Friction Reducer
A friction reducer in oil and gas operations is a chemical additive, generally supplied as a liquid emulsion or slurry, that is incorporated into hydraulic fracturing slickwater or drilling fluids to reduce the turbulent friction pressure losses that occur when high-viscosity or high-velocity fluid flows through pipes, wellbore tubing, perforations, or fracture channels, enabling higher pump rates and greater hydraulic horsepower utilization than would be achievable at the same pump pressure without the additive; in hydraulic fracturing operations, friction reducers are the primary rheological additive in slickwater fracturing fluids (which are otherwise nearly as thin as fresh water, with viscosity of 1 to 3 centipoise), where they dramatically reduce the pipe friction pressure losses during high-rate injection (at pump rates of 50 to 150 barrels per minute down the wellbore) that would otherwise limit the achievable injection rate to a small fraction of what is needed to create the long, complex fracture networks required for economic shale gas and tight oil production; friction reducers used in hydraulic fracturing are almost universally high-molecular-weight polyacrylamide (PAM) or polyacrylamide-co-acrylic acid (PAMAA) polymers, which function by damping the turbulent eddies in the flowing fluid through a viscoelastic mechanism (the Toms effect, first described by Boris Toms in 1948) that reduces the energy dissipated in turbulent flow without significantly increasing the effective viscosity of the fluid at the proppant-transport shear rates in the fracture.
Key Takeaways
- The Toms effect (also called the polymer drag reduction effect) is the physical mechanism by which high-molecular-weight polymers reduce turbulent friction pressure losses, first observed by B. A. Toms in 1948 when he found that dilute solutions of poly(methyl methacrylate) in monochlorobenzene at concentrations of a few hundred parts per million showed dramatically lower pipe friction than pure solvent at the same flow rate: the mechanism involves the elongation and alignment of polymer molecules in the turbulent flow field, where the stretched polymer chains interact with and suppress the turbulent eddies in the buffer layer between the viscous sublayer and the fully turbulent core, reducing the energy transfer from the mean flow to the turbulent fluctuations; friction reduction of 50 to 80 percent is commonly achieved in pipe flow at polymer concentrations of 0.1 to 1.0 gallons per thousand gallons (0.01 to 0.1 percent by volume) for high-molecular-weight polyacrylamides (molecular weight of 10 to 30 million daltons); the friction reduction efficiency increases with molecular weight (longer chains are more effective at damping turbulence) and with polymer concentration up to an optimum, beyond which the increased viscosity contribution from the polymer overcomes the turbulence-damping benefit.
- Slickwater fracturing fluid design with friction reducers balances friction reduction in the wellbore tubing against proppant transport and fracture complexity objectives in the formation: in the wellbore (where the fluid flows at high velocity through 4.5 to 5.5 inch tubing at rates of 80 to 120 barrels per minute), the friction reducer is the primary additive because the turbulent flow in the straight pipe section is where 80 to 90 percent of the total surface treating pressure is lost to friction; in the fracture (where flow velocities are much lower and the geometry is tortuous), the same polymer concentration provides little friction reduction but may contribute some viscosity to proppant suspension; the optimum friction reducer concentration for slickwater is typically 0.25 to 1.0 gallons per thousand gallons (0.25 to 1.0 gpt), with the lower end used in lower-temperature wells where the polymer retains more molecular weight integrity and the higher end used in hotter wells where thermal degradation reduces the effective concentration; exceeding the optimum concentration not only wastes chemical cost but can generate excessive friction (because the viscous contribution of the polymer starts to dominate over the turbulence-damping effect at high concentrations).
- Friction reducer compatibility with produced water and formation water is a critical consideration in hydraulic fracturing operations because many polyacrylamide-based friction reducers precipitate or lose effectiveness in high-salinity brines (above 50,000 to 100,000 ppm total dissolved solids), limiting the use of produced water as the base fluid for slickwater fracturing without treatment or specially formulated salt-tolerant friction reducers: conventional anionic polyacrylamides (with negatively charged carboxylate groups along the polymer backbone) precipitate in the presence of divalent cations (calcium, magnesium, and barium) that bridge between the negatively charged polymer chains and cause the polymer to come out of solution as a white floc; salt-tolerant friction reducers (formulated with non-ionic or cationic polymer backbones, or with special stabilizing agents that prevent precipitation) have been developed specifically for use with produced water and high-TDS formation water, enabling water recycling programs that reduce the freshwater consumption and disposal costs of hydraulic fracturing programs in water-constrained regions; the Permian Basin, Marcellus Shale, and Eagle Ford operators have all invested heavily in salt-tolerant friction reducer formulations that enable produced water reuse fractions of 50 to 100 percent of the total fracturing fluid volume.
- Environmental and regulatory aspects of friction reducers have become increasingly important as fracturing chemical disclosure requirements expanded after 2010, with operators required in many jurisdictions to disclose friction reducer components to FracFocus.org and other public registries: most friction reducers are based on polyacrylamide, which is classified as non-hazardous and approved for industrial water treatment applications by the US EPA; however, some friction reducer formulations contain trace amounts of residual acrylamide monomer (a known neurotoxin and potential carcinogen) remaining from the polymerization process, with US EPA setting a maximum residual acrylamide concentration of 0.05 percent in polymers used in drinking water treatment that some operators apply as a benchmark for oilfield friction reducer quality; "green" or "environmentally preferred" friction reducers based on guar gum (naturally occurring polysaccharide) or other bio-derived polymers have been developed and marketed as alternatives to synthetic polyacrylamides, though their friction reduction efficiency is generally lower than synthetic PAM at equivalent concentrations, requiring higher treatment rates to achieve comparable pipe friction performance.
