Expert Knowledge
Wax, parrafine and wax deposits in crude oil
Wax and paraffine in crude oil
The groups of waxes and paraffins are often combined when crude oil is concerned. Under the same circumstances both groups lead to the same problems. Thus, the terms “wax” and “paraffine” are used synonymous.
Crude oil is a very heterogeneous alloy of several thousands of single components of mainly hydrocarbons. The amount of heteroatoms like sulfur, oxygen, nitrogen and metalorganic compounds differs from oil to oil. The crude contains many different fractions from soluted gases (gaseous under standard conditions = atmospheric pressure and room temperature) up to high molecular compounds like resins and asphaltenes which can be classified and separated by their chemical and physical properties.
Paraffines are a part of the homologeous series of alkanes – the saturated series of hydrocarbons – with the general formula CnH2n+2. Paraffines are classified as the part from about C18. Due to the numerous possibilities of branched iso-alkanes the physical properties of paraffins are wide-spreaded.
Waxes are a very heterogeneous group of higher hydrocarbons without a specific composition, often heteroatoms are contained. Waxes are defined by their mechanical and physical properties. (see wax on Wikipedia)
Problems with wax in crude oil
The crude oil composition varies strongly from reservoir to reservoir. The spectrum of extracted oil reaches from thin, low-viscous and straw-coloured oils with very low amounts of wax to deep black, highly viscous (solid at room temperature) oils with high amounts of wax. A classification in paraffinic-waxy and naphtenic oils helps to estimate the behaviour of the oil but for accurate informations about the oil extensive tests under realistic conditions has to be executed.
If crude oil is cooled – in the borewell, at oil conditioning or during pipeline transport – below its WAT wax crystals start to form. These crystals can form deposits when coontacting any kind of surface like walls, valves etc. Over short periods of time these deposits can plug the equipment. During a continouos oil prodution they can be hold at a nearly constant level. Shear forces on the bounding surface of the wax layers – transmitted by the transpoted fluid – are increasing if the diameter is reduced by deposits. The soft wax layers are sheared of. A varying or stopping prodution flow can lead to formation of massive wax plug. These plugs can only be removed with high efforts: physically (by heating or by pressure pulses) or chemically (by using dissolvers). The economic costs and the production loss very high are in each case.
Control and avoidance of wax deposits
Physical, mechanical or chemical
Considering that the removal of wax deposits is a very time-consuming and expensive process the prevention of the formation or if prevention is not possible the control of wax deposits is a main objective. This can be done like the removal in three different ways:
- mechanical: deposits in pipelines can be removed by pigs (technical devices for inspection and cleaning of pipes) and at borewells special inserts for deposit removal are in use. These devices can be used regularly if certain parameters (time, dropping feed rate, pressure loss etc) are exceeded.
- physical: due to the fact that the wax deposition is a temperature-dependant process, the oil or the production devices can be heated to stay above the WAT of the oil. So the wax crystal formation is avoided.
- chemical: by adding chemicals already at the borewell the chemical properties of the crude can be influenced. The WAT can be shifted to higher temperatures that are near or better above the operating temperature. Also the growing of wax crystals can be reduced by using kinetic inhibitors which do not suppress the crystal formation but reduce the speed of the crystal growth rate. Another way is to allow the formation of crystals but to interfere in the agglomeration process with anti-agglomerats, the wax does crystallise but the crystals can not combine to bigger particles.
Wax Appearance Temperature (WAT) and Wax Deposition Temperature (WDT)
In crude oil paraffins and waxes are soluted in the low-molecular, liquid components. If the oil is cooled down the limit of solubility of these high-molecular compounds is under-run at a specific temperature. The component whose solubility is reached starts to crystallise, first micro-crystals will begin to formate. This process leads to a phase separation, the initial single phase alloy changes to a two-phase mixture. If the composition is not altered, crystallisation is a temperature-only process. Pressure dependency is also influencing the crystallisation, but it is of much lesser influence than temperature, so under test conditions with only small pressure changes it can be neglected.
Wax Appearance Temperature WAT
Wax Deposition Temperature WDT
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