The present invention relates to an improved method and additive for reducing the viscosity of crude oil and in particular to an improved method of reducing the viscosity of different types of crude oil over a range of temperatures.

Many crude oil deposits are of such high viscosity that they are very difficult to exploit without the aid of chemicals that allow the crude oil to be produced and transported to suitable locations for processing and/or shipping.

Crude oil (or crude petroleum oil) is found in nature in both surface and subsurface deposits. These deposits contain different oils which have different characteristics. For example, some crude oils contain high levels of waxes, while others contain high levels of naphthenic compounds, among many other organic components. Therefore, viscous petroleum products containing different components are extracted from different oil fields, and the presence of high levels of these components increases the viscosity of crude oils.

When crude oil is extracted from subterranean reservoirs it is generally at a temperature which allows it to flow either naturally, or with the assistance of pumps, to the surface. In most cases, the temperature of crude oil at the surface is sufficiently high to maintain the low viscosity necessary to allow the crude oil to flow freely from the producing wells to the gathering or production system.

However, when crude oil is produced and passed through surface pipe-work, it generally loses heat. If the temperature is reduced sufficiently during this process, wax and/or other components precipitate out of the crude oil solution. Alternatively, as a consequence of the temperature reduction, the components reach such a high viscosity that the crude oil flows less readily. In fact, in many cases where viscous crude oils are produced and the temperature is lowered sufficiently, the crude oils will not flow at ambient temperatures.

Present solutions to the problem of crude oil mobility reduction involve the addition of various different chemicals which increase the mobility and flow characteristics of the crude oil. The choice of chemicals that are used is dependent on whether the precipitation or the viscosity increase mechanism is involved in reducing the mobility of the crude oil.

When the reduction in flow is caused by wax or asphaltene problems, additives such as inhibitors, pour point reducers, or solvents may be added to increase the flow of the crude oil . These additives either change the composition of the crude oil or interfere with the mechanism of deposition. For example, solvents can change the amount of aromatic components in the crude oil solution or they can interfere with the mechanism of deposition. Whereas inhibitors can prevent precipitation of material from a specific crude- oil at a specific temperature and pour point reducers can change the temperature at which crude oil stops flowing when cooled. In general, the effect of these additives is to reduce the viscosity and increase the mobility of the crude oil. However, as alluded to above, it is often necessary to choose a chemical additive that is specific to the type of crude oil and to the mechanism of mobility reduction.

In the event that specific chemicals are too expensive to use for increasing the mobility of crude oil, a generally applicable system can be used. This system involves using a surfactant and a divalent inorganic ion with a high shear apparatus to form an emulsified system referred to as Orimulsion (for which patents already exist) . However, Orimulsions are water in oil emulsions (oil continuous phase) which can prove difficult to separate into oil and water. Such Orimulsions are normally used as fuel rather than being refined to produce more valuable petroleum products.

Currently, chemicals that are employed to change crude oil flow characteristics are expensive and must be used in large quantities, making the cost of using these chemicals significant. Furthermore, most of the above treatments have the drawback that they are specific to the particular mechanism (precipitation or viscosity increase) by which the crude oil flow properties are decreased.

From the processes described above, it is apparent that present methods for reducing the viscosity and increasing the flow characteristics of crude oil suffer from several drawbacks and disadvantages.

It is an object of the present invention to obviate, or at least mitigate, some of the disadvantages associated with the prior art.

Therefore, it is an object of the present invention to provide a method for reducing the viscosity and increasing the mobility of crude oil.

It is a further object of the present invention to provide a method and additive for reducing the viscosity and increasing the mobility of crude oil which is not specific to the mechanism of viscosity increase.

A still further object of the present invention is to provide a method and additive for reducing the viscosity of crude oil which facilitates the recovery of the crude oil.

According to a first aspect of the invention, there is provided a method for reducing the viscosity of crude oil comprising the step of adding at least one additive to a crude oil, wherein the at least one additive comprises at least one polyvinyl alcohol, and wherein the resultant mixture is a dispersion mixture. In the context of the invention, references to viscosity of crude oil should be interpreted to mean viscosity of the crude oil itself and/or viscosity of a mixture in which, crude oil is present.

The method may comprise the additional step of mixing the at least one additive with the crude oil by agitation.

