Acrylamide Polymer
An acrylamide polymer is a synthetic polymer built from acrylamide monomers (CH2=CH-CO-NH2), forming linear or slightly branched chains with a nonionic character at neutral to mildly alkaline pH because the amide group (-CO-NH2) does not carry a net electrical charge under typical oilfield conditions. Polyacrylamide (PAM) is the most common acrylamide homopolymer, produced by free-radical polymerisation of acrylamide in aqueous solution and available over an extremely wide range of molecular weights, from tens of thousands to more than twenty million Daltons. The molecular weight of the polyacrylamide determines its primary function: very high molecular weight PAM (5 to 20 million Daltons) acts as a flocculant for fine suspended particles in drilling waste water and produced water treatment systems; intermediate molecular weight PAM (1 to 5 million Daltons) is used as a friction reducer in hydraulic fracturing slickwater fluids; and low molecular weight PAM (100,000 to 500,000 Daltons) provides mild dispersing effects in concentrated clay or cement slurries. The acrylamide polymer occupies a distinct niche from the acrylamide-acrylate copolymers (PHPA) used for shale inhibition: the nonionic character of pure polyacrylamide means it does not interact as strongly with clay surfaces as anionic PHPA and is therefore less effective for shale encapsulation, while its ability to reduce fluid friction and flocculate suspended particles makes it valuable in other parts of the oil and gas production and completion workflow.
Key Takeaways
- The Toms effect (drag reduction by polymers in turbulent flow) is the physical mechanism by which polyacrylamide reduces friction in hydraulic fracturing slickwater systems. In turbulent flow at the high pump rates used in Montney and Duvernay slickwater fracturing (typically 10 to 20 cubic metres per minute through 90 to 114-millimetre tubing), the formation of turbulent eddies accounts for a large fraction of the total pressure drop along the tubing. Long PAM chains dissolved in the carrier water at concentrations of 0.02 to 0.05 percent by weight align with the flow in the turbulent boundary layer and partially suppress the formation of eddies, reducing the turbulent drag by 40 to 75 percent compared to fresh water at the same flow rate. This friction reduction allows the surface pump pressure to drive fluid at the desired rate at lower wellhead pressure, reducing the power required from the pump engines, or alternatively allowing higher flow rates with the same engine power that could not be achieved with plain water. Slickwater friction reducers (FR) are almost universally polyacrylamide-based (nonionic or slightly anionic PAM) because of their cost-effectiveness, wide availability, and performance across the range of water salinities and temperatures encountered in WCSB fracturing programs.
- Flocculation by high-molecular-weight polyacrylamide occurs through the bridging mechanism: individual PAM chains, with contour lengths of several micrometres, adsorb at one or more points onto the surface of a fine particle and simultaneously at other points onto an adjacent particle, bridging the two particles together. As bridging continues among many particles, the individual fine particles are gathered into loose, open-structured flocs with diameters of 50 to 500 micrometres, which settle or float significantly faster than the individual submicrometre or micrometre-sized particles would on their own. In drilling waste water treatment, high-MW PAM flocculants are added to the clarifier or settling pit at concentrations of 1 to 5 mg/L (parts per million by weight), and the flocculated solids settle within minutes to hours rather than days required for unaided sedimentation. In produced water treatment for disposal or re-injection, PAM flocculants are used in conjunction with coagulants (alum, ferric sulphate, or polyaluminium chloride) that first neutralise the electrical charges on colloidal particles, and then PAM bridges the neutralised particles into settleable flocs.
- Environmental considerations for acrylamide polymers centre on the residual unreacted acrylamide monomer remaining in the commercial polymer product, not the polymer itself. Polyacrylamide is essentially non-toxic to aquatic organisms and mammals at concentrations used in oilfield applications; it is biodegraded by soil and aquatic bacteria over weeks to months under aerobic conditions and is approved for use in drinking water treatment at low concentrations in many regulatory jurisdictions. The acrylamide monomer (CH2=CH-CO-NH2), by contrast, is a well-established neurotoxin and probable carcinogen; occupational exposure limits for acrylamide are below 0.03 mg/m³ air. Commercial PAM products used in oilfield applications are required to contain less than 0.1 percent residual monomer by regulatory standards in the US and Canada, and high-purity grades used in food processing and water treatment contain less than 0.05 percent. Handling precautions for concentrated PAM solutions focus on preventing skin and mucous membrane exposure to the residual monomer rather than the polymer.
- Salt sensitivity of nonionic polyacrylamide is much lower than that of anionic polymers (polyacrylate, PHPA). Because PAM has no charged groups in its nonionic form, it does not interact with dissolved cations (Ca2+, Mg2+, Na+, K+) through electrostatic effects, and its aqueous solution viscosity changes very little across a wide range of salinities from fresh water to 100,000 ppm total dissolved solids. This salt tolerance makes nonionic PAM friction reducers useful in hydraulic fracturing operations where the available water supply is saline (produced water re-use for fracturing) or where the formation brine will immediately mix with and dilute the fracturing fluid near the perforations. Anionic PAM or anionic PHPA, by contrast, would lose viscosity and friction-reducing performance in the presence of high divalent cation concentrations because the Ca2+ and Mg2+ ions compress the polyion's charge cloud (screening electrostatic repulsion between charged segments), causing the chain to adopt a more compact configuration with less hydrodynamic volume and less friction-reducing ability.
