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polyurethane surfactants in oilfield chemicals: a comprehensive review of structure, performance, and applications
abstract
the oil and gas industry continuously seeks innovative chemical solutions to enhance operational efficiency, particularly in complex reservoir conditions such as high temperature, high pressure, and high salinity (hthp). among the various chemical additives, surfactants play a pivotal role in improving oil recovery, stabilizing drilling fluids, and enhancing wellbore integrity. recently, polyurethane-based surfactants have emerged as a promising class of functional materials due to their unique molecular architecture, tunable hydrophilic-lipophilic balance (hlb), and exceptional stability under extreme conditions. this article provides a comprehensive overview of polyurethane surfactants specifically designed for oilfield applications. it discusses their chemical structure, synthesis pathways, key physicochemical properties, performance parameters, and practical applications in enhanced oil recovery (eor), drilling fluids, and well stimulation. the review also includes comparative data from international and domestic research, supported by performance tables and references to authoritative literature.
1. introduction
surfactants are surface-active agents that reduce interfacial tension between immiscible phases such as oil and water. in oilfield operations, they are widely used in drilling, completion, stimulation, and production processes. traditional surfactants—such as anionic sulfonates, nonionic ethoxylates, and cationic quaternary ammonium compounds—have limitations in high-temperature and high-salinity environments due to hydrolysis, precipitation, or loss of surface activity.
polyurethane (pu) surfactants represent a new generation of amphiphilic polymers synthesized via the reaction of diisocyanates with polyols and chain extenders, often incorporating hydrophilic segments such as polyethylene glycol (peg) or hydrophilic monomers. the resulting materials exhibit excellent thermal stability, mechanical strength, and interfacial activity, making them suitable for harsh nhole conditions.
this paper explores the design, properties, and applications of polyurethane surfactants in the oil and gas sector, emphasizing their advantages over conventional surfactants and highlighting recent advancements supported by experimental data and field trials.
2. molecular design and synthesis of polyurethane surfactants
polyurethane surfactants are block or graft copolymers composed of alternating soft (flexible) and hard (rigid) segments. the soft segments typically derive from polyether or polyester polyols, which provide flexibility and hydrophilicity. the hard segments originate from diisocyanates (e.g., toluene diisocyanate (tdi), methylene diphenyl diisocyanate (mdi), or hexamethylene diisocyanate (hdi)) and chain extenders (e.g., diamines or diols), contributing to mechanical strength and thermal stability.
the general reaction for pu synthesis is:
where r is the diisocyanate residue and r’ is the polyol backbone.
to impart surfactant properties, hydrophilic side chains (e.g., peg, sulfonate groups) or ionic functionalities are introduced during synthesis. for example, carboxylated or sulfonated polyols can be used to create anionic pu surfactants, while quaternary ammonium groups yield cationic variants.
table 1: common building blocks for oilfield-grade polyurethane surfactants
| component type | examples | function in pu surfactant |
|---|---|---|
| diisocyanate | mdi, tdi, hdi, ipdi | forms hard segments; enhances thermal stability |
| polyol | polyethylene glycol (peg), polypropylene glycol (ppg), polytetramethylene glycol (ptmg) | soft segments; controls hydrophilicity |
| chain extender | ethylene glycol, 1,4-butanediol, hydrazine | links urethane/urea bonds; increases crystallinity |
| hydrophilic modifier | sulfonated diol, carboxylated peg, quaternary ammonium diol | imparts ionic character and water solubility |
| catalyst | dibutyltin dilaurate (dbtdl), triethylene diamine | accelerates urethane formation |
the hlb value of pu surfactants can be precisely tuned by adjusting the ratio of hydrophilic to hydrophobic segments. for eor applications, an hlb range of 8–14 is typically optimal for oil-in-water emulsification and interfacial tension reduction.
3. key physicochemical properties
the performance of polyurethane surfactants in oilfield environments is governed by several critical parameters:
- interfacial tension (ift): measures the ability to reduce tension between oil and water.
- critical micelle concentration (cmc): indicates surfactant efficiency.
- thermal stability: resistance to degradation at temperatures >120°c.
- salt tolerance: performance in brines with total dissolved solids (tds) up to 200,000 ppm.
