The effect of partially hydrolyzed polyacrylamides (HPAMs) used for polymer flooding is unsatisfactory under the conditions of high temperature and high salinity. In order to improve the viscosifying ability of HPAM, branched macromolecular skeleton monomer is used to change the linear backbone structure. A new branched hydrophobically modified polyacrylamide (BHMPAM) was synthesized by the free radical copolymerization of functionalized branched macromolecular skeleton monomer, acrylamide (AM), acrylic acid (AA), and hydrophobic monomer hexadecyl-allyl-dimethyl ammonium chloride (C16DMAAC). The properties of polymer solution were characterized; the results of the experiments showed that BHMPAM exhibited the properties of pseudoplastic fluid, and the viscosity of BHMPAM was 345.9 mPa
Currently, the common used oil displacement agent is partially hydrolyzed polyacrylamide (HPAM), the viscosity of which, however, descends drastically with the increase of temperature and salinity [
Compared with the traditional linear polymer, branched macromolecule, as a new highly branched polymer whose structure is similar to three-dimensional sphere, owns its unique features, including shear tolerance, rheology, and thermal stability [
Ethylene diamine (≥99.0%), methyl acrylate (≥99.0%), methanol (≥99.0%), sulfuric acid (95–98%), maleic anhydride (≥99.0%), acrylic acid (≥99.0%), dimethyl sulfoxide, acrylamide (≥99.0%), sodium hydrogen sulfite (≥58.5%), and ammonium persulfate (≥98.0%) were purchased from Chengdu Kelong Chemical Reagent Factory. C16DMAAC is laboratory homemade. HPAMs (viscosity average molecular weight is
The ionic composition of synthetic formation water.
Synthetic formation water | Composition | Na+ and K+ | Ca2+ | Mg2+ |
|
|
|
Cl− | TDS |
Content (mg/L) | 3091.96 | 276.17 | 158.68 | 14.21 | 311.48 | 85.29 | 5436.34 | 9374.12 |
The methods of synthesis and modification of branched polyamidoamine macromolecular skeleton monomer refer to the literature [
The mechanical stirrer and N2-inlet/outlet installation were equipped in a 250 mL, three-neck round-bottomed flask, in which acrylamide and acrylic acid were added at the mass ratio of 4 : 1 with distilled water as solvent, and then a certain quantity of hydrophobic monomer and functionalized branched macromolecular skeleton monomer was added. The initiators (sodium bisulfite and ammonium persulfate) were added at 45°C, and then solution reacted for 12 h. After reaction, the products were purified by methanol several times and then dried for several days under vacuum at 50°C, so BHMPAM were obtained. The result of measuring BHMPAM’s viscosity average molecular weight was
The structure of BHMPAM compared to HPAM.
Referring to China’s National Standards about determination of the dissolution rate of powdered polyacrylamide (GB12005.8-1989), dissolving time was conducted on a DDS-11A conductivity meter at (
For the dissolution of the polymers, a stock solution of 5000 mg/L was prepared by stirring the synthetic formation water (or distilled water) with a mechanical stirrer until a vortex was established. The polymer powder was poured slowly into the vortex and stirred for at least 3 h. This stock solution was subsequently diluted to obtain the desired concentration.
All viscosities were measured at a shearing rate of 7.34 s−1 with Brookfield DV-III at 65°C.
Rheological measurements were conducted on a HAAKE RS600 Rotational Rheometer (Germany) at 65°C. Shear viscosity range was 0.01–200 s−1.
Storage modulus (
The results of dissolving time are shown in Table
The solubility of BHMPAM and HPAM.
Sample | Granularity ( |
Stirring rate (r/min) | Temperature (°C) | Concentration (mg/L) | Time (h) |
---|---|---|---|---|---|
BHMPAM1 | 350~833 | 90 | 30 | 5000 | 2.5 |
BHMPAM2 | 350~833 | 90 | 30 | 5000 | 4.3 |
HPAM1 | 350~833 | 90 | 30 | 5000 | 1.6 |
HPAM2 | 350~833 | 90 | 30 | 5000 | 2.4 |
(1: the solvent is distilled water; 2: the solvent is synthetic formation water.)
As shown in Table
The viscosity of BHMPAM increased swiftly with the increasing polymer concentration when the polymer concentration was between 1000 and 1500 mg/L, as shown in Figure
Viscosity versus polymer concentration of BHMPAM compared to HPAM in synthetic formation water.
The viscosity of polymer was constituted by the structural viscosity and bulk viscosity [
In general, the viscosity of polymer solutions shows a temperature dependent behavior. As shown in Figure
Viscosity versus temperature of BHMPAM compared to HPAM in synthetic formation water (concentration of both polymers: 1750 mg/L).
The influence of temperature on the viscosity of BHMPAM appeared in two aspects [
As shown in Figure
Viscosity versus shearing rate of BHMPAM compared to HPAM in synthetic formation water (concentration of both polymers: 1750 mg/L).
When the BHMPAM solution concentration exceeded associating concentration, the hydrophobic association formed mainly between different polymer chains, which generated supramolecular aggregates. The hydrophobic association may present a reversible equilibrium state with both formation and destruction under shearing. With the increasing of shearing rate, the destruction rate of the association of aggregates was greater than the rate of formation, so that the size of the aggregates in the solution decreased, and the viscosity decreased with the increasing of shear rate and the polymer solution performed shear thinning [
BHMPAM’s viscosifying is superior to HPAM under shearing. On one hand, the hydrophobic association formed network structure in solution; on the other hand, BHMPAM’s degree of branching was so high that the hydrodynamic radius was relatively small, while compared to the same molecular weight of straight chain HPAM. The results were absolutely obvious; that is, the entanglement of molecular chains increased and the chain was hard to cut.
The
Viscoelasticity versus vibrational frequency of BHMPAM compared to HPAM in synthetic formation water (concentration of both polymers: 1750 mg/L).
Solution structure was formed by entanglement or intermolecular interaction. The deformation of solution structure would consume energy under imposed stress; at this time, the polymer solution would present viscosity. Also, the deformation of solution structure was a relaxation process, and the elasticity of polymer would be triggered when the deformation lagged behind imposed stress [
A new branched hydrophobically modified polyacrylamid (BHMPAM) was synthesized by the free radical copolymerization; this method is simple and convenient and can be easily implemented into industrial production. The structure of main chain of BHMPAM is highly branched, which enhances the temperature tolerance and the salinity tolerance of polymer aqueous solution, and BHMPAM shows the property of shearing thinning which is beneficial for injection. BHMPAM shows higher viscosifying ability, and the viscoelasticity, especially elasticity of BHMPAM, increases dramatically compared with HPAM, which makes BHMPAM able to meet the requirements for oil displacement agent used in high temperature and high salinity reservoir.
This financial support by the special fund of China’s central government for the development of local colleges and universities—the project of National First-Level Discipline in Oil and Gas Engineering is gratefully acknowledged.