- High-concentration polyacrylamide gels (crosslinked with N,N'-methylenebisacrylamide or chromium(III) acetate) are used as conformance control agents in mature oilfield waterfloods to divert injected water from high-permeability channels (thief zones) into lower-permeability, more oil-rich zones. The gel is injected as a solution that penetrates the thief zone, then crosslinks in situ to form a rigid gel that partially or completely blocks further fluid flow through that zone, diverting subsequent injected water into adjacent productive zones. The crosslinked PAM gel must be mechanically stable enough to resist the shear stress of flow through the pore throats at the injection pressure, yet conformable enough to enter pore spaces in the thief zone. Gel strength, syneresis (liquid expulsion from the gel over time), and thermal stability at reservoir temperature are the critical performance parameters evaluated in laboratory testing before field application of crosslinked PAM conformance treatments.
Polyacrylamide as a Friction Reducer in Slickwater Fracturing
Slickwater hydraulic fracturing fluids consist primarily of water (97 to 99.5 percent by volume), polyacrylamide friction reducer (0.02 to 0.05 percent), and a biocide or scale inhibitor (less than 0.01 percent). The very low polymer concentration (200 to 500 mg/L) is sufficient to reduce pipe friction by 40 to 75 percent because the drag reduction mechanism is triggered by polymer chain extension in the turbulent boundary layer and does not require high viscosity. The advantage of slickwater over more viscous gel fracturing fluids (crosslinked guar, linear gel) is that slickwater creates complex, multi-strand fracture networks in brittle rock (such as Montney siltite and Duvernay shale) rather than single, wide planar fractures, accessing a larger stimulated rock volume and more natural fractures. The trade-off is that slickwater's low viscosity provides less proppant transport capability than gel fluids, which is managed by using very fine proppant (40/70 or 100 mesh sand) that can be transported by the turbulent flow of slickwater even without gel viscosity.
In cold weather conditions common in the WCSB from October to April, slickwater pump trucks must ensure the PAM friction reducer dissolves and activates at low water temperatures. High-MW nonionic PAM dissolves more slowly in cold water (below 5 degrees Celsius) because molecular diffusion is slower and the polymer hydration time is extended. Friction reducer suppliers provide emulsion formulations of PAM (PAM polymer pre-dissolved in oil as an inverse emulsion) that disperse rapidly in cold water without the hydration delay of dry powder PAM, enabling effective friction reduction even when mixing water is drawn from surface tanks at near-freezing temperatures.
Fast Facts
Polyacrylamide was first synthesised in the 1890s and commercialised in the 1950s by American Cyanamid and Dow Chemical under trade names including Cyanamer and Purifloc. Its application as a petroleum drilling fluid additive began in the 1960s, initially as a high-molecular-weight flocculant for waste mud pits, with the friction-reducing (slickwater) application developing in the 1990s when horizontal multistage fracturing of tight formations became widespread in North America. Slickwater fracturing of the Barnett Shale in Texas in the mid-1990s by Mitchell Energy popularised the slickwater technique; the ability to pump large volumes of water at high rates with minimal friction pressure was a key enabling technology. In Canada, slickwater fracturing of the Montney and Duvernay formations in BC and Alberta accounts for the majority of high-molecular-weight PAM consumption in the oilfield sector, with annual PAM usage for WCSB fracturing estimated in the tens of thousands of tonnes per year. Water re-use for fracturing (using produced water instead of fresh water) is increasingly required by regulators and encouraged by economics, and salt-tolerant PAM friction reducers have been specifically developed for high-salinity produced water to allow re-use without desalination.
Synonyms and Related Terminology
Acrylamide polymer is also called polyacrylamide, PAM, or (when used specifically in fracturing) friction reducer (FR) or slick water additive. Related terms include slickwater (a hydraulic fracturing fluid consisting primarily of water with a polyacrylamide friction reducer at very low concentration; used in brittle shale and tight siltstone formations to create complex fracture networks; the dominant fracturing fluid for Montney, Duvernay, Cardium, and Viking tight formations in the WCSB), friction reducer (a polymer additive, typically polyacrylamide, added to hydraulic fracturing or water injection fluids to reduce turbulent pipe friction at the pump rates used in high-volume fracturing; reduces surface pump pressure by 40 to 75 percent, enabling higher flow rates with the same engine power), drag reduction (the reduction of turbulent flow pressure drop achieved by dissolving long-chain polymers in the flowing fluid; the Toms effect by which polyacrylamide and other flexible polymer chains suppress turbulent eddy formation in the boundary layer; the physical mechanism underlying friction reducer performance in slickwater fracturing), flocculant (a polymer additive used to aggregate fine suspended particles into larger flocs that settle or float more rapidly; high-molecular-weight polyacrylamide is the most widely used flocculant in oilfield produced water treatment and drilling waste water clarification), and conformance control (the set of operations used to improve the distribution of injected water or gas across a reservoir during secondary and tertiary recovery; crosslinked polyacrylamide gels are injected into high-permeability thief zones to divert subsequent injection into lower-permeability, more productive zones).
How Polyacrylamide Friction Reducer Enabled a High-Rate Duvernay Completion
A Calgary-based operator was designing a multistage hydraulic fracturing program for a 2,600-metre Duvernay Formation horizontal well in the Kaybob area of west-central Alberta. The well design called for 18 fracture stages at a target pump rate of 14 m³/min of slickwater fluid. Pre-fracture hydraulic modelling indicated that a 14 m³/min rate through the 89-millimetre production tubing and 11.4-centimetre wellhead connection would generate a tubing friction pressure of approximately 31 MPa (4,500 psi) if plain fresh water were used, requiring surface pump pressure of approximately 93 MPa (13,500 psi) to achieve the target bottomhole treating pressure of 62 MPa.