Preferably, the additive comprises aqueous polyvinyl alcohol .

The additive may be added by injection.

Preferably, the method is operated over the temperature range of 10° to 80°. More preferably, the method is operated over the temperature range of 10° to 40°.

The additive may be added such that it is present in the dispersion mixture in a volume range of 10 %vol/vol to 40 %vol/vol as a percentage of the dispersion mixture volume.

Preferably, the additive is added such that it is present in the dispersion mixture in a volume range of 15 %vol/vol to 30 %vol/vol as a percentage of the dispersion mixture volume.

More preferably, the additive is added such that it is present in the dispersion mixture in a volume range of 20 %vol/vol to 25 %vol/vol as a percentage of the dispersion mixture volume. The additive may comprise a solution of aqueous polyvinyl alcohol resin with concentration in the range 0.1% to 2%. Preferably, the additive comprises a solution of aqueous polyvinyl alcohol resin with concentration of approximately 0.5%.

Preferably, the dispersion mixture comprises a continuous aqueous phase and a discontinuous non-aqueous phase.

The method may comprise the further step of adding a second additive selected to decrease the pour point of the dispersion mixture. Preferably, the second additive is a wax crystal modifier.

According to the second aspect of the invention, there is provided a method of transporting crude oil from a first location to a second location, the method comprising the steps of: reducing the viscosity of crude oil at the first location by the method according to d in any of Claims 1 to 14; separating the dispersion mixture at the second location.

Preferably, the dispersion mixture comprises a continuous aqueous phase and a discontinuous non-aqueous phase.

The method may comprise the further step of separating the aqueous and non-aqueous phases.

The method may comprise the step of adding a polymer breaker to the dispersion mixture. Preferably, the polymer breaker is a periodate salt. More preferably, the polymer breaker is sodium periodate.

The method may comprise the further step of reforming the dispersion mixtiαre from the separated aqueous and non- aqueous phases by agitating the mixture.

According to the third aspect of the invention, there is provided an additive for reducing the viscosity of crude oil, the additive comprising at least one polyvinyl alcohol and at least one wax crystal modifier.

In its fourth aspect, the invention relates to use of at least one polyvinyl alcohol as a viscosity reducing additive in a exude oil mixture.

According to the fifth aspect of the invention, there is provided a improved system for transporting crude oil from a first location to a second location, the system comprising a conduit having a crude oil mixture flowing therein, wherein the crude oil mixture is a dispersion mixture of crude oil and at least one additive and the at least one additive comprises a polyvinyl alcohol.

The method for reducing the viscosity of crude oil provides a way of increasing the mobility of crude oil in, for example, pipelines. In achieving this effect, the method produces a "water-wet" oil in water dispersion mixture. In a sense, the dispersion mixture which is formed can be visualised as waxy ball-bearings, surrounded by water. Inside a pipeline or conduit, the crude oil forms small globules and (as described above) behaves like waxy ball- bearings inside a water sheath, and the mixture moves freely in the pipeline.

The method described in the examples uses an aqueous solution of a polyvinyl alcohol (PVA) which is generally, but not necessarily, made at high concentration and diluted. As an alternative, the polyvinyl alcohol can be produced on-site immediately before addition to the crude oil stream.

The mechanism of adding the additive is by injection into an area of mixing which does not emulsify the aqueous polyvinyl alcohol solution with the crude oil, but produces a dispersion mixture. The dispersion mixture so-formed has a continuous aqueous (or water) phase and a discontinuous non-aqueous (or oil) phase (i.e. an oil in water dispersion) . This is not an emulsion, nor does it contain an emulsified phase, rather, it is a dispersion mixture.

In addition, the aqueous:non-aqueous ratios that are used in the present invention would ordinarily produce a continuous non-aqueous (or oil) phase and a discontinuous aqueous (or water) phase (i.e. a water in oil dispersion) . Somewhat surprisingly, this is the opposite of the properties dispersion mixture of the present invention as described above. The dispersion mixture is therefore considered to be "water-wet", as opposed to "oil-wet". Consequently, the present invention allows the aqueous layer to be separated from the non-aqueous layer. For each particular crude oil there is an amount of additive which preferentially can be added and which will remain dispersed, and which will not immediately drop out of solution when mixing is stopped. This additive amount is generally between 5% and 25% by volume of the mixture so-made with the specific crude oil.

