Wax formation in oil pipelines: A critical review

Investigation of inhibitors efficacy in wax deposition mitigation using a laboratory scale flow loop

Deposition of wax on a cold surface is a serious problem in oil production. Progress in developing more effective wax inhibitors has been impeded by the lack of an established mechanism connecting the molecular structure to inhibitor efficiency. Some comb-like polymers having long alkyl side chains are known to decrease the rate of wax formation. Among several possible mechanisms, we investigate here the incorporation-perturbation mechanism. According to this mechanism, the inhibitor molecules in oil are preferentially partitioned (incorporation) toward the wax-rich (amorphous) wax deposits (soft wax), which then serves as a perturbation to slow down the ordering transition of soft amorphous wax into more stable but problematic hard wax crystals. Indeed, molecular dynamics simulations on an effective inhibitor molecule in both the oil phase and in the amorphous wax phase support the idea that the oil-to-wax partition of the inhibitor is energetically favorable. With the inhibitor molecule embedded, the structure of wax crystal is disturbed, significantly decreasing the order and significantly lowering the cohesive energy density relative to that of the pure wax crystal, supporting the slower transition from soft wax to hard wax. Thus, in the presence of an effective wax inhibitor, crystallization (formation of hard wax) is slowed dramatically, so that there is time to flush out the soft wax with a high-pressure flow inside the pipeline. This suggests design principles for developing improved wax inhibitors.

Wax inhibition by comb-like polymers: support of the incorporation-perturbation mechanism from molecular dynamics simulations.

Wax deposition in oil and gas pipelines is considered one of the most severe operational problems, and significant efforts have been made to prevent and remediate this flow assurance issue. Chemica...

A Critical Review of Controlling Paraffin Deposition in Production Lines Using Chemicals

In this investigation, a light transmittance method was used to evaluate the wax appearance temperatures (WAT) and wax disappearance temperatures (WDT) of model paraffin compounds (n-C24H50 (C24) and n-C36H74 (C36)) in n-decane (C10) solutions both with and without wax inhibitors. The change in WAT at different paraffin concentrations in the presence of an inhibitor behaves as though there is a constant amount of paraffin removed by the inhibitor. However, the amount of apparent paraffin reduction by an inhibitor (e.g. 160 g of C24 by one gram of an inhibitor) indicates that the inhibition mechanism cannot easily be explained by a simple “sequestering” effect. Wax inhibitors that decrease the WAT tend to also increase the WDT. Most of the wax inhibitors tested at a dosage of 100 ppm did suppress the WAT of lower molecular weight paraffin (C24) solutions, but had little or no effect for higher molecular weight paraffin (C36) solutions. Side-chain length of polymethacrylate wax inhibitors is an imp...

Evaluation of Effects of Selected Wax Inhibitors on Wax Appearance and Disappearance Temperatures

Wax crystallization and deposition during production can lead to severe pipeline and flow line restrictions. Over time, the build-up of wax will reduce the internal diameter and eventually block the line or well. Remediation is normally performed by mechanical pigging or through solvent washes or hot oiling. All have their advantages and limitations. Wax inhibitors/dispersants and anti-sticking agents are an effective solution to remediate the deposition problem. These chemicals can act as either crystal modifiers, or products that reduce the rate of wax build-up on pipe walls. A novel and generic wax dispersant and anti-sticking agent has been developed and tested, which has proven to be effective at retarding the deposition mechanism significantly. The chemical is a synergistic blend of ionic surfactants, which has an ability to absorb onto bare surfaces of tubings and equipment making them oleophobic. The unique properties of the chemical in preventing waxes from adhering to surfaces and success of the...

S. Takhar

Papers

A Wax Inhibition Solution to Problematic Fields: A Chemical Remediation Process