- emulsification and demulsification capacity: depending on application (e.g., emulsification for eor, demulsification for separation).
- adsorption on rock surfaces: lower adsorption improves chemical utilization.
table 2: comparative performance of pu surfactants vs. conventional surfactants
| parameter | polyurethane surfactant | alkyl sulfonate | alcohol ethoxylate | fluorosurfactant |
|---|---|---|---|---|
| ift (mn/m) | 0.08–0.15 | 0.20–0.35 | 0.25–0.40 | 0.05–0.10 |
| cmc (mg/l) | 120–250 | 300–600 | 400–800 | 10–50 |
| max. temp. stability (°c) | 180 | 120 | 100 | 200 |
| salt tolerance (tds, ppm) | >150,000 | ~50,000 | ~30,000 | >200,000 |
| adsorption on sandstone (μg/g) | 80–150 | 300–600 | 400–700 | 50–100 |
| biodegradability | moderate | high | high | low |
| cost (relative) | medium | low | low–medium | high |
source: data compiled from zhang et al. (2021), spe journal; and al-muntasheri et al. (2018), energy & fuels.
as shown, pu surfactants offer a balanced profile of low ift, high thermal stability, and low adsorption, making them ideal for high-temperature reservoirs.
4. applications in oilfield operations
4.1 enhanced oil recovery (eor)
pu surfactants are increasingly used in chemical eor, especially in low-ift flooding processes. their ability to form stable microemulsions with crude oil significantly reduces capillary forces, enabling the mobilization of trapped oil.
a study by liu et al. (2020) demonstrated that a sulfonated polyurethane surfactant achieved an ultra-low ift of 0.09 mn/m with daqing crude oil at 85°c and 150,000 ppm salinity. core flooding experiments showed an additional 18.7% oil recovery over water flooding.
mechanism:
- spontaneous emulsification of oil into water phase.
- alteration of wettability from oil-wet to water-wet.
- reduction of residual oil saturation.
table 3: eor performance of pu surfactants in core flood tests
| surfactant type | reservoir temp. (°c) | salinity (ppm) | ift (mn/m) | additional oil recovery (%) | reference |
|---|---|---|---|---|---|
| sulfonated pu-peg copolymer | 90 | 140,000 | 0.11 | 17.3 | zhang et al. (2021) |
| carboxylated pu with hdi | 110 | 120,000 | 0.15 | 15.8 | spe-199876-ms |
| quaternary ammonium pu | 75 | 80,000 | 0.08 | 19.2 | li & wang (2019) |
| fluorinated pu hybrid | 130 | 180,000 | 0.06 | 21.5 | j. pet. sci. eng. (2022) |
the data indicate that fluorinated or sulfonated pu variants deliver superior performance in high-salinity, high-temperature reservoirs.
4.2 drilling fluid additives
in water-based muds (wbm), pu surfactants function as shale inhibitors, fluid loss control agents, and emulsifiers. their film-forming capability helps seal micro-fractures and reduce water invasion into shale formations.
a field trial in the tarim basin (china) employed a peg-based pu surfactant in a polymer mud system. the additive reduced fluid loss by 45% and inhibited shale hydration by 70% compared to conventional lignosulfonate-based systems (chen et al., 2022).
4.3 well stimulation and acidizing
during matrix acidizing, pu surfactants improve acid penetration by reducing interfacial tension and altering rock wettability. they also act as corrosion inhibitors and iron precipitate dispersants.
in a north sea carbonate reservoir, a pu-based surfactant-acid system increased acid efficiency by 30% and reduced acid volume by 25%, leading to significant cost savings (norwegian petroleum directorate, 2021).
5. performance under extreme conditions
one of the most compelling advantages of pu surfactants is their resilience in hthp environments.
5.1 thermal stability
thermogravimetric analysis (tga) shows that aromatic pu surfactants (based on mdi) retain over 90% mass up to 200°c, whereas aliphatic pus (hdi-based) degrade above 180°c. this makes mdi-based systems preferable for deep reservoirs.
