Álcool polivinil (PVA) para agentes de recuperação de óleo melhorados

Polyvinyl alcohol (PVA) is a water-soluble synthetic polymer prepared through the alcoholysis of polyvinyl acetate. The resulting macromolecule bears a carbon–carbon backbone densely substituted with secondary hydroxyl groups, rendering it fully non-ionic and chemically distinct from the partially hydrolysed polyacrylamides (HPAM) that dominate bulk polymer flooding. In enhanced oil recovery (EOR), PVA functions as a mobility-control and conformance-improvement agent, with primary application in high-salinity, high-hardness reservoirs where anionic polymers undergo phase separation or rapid viscosity loss. The absence of charged groups eliminates electrostatic interactions with dissolved divalent cations, so injection brines containing Ca²⁺ concentrations exceeding 10,000 mg/L can be used without precipitation. This characteristic, combined with a fully synthetic structure devoid of polysaccharide backbones susceptible to enzymatic degradation, allows PVA-gelled barriers to retain mechanical integrity in formations with robust microbial activity and bottomhole temperatures up to 95 °C when a suitable oxygen scavenger is co-injected.

How Does Polyvinyl Alcohol Maintain Mobility Control in Formations with High Divalent Cation Concentrations?

Conventional HPAM derives its viscosifying power from carboxylate anions generated by partial hydrolysis; in brines containing even moderate amounts of Ca²⁺ or Mg²⁺, intramolecular charge screening collapses the polymer coil, and above a critical cation threshold—typically 400–800 mg/L of Ca²⁺ for 25 mol % hydrolysed polyacrylamide—insoluble calcium polyacrylate precipitates form, plugging the near-wellbore region. PVA, by contrast, retains a fully extended random-coil conformation in solutions with total dissolved solids (TDS) surpassing 200,000 mg/L, as its hydroxylic hydration sphere is indifferent to ionic strength. Intrinsic viscosity measurements (ISO 1628-1:2021) on a fully hydrolysed PVA grade (molecular weight 75,000 g mol⁻¹) in synthetic seawater at 35,000 mg/L TDS and 60 °C yield [η] = 0.79 dL g⁻¹, only 4 % lower than the value in deionised water, whereas an HPAM of comparable molecular weight collapses to less than 20 % of its fresh-water viscosity. This brine-insensitivity allows PVA to be injected as a low-concentration linear flood for mobility correction in harsh environments, though economic considerations often push operators towards crosslinked-gel strategies where smaller polymer masses achieve the required permeability reduction. PVA grades tailored for EOR are characterised principally by their degree of hydrolysis and 4 wt % aqueous solution viscosity at 20 °C (Brookfield LV, 30 rpm, ASTM D2196-20). A widely adopted designation is the four-digit code; for example, PVA 1799 indicates a degree of polymerisation of approximately 1700 and a hydrolysis level of 99 mol %, while PVA 1788 corresponds to 88 mol % hydrolysis. The fully hydrolysed grades—≥98 mol %—exhibit maximum hydrogen-bonding density, which strengthens crosslinked gel networks and raises thermal resistance, but their aqueous solutions gel slowly even in the absence of a crosslinker when cooled below 30 °C, demanding heated storage tanks at the wellsite. Typical specification limits for a gel-grade PVA 1799 include: viscosity of a 4 % aqueous solution 25–31 mPa·s at 20 °C, pH 5–7, ash content ≤0.5 %, and volatile matter ≤5.0 % by weight. Partially hydrolysed grades (e.g., PVA 1788, viscosity 20–26 mPa·s under the same conditions) retain higher water solubility at ambient temperature and produce less viscous linear solutions, facilitating deep matrix penetration before gelation is triggered by a delayed crosslinker.

Crosslinking Gelation Time as a Function of Borate Loading and pH at 80 °C

In conformance-control operations, PVA is commonly crosslinked with sodium tetraborate (borax) to form a three-dimensional network via monodiol complexation between borate ions and adjacent hydroxyl pairs. The gelation time, defined by the Sydansk bottle-test method (adapted from API RP 63), is acutely sensitive to pH and borate-to-polymer ratio. At 80 °C, a formulation containing 2.5 wt % PVA 1799 and 0.3 wt % borax displays a gelation onset of 5–7 h when the system pH is maintained at 8.5 ± 0.3 using a sodium carbonate buffer. Elevating pH to 10.0 shortens the gel point to 40–60 min, which introduces a significant processing hazard: premature near-wellbore crosslinking can raise injection pressure beyond the fracture gradient and permanently damage the formation. The tight operating window mandates continuous in-line pH monitoring and the use of segregated polymer- and crosslinker-streams that combine only at the wellhead through a static mixer, avoiding any tank residence time of mixed fluid. Progressive-cavity pumps (e.g., Seepex BN series) are the preferred surface equipment because their low-shear, pulsation-free delivery limits mechanical chain scission; gear pumps operating at 2500 rpm have been observed under field conditions to reduce PVA solution viscosity by 10–15 % per pass (ISO 3219:2021, cone-plate geometry at 1000 s⁻¹), a loss that can double the required polymer concentration to meet target injectivity. The pre-drying of PVA powder is mandatory when storage relative humidity exceeds 60 %. Moisture absorption inflates the apparent weight, leading to under-dosing and unpredictable crosslinking kinetics, and can initiate caking in the hopper of the dry-polymer eductor, causing feed interruptions. Field blending units are typically fitted with desiccant-bed air dryers on the hopper vent and jacket heaters that maintain the powder at 35–40 °C.