The concentration of aqueous polyvinyl alcohol and the shear, or mixing force, required to form the dispersion is dependent on the particular crude oil being treated and the temperature at which the dispersion is made.

All polyvinyl alcohol materials, irrespective of saponification value, are functional in respect of crude oil viscosity reduction. Therefore, any PVA can be used to reduce the viscosity of crude oil. However, for each particular crude oil there is a PVA with a molecular weight and saponification value which is most effective. Similarly, the concentration at which the additive is effective in an aqueous solution is also crude oil specific. Furthermore, the mixing shear required to create a mixed phase system is particular for the crude oil being treated.

In certain cases, further improvements maybe made through the addition of wax crystal modifiers to the crude oil before mixing with the additive. Wax crystal modifiers interfere with crystal formation in fluids. In effect, wax crystal modifiers prevent agglomeration within oil globules and thus prevent the formation of precipitates. Therefore, depending- on the type of crude oil and desired effect, wax crystal modifiers can act as cold flow improvers, pour point depressants, viscosity reducers, paraffin deposition inhibitors, and the like. A variety of wax crystal modifiers are known in the art, and are generally identified by their function, such as cold flow improver, pour point depressant, viscosity reducer or paraffin deposition inhibitor.

For example, the dialkylalkenylsuccinates of U.S.Patent No. 2,561,232 (Rudel et. al.), and assigned to Standard Oil Development Company, have been known as effective pour point depressants for petroleum derived liquids since at least as early as 1951. Also, U.S. Patents Nos. 3,574,575 and 3,634,052, both of which were assigned to Mobil Corp., and the text, Chemical Additives for Fuels: Developments Since 1978, edited by M.T. Gillies (Noyes Data Corporation 1982) , pages 115-152, all of which are incorporated herein by reference, disclose other types of wax crystal modifier.

Recently, polymer compositions comprising dispersions of at least one olefinically unsaturated compound and containing aliphatic side chains of at least ten carbon atoms (e.g., a polyacrylate or a polyethylene vinyl acetate related product) , in a continuous liquid phase comprising at least two surfactants and a liquid polyol was disclosed anonymously in Research Disclosure (July 1995), page 501 (entry 37550) , as being effective pour point depressants in crude oil (petroleum) and certain fuel oils. These too are suitable for use in combination with the method of the present invention as wax crystal modifiers. The method of the present invention overcomes many of the drawbacks associated with the prior art and, in fact, offers several advantages. For example, a transportation system for transporting crude oil that has been treated by the method of the present invention does not require to be heated or thermally insulated keep the viscosity suitably low for the crude oil to be effectively and economically transported.

The polyvinyl alcohol additive can be rendered ineffective by the acidition of a polymer breaker. For example, a polyvinyl alcohol additive dispersed in crude oil is rendered ineffective by being broken by a periodate salt either: in solid form or in aqueous solution. After the addition of the polymer breaker, the disrupted polyvinyl alcohol and the water (as the aqueous phase) can be more easily separated from the crude oil (as the non-aqueous phase) . The crude oil from this separation has a water content which is sufficiently low that the crude oil can be refined to generate valuable petrochemicals.

The method as described can confer sufficient stability onto the so-formed dispersion mixture to allow transportation of the crude oil. Also, the dispersion mixture is stable for" short periods during which there is cessation of movement within the system.

In the event that the additive does separate from the dispersion mixture due to excess addition of chemical or prolonged cessation of movement of the crude oil mixture, the dispersion mixture can be reformed by mixing the aqueous and non-aqueous phases, even under low shear conditions, therefore again reducing the viscosity of the crude oil mixture.

The method described involves the application of an additive which, when added to crude oil, increases the mobility, and decreases the viscosity, of the resulting mixture over a wide range of temperatures . The additive is added to the crude oil in a specific way such that a dispersion mixture, and not an emulsion, is formed. Also, the dispersion mixture formed has a continuous aqueous (or water) phase and a discontinuous non-aqueous (or oil) phase. This method of treating crude oil can be used regardless of the mechanism (precipitation or increased viscosity) by which the mobility of the crude oil is reduced.

Example embodiments of the present invention will now be described.

