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 "paraffin" 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 1.
Wax appearance temperature (WAT) and measuring methods:
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.
The temperature at which the first wax crystals start to formate from a crude or a crude oil product is called Wax Appearance Temperature, WAT. It is given in °C (°F) and is an important value for the transportation stability of oil.
these days, the determination of the wax appearance temperature under static conditions is very accurate and needs only small sample amounts and little time. Attention should be payed to the temperature control of the probe, it must be done very slowly because the thermal equillibrium has a great influence on the measured WAT. A too fast cooling leads to supercooling, crystallisation will start late so the measured WAT would be too low. Commonly used methods are:
Cold Finger: A temperature controlled metal cylinder (finger) is inserted in the sample and is cooled down slowly. At fixed time intervals the finger is taken out and examined for wax depositions. By this simple design many tests can be run simultaneously with only little efford. The accuracy of this method is low compared to others and the expenditure of time for a single test is high, but for screening tests this method has its advantages. Another feature of the cold finger is the measurement of the total amount of wax in a sample.
Differential Scanning Calorimetry, DSC2: With this method the amount of heat that is emmited or absorbed by a sample can be compared with a reference sample that is subjected to the same temperature profile.
The process of crystallisation is always accompanied by a change in heat flux. Phase changes like crystallisation are always exothermic (heat releasing) or endothermic (heat absorbing). This effect leads to a slight variance in the thermal behaviour of the sample compared to the inert reference. These changes in heat flux by crystallisation process allow a very accurate measurement of the WAT.
Cross Polarisation Microscopy, CPM: A completely molten and temperature controlled sample is applied as a thin layer on the sample holder of a light-optical microscope. Below and over the sample light-polarizers3 are installed. The polarizers are adjusted in way that no light can penetrate the second filter. This is achieved if the planes of polarisation are at right angle. The light is polarised at the first filter (primary polariser), penetrates the sample and is blocked at the second filter (secondary polariser, analyser) due to the fact, that polarised light can only pass through a polarizer if the planes of polarition are equal.
The CPM takes advantage of the fact that many crystals can turn the plane of polarisation of light. This effect is called optical anisotropy. In the slowly cooled sample crystallisation starts when the WAT is reached. The emerging crystals are anisotropic to light and turn the plane of polarisation. On the before dark secondary polariser now little points of light start to appear. This method is very accurate but time- and work-consuming.
Wax Flow Loop: In a test loop an amount of sample is pumped through a thermostated pipeline section. In this section the wall temperature of the pipeline can be adjusted by heating or cooling. Pressure- and temperature measurements before and after the section show changes resulting from wax depositions. If the wall has a temperature below the WAT of the sample fluid wax depositions will occure. These deposits leads to a reduced diameter of the pipeline and by this to a pressure drop that can be detected. In addition the wax layers on the wall have a thermal insulation effect which is showing in changes in the differential temperature.
The accuracy of this method is not as high as the DSC or the CPM but in opposite to those methods the sample can be examined under flow conditions.
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:
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 and Wax Flow Loop:
The WAT can statically be measured very accurate and with low effort. In daily work it is observable that falling below the WAT does not automatically lead to problems with wax depositions. The wax layers are soft during their formation and vulnerable to shear forces exerted by the flowing fluid.
With the Wax Flow Loop the behaviour of oil and oil products in the production system can be examined with a flowing medium. The variable flow rates allow simulations of different application-fields. The wide-range temperatured test pipeline allows simulation of extreme enviromental conditions like cold regions or sub-sea. Not only the behaviour of the flowing medium can be examined but also inhibitors of any kind can be tested for their performance, compared relatively and classified in absolute ways.
An exchangeable test pipeline with different diameter and length options allows measurements over a wide range of flow conditions. Studys show that the nature of the inner surface (texture, material) has only very little (if any) effect on the wax deposit formation