When PVA Replaces Partially Hydrolysed Polyacrylamide in Polymer Flooding of High-Temperature Carbonate Reservoirs

Carbonate formations with matrix temperatures of 80–95 °C and injection brines rich in Ca²⁺ and SO₄²⁻ present a hostile environment for conventional HPAM floods: thermal hydrolysis accelerates, generating additional acrylate sites that precipitate as calcium salts, and the shear forces experienced in downhole chokes and perforations can reduce HPAM molecular weight irreversibly. PVA, with its fully hydrolytically stable acetate-derived backbone, exhibits no autocatalytic hydrolysis even after 180 days of ageing in synthetic brine at 95 °C containing 50 ppm dissolved oxygen when 100 ppm of sodium bisulphite is present as an oxygen scavenger. Core-flood experiments on Berea sandstone (100 mD brine permeability) at 85 °C with a 1.5 wt % PVA 1799 solution crosslinked with 0.15 wt % borax yielded a residual resistance factor (Frr, ASTM D5080-22) of 12 ± 2, sufficient to divert subsequent chase water into lower-permeability matrix. Published data for this specific configuration in fractured dolomite formations is limited, but analogous field trials in sandstone reservoirs with high-contrast permeability streaks have demonstrated water-cut reductions from ~92 % to ~71 % within six months of treatment. A comparative overview of PVA against alternative EOR polymers in a typical synthetic seawater environment (35,000 mg/L TDS, 70 °C) is given below. The cost index is normalised such that conventional HPAM = 1.00.

Parameter (test method)	PVA 1799 (fully hydrolysed)	HPAM (25 mol % hydrolysis)	Xanthan gum (biopolymer)
Intrinsic viscosity [η] in brine (dL/g, ISO 1628‑1)	0.79	18.5	82
Viscosity loss after shearing 30 min at 10,000 s⁻¹ (%, ISO 3219)	2	82	28
Calcium tolerance (mg L⁻¹ Ca²⁺ without precipitation)	>10,000	400	2,000
Thermal half-life at 85 °C, 50 ppm O₂ (days, viscosity decay)	>365	90	60
Risk of biodegradation	None	Moderate	High
Relative cost index	3.4	1.00	2.2
Crosslinker required for conformance gel	Borate, aldehyde, or organotitanate	Chromium(III), aluminium citrate, or organic crosslinker	Not typically crosslinked; used as linear biopolymer

Partially Hydrolysed Grades for Low-Viscosity Injection and Deep Profile Modification

When treatment objectives shift from near-wellbore shut-off to deep matrix diversion, the higher solution viscosity of fully hydrolysed PVA 1799 can limit injection depth. Partially hydrolysed PVA 1788 (88 mol % hydrolysis) provides a 4 % solution viscosity of 20–26 mPa·s, approximately 20 % lower than that of the fully hydrolysed grade, and the lower hydroxy density retards borate crosslinking, extending gelation time by a factor of 1.5–2.0 at equivalent crosslinker loading and pH. Deep placement is achieved by injecting a slug of PVA 1788 with a delayed gelation trigger—often a chelated borate complex that releases reactive borate ions only upon thermal dissociation—allowing the fluid to travel tens of metres into the reservoir before the network forms. The gel formed from partially hydrolysed PVA exhibits ultimate compressive strength roughly 30 % lower than that of the fully hydrolysed analogue (measured at 20 % strain on a universal testing machine), yet this is often sufficient for thief-zone blocking in unconsolidated sands where excessive gel stiffness causes fracture propagation. Incompatibilities must be observed. Aminic crosslinking agents—hexamethylenetetramine or urea-formaldehyde precondensates—induce immediate precipitation of PVA from saline solutions via pH shift and hydrogen-bond disruption, rendering them unusable. Oxygen ingress must be minimised throughout the preparation chain because dissolved oxygen accelerates chain scission catalysed by trace heavy-metal ions; a target of <20 ppb dissolved O₂ in the injection brine, monitored by an optical sensor, is maintained by continuous nitrogen blanketing of mixing tanks and sodium bisulphite dosing. Clean-up from the formation after a PVA treatment relies on oxidative breakers, most commonly sodium persulphate or proprietary peroxygen compounds activated at temperatures above 60 °C, since the polymer backbone cannot be thermally depolymerised at attainable reservoir temperatures. The transport and storage of PVA powder in bulk bags or silos follows standard hygroscopic solid-handling protocols. Lot-to-lot variability in ash content and residual acetate groups can shift the effective crosslinking density; blending on-site from homogenised supersacks with certificate-of-analysis-controlled hydrolysis tolerance (±0.5 mol %) is recommended. Despite a per-kilogram cost roughly three times that of HPAM, the reduced polymer quantity required for gel placement—often 1.5–3 wt % versus 5–7 wt % for HPAM-gelled plugging systems—and the elimination of biocide packages bring the total chemical expenditure for a treatment to within 1.5–2.0 times the HPAM baseline, a figure that has proven acceptable in offshore and remote-location operations where failure of a polymeric plug entails intervention costs an order of magnitude higher.