FR2537883A1 - Method of conveying, desalting and dehydrating a heavy petroleum oil - Google Patents

Abstract

Conveying a heavy petroleum oil in the form of an oil-in-water emulsion. Improvement in the water-oil separation by the use of methanol. The emulsion 1, to which methanol 2 has been added, is heated 4 and then settled 7. The oil phase is separated off 9, the water 14 is separated from the methanol 13 and the latter can be recycled.

Description

The invention relates to a combined process for transporting a heavy petroleum oil and for desalting and dehydration thereof.

Salt is one of the elements that slow down the development of heavy oils.

Salt is present in variable quantities, usually 100 to 1500 ppm by weight, and in forms which are difficult to extract. It is present at all stages of petroleum exploitation, in particular downhole, wellhead, transport by pipeline or by sea.

Its presence in the form of crystals or ionized salts in water is a cause of inconvenience, in particular blockages (strainers) 7, corrosions and parasitic reactions. Refiners therefore usually require that the crude oil contain less than 10 ppm of salts before undergoing refining treatment.

The removal of heavy oil salts is made difficult - by high viscosities - by densities which are close to I or greater than i and which therefore hinder the dehydration of crude oil by natural decantation.

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Thus, the oil1,, Laguna Once ", used in the example below, has a viscosity of 643 poises [64, 3 Pa.s3 at 20 OC and a density of 0.987 at 25 OC.

To remove these salts, transfer the salt crystals to a solvent medium where they are soluble and then separate the solvent.

Water, when it is of good quality and available in sufficient quantity, is a good solvent.

Three stages characterize classic desalting - the mixing of oil with water intended to cause diffusion
salts from oil to water - application of gravitational forces (quiescent or centrifuged
tion) intended to cause the coalescence of droplets - the separation of the two phases.

For conventional crude oils, the technique of emulsion of water in oil is used for diffusion. Oil naturally comes out of the well in this form. Water is generally added to reach water percentages of the order of 7 to 9%. The emulsion is redone, for example by passing through pumps. Coalescence is achieved by chemical treatment and / or by an electrostatic field.

The temperature is chosen to decrease the viscosity (<5 mPa.s), promote diffusion and increase the decantation speed.

These techniques are difficult to transpose to heavy crude oil, diluting the latter with lighter hydrocarbons makes it possible to reduce its viscosity and its density, but this result is only obtained by using rare and expensive light cuts. Their low efficiency requires high solvent levels which have the effect of oversizing the devices. If these cuts are not available on site, it is necessary to dehydrate and desalt to separate the solvent by distillation: operation costly in energy because of the relatively high temperature levels that must be reached, to separate the products.

Another problem facing oil tankers is that of transporting heavy oils from production sites to refineries, this transport involving, as appropriate, the passage through the casing before "the Christmas tree", the passage in oil pipelines, in ship tanks and / or in intermediate storage tanks.

It has already been proposed to transport heavy oils in the form of oil-in-water emulsions; indeed these emulsions have a viscosity considerably lowered compared to that of heavy oils and completely compatible with the requirements of transporters. Thus viscosities of, for example, 30 to 100 cSt (1 cSt = 1 mm2 / s) at 20 OC are commonly obtained, while the usual requirements, for oil pipelines, are: <200 cSt at 20 OC .

During this transport in the form of an oil-in-water emulsion, at least part of the salt passes from the oil into the aqueous phase, so that a desalting effect is observed. A process of this type is described in US Pat. No. 3,491,835, which claims more particularly the recycling of the aqueous phase, separated at the end point of transport, to the places of production.

An essential problem to solve if we want to take full advantage of
of the advantages of this transport technique, the effect of desalting and dehydration of the oil, is however that of properly breaking the emulsion at the end point of transport.

The invention aims precisely to solve this problem.

The method of the invention therefore comprises the following steps:
A) placing heavy oil in oil emulsion in water
B) transport of the emulsion thus formed
C) rupture of the emulsion, making it possible to collect the oily phase and the aqueous phase separately.

The oily phase obtained has a considerably reduced salt and water content compared to the starting oil.

The present process is characterized in that the breaking of the emulsion is carried out in the presence of methanol. It has in fact been found that the presence of methanol not only promotes the separation of the phases by lightening the aqueous phase but also improves dehydration and desalting.

