Classification, synthesis and application of polyacrylamide
Classification and introduction of polyacrylamide
Industry will contain more than 50% of the acrylamide monomer polymers collectively referred to as polyacrylamide. Its structure is easy to form hydrogen bonds between amide groups, so it has good water solubility and high chemical activity.
Synthesis of polyacrylamide
1. Aqueous solution polymerization: persulfate system (optimal), organic peroxide system, bromate or aluminate system, metal ion system and other initiators are often used.
2. Reverse emulsion polymerization method: W/O (water in oil) emulsifier is used. In order to achieve better emulsifying effect, HLB value of emulsifier after compound is needed, such as HLB value of water phase system is close to that of water phase system.
3. Reverse microemulsion polymerization: emphasis on offshore oil recovery.
Suspension polymerization: a new method.
5. Precipitation polymerization: choose the right solvent, dissolve AM monomer in it, generate polymer and precipitation, can directly get the product powder.
Application of polyacrylamide in oil field
1.PAM as oil displacement agent; 2.HPAM is used as drilling fluid regulator to adjust its rheology, carry cuttings, lubricate drill bits, reduce fluid loss and control water loss; 3.PAM can be used as a water blocking agent without cross-linking, but can also be used with metal sulfate, aluminum salts and other cross-linking, and can also form polymer networks with some resins. 4.PAM as fracturing fluid additive.
Influence factors of polyacrylamide viscosity
1 concentration: approximate pair-number relationship, for polymer polyacrylamide is only a few percent concentration, the solution is very thick. Concentrations above 10% are difficult to handle, and viscosity decreases but not significantly at higher temperatures.
2. PH: non-ionic, PH from acid to alkali transition, amide group into carboxyl group, viscosity increases. When PH is above 10, hydrolysis occurs and viscosity increases rapidly.
3. Shear force: when the shear force is very low, the viscosity is independent of the shear force. When the shear force increases above the critical value, the viscosity decreases significantly.
4. Molecular weight: when the molecular weight is low, the viscosity increases not obviously with the molecular weight, but when the molecular weight increases to a certain value, the viscosity increases significantly. There is an inflection point with the increase of molecular weight of PAM, that is, when the molecular weight of PAM increases to a certain value, the viscosity increases sharply.
Stability of polyacrylamide
When placed at 50℃ or lower, the molecular weight did not change significantly and the viscosity did not decrease significantly. In molecular weight greater than 1.5X106, viscosity loss and molecular weight reduction will occur simultaneously when placed at 75℃ or higher.
The decrease in viscosity is caused by the gradual decrease in hydromechanical volume due to the change of chain phenomenon, and it is generally believed that the oxidation fracture of polymer chain plays an important role. The solution is exposed to high temperatures for weeks or months, resulting in severe viscosity loss.
Flocculation of polyacrylamide
Its molecular chain is very long, and the amide group will form hydrogen bonds with many substances, forming a "bridge", which is conducive to the particle sinking. The viscosity of partially hydrolyzed PAM increases rapidly when aqueous solution of alumina is added.
Hydrolysis of polyacrylamide
Hydrolysis is converted to polymers containing carboxyl groups, known as partially hydrolyzed polyacrylamide. The rate in neutral medium is very low, generally in alkaline (sodium carbonate, sodium hydroxide). PAM industrial production is often used in acrylamide polymerization before adding alkali solution, or in the polymerization of PAM colloid into the alkali manufacturing part of the hydrolysis of polyacrylamide. Anionic PAM with hydrolysis degree of 30% mole was easily obtained. To prepare products with high degree of hydrolysis (more than 70%), copolymerization of acrylamide and sodium acrylate is used.
Hydroxylmethylation of polyacrylamide
Polyacrylamide reacts with formaldehyde to form hydroxymethylated polyacrylamide. The hydroxylmethylation reaction of PAM and formaldehyde can be carried out under both acidic and alkaline conditions. The reaction rate is very fast under alkaline conditions (PH=8~10), and much slower under acidic conditions. Adjust PH of PAM solution to 10.2, add formaldehyde, stir at (32±2) ℃ for 2h, then adjust to PH7.5, add to
The product is hydroxymethylated PAM after heating at 165℃ for 15min in a drum dryer.
Sulfomethylation of polyacrylamide
PAM reacts with NaHSO3 and formaldehyde under basic conditions to form anionic derivative, sulfomethylated polyacrylamide. NaHSO3 can also be added to hydroxymethylated polyacrylamide solution injection, reaction to obtain sulfonomethylpolyacrylamide. This reaction is called sulfonomethylation. The reaction is performed in an alkaline medium (PH=10~13) at a temperature of 50~68℃.
