Oilfield Chemistry: Chemical Flooding Technology
Increase the reach factor
• Influencing factors: heterogeneity, water-oil flow ratio
• The main method to improve the spread coefficient: change the fluidity of the oil displacement agent and oil.
Flow is a measure of the ability of a fluid to pass through a porous medium
Main methods to improve the efficiency of oil washing
Ø Change the wettability of the rock surface
Ø Reduce the adverse effects of capillary resistance
The fluidity of the oil displacement agent is much greater than the fluidity of the oil. During oil displacement, the oil displacement agent bursts into the oil well along the high permeability layer. In order to improve the sweep coefficient of the oil displacement agent, it is necessary to reduce the fluidity of the oil displacement agent and (or) increase the fluidity of the oil.
The main method is to change the wettability of the rock surface and reduce the capillary resistance.
Chemical flooding (chemical flooding)
Ø Polymer flooding method (polymer flooding)
Ø Surfactant flooding method (surfactant flooding)
Ø Alkali flooding (alkali flooding)
Miscible flooding (miscible flooding)
Ø Hydrocarbon miscible flooding (hydrocarbon miscible flooding)
Ø Non-hydrocarbon miscible phase flooding (non-hydrocarbon phase flooding)
Thermal oil recovery (thermal recovery)
Steam drive method (steam drive)
Ø Oil layer in-situ combustion method (burning oil layer)
Ø 1 polymer flooding
Ø 2 Surfactant flooding
Ø 3 Alkali flooding
Ø 4 compound drive
Polymer flooding refers to the method of using polymers as oil displacement agents to enhance crude oil recovery. The method of using water-soluble polymers to increase water phase viscosity, improve fluidity ratio, and stabilize the displacement front is a method.
Ø Also known as:
v polymer solution flooding
v polymer enhanced water flooding
v Thickening water flooding and tackifying water flooding
Mainly to improve the sweep coefficient, suitable for heterogeneous heavy or heavy reservoirs. The higher the penetration rate, the polymer with large relative molecular weight will not clog the formation. When the penetration rate is lower than 20mD, use a polymer with low relative molecular weight.
Second, polymer flooding is the most commonly used polymer
Polymers used in oil layers have specific requirements: good tackifying properties, high thermal stability, good chemical stability, shear resistance, and little adsorption in oil layers. The polymer main chain should be a carbon chain (good thermal stability), with a certain amount of negative ionic groups (good tackifying effect) and a certain amount of non-ionic hydrophilic groups (good chemical stability).
HPAM for oil displacement has the following basic characteristics:
(1) High molecular weight: The molecular weight of HPAM for general oil displacement is 10 million to tens of millions.
(2) Polydispersity: The molecular weight of HPAM is heterogeneous and is a mixture of homologous polymers with different molecular weights.
(3) Diversification of geometric structures: polymer geometries are wired, branched, and body-shaped.
(4) Stable physical and chemical properties: HPAM has stable chemical properties and special physical properties to meet the requirements of oil displacement.
The thickening of water by polymers is caused by the following reasons:
(1) Beyond a certain concentration, polymer molecules become entangled with each other to form structures, resulting in structural viscosity.
(2) Hydrophilic groups in the polymer chain are solvated (hydrated) in water.
(3) Ionic polymers dissociate in water to form a diffusion double layer, generating many segments with the same charged symbol. Polymer molecules form loose random coils in water, which have good tackifying ability.
1. Definition: The effect of salt on the viscosity of a polymer solution that has a specific effect (viscosity reduction).
2. Reason: After the salt is added, the diffusion electric double layer formed by the carboxyl group and sodium ions is compressed by the salt, which reduces the negative electricity of the chain segment, and the HPAM molecules form a tight random coils, thus greatly reducing the thickening ability of water.
It is best to mix the polymer with water, and inject fresh water before and after the plug.