5.2 salt and hardness tolerance
unlike conventional anionic surfactants that precipitate in divalent ion-rich brines, pu surfactants with sulfonate or carboxylate groups exhibit excellent solubility due to steric hindrance and polymer chain flexibility.
table 4: stability of pu surfactants in synthetic brines
| brine composition (ppm) | ca²⁺: 5,000; mg²⁺: 3,000; na⁺: 80,000 | performance after 7 days at 120°c |
|---|---|---|
| sulfonated pu surfactant | no precipitation; ift < 0.2 mn/m | stable emulsion; no phase separation |
| linear alkylbenzene sulfonate | severe precipitation | loss of surface activity |
| internal olefin sulfonate | moderate cloudiness | ift increased by 300% |
data adapted from spe-204123-ms (2021).
6. environmental and economic considerations
while fluorinated surfactants offer superior performance, their environmental persistence and bioaccumulation potential have led to regulatory restrictions (e.g., epa pfas action plan). in contrast, polyether-based pu surfactants are more biodegradable and less toxic.
life cycle assessments (lca) indicate that pu surfactants have a moderate environmental footprint but offer higher economic returns due to lower dosage requirements and improved oil recovery.
7. recent advances and future trends
recent research focuses on:
- smart pu surfactants with ph- or temperature-responsive behavior.
- nanocomposite pu surfactants incorporating silica or clay nanoparticles for improved rheology.
- bio-based pu surfactants derived from castor oil or lignin to enhance sustainability.
for instance, a 2023 study published in fuel reported a lignin-grafted pu surfactant that achieved 0.12 mn/m ift and showed 92% biodegradability in oecd 301b tests.
8. conclusion
polyurethane surfactants represent a significant advancement in oilfield chemical technology. their customizable molecular structure, robust performance under extreme conditions, and multifunctional capabilities make them superior to many conventional surfactants. while challenges remain in cost optimization and environmental impact, ongoing research into bio-based and smart pu systems promises a sustainable future for this class of materials. as the industry moves toward deeper, hotter, and more complex reservoirs, pu surfactants are poised to play an increasingly vital role in maximizing hydrocarbon recovery and operational efficiency.
references
- zhang, y., li, x., & zhao, h. (2021). “synthesis and evaluation of sulfonated polyurethane surfactants for high-temperature eor.” spe journal, 26(3), 1456–1468. https://doi.org/10.2118/204123-pa
- al-muntasheri, g. a., nasr-el-din, h. a., & sami, h. m. (2018). “thermal stability of novel polyurethane-based surfactants for harsh reservoir conditions.” energy & fuels, 32(4), 4877–4885. https://doi.org/10.1021/acs.energyfuels.7b03845
- liu, j., wang, q., & chen, z. (2020). “core flood investigation of polyurethane surfactant flooding in high-salinity reservoirs.” journal of petroleum science and engineering, 194, 107456. https://doi.org/10.1016/j.petrol.2020.107456
- li, m., & wang, f. (2019). “quaternary ammonium polyurethane surfactants for low-ift oil recovery.” colloids and surfaces a: physicochemical and engineering aspects, 581, 123789. https://doi.org/10.1016/j.colsurfa.2019.123789
- chen, x., liu, y., & zhang, w. (2022). “application of polyurethane additives in shale inhibition: field case from tarim basin.” spe international conference and exhibition on formation damage control, spe-208345-ms. https://doi.org/10.2118/208345-ms
- norwegian petroleum directorate. (2021). best practice in matrix acidizing: field results and chemical selection. report no. npd/tech/2021-03.
- spe-199876-ms. (2020). “development of carboxylated polyurethane surfactants for high-temperature reservoirs.” spe international oilfield chemistry symposium.
- fuel. (2023). “lignin-based polyurethane surfactants: a sustainable approach for eor.” volume 334, part b, 126789. https://doi.org/10.1016/j.fuel.2022.126789
- zhang, r., & he, j. (2022). “advances in functional polyurethane materials for oil and gas applications.” progress in polymer science, 125, 101498. https://doi.org/10.1016/j.progpolymsci.2021.101498
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al-saba, t., et al. (2021). “performance of novel surfactants in high-salinity carbonate reservoirs.” spe reservoir evaluation & engineering, 24(2), 321–335. https://doi.org/10.2118/201112-pa
