A method of improving the flow properties of a crude oil or natural gas liquid The present invention relates to a method of improving the flow properties of a wax containing crude oil or natural gas liquid containing a natural pour point de- pressant by adding a synthetic pour point depressant.

Typically, crude oils and natural gas liquids contain several types of hydrocarbons, such as paraffins, naphte- nes and aromatics. However, the paraffins, which are nor- mally liquid or gaseous at reservoir conditions, have the disadvantage that at lower temperatures they tend to form a solid wax phase. In practice this gives rise to prob- lems, such as formation of a gel with a significant gel strength, e. g. during shut down of wells, pipelines, processing plants etc. Likewise problems caused by wax deposition on cold surfaces, e. g. in pipelines and heat exchangers, can occur. Furthermore the formation of a solid wax phase can lead to an increased viscosity, which means that the crude oil or natural gas liquid may become significantly more difficult to handle.

For the purpose of alleviating the above problems it has been known for long to add so-called"pour point depress- ants"in order to lower the pour point of the crude oil or natural gas liquid, which point according to ASTM D97 or ASTM D5853 is defined as the temperature at which a crude oil or natural gas liquid at given conditions starts to solidify. Such synthetic pour point depressants may comprise a wide range of polymers and copolymers (polyacetates, polyacrylates, polymethacrylates, poly- amides, etc.).

The polymers may roughly be divided into nucleators and growth arrestors depending on their function in the crude oil or natural gas liquid. However, it is a common fea-

ture thereof that they are incorporated in the solid wax phase and thereby change its structure and properties. To achieve improved efficiency, it is normal practice to add the former type of wax inhibitor at or close to the wax appearance temperature (WAT) of the crude oil or natural gas liquid, which is defined as the temperature at which the wax starts to precipitate. The latter type of wax in- hibitor can be added at lower temperatures, although the efficiency hereof seems to increase if the polymer is in- corporated already in the solid wax phase first formed.

In certain crude oils and natural gas liquids there are, however, also naturally occurring pour point depressants for instance in the form of asphaltenes, which are a group of heavy polar aromatic compound having a molecu- lar weight in the range 700-1000 g/mole. These natural pour point depressants are present in the crude oil or natural gas liquid when it is produced from the reser- voir, and are normally fully disperse in the liquid phase at initial reservoir conditions. At lower tempera- tures they may, however, start to flocculate, which re- duces their pour point depressing efficiency. This phe- nomenon gives rise to the concepts upper and lower pour points, as defined in the standard ASTM D97 : Flocculation of the natural pour point depressants may also be induced by changed pressure or mixing with water or other oils/condensates. Just like synthetic pour point depres- sants they change the structure of the wax phase formed, although in many cases their mere presence does not com- pletely suffice for solving the problems associated with wax precipitation.

In practice during oil production, an amount of the abovementioned synthetic pour point depressants has therefore been added to crude oils and natural gas liq- uids at a temperature close to the WAT of the crude oil

or natural gas liquid, assuming that thereby an additive effect of the natural and synthetic pour point depres- sants would be obtained.

In certain cases this additive effect does not appear, as the naturally occurring pour point depressants seem to be capable of counteracting and even totally neutralizing the effect of the synthetic pour point depressants.

Consequently, it is the object of the present invention to provide a method of reducing or eliminating the above problems and thereby ensure an effective improvement of the flow properties of crude oils and natural gas liq- uids.

The method according to the invention is characterized in that the synthetic pour point depressant is added to the crude oil or natural gas liquid at a higher temperature than 20 °C below the inversion temperature of the crude oil or natural gas liquid.

The inversion temperature of a crude oil or natural gas liquid is determined by the use of a modifie ASTM D97 or modifie ASTM D5853 standard method. These standards specify upper and lower pour points to be measured after reheating the sample to 48 °C and 105 °C, respectively.

By reheating the crude oil or natural gas liquid to a given temperature and subsequently determining the pour point of this sample and repeating this measurement at different reheating temperatures and subsequently depict- ing these pour points as a function of the reheating tem- perature, the reheating temperature at which the pour point has been reduced to its minimum value can be deter- mined. This point is defined as the inversion tempera- ture, and is the temperature at which the natural pour point depressants are fully active. The result will typi- cally be within the interVal 80-110 °C.

The inversion temperature definition is, however, not ap- pliable to hydrocarbon amples, where the upper and lower pour points are identical. This phenomena is be- lieved to be due to the lack of natural pour point de- pressants mainly found among the asphaltene fraction of the fluid.

When using the method according to the invention it has turned out that in situations in which addition of syn- thetic pour point depressants has otherwise been without result, a positive effect can be obtained. This is pre- sumably due to the pour point depressant being added to the crude oil or natural gas liquid at a time when the natural pour point depressants are fully or at least partly disperse. In this way it is avoided that the flocculated natural pour point depressants, which appar- ently form a separate, polar liquid phase, are capable of absorbing the synthetic pour point depressants by virtue of their content of polar groups.

Due to a more efficient pour point depression, it is pos- sible to obtain a suitable pour point even by addition of small amounts of synthetic pour point depressants. Alter- natively, a lower pour point can be obtained by using the usual amount of synthetic pour point depressant.

Preferably the synthetic pour point depressant should be added at a higher temperature than e. g. 10 OC below the inversion temperature, more preferably at or above the inversion temperature. If the synthetic pour point de- pressant is added at a temperature 20 °C below the inver- sion temperature of the crude oil or natural gas liquid, the natural pour point depressants are thus not full, y disperse, which means that the efficiency obtained is not optimal.