Example 1

A crude oil sample of southern hemisphere origin has a viscosity of less than lOOOcP at temperatures above 600C, which is close to the production temperature for the oilfield from which the crude oil sample has been taken. At this temperature the crude oil is capable of being moved by pumping systems, and is pumped along a pipeline to a suitable production separation facility.

After entering the production separation system, the crude oil cools and. enters the storage system at a temperature of approximately 400C. The viscosity of the crude oil is now between 250OcP and 550OcP. At this temperature and viscosity, the crude oil cannot be pumped aiong a pipeline to the shipping point without uneconomically large pumping systems, high pressure pipelines or the provision of a fully thermally insulated, or heated, pipeline.

By the addition of various amounts of a polyvinyl alcohol solution in water" (together with a system capable of dispersing, but not emulsifying, the polymer in the crude oil) the viscosity of the oil is reduced to values of 50OcP or less, at temperatures of approximately 300C.

For the particular crude oil in this example, the amount of additive required is between 20% and 25% of an aqueous solution containing 0.5% of a polyvinyl alcohol which had a viscosity of 5OcP at 4% wt/wt in water and saponification degree of 86 mol% to 89 mol%.

When concentrations of less than 20% of the above 0.5% PVA solution are added, the viscosity reduction is lower, and the viscosity increases with reducing concentrations of additive over the temperature range referred to above. For concentrations higher than 25% the mixture rapidly sheds additive immediately after mixing, and the residual amount of additive dispersed in the crude oil phase stabilises at between 20% and 25%.

At the optimum concentration addition the resulting dispersion mixture of crude oil and additive is stable for up to 24 hours, and any additive that separates after this time is very easily re-dispersed by simple agitation. Table 1 below details the viscosity and the amount of free water present in a dispersion mixture of the polyvinyl alcohol additive and the crude oil sample of the present example at a temperature of 300C. From the table it is apparent that the viscosity drops rapidly and the percent water content rises markedly with increasing quantities of additive.

Table 1

When higher than optimum amounts of additive are added and dispersed in the crude oil, and the resulting mixture is left to stand with no agitation in cylindrical glass vessels at fixed temperature, the water phase sediments within the crude oil column such that the amount of water held in the top of the oil column is lower than that held at the bottom of the column, in addition to that separating out of the oil phase.

In this example the amount of water present, as determined by coulometric Karl Fischer titration was found to be less than 2% wt/wt at the top of the oil column. The amount of water present at the bottom of the oil column but above the interface of the oil and any separated water- was over 20% wt/wt. In this test, the total additive applied was 30% and the volume of separated aqueous phase found at the bottom of the column after 24 hours was 8%.

Example 2

Native crude oil with the viscosities as illustrated in Table 2 below (two samples from same field, different wells) contain less than 0.3% wt/wt water as determined by Coulometric Karl Fischer method.

Table 2

After dosing crude oil Sample 1 with additive at 23% (0.5% polyvinyl alcohol as previously specified above) and dispersing, the viscosities of the fluid at various temperatures are as illustrated in Table 3.

Table 3 In a further experiment, the crude oil Sample 1 containing 25% additive dispersion is placed in a beaker with a 20mrn stirring bar, in an incubator at fixed temperature of 300C, and is stirred at 120 rpm using a magnetic stirrer. This simulates the effect of movement of the crude oil in a low flow pipeline situation in which turbulent flow is not present.

The viscosity of the mixture is measured at various times as illustrated in Table 4 below. All apparatus is enclosed within the incubator to ensure that the temperature remains constant throughout the test period.

Table 4

The results show that when the mixture is under constant movement trie viscosity deviates little from the initial viscosity measured immediately after creation of the dispersed additive/oil mixture.

In circumstances where the crude oil and additive dispersive mixture is not constantly under movement, the viscosity of the mixture increases as the aqueous phase sediments to the lower part of the crude oil phase.

In another experiment, the crude oil Sample 1 containing 25% additive dispersion is placed in a beaker in an incubator; at fixed temperature. This is done to simulate the effect of no movement of the crude oil in a shut down pipeline situation.

The viscosity of the mixture is measured at various times. All apparatus is enclosed within the incubator to ensure temperature remains constant throughout the test period.

