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Abstract
Pour point depressant (PPD) activity mechanisms include solubilization, morphology alteration, and steric/entropic repulsion. A wealth of complex intermolecular interactions governs PPD efficacies in real crude compositions. The current study utilizes single-component model waxes (n-C24H50, n-C28H58, n-C32H66 or n-C36H74) dissolved in dodecane to reveal chain-length dependent PPD efficacies. Wax appearance temperature depression, effected by PPD, diminishes as paraffin chain-length increases. Stronger London van der Waals attractions between longer wax components effectively diminish PPD solubilization activity. Controlled-temperature centrifugation reveals partitioning of PPD polymers and paraffin waxes between solid and liquid phases. PPD polymers typically contain a polydisperse molecular weight distribution. The highest molecular weight PPD polymers partition preferentially to solid phases, including PPD aggregates and PPD-modified wax crystals. The lowest molecular weight PPD polymers remain preferentially soluble in the liquid phase, binding only weakly to precipitated wax. The presence of precipitated wax counteracts the solubilization activity of PPD polymer. At high precipitated wax fractions, PPD shows no wax solubilization activity. As the precipitated wax fraction decreases, the wax solubilization activity of the PPD progressively increases. The wax solubilization activity of the PPD attains a maximum at the WAT. Finally, the results are consistent with a single optimal polymer molecular weight for PPD activity occurring at a single temperature. PPD polymers larger than the optimal MW undergo a coil-to-globule transition prior to wax crystallization, deactivating the polymer. Polymers smaller than the optimal molecular weight show weaker binding to wax crystals, consistent with a smaller Gibbs free energy of binding, and partition preferentially to the liquid phase. Modern PPD formulations should utilize polymers that are tailored according to molecular weight. Optimally tailored PPD polymers should be less polydisperse in nature than current PPD formulations.
Highlights
The presence of paraffin wax in produced petroleum fluids gives rise to several commonly encountered flow assurance problems
Co-crystallization and adsorption may not necessarily be mutually exclusive terms, as interfacial adsorption may readily occur in a structured, crystallographic manner for pendant aliphatic moieties on thin paraffin wax crystal side faces constituted by relatively high-interfacial-energy, orthorhombically structured –CH2– groups
Efficacy of five types of Pour point depressant (PPD) are evaluated for simple model oils containing single component waxes in n-dodecane
Summary
The presence of paraffin wax in produced petroleum fluids gives rise to several commonly encountered flow assurance problems. Paraffin waxes in petroleum fluid comprise linear, branched, and cyclic longchain saturated aliphatic hydrocarbons. The solubility of paraffin components in petroleum fluids increases strongly with increasing temperature. Paraffin components typically exist in a dissolved state in high pressure, high temperature (HPHT) petroleum reservoirs. As petroleum fluid flows along the length of a production string, thermal losses to the surrounding environment may cause the fluid temperature to fall below the wax appearance temper ature (WAT). Production problems originating from paraffin wax include wax deposition during continuous flow, pipeline plugging attributed to gelling during flow shut-in, increased fluid viscosity(Kelland, 2014), and emulsion stabilization (Rodriguez-Fabia et al, 2019)
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