Polyalkalene Glycol

Key Takeaways

• Cloud point behavior of PAG drilling fluid additives is the key mechanism by which PAG provides thermally responsive shale inhibition in water-based mud systems, with the cloud point temperature being the temperature above which the PAG molecules become insoluble in water and aggregate into a separate PAG-rich phase that deposits on and plugs the exposed shale surface: at surface mixing and circulation temperatures (typically below 40 to 60 degrees Celsius), the PAG is dissolved in the water phase of the mud and provides moderate shale stabilization through adsorption onto clay surfaces; as the mud enters the high-temperature region near the bit (where bottomhole circulating temperatures may exceed 100 to 150 degrees Celsius in deep wells), the PAG above its cloud point precipitates from solution and forms a viscous or solid phase that deposits on the shale wellbore wall, plugging the micro-fractures and pore throats through which mud filtrate would otherwise invade and cause clay swelling; when the bit cuttings are carried up the annulus into the cooler upper wellbore region below the cloud point temperature, the deposited PAG re-dissolves and the shale cuttings are released into the mud system with minimal PAG loss from the circulating volume; the cloud point temperature of a PAG copolymer is adjusted by changing the EO-PO ratio (higher EO content raises the cloud point toward or above 100 degrees Celsius, while higher PO content lowers it toward ambient), allowing PAG mud systems to be designed for the specific bottomhole temperature of each well to achieve the desired thermally responsive plugging behavior.
• PAG lubricity enhancement in water-based mud reduces torque and drag in extended-reach and horizontal wells by adsorbing onto steel tool joint surfaces and the wellbore wall to form a boundary lubrication film that reduces the coefficient of friction between the rotating drillstring and the wellbore contact points: in a vertical well with low side forces on the drillstring, mud lubricity has little effect on drilling performance, but in a horizontal or extended-reach well where the drillstring rests on the low side of the wellbore under the full weight of the horizontal pipe, the friction between drillstring and wellbore wall is the primary source of drag that limits how far the drill bit can be pushed laterally from the kick-off point; PAG reduces this friction by providing a polymer lubricant film at the contact points, with the ether oxygen groups bonding to the metal oxide surface of the steel tool joint and the hydrophobic alkyl backbone of the PO units reducing water adsorption at the contact surface and lowering the effective coefficient of friction; typical lubricity coefficients of PAG-treated WBM are 0.10 to 0.15 (compared to 0.20 to 0.30 for untreated WBM and 0.05 to 0.10 for OBM), representing a meaningful improvement in drag reduction that can extend the reachable total depth of an extended-reach well by several hundred to several thousand feet compared to the same well drilled with untreated WBM.
• PAG as a completion and workover fluid base provides a clean, water-soluble viscosifier that can be formulated at various densities and viscosities for packer fluid applications, gravel pack carrier fluids, and sandstone acid stimulation viscosified carrier fluids, where the PAG's ability to be broken down by dilution with produced water (below the cloud point) or thermally degraded at high temperatures provides natural cleanup without the need for enzyme or oxidizer breakers required for conventional HEC polymer fluids: the PAG packer fluid maintains the annulus between the production tubing and casing at a controlled density to balance the wellbore pressure and prevent the packer from unsetting due to unbalanced pressure during well operations; the PAG viscosifier in gravel pack carrier fluid provides the viscosity needed to transport the gravel through the perforation tunnels and into the formation at the carrier velocity required for uniform gravel placement, with the polymer breaking naturally as the fluid contacts the low-pH, high-temperature formation water rather than requiring a breaker treatment that might be ineffective if the gravel pack fails to achieve the designed placement geometry; the biodegradability and low environmental impact of PAG fluids makes them preferred for operations in environmentally sensitive areas and for injection into formations where conventional polymer residue would cause formation damage.
• PAG synthetic lubricants for surface oil and gas compression and processing equipment replace mineral oil lubricants in applications where the thermal and oxidative stability of mineral oil is insufficient, including natural gas compressor cylinders operating at high discharge temperatures, gas turbine bearings exposed to jet fuel and combustion products, and heat exchanger systems where thermal stability at 200 to 300 degrees Celsius is required: PAG lubricants have a naturally high viscosity index (resistance of viscosity to temperature change) that makes them effective lubricants across a wide temperature range from cold-start at -40 degrees Celsius to high-temperature operation above 200 degrees Celsius, reducing the need for viscosity index improver additives that can shear degrade in service; PAG lubricants are miscible with natural gas (unlike mineral oils which can form aerosol mists that carry-over into the gas stream and contaminate downstream equipment), which in some gas compressor applications simplifies the lube oil management since the PAG dissolves into the gas phase and is recovered in the gas dryers or dehydration unit rather than accumulating as an aerosol in pipeline segments; the water solubility of PAG lubricants that is beneficial in drilling and completion applications is a disadvantage in compression equipment where condensation water could extract PAG from the lubricating film, requiring careful selection of PAG molecular weight and EO-PO ratio to minimize water miscibility in surface equipment applications while maintaining the required thermal stability and lubricity.
• Environmental acceptability of PAG drilling fluid additives has been evaluated under the North Sea OSPAR PLONOR (pose little or no risk to the environment) framework and under US EPA NPDES permit criteria for offshore discharge, with most PAG types meeting biodegradability (greater than 60 percent in 28-day BOD test for biodegradable assessment) and toxicity thresholds (LC50 greater than 30,000 mg/L for marine organisms) that qualify them for offshore discharge without the restrictions applied to diesel or mineral oil-based additives: the environmental acceptability of PAG has made it the dominant shale inhibitor and lubricant additive in North Sea water-based mud systems designed for offshore cuttings discharge compliance, replacing earlier glycol and glycerol-based additives that had lower biodegradability or higher aquatic toxicity; the OSPAR assessment requires that the specific PAG formulation (not just PAG generically) be characterized for toxicity and biodegradability using standardized test methods, since the toxicity and biodegradability of PAG copolymers varies with molecular weight and EO-PO ratio, with higher-molecular-weight PAGs generally having lower biodegradability rates that may not meet the OSPAR threshold for high-volume marine discharge operations; drilling contractors and operators selecting PAG mud additives for North Sea or other offshore discharge applications routinely verify the OSPAR Harmonized Mandatory Control System (HMCS) compliance status of the specific product from the supplier's chemical notification database before use.