Oilfield waxes

Oilfield waxes are a natural constituent of crude oils and condensates consisting of mainly heavier (>C17) paraffinic hydrocarbons. These may be straight, branched chain or cyclic, and they affect production in two ways.

Firstly they can have an adverse affect on the viscosity of the oil. This has important implication to pipelines, either in-field or export. They can impart non-Newtonian behaviour, ie that the viscosity of the oil depends, in addition to the temperature, upon the shear rates applied to it. A good common example of this is household non-drip paint. Here, under low shear the paint does not flow (ie non-drip), but at high shear when applied by paintbrush, the paint flows naturally to cover the surface. Such behaviour in production operations normally manifests itself during shut-downs or later in field life when production rates drop off. Following a shut-in, when flow restarts, the initial shear rates may be very low. At low shear rates the apparent viscosity may be high, in some cases so high that the available pressure from the pumps is insufficient to start flow. This is particularly a problem in subsea lines where the fluid temperature falls, compounding the high viscosity.

Secondly, wax crystals can be deposited from bulk. In a pipeline system, this may reduce the internal dimensions, but the real effect upon flow through the line is an increase in the surface roughness of the pipewall. This causes an increase in the energy needed to pump the fluids through the line. Thus, for a given pumping pressure, the volume throughput would be less in lines where wax deposition has taken place. Wax deposition has also been noted in risers, manifolds, at wellheads and in separators in addition to pipeline systems.