Paraffin Inhibitors
Paraffin inhibitors, also known as wax inhibitors, are used to protect against wax deposition in the wellbore, production tree, and subsea pipelines/flowlines from plugging. Waxes are solids made up of long-chain (>C18), normal or branched alkane compounds that are naturally present in crude oils and some condensates.It has been established conclusively that normal alkanes (n-paraffin waxes) are predominantly responsible for pipeline wax deposition. Paraffin inhibitors also have waxy structures that can co-deposit with wax crystals and therefore interrupt the growth and prevent agglomeration or deposition with additional functional groups.
Chemicals that affect the wax appearance temperature (WAT) are usually referred to as wax inhibitors or wax crystal modifiers. Chemicals that affect the pour point are referred to as pour point depressants (PPDs) or flow improvers. Since both classes of chemicals must somehow interact with the wax crystallization process, there are overlaps in the chemistry and mechanisms of the two classes. Thus, most wax inhibitors also function as PPDs. Wax inhibitors and PPDs must be deployed to pipelines before the bulk temperature drops below the WAT.
It is important to note that many applications with wax inhibitors do not totally prevent wax deposition in the whole flowline but can reduce the frequency for wax removal treatments such as pigging or hot oiling.
Several studies suggest that effective wax inhibitors create weaker deposits, which are more susceptible to removal by shear forces in the flow field. At the microscopic scale, wax inhibitors have been shown to have a dramatic effect on wax crystal morphology: rather than the usual platelike growth exhibited by pure wax, highly branched microcrystalline meshes are observed.
Chemistry
The main classes of wax inhibitors and PPDs can be summarized as follows:
- Ethylene polymers and copolymers
- Comb polymers
- Miscellaneous branched polymers with long alkyl groups
The comb polymers are most effective as wax inhibitors but can work synergistically with ethylene copolymers, such as EVA copolymer, and surfactants. As the name suggests, comb polymers resemble a comb in that they have a polyvinyl backbone with many long-chain side groups
High concentrations of PPDs or wax inhibitor polymers can be difficult to handle in cold climates because of their high solidification temperature. Oil-in-water emulsification can be used to prevent them from gelling or allow more than one production chemical to be deployed in the same line.
Product selection
To determine the potential for wax deposition of a crude or condensate, the WAT or cloud point is first measured. There are a variety of ways to do this, with differential scanning calorimetry (DSC) being the most commonly used method in the industry.
The rate of deposition of wax and paraffin inhibitor performance are usually determined using the cold-finger technique. A more elaborate apparatus for measuring wax-deposition rates is a flow loop.
Measurement of the pour point of a waxy crude is traditionally carried out using the simple
ASTM D-97 (or IP 15) pour-point test, although one should also measure viscosity. The repeatability of the test can be poor but allows a ranking of PPD efficiency. The dosage of PPD needed in a laboratory test will not be representative and will almost always be higher than that necessary
for field use.
Applications
The injection of these chemical inhibitors is dependent on the composition of the produced fluids. Injection can occur continuously at the tree, pipeline, manifold, and other critical areas while the production flow is hot and above WAT, and to batch treatments at production start-up and shut-down processes. The wax content, pour point, and other factors are determined prior to beginning production to determine the chemical(s) needed, if any, and the best method for treatment. For a 10,000-BOPD (Barrels of Oil Per Day) well, the paraffin inhibitor could be injected at a rate of 30,000 gal per year (enough to ensure a 200-ppm concentration in the produced fluid flow).
Wax dispersants
Wax dispersants are surfactants that adsorb onto pipe surfaces, reducing the adhesion of waxes to the surface. This could be by changing the wettability of the surface to water wet or by creating a weak layer on which wax crystals grow and are later sheared off by turbulent fluid flow. Some wax dispersants probably function by adsorbing and water wetting the surface of the pipe. Wax dispersants also will adsorb to growing wax crystals, thereby reducing the tendency for them to stick together. A good dispersant formulation will also function to penetrate accumulated deposits of wax, adsorbing on individual particles and enabling them to move freely into the surrounding oil.
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