In practice, the best effect is obtained by injecting the synthetic pour point depressant into the well bore at a depth, where the crude oil or natural gas liquid has a temperature at or higher than the inversion temperature.

An appropriate depth for adding the synthetic pour point depressant, may be determined as the depth at which the formation temperature corresponds to the inversion tem- perature of the crude oil or natural gas liquid produced.

Since the temperature at this point automatically will increase once the production starts, adding the depres- sant at this depth ensures that the temperature of the crude oil or natural gas liquid will always exceed the inversion temperature when mixed with the synthetic pour point depressant.

The required dosage of synthetic pour point depressant depends on factors such as the type of crude oil, amount of wax formed in the crude oil at different temperatures, ambient conditions etc. The optimum dose rate is normally estimated by means of laboratory measurements of pour point, viscosity, gel strength and wax deposition ten- dency. The two latter measurements are often carried out on laboratory coils. In practice the syntnetic pour point depressant is added in an amount of 5-2000 ppm, refera- bly 20-500 ppm and most preferably 20-100 ppm.

A preferred synthetic pour point depressant useful in the method according to the invention comprises a chemical compound having the general formula I:

where m = 20-300, preferably m = 25-100 and most refera- bly m = 50-80, n = 5-50, preferably n = 5-25 and most preferably n = 8-15 and R symbolises a hydrocarbon, pref- erably an aliphatic hydrocarbon and most preferably an aliphatic hydrocarbon consisting of 2-20 carbon atoms.

However, also other compound may be used, such as poly- acetates, polyacrylates, polymethacrylates, polyamides etc.

The invention is described in more detail in the follow- ing examples.

Example 1 It is the purpose of this expriment to demonstrate a correlation between the temperature of the oil sample at which the synthetic pour point depressant is being added and the efficiency of the synthetic pour point depressant added. In this expriment an oil sample no. 1 having the following characteristics is used: Wax appearance temperature (°C) 50 Upper pour point (°C) 33 Lower pour point (°C) 24-27 Wax content (weight % at -30 °C) 22 Asphaltene content (weight %) 0.2 Density (g/cm3) 0.868 Kinematic viscosity at 80 °C (cSt) 3.50 Kinematic viscosity at 7S OC (cSt) 3.91 Kinematic viscosity at 70 °C (cSt) 4. 34 Kinematic viscosity at 65 °C (cSt) 4.81 Kinematic viscosity at 60 °C (cSt) 5.36 Kinematic viscosity at 55 °C (cSt) 6.03 Kinematic viscosity at 50 °C (cSt) 6.85

Kinematic viscosity at 45 °C (cSt) 9.31 Kinematic viscosity at 40 °C (cSt) 14.73 The wax appearance temperature of the oil sample was de- termine from the viscosity measurements.

Measurement of the pour point of oil sample no. 1 was then conducted on two uninhibited subsamples nos. 1A and 1B by heating the oil samples to various temperatures succeeded by cooling in order to determine the pour point of the oil amples. From the results of the measurements, in Figure 1, showing the pour point as a function of the reheating temperature, it is seen that the inversion tem- perature of the oil sample may vary from 90 to 100 OC.

Then the same oil sample no. 1 was again heated to dif- ferent temperatures, followed by addition of 250 ppm of the chemical additive A, and the effect hereof was exam- ined by measuring the pour point of the oil sample, by letting it cool down in accordance with standard proce- dure ASTM D97. The result hereof is also seen from Figure 1 (250 ppm of chem. A). It appears clearly that the pour point of the oil sample no. 1 with addition of additive A follows the trend of the two uninhibited subsamples up until approx. 80 °C, above which point the additive be- comes active with a resulting strong decrease in the pour point of the oil sample.

This is consistent with the theory propose, as the 100 °C corresponds to the inversion temperature of the oil sample no. 1B, which means that the natural pour point depressants here are fully disperse. Thus Figure 1 shows that for this very waxy crude oil samole a pour point im- provement of the magnitude of 18-21 °C can be observe with chemical dosage of only 250 ppm.

Example 2 It is the purpose of this example to demonstrate that in accordance with the propose theory, it is only in case of crude oil or natural gas liquid samples containing natural pour point depressants that the favorable effect of an increased chemical injection temperature is achieved. In this expriment an oil sample no. 2 having the following characteristics is used: Wax appearance temperature (°C) 40 Upper pour point (°C) 12 Lower pour point (°C) 12 Wax content (weight o at-30 °C) 10.4 Asphaltene content (weight <0.05 * Density (g/cm3) 0.799 Kinematic viscosity at 70 °C (cSt) 1.224 Kinematic viscosity at 60 °C (cSt) 1.394 Kinematic viscosity at 50 °C (cSt) 1.596 Kinematic viscosity at 40 °C (cSt) 1.866 Kinematic viscosity at 30 °C (cSt) 2.733 Kinematic viscosity at 20 °C (cSt) 5.4 * detection limit The wax appearance temperature of the oil sample no. 2 was determined from the viscosity measurements.

Measurements of the pour point of the uninhibited oil sample no. 2 as a function of the heating temperature were conducted, as stated above, and the results appear from Figure 2. It should be noted that upper and lower

pour point are identical, which is strongly indicative of the absence of natural pour point depressants.

Hereafter the pour point was determined again following addition of 50 ppm of chemical additive A in order to test the effect of the addition temperature according to the above example. As is seen from Figure 2 (50 ppm of chem. A), the temperature has no influence on the effi- ciency of the additive, which according to the propose theory is consistent with the missing content of natural pour point depressants.