Amine methylation of polyacrylamide
Polyacrylamide and dimethylamine, formaldehyde reaction can form dimethylamine N- methyl acrylamide polymer. The reaction is called the Maniki reaction.
Hofmann degradation reaction of polyacrylamide
Polyacrylamide and sodium hypochlorite or sodium hypobromate reaction under alkaline conditions can be prepared cationic polyethylene imine. The reaction involves adding dilute polyacrylamide solution to a solution containing NaOH and NaClO under agitation, keeping it at room temperature for 1h, and then neutralizing it with hydrochloric acid until PH=8. At this point, due to the high concentration of salt in the solution, polyethylene imine is separated by gelatinous precipitation. Can be used to improve the dry strength of paper.
Crosslinking reaction of polyacrylamide
Polyacrylamide aqueous solution was heated under acidic conditions to form insoluble crosslinked PAM gel through imination reaction. This crosslink can be broken by adding alkali (PH=10~12) and hydrolysis occurs. Crosslinking is produced by heating polyacrylamide or formaldehyde or methylene diacrylamide under acidic conditions.
Polymer flooding has a significant effect on improving oil recovery (EOR), and polyacrylamide can play an important role in regulating the rheological properties of injected water, increasing the viscosity of driving fluid and improving the sweep efficiency of water flooding.
Water is a good solvent for partially hydrolyzed polyacrylamide, adding salt to it will reduce the solubility of HPAM, and different concentrations of methanol can be added to adjust the solubility of HPAM.
1. The charge shielding effect of univalent inorganic salts on HPAM leads to the size shrinkage of HPAM wire, and the effect tends to a limit value with the increase of salt concentration. For strong electrolytes, salt effect is independent of anion type.
2. Charge shielding and crosslinking of HPAM occur simultaneously with Ca ion. In a certain range of Ca ion concentration, the apparent molecular weight increases, but the size of the wire pellet continues to shrink, and the effect of intramolecular crosslinking is more important than that of shielding.
3. The solubility of HPAM in water is better than THAT of PAM, and the conformation of HPAM is longer than that of PAM after the repulsion effect of charge caused by salt effect is eliminated.
Oil displacement mechanism of polyacrylamide
The mechanism is mainly through reducing the water-oil fluidity ratio (increase the viscosity of water, reduce the fluidity of water), reduce the fingering of water, achieve piston displacement, in order to improve the sweep index of displacement agent, so as to improve the oil reservoir recovery.
Change the wettability of rock surface: the polymer has more hydrophilic groups, so that the wettability tension and cohesion of oil are reduced, to achieve the effect of oil displacement; Shear and tensile action; Viscoelastic benefit; Emulsification entrainment.
Degradation of polymers during subsurface flooding
1.HPAM is very sensitive to the shear rate of more than 5000 revolutions per second, and is also very sensitive to time. Under the shear action, it is easy to break the chain and degrade to about 1/4 of its original size.
2. With the increase of the flow rate, the viscosity of the polymer showed a significant downward trend, mainly because there were fewer and fewer macromolecules contributing to the viscosity, and the polymer molecules were subjected to a strong force, and more small molecules were generated by fracture, resulting in a large decrease in viscosity. As it flows through the core, there is some degree of retention and adsorption, and the concentration inevitably decreases, affecting the viscosity.
3. The faster the flow rate is, the faster the chain breaks, which occurs in the middle of the chain.
4. Temperature has little effect on the degradation of polymer, but considering the change of HPAM with temperature, it is considered that the polymer molecular chain tends to stretch, the molecular chain rigidity is strengthened, and it is easier to fracture under the action of external force.
5. The degree of degradation is related to the injection rate of HPAM solution.
The main cause of degradation can only be tensile action. The polymer will creep at a certain temperature and under a certain external force, but when the polymer enters the core at a higher rate, the diameter of the core pore is very uneven. Some can let the polymer through, some can not pass, or is difficult to pass, at the same time the direction of the pore is random, the pore wall is rough, so the direction of the polymer flow is constantly changing in a variety of angles. At this point, the polymer is suddenly stress from different directions, the larger stress will cause larger oscillation and deformation in the polymer, and for molecular chains are relatively soft polyacrylamide molecular chain in a short period, the deformation is always far lags behind that of the stress, segment will be in a continuous collision is becoming more and more stiff, Thus, under continuous and large stress shocks, the stress will increase to the point that the molecular chain will be pulled apart. At the same time, because of the large degree of freedom at both ends of the chain, the mechanical relaxation is faster. So, the bonds in the middle are going to break first. If the polymer is still large enough, a secondary fracture occurs until it can pass through the core or the stress is reduced by slower and slower speeds.
Under the same shear force, the higher the polymer concentration is, the greater the viscoelasticity is, and the oil displacement efficiency increases.