By increasing the viscosity of water and retention in the porous medium, the polymer can reduce the penetration rate of the porous medium to water, reduce the water-oil fluidity ratio, increase the sweep coefficient, and improve the crude oil recovery.
Fourth, the flow characteristics of polymers in porous media
The penetration rate of the polymer solution through the porous medium decreases, which is expressed by the residual resistance coefficient.
The reason for the formation of residual resistance is the retention of the polymer in the porous medium. There are three ways to remain in the porous medium: adsorption, mechanical capture and physical blockage.
1. Adsorption
Polymer adsorption refers to the phenomenon that polymers accumulate on the surface of rock pore structures through dispersion force, hydrogen bonding, and electrostatic action. NaCl concentration increases, adsorption capacity increases; molecular weight increases, adsorption capacity increases; temperature increases, adsorption capacity decreases. See Table 3-1
The factors affecting the adsorption capacity are:
(1) The composition and structure of the rock. Clay minerals > carbonate > sandstone,
Montmorillonite > illite > kaolinite;
(2) The type, molecular weight and concentration of the polymer. The same polymer with amide group, M is large, and the adsorption capacity is large;
(3) The influence of salt content and pH in water. The lithology is the same, the polymer is the same, the higher the salt content, the greater the adsorption capacity, and the larger the pH value, the smaller the adsorption capacity;
(4) As the temperature increases, the molecular motion intensifies and the adsorption capacity decreases.
(5) Dynamic adsorption is less than static adsorption (with high-speed flow desorption).
2. Mechanical capture
Polymer trapping refers to the phenomenon in which random coils of polymer molecules smaller than the diameter of the pore throat are left outside the pore throat through "bridging" (Figure 4-2).
The polymer structure is soft, and the length is consistent with the flow line when flowing at high speed, and it is easy to enter the pores smaller than the diameter of the polymer. After the flow rate is reduced, the stress-relaxed polymer returns to its original state and is trapped in the pores.
Mainly due to the structure of the polymer, the flexible polymer coil is large, and the trapping effect is large; the pores are small and easy to trap.
The capture of the low permeability layer is the main mechanism of retention (reduced penetration rate), and the adsorption of the medium permeability layer is the main mechanism of retention (reduced penetration rate).
3. Physical blockage
Physical blockage refers to the blockage caused by the chemical reaction between various water-insoluble substances in the polymer solution or the polymer solution and the fluid in the formation or formation to form sediment. Physical blockage retention is irreversible.
Three kinds of retention tend to occur simultaneously, especially adsorption and mechanical capture. Appropriate retention amount is conducive to chemical oil flooding. If the retention amount is too large, the polymer cannot flow to the expected position, which affects the ripple coefficient and is unfavorable to chemical oil flooding. If the retention amount is too large, it will cause formation damage.
4. Rheological properties of polymer solutions in porous media
At the high shear rate near the injection well, it exhibits expansive flow, and the viscosity increases with the increase of shear rate.
V. Requirements for selecting polymers for oil displacement
Basic principles:
(1) Good viscosity. Adding a small amount can increase the viscosity of the solution;
(2) Good thermal stability. Viscosity does not decrease significantly at formation temperature;
(3) Good chemical stability. No chemical reaction with formation water and injected water; good compatibility, no precipitation with formation Ca2 +, Mg2 + plasma;
(4) less retention. Less adsorption in the formation;
(5) Strong shear resistance. Less mechanical degradation through pumps and boreholes;
(6) Wide range of sources, low price.
There are currently three types of polymers commonly used:
1. Synthetic polymers. Such as polyacrylamide PAM, partially hydrolyzed polyacrylamide HPAM, polyvinyl alcohol, etc.;
2. Natural polymers. Natural vegetable gum and its derivatives: hydroxylated gua gum, modified alginate and cellulose derivatives, such as carboxymethyl cellulose CMC and hydroxyethyl cellulose HEC, etc.;
3. Biopolymer. Biopolysaccharide polymer compound XC
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