- Friction reducers in drilling fluids are used to reduce the torque and drag experienced by the drill string as it rotates and slides against the wellbore wall in extended-reach and highly deviated wells, with different chemical classes used than in fracturing: in drilling, torque-and-drag friction reducers function primarily as boundary lubricants (reducing the coefficient of friction between the rotating drill string and the stationary casing or formation face) rather than as turbulent drag reducers (because the flow in the annulus is generally in the laminar or transitional regime, not the turbulent regime where the Toms effect is operative); common drilling friction reducers include fatty acid derivatives, glycol-based lubricants, graphite particles, and synthetic ester-based lubricants that adsorb on metal surfaces and reduce the metal-on-metal contact friction; in oil-based drilling fluids, the base oil itself provides lubrication and additional friction reducers may not be required, while in water-based muds used in long horizontal sections, lubricity additives (friction reducers) reduce the equivalent circulating density of the mud by reducing the axial pipe drag that increases the effective hydrostatic pressure in the annulus.
Fast Facts
The first commercial application of friction reducers in hydraulic fracturing began in the early 2000s as operators experimenting with slickwater fracturing in Devonian shales (particularly the Barnett Shale in Texas, pioneered by Mitchell Energy) found that high-molecular-weight polyacrylamide dissolved in the fracturing water at concentrations of less than 1 gallon per thousand gallons reduced treating pressure by 50 to 70 percent while maintaining the low viscosity needed to create complex fracture networks. This discovery made high-rate slickwater fracturing economically feasible in low-permeability formations, directly enabling the shale revolution that transformed global natural gas and oil production from 2005 onward.
What Is a Friction Reducer?
A friction reducer is a chemical additive (typically a high-molecular-weight polyacrylamide polymer) used in hydraulic fracturing slickwater fluids and drilling fluids to reduce turbulent or adhesive friction losses that would otherwise limit pump rates or increase treating pressures, enabling higher injection rates, lower surface treating pressures, and more efficient use of hydraulic horsepower. In slickwater fracturing, friction reducers at concentrations of 0.25 to 1.0 gpt reduce pipe friction pressure by 50 to 80 percent through the Toms polymer drag reduction effect. In drilling, lubricity additives functioning as friction reducers reduce drill string torque and drag in deviated and extended-reach wells.
Synonyms and Related Terminology
Friction reducer is also called a drag reducer, friction reducing agent, slickwater additive, or lubricity additive depending on the application context. Related terms include slickwater fracturing (the hydraulic fracturing technique that uses low-viscosity, nearly water-like fracturing fluid (water plus friction reducer at 0.25 to 1.0 gpt, plus biocide and scale inhibitor) pumped at very high rates (80 to 150 bbl/min) to create complex fracture networks in low-permeability shale formations, with the friction reducer enabling the high pump rates that drive fracture complexity by reducing the pipe friction losses that would otherwise require impractically high surface treating pressures), polyacrylamide (PAM, the high-molecular-weight (10 to 30 million dalton) synthetic polymer that is the active ingredient in most hydraulic fracturing friction reducers, functioning through the Toms polymer drag reduction effect to suppress turbulent eddies in high-velocity pipe flow and reduce friction pressure losses by 50 to 80 percent at concentrations of less than 1 gallon per thousand gallons of fracturing fluid), hydraulic fracturing (the well stimulation technique that injects pressurized fluid into a low-permeability formation to create and propagate fractures that increase the effective drainage area and production rate of the well, with friction reducers being the essential enabling additive for slickwater fracturing that has made tight oil and shale gas production economically viable in low-permeability formations), torque and drag (the rotational and axial forces experienced by the drill string as it rotates and slides against the wellbore wall in deviated and extended-reach wells, which are reduced by drilling friction reducers (lubricity additives) that decrease the coefficient of friction between the drill pipe and the casing or formation face, enabling longer horizontal sections and higher weight-on-bit in highly deviated trajectories), and produced water reuse (the recycling of water produced from oil and gas wells as the base fluid for new hydraulic fracturing operations, which requires salt-tolerant friction reducers that maintain their drag reduction effectiveness in high-salinity brines rather than precipitating as conventional anionic polyacrylamides do in the presence of divalent cations).
Why Friction Reducers Are Among the Most Economically Significant Chemical Additives in Modern Oilfield Operations
Without friction reducers, slickwater fracturing at 100 barrels per minute in a 5.5-inch wellbore would require surface treating pressures 3 to 4 times higher than with friction reducer treatment, exceeding the pressure ratings of standard wellhead equipment and making the operation either impossible or prohibitively expensive. The $0.50 to $2.00 per barrel cost of friction reducer in the slickwater is offset against the alternative of either accepting lower injection rates (which reduces fracture complexity and well performance) or using gel-based fracturing fluids (which are 5 to 10 times more expensive per barrel). The shale revolution, which has made the United States the world's largest oil and gas producer, was enabled in large part by the practical feasibility of high-rate slickwater fracturing that friction reducers made possible at economical chemical cost. Few oilfield chemicals have contributed more directly to a transformation in global energy supply.