By methanol is meant not only pure methanol but also methanol of commercial quality which can contain up to, for example, 10% of other compounds, in particular methanol or heavier alcohols.

Details are given below on the preferred conditions for carrying out the transport of petroleum oil-in-water emulsion.

A) - Putting heavy oil into an oil emulsion in water.

The oil to be desalted and transported can be emulsified by passing a mixture of oil, water and emulsifier through any mixing device, for example downhole, at the wellhead or on a transmission line. The treatment can be repeated if necessary in the final stage on the refining site.

The oil, before entering this equipment, can be preheated to a temperature of around 80 to 200 C, depending on the quality of the crude oil (viscosity, density, asphalt content, salt content), for example between 80 and 100 C at the bottom of the hole and at least 140 C at the surface.

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For extra heavy crudes with a density greater than d45 = I, 000 (less than 100API), it may be necessary to raise the # temperature to 190200 C-or above. The risks of vaporization of the organic phase, unlike traditional crudes, are not to be feared. These temperatures can be obtained economically by exchange with medium pressure steam in a preheater.

Crude oil is usually in the form of a water-in-oil emulsion; so as to avoid the vaporization of this water and the deterioration of the tubes, the emulsification operation is preferably carried out at a low pressure (4 to 5 bars). The salt content of this constitution water is very high compared to the water provided for transport. Preferably, a fraction (for example 10 to 30%) of 1 "of water is injected which must be used to make the emulsion either at the suction of the crude oil pump or upstream of the preheater. temperature rises in the exchange train, the diffusion of water and salts is initiated to accelerate in the pipes thanks to the agitation produced by the changes of direction due to the pins.

 This high temperature promotes the solubilization of magnesium and calcium chlorides, among others.

The efficiency of desalting is a direct function of the viscosity of the organic phase and therefore of its temperature. We will therefore try to reach viscosities of the order of 10 mm 2 / sce which is possible for temperatures of the order of 170 to 180 C. This accessibility to the salt crystals will likewise be facilitated by the reduction in viscosity of the continuous phase: water. The water will come into contact with the oil brought to a high temperature.

At a temperature of about 80 to 100 4C the viscosity of water, which is 1 cSt at 20 OC, is reduced to about 0.35 cSt (lcSt = 1 mm2 / s)
The temperature adjustment is preferably such that the temperature at the outlet of the emulsifier does not exceed 70 to 75 ° C., temperatures above which risks of instability may appear. In certain cases, at the bottom of a well for example, heat can even be introduced in the form of vapor.

But simply reducing viscosity is not enough. The solutions must be shaken and thoroughly mixed to improve the transfer.

This stirring is obtained by using effective stirring devices known in the art.

The emulsifying agent can be a colloidal substance of mineral origin such as the finely divided clays commonly used in drilling operations and which would reinforce the role of natural substances contained in certain waters, for example river water, or preferably active substances called surfactants which can be anionic, cationic, nonionic or zwitterionic.

Examples of non-ionic agents are the compounds formed by the reaction of methylene oxide with, for example, an alcohol, an alkyl phenol, an ester, an amide or an alkyl sulfate.

Examples of anionic agents are sulfonates, for example alkylarylsulfonates, sodium, potassium or ammonium alkylsulfates and carboxylates.

Examples of cationic agents are quaternary ammonium salts with a long hydrocarbon chain.

As examples of zwitterionic surfactants, mention may be made of alkylcarboxybetaines, alkylamidosultaines, and also carboxy betalnes derived from naphthenic acids.

These agents are very well known to specialists and the invention is not limited to the use of particular categories of these agents. A preferred agent, however, is tall oil.

It may be the tall oil known as crude oil before distillation or the tall oil known as tall oil pitch recovered at the bottom of the crude tall oil rectification column.

This agent has indeed several advantages: low price, high efficiency even at low content of 0.5 to 1.5% by weight, and preferably I%, its property at the time of the emulsion rupture to separate preferentially towards the organic phase which represents an advantage at the level of the weight balance but also reduces the organic load of the aqueous phase which makes it unnecessary most of the time its treatment.

To be able to be used, tall oil or tall oil pitch must be used in the form of # salt obtained from soda, for example
For example, 0.25 grams of sodium hydroxide and 1 g of tall oil can be used for 80 g of water and 120 grams of heavy crude.

