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PAM micro-gels are, in general, also cross-linked by organic cross-linkers.

Micro-gels are special colloidal gel particles or polymers, which have been shown to be able to change their gel configuration in response to external stimuli.[290] Unlike macro-gels, micro-gel or sub-micro-gels have been designed to activate certain properties at a pre-determined “in-depth” location within in the reservoir (see Figure 13). For instance, micro-gel systems typically contain expandable polymeric particles possessing both labile and stable cross-linkers. Under the stimuli of pH or temperature extremes, the labile cross-linker will degrade to decrease the degree of crosslinking where the polymeric particles can expand to many times their original volumnar size (see Figure 14), which can block pore throats and direct injection water into unswept oil-rich zones.

Inverse-emulsion, micro-emulsion, suspension, and precipitation polymerization methods are the most common approaches to establish control the particle sizes within a narrow range, from milli- to nanometer size scales.

Figure 13. Illustration of the fundamental difference between macro-gels and micro-gels. [291]

Figure 14. Proposed swelling mechanism of an expandable polymer particle.

An example of a micro-gel joint research and development by an industry consortium (BP, Chevron Texaco, and Nalco Company) was called the Bright Water project. The micro-gel product has been in practical application since 2001 and has been applied in about 60 wells. In preparation of the product, the initial polymeric “kernel”

particles sizes of 0.1 to 3 um were first synthesized through an inverse emulsion polymerization process.[292-296] Through esterification between carboxylic and hydroxyl groups under an azeotropic, thin film evaporation technique to remove water,

the polymeric micro particles were internally cross-linked with labile cros-slinker. Upon adding light mineral oil, surfactants, brine, and completely mixing, a dispersed kernel particle suspension was obtained, without aggregation, in brine.

The suspension contained particles within a 0.5 to 1 micron size range, which easily pass through the reservoir channel. When the micro-gel particles reached the desired location, decomposition of the labile cross-linker in the kernel particles lead to particle absorption of water and volume expansion. Furthermore, the particles were observed to interact with each other on a larger scale that was from 10 to 30 times larger than the original single particle size.[297] In high permeability, porous media, the micro-gel could be enhanced by increasing micro-micro-gel concentration.[298]

Ensuing after the successful application of Bright Water, many similar swellable polymers or DDG[207-209, 299-301] were developed. For example, Moradi-Araghi et al.[302] invented stable, cross-linked water-soluble swollen polymers comprising expandable micro polymer particles using labile, water soluble diacrylates, such as PEG200 diacrylate, crosslinking agents, stable crosslinking agent monomers, e.g., methylene bisacrylamide, and a tertiary cross-linker such as formaldehyde or

hexamethylenetetramine that can become activated once the labile cross-linker degrades under pH or temperature. The micro-gel system could be capable of being further cross-linked to form an expanded and stable gel. The polymer particles were prepared from AM and AMPS through inverse-emulsion polymerization where the particles were co-cross-linked with methylene bis-acrylamide as the stable cross-linker and either PEG-200 or PEG-400 diacrylate as the labile cross-linker.

There are several advantages to conducting acrylamide polymerizations by the inverse emulsion process. However, with these advantages there are also several

disadvantages, including the presence of oil and the eventual phase separation of the oil for inherently unstable emulsions. The oil is also an undesirable field contaminant in most applications. Therefore, the oil is typically removed post-polymerization, replaced by an alternative oil phase, or simply emulsified with the micro-gel dispersion during the field application.

Potential problems associated with the oil phase may be bypassed by carrying out the reaction in supercritical gas fluids, from which powdered products can be obtained directly. Supercritical fluids offer advantages in that they exhibit “liquid-like” densities and solvency power while having “gas-like” viscosities.[303] Currently, many polar or hydrophilic molecules, such as water, proteins, amides, ionic species, sugars, etc., exhibit very poor solubility in supercritical carbon dioxide (ScCO2)and only limited research efforts have been made towards polymerization of water-soluble vinylic monomers containing amides in ScCO2, including inverse emulsion polymerization of

acrylamide,[304] polymerization of N-ethylacrylamide, and dispersion copolymerization of N,N-dimethylacrylamide.[305]

Inverse-emulsion polymerization in ScCO2 requires that the monomer be relatively insoluble in ScCO2. Adamsky et al.[304] reported the inverse-emulsion polymerization of AM monomer in ScCO2. The key point for the emulsion

polymerization was to choose the proper emulsifying agent. Adamsky et al. synthesized an emulsifier with a carbon dioxide compatible, fluorine and also amido groups. (see

Figure 15) By virtue of the CO2-soluble amphiphiles, acrylamide was successfully polymerized using inverse-emulsion polymerization in ScCO2.

F CF CF₂ O CF NH₂ CF₃ CF₃

O

14

Figure 15. The surfactant of inverse-emulsion polymerization of acrylamide in ScCO2.

[304]

Ohde et al.[303] synthesized polyacrylamide nanoparticles by inverse emulsion polymerization in ScCO2. The chosen surfactant was a perfluoropolyether-phosphate and the initiator was potassium persulfate. Through the inverse emulsion method, the

nanoparticles were obtained with a size distribution between 50 and 200 nm.

Table 3

The change of injection characteristic parameters before and after the polymer microsphere treatment.[306]

Liu et al.[306] presented an oilfield application of polymer microspheres flooding in reservoir Q in Bohai Bay, China, which was a heterogeneous heavy oil reservoir

suffering from fast water breakthrough with poor recovery. The pilot tests were done in four wells. Three types of polymer microsphere were used with the concentration of 5000, 3000, and 5000mg/L, and injection pore volume of 0.01PV, 0.005PV and 0.015PV.

Good pore occlusion was observed, as shown in Table 3. Injection pressure, pressure index and filling degrees were greatly raised. Filling degree refers to measure the sealing pore volume and the unsealing pore volume. Additionally, the oil production decline rate was reduced from 7.0% to 4.9%, and water cut was reduced by 2.8% on average.

Crosslinking polymer systems based on multiple cross-linkers were also adopted due to desire of multiple properties.[254, 283, 307]

3.4 Polyacrylamide Gels Modified by Combined Crosslinker and Hybrid