Polyol

Key Takeaways

• Shale inhibition by polyols operates through three synergistic mechanisms that together reduce the swelling pressure and dispersion rate of reactive clay minerals: (1) hydrogen bonding between the hydroxyl groups and the clay surface silanol groups reduces the clay surface energy and inhibits water molecule intercalation into the clay interlayer space that drives osmotic swelling; (2) the polyol's multi-dentate binding (attachment through multiple hydroxyl groups simultaneously) creates a more stable adsorption than monodentate inhibitors such as potassium ions, which are displaced from the clay surface by water more readily; and (3) cloud point polyols add a phase-separation mechanism where the polymer deposits a hydrophobic film on the shale surface at downhole temperatures, physically blocking water from reaching the clay mineral sites; the inhibition efficiency of polyols is typically evaluated by the linear swell test (immersing compacted shale pellets in a polyol-water solution and measuring the pellet height increase as a function of time), with inhibition values (percent reduction in swell relative to distilled water) of 40 to 70 percent achieved by effective polyol formulations compared to 80 to 95 percent inhibition by oil-based muds (the benchmark for reactive shale inhibition).
• Cloud point glycols (CPGs) represent the most sophisticated polyol inhibitor class, using the inverse solubility behavior of ethylene oxide-propylene oxide copolymers to create a temperature-activated delivery mechanism: at surface temperatures (typically 15 to 30 degrees Celsius), the CPG is fully dissolved in the aqueous mud base fluid, maintaining a clear, low-viscosity solution that can be processed by the surface mud handling equipment; as the mud circulates down the drillstring and heats up through the geothermal gradient to the bottomhole temperature (typically 80 to 150 degrees Celsius for most well applications), the CPG reaches its cloud point and phase-separates from solution as a polymer-rich droplet phase that wets the hydrophilic clay surface preferentially; the deposited CPG film reduces water activity at the shale surface and provides mechanical protection of the wellbore wall; upon returning to surface in the annulus, the CPG re-dissolves as the mud cools, recycling the inhibitor back into solution for the next circulation pass; the cloud point temperature is tuned by adjusting the ethylene oxide to propylene oxide ratio in the copolymer (higher propylene oxide content lowers the cloud point, because propylene oxide units are more hydrophobic than ethylene oxide units) to match the expected bottomhole temperature range.
• Lubricating properties of polyols in water-based drilling muds reduce the coefficient of friction between the drill string and the borehole wall or casing, decreasing the torque and drag forces that limit the reach of deviated and horizontal wells and reduce drilling efficiency in tight clearance completions: polyols adsorb at the metal-rock interface through their hydroxyl groups and reduce the shear stress required for relative motion between the surfaces, with lubricity improvement of 20 to 40 percent (measured as reduction in the coefficient of friction on a lubricity meter relative to unmodified water mud) achievable at polyol concentrations of 2 to 5 volume percent; the mechanism of polyol lubrication is distinct from the film lubrication provided by oil (which forms a continuous oil film between the sliding surfaces) and more analogous to boundary lubrication (where the adsorbed polyol monolayer reduces the adhesion between asperities on the opposing surfaces); polyols provide particularly useful lubrication in polymer-based water muds (PHPA, xanthan gum) where the polymer viscosifier does not itself provide significant lubricity, and where the addition of traditional lubricants (diesel, mineral oil, graphite) is restricted by environmental regulations for offshore discharge.
• Regulatory acceptability and environmental fate of polyols are critical considerations for offshore drilling operations subject to produced water discharge regulations: glycerol (the simplest three-carbon polyol) is rapidly biodegraded in seawater (BOD28/ThOD ratio of greater than 0.6, classified as "readily biodegradable" under OECD 306 test), has low aquatic toxicity (LC50 greater than 1,000 mg/L for mysid shrimp), and is therefore acceptable for discharge in most offshore jurisdictions including the North Sea (under OSPAR regulations), the Gulf of Mexico (under EPA VGP permit), and Australia; glycols (polyethylene glycol, PEG) have more variable biodegradation rates depending on molecular weight (low-MW PEG 200-600 is readily biodegradable, higher-MW PEG 1000-8000 degrades more slowly) and may require CEPS (chemical evaluation for platform discharge) approval in some jurisdictions; cloud point glycols (EO-PO copolymers with higher propylene oxide content) can be persistent in the marine environment due to their hydrophobic character at ambient temperature, and some formulations have been restricted from discharge in the North Sea; polypropylene glycol (PPG) is typically restricted from overboard discharge due to persistence and bioaccumulation potential; the trend in offshore drilling chemistry is toward glycerol and low-MW glycol formulations that combine adequate shale inhibition performance with the environmental profile required for zero-discharge-exemption in sensitive areas.
• Polyols in hydraulic fracturing fluids serve as friction reducers in water-based frac fluid systems and as clay stabilizers in the fracturing fluid that contacts reactive clay-bearing formations during fracturing: propylene glycol at 0.1 to 1.0 volume percent in slickwater frac fluid reduces the turbulent flow friction pressure in the frac pipe and wellbore (by an effect distinct from polyacrylamide-based friction reducers, which operate through viscoelastic drag reduction), while also stabilizing fine-grained clay minerals in the formation face and induced fracture walls against mobilization and pore throat plugging during production of the frac fluid back through the fracture; in carbonate formations stimulated with acid fracturing, polyols are incorporated in the diverting acid formulation as viscosifiers that allow gelled acid to be placed preferentially in the lower-permeability matrix rather than being lost entirely to the high-permeability fracture or wormhole system; sorbitol (a six-carbon polyol derived from glucose reduction) is used in some high-temperature acidizing systems as a temperature-stable corrosion inhibitor intensifier that improves the performance of conventional amine-type corrosion inhibitors in 15 to 28 percent HCl acid at temperatures of 120 to 175 degrees Celsius, where standard inhibitors are partially decomposed by the acid-catalyzed elimination of the amine functional group.