Another preferred emulsifier is an asphalt fraction from a deasphalting unit, a fraction preferably having the following characteristics: density between 1.04 and 1.1 at 50 ° C. and asphaltenes content between 30 and 60% by weight. This type of natural emulsifier more specially adapted to heavy crudes of low or very low asphaltene content has a certain number of advantages - insensitivity to variations in salinity (since it can withstand the salt content of formation water) - low price and abundant availability - it enhances the stability of the emulsion when a conventional type of emulsifier is used - it increases the organic phase which is favorable for the subsequent separation of the phases.

The water can be of any origin. However, water with a salt content (dry mineral residue) at most cicada is preferred at 50 ppm weight / liter.

River water is preferred for this reason.

The pH is chosen according to the surfactant chosen. With non-ionic and zwitterionic surfactants the pH is practically indifferent; with anionic surfactants (carboxylates or sulfonates), - the use of which is preferred, a pH of 9 to 12 is preferred. On the contrary, with cationic surfactants, an acid pH will be adopted.

The presence of colloidal matter in suspension is not annoying; it can even stabilize the emulsion. We must therefore be satisfied with removing particles with a diameter greater than 0.1 mm in a lower #.

The oil in water emulsion can contain a variable proportion of water, from 30 to 80% by weight, preferably 35 to 50%, the complement corresponding to the oil.

B) - Transport of 11 emulsions
The salinity as well as the water content of heavy crudes can be very variable and very high, depending on the deposit, wells, their lifespan and the method of exploitation. However, some heavy crudes contain harmful elements whose harmfulness is all the greater when these elements are more soluble in water. However the continuous aqueous phase must remain during t #out neutral transport if one wants to minimize corrosion.Among these elements one can include, hydrogen sulfide, mercaptans, carbonyl sulfide (COS), carbon dioxide carbon, naphthenic acids, sediments (iron sulfide, vanadi # um or aluminum compounds) '. Consequently, it may be necessary for certain types of crude oil to change the water of the emulsion before long distance transport is accomplished. This dehydration separation operation will advantageously be carried out near the oil field. When the separated aqueous phase contains emulsifiers from an enhanced recovery operation, it is advantageous to return it to the deposit.

 The crude oil is ready for a new final emulsification in the case of long distance transport.

After the manufacture of the oil-in-water emulsion, there is at least 1 day #, most often 15 to 90 days before the emulsion is destroyed. The desalting effect, initiated in the #making of the emulsion, continues during transport.

For some very salty crudes and heavy cuts, a change was observed in the size of the oil particles, which are preferably between 3 and 10 microns due to osmotic transfers of water in the oil.

In this case, in order to maintain the #stability of the emulsion, it can be passed again in mixing devices arranged on the path followed by
The emulsion transport temperature will preferably be between +10 and +50 C. Temperatures that are too high (80 OC) or too low (0 C) should be avoided; in the latter case, additives preventing freezing make it possible to descend lower, as well as the use of salt water to produce the emulsion.

C) - Rupture of the emulsion - dehydration
At the end of the transport; it is necessary to break the emulsion, coalesce the droplets and separate the aqueous phase from the organic phase.

This result is # t obtained according to the invention by adding methanol with heating of the emulsion to 50 to 100 OC, preferably 80 to 90 OC.

Separation can also be promoted by adding a diluent for the aqueous phase or the organic phase, in addition to methanol.

One can for example dilute the organic phase either by a lighter compound, for example a cut of hydrocarbons of density 0.7 to 0.8, or by a heavier compound such as one of those mentioned in the American patent n ° 3.878 .090. Among the compounds heavier than crude oil, it should be remembered the potential natural emulsifier which is the asphalt obtained by deasphalting the vacuum distillation residue of heavy crude oil.

Other factors favoring the separation of the phases are - the use of centrifugation, in preference to simple decantation; - the modification of pH, in particular when the surfactant is of the typeionic type. For example, if the surfactant was of the anionic type, the pH is brought to 3-6.

- the possible contribution of demulsification additives in particular of # surfactants.

Methanol can be used in a proportion of 5 to 200% of the weight of the emulsion strip, preferably 20 to 40, of this weight.

After separation of the phases by concentration and or centrifugation, the solvent is separated by distillation.

The distillation of methanol is easy and accepts low-level thermal energy if it is done under vacuum. Methanol losses are negligible.

