Oil is a high-carbon energy source that is widely used today. It is widely used in the world as a fuel and petrochemical industry due to its abundant content, wide application and mature mining methods. Although oil is abundant today, the oil recovery rate has never reached 100%. This is because the formation of oil is related to biological sedimentation and geological changes. The physical and chemical properties of oil minerals vary from one oil field to another, especially in terms of viscosity and fluidity. Industrial production focuses on efficiency. As the foundation of the oil industry, oil mining pays special attention to mining efficiency. However, directly mining crude oil with high viscosity and poor fluidity brings many difficulties to mining.
With the increasing demand for oil and the gradual reduction of oil reserves, it is becoming more and more important to improve the recovery rate of oil fields. After primary oil recovery (relying on the natural energy of the formation to produce oil) and secondary oil recovery (using water injection and gas injection technology to supplement the energy of the reservoir to produce oil), the residual oil in the reservoir is still more than 50%, so tertiary oil recovery technology has been widely studied.
Tertiary oil recovery refers to the method of exploiting the discontinuously distributed residual oil trapped in the reservoir voids by other physical, chemical or biological methods. It can be divided into four types: thermal drive, miscible drive, chemical drive and microbial oil recovery. Among them, chemical drive includes alkali drive, polymer drive, surfactant drive and composite drive. When using chemical drive, it is necessary to consider multiple factors such as formation water salinity, crude oil pH value and viscosity, reservoir temperature and heterogeneity. Among chemical oil recovery agents, surfactants have unique surface activity. Whether as a main agent or an auxiliary agent, they have become a very potential oil recovery method and have attracted the attention of researchers.
Surfactants refer to substances that can significantly change the interface state of their solution system by adding a small amount. Surfactants are amphiphilic and contain both hydrophilic and hydrophobic groups. Hydrophilic groups are usually polar groups, such as carboxylic acids, sulfonic acids, amines, etc.; while hydrophobic groups are usually non-polar groups, such as alkanes or long chains of aromatic hydrocarbons. Surfactants can be divided into ionic and non-ionic types. Ionic types include anionic, cationic and zwitterionic types. These types of surfactants are used in tertiary oil recovery.
Nonionic surfactants refer to a type of surfactant that does not produce ions in aqueous solution. Commonly used nonionic surfactants include fatty alcohol polyoxyethylene ethers, alkylphenol polyoxyethylene ethers, oleic acid polyoxyethylene esters, etc. This type of surfactant has good emulsification ability, strong salt resistance, and low critical micelle concentration. However, its turbidity point is low, and it cannot be used in formations that exceed its turbidity point. It is not alkali-resistant and has a high price, which limits its application.
Anionic surfactants are the most common surfactants in tertiary oil recovery. Commonly used anionic surfactants include petroleum sulfonates, alkylbenzene sulfonates, petroleum carboxylates, etc. Anionic surfactants have a high turbidity point and a small amount of adsorption on the sandstone surface, which can increase the surface charge density of the rock. However, its salt resistance is poor, the critical micelle concentration is high, and it is not suitable for underground environments with high temperature and high salt.
Commonly used cationic surfactants are quaternary ammonium surfactants. Since cationic surfactants are easily adsorbed or precipitated by the formation, they have poor ability to reduce the interfacial tension of oil and water and are generally not suitable for tertiary oil recovery.
Zwitterionic surfactants refer to a type of surfactant that has both anionic and cationic groups in the hydrophilic group. It mainly includes amine oxide type, imidazoline type, betaine type, amino acid type, etc. The outstanding feature of zwitterionic surfactants is that they have an isoelectric point, and their own charge changes with the pH value of the solution. Zwitterionic surfactants have good cleaning, emulsification and foaming properties, and do not react with divalent cations in the formation. However, due to the complex preparation process and high production cost of zwitterionic surfactants, they can only be used in certain specific environments.
The order of oil recovery effects of the above four types of oil recovery surfactants is: non-ionic > anionic > cationic > zwitterionic. Studies have shown that when the water content is lower than 70%, the injection of 0.2% surfactant by mass is more effective. When the mass fraction of the surfactant exceeds 0.3%, the interfacial tension will rise.
With in-depth research on surfactants for oil displacement, a series of new surfactants with excellent properties have been developed. These surfactants not only have better surface activity, but also have significantly improved temperature and salt resistance. improve.
Requirements for surfactants in tertiary oil recovery:
(1) It has high surface activity at the oil-water interface, has appropriate solubility, cloud point, and pH value, and reduces the adsorption of crude oil by rock formations.
(2) The amount of adsorption on the rock surface should be small.
(3) There should be a large diffusion speed in the formation medium.
(4) When the concentration in water is low, it should have strong oil displacement ability.
(5) It should have the ability to prevent the occurrence of side reactions of other chemical agents, which is the so-called "retardant property".
(6) The surfactant used for water injection should take into account its relationship with the mineral components of the formation, the composition of the formation water injection water, the formation temperature, and the depletion degree of the oil reservoir.
(7) It should have the ability to withstand high temperature and high salt concentration in the formation.
(8) It has high economic value and has an advantage in input-output ratio.
A surfactant formed by chemically bonding two or more identical or nearly identical surfactant monomers together at or near the hydrophilic head group with a linking group is called a gemini surfactant. Gemini surfactants are more easily adsorbed on the gas/liquid surface, thereby more effectively reducing the surface tension of the aqueous solution. A low concentration of gemini surfactant solution can obtain a lower surface tension. Compared with ordinary surfactants, the adsorption amount of gemini surfactants on the rock surface can reach the lowest, and it also has higher stability under high temperature and high salt conditions.
Wuhan Bolachem New Material Co., LTD.'s PT-420/PT-440/PT-465/PT-2502 series products are acetylene glycol polyoxyethylene ethers, which are typical Gemini non-ionic surfactants with super wettability. They can be used as good imbibition aids in low-permeability reservoirs, which can change the wettability of rocks and make them hydrophilic. Water will invade the matrix rock along the smaller pore throats, and the absorbed water will displace the crude oil from the low-permeability matrix rock along the larger pore throats. After the fracture oil is driven out, it will be compensated by the injected water. Due to the capillary imbibition effect, water can replace and displace more crude oil in the matrix rock into the fracture system, thereby greatly improving the oil recovery rate.
