temperature. In the plugging performance test, the main observation is the change of permeability before and after steam injection, and the change of the resistance factor is generally used to evaluate the plugging effect. Due to the difference in crude oil properties, the microscopic oil displacement mechanism should be studied at the same time to help screen and adjust the formula. 

The principle of action is mainly due to the low density and viscosity of steam, good fluidity, and it is easy to cause gravitational separation and lead to overlap and steam channeling, which will make the sweep coefficient and displacement effect worse. The use of foaming agent can produce foam in the formation. Because the foam is a low-density dispersion system, it has a higher apparent viscosity, and has a tendency to expand underground and forms a gas resistance effect in the steam channeling zone. It can be used in steam channeling. A foam plugging layer is formed on the channel to control the steam migration rate, improve the sweeping effect, and increase the steam efficiency, thereby increasing the recovery factor.