When a halogenated derivative of methane is used to add weight to the organic phase, preference is given to carbon tetrachloride or trichloromethane. Indeed the first forms with methanol an azeotrope at a temperature of 55.7 OC and the second forms an azeotrope with methanol at a temperature of 53.5 OC, which promotes the distillation of methanol and therefore allows the use of sources of low heat during this distillation.

The desalting treatment of the invention followed by the fractionation of the phases in the presence of methanol can be repeated, for example at the outlet of the well if the crude oil has a too high saline or formation water content (a content of 20% water is frequent) and if the crude contains elements harmful to transport such as C02, sulfur products such as mercaptans and others
It can also be renewed if particularly desalting is required, below 10 ppm for example in the refining zone.

It is thus possible to obtain water contents in the crude as low as 0.01 to 0.05% by volume and salt contents as low as 5 to 10 ppm by weight. The losses of hydrocarbons in water are very low, for example less than 10 ppm by weight.

The heavy oils to which the process applies more particularly have a density greater than 0.933 (<20 0API) and a viscosity greater than 10,000 cSt (1 cSt = 1 mm2 / s) at 25 OC. They commonly contain 5 to 15% by weight of asphaltenes (dosage with n-heptane).

EXAMPLE
An emulsion is prepared on the field by mixing at 140 OC 60 parts by weight of a Venezuelan crude with 40 parts by weight of water (flight content: 250 mg / l), 0.5 parts by weight of tall oil, as emulsifier, and 0.125 parts by weight of soda The viscosity of the emulsion is 40 centipoise (1 poise = 0.1 Pa.s) at 20 OC.

The Venezuelan crude was a Laguna Once d2S0C = 0.987
4 asphaltenes (heptane test) = 8.7% by weight salts = 80 mg / kg acid number = 4.3 viscosity at 20 OC = 643 poises (1 poise = 0.1 Pa.s)
The emulsion was transported in a pipe simulating an oil pipeline (ref.

1 of the attached figure). On arrival, industrial grade methanol (line 2) was added in equal weight relative to the water of the emulsion, and the resulting mixture was stirred by passage through a mixing valve (3), at room temperature. This mixture then passes through a preheater (4) with a turbulent flow regime under a pressure of 4 bars; the mixture is heated by condensed vapors or low pressure vapors (5,6); it leaves it at 80-90 ° C., for example 85 ° C., and passes through a decanter (7) maintained at substantially the same temperature of 85 ° C. The light phase, from den d850C sity d45 4 = 0.870, is drawn off from the top (line 8); it contains approximately 50% water and 50% methanol. It is easily separated from the oil phase, which is drawn off (line 9) from below (d45 C = 0.960) and whose viscosity of 300 centipoises (1 poise = 0.1 Pa.s) at 85 OC allows easy pumping . This oil phase contains only 40 ppm by weight of salts and 0.05% by weight of water.

 The light phase is distilled, preferably under reduced pressure in column 10, so as to consume only one form of energy (11,12) at low level; methanol is easily collected at the top (13) and the water at the bottom (14). Methanol is condensed (15) and can be recycled (2). The losses are negligible since the recovered water contains only 10 ppm by weight of hydrocarbons and 150 ppm by weight of methanol. However, additional methanol can be admitted via line 16

Claims (8)

 methanol.  characterized in that step c) is carried out in the presence of  CLAIMS 1. Combined method of transporting a heavy petroleum oil and at least partial desalting thereof, characterized by steps a) - placing the heavy oil as an oil emulsion in water, b) - transport of the emulsion c) - rupture of the emulsion and separation of the water and oil phases, ca 2. Method according to claim 1, in which the amount of methanol is from 5 to 20% by weight of the water of the emulsion. 3. The method of claim 1, wherein the amount of methanol is 20 to 40% by weight of the water of the emulsion.  4. Method according to one of claims 1 to 3, in which step c) is carried out at a temperature of 50 to 100 OC, 5. Method according to one of claims 1 to 3, wherein step c) is carried out at a temperature of 80 to 90 OC. 6. Method according to # one of claims 1 to 5, in which the emulsification is carried out at a temperature of 80 -200 OG. 7. Method according to one of claims 1 to 6, in which an emulsifying agent is used to produce the emulsion of step a). 8. The method of claim 7, wherein the emulsifying agent is tall oil.