The water; content of the top of the oil phase is determined by Karl Fischer analysis of samples withdrawn immediately before measurements for viscosity are made. The results from these analyses are illustrated in Table 5.

Table 5 Note that the original crude oil sample, without any additive, had a water content of 0.2%, a viscosity of 12165cP, and OmL of free water at 30.0°C. The viscosity of the mi>cture, however, never returns to values as high as those found in the original crude oil.

Example 3

In another" experiment crude oil Sample 1 is mixed with 25% additive and agitated at 120 rpm for 24 hrs at 3O0C. The viscosity is measured as illustrated in Table 6 below, before the addition of 0. Ig solid sodium periodate and mixing/.

Table 6

After 16 h.rs the top 75% of the oil column was removed and a water content by coulometric Karl Fischer determination found 3% wt/wt water present in the sample. The polymer additive can be removed, or broken, by a polymer- breaker. Also, the water content of the crude oil containing additive is reduced by the addition of the polymer1 breaker and with appropriate mixing.

As is widely known, the addition of a periodate salt will break 1,2, glycol head to head linkages in a polyvinyl alcohol polymeric material, where these linkages occur. This leads to the formation of lower molecular weight polymexr chains which are no longer effective as a crude oil mobility enhancing additive.

When a crude oil and additive system is dispersed and the resulting mixture is treated by the addition of a sodium periodate salt at a concentration of 0.1% wt/vol, the periodate salt will reduce the polyvinyl alcohol polymer units to a lower molecular weight. The cleaved additive is then no longer able to remain as a dispersed phase and water separates quickly from the crude oil mixture.

Within a few hours, the aqueous phase sediments to the bottom of the oil column where it separates from the crude oil and can be clearly seen as a distinct, separate aqueous phase.

Example 4

In this example, tests are performed on a sample of crude oil of European origin with a pour point (the point at which the crude oil solidifies) of 230C. This material does not respond to treatment with additive in that the crude becomes solid in both the treated and the untreated samples . Pour point measurements are made with various concentrations of PA082003 wax crystal modifier chemical from Baker Petrolite Limited and also in the presence and absence of 25% volume water containing various concentrations of 1 percent polyvinyl alcohol (PVA) Gohsenol GH 23 (10 % solution of aqueous polyvinyl alcohol resin) from Nippon Gohsei . The pour point is determined by allowing the oil to cool and tilting the container periodically to see if the oil flows. The results from the experiments are shown in Table 7 below. It is apparent from these results that the addition of polyvinyl alcohols markedly decreases the pour point of the crude oil when at least some wax crystal modifier is present.

Crude Oil Pour point Readings Concentration PA082003 0% PVA (0C) 1.0% PVA (°C) Wax Crystal modifier (mg/kg) 0 23 23 100 20.5 15 500 20 13 1000 19 11 2000 18 5

Table 7 Example 5

In a further experiment, tests are performed on a second crude oil with a range of different crystal modifiers and polyvinyl alcohol Gohsenol GH23 (10 % solution of aqueous polyvinyl alcohol resin) . The chemicals being tested are A5603C and A5445 from Weatherford and PAO 83110 and the previously tested PAO82003 from Baker Petrolite. The crude oil is heated to the temperature of interest and the additives are dosed at 20% of a 1% solution of polyvinyl alcohol and 2000 ppm of wax crystal modifier. The viscosity of the resultant mixture in centipoise cP is determined using a Brookfield viscometer. The results of this experiment are illustrated in Tables 8a to 8e below. From these results it is shown that the polyvinyl alcohol additive decreases the viscosity of crude oil over a range of different temperatures. It is also shown that wax crystal modifiers can help to enhance the effect of the polyvinyl alcohol additives.

Table 8a

Table δb

Table 8c Table 8d

The present invention provides a method for reducing the viscosity and increasing the mobility of different crude oils over a variety of temperatures. Furthermore, the method of the present invention can be used regardless of the mechanism by which the crude oil has become less mobile (precipitation or viscosity increase) .

The present invention also provides a method for transporting crude oil, and a method for transporting crude oil in a pipeline.

The method of the present invention overcomes many of the drawbacks associated with the prior art and, in fact, offers several advantages. In particular, the present invention does not require the use of large amounts of specific (and expensive) chemical additives for specific type of crude oil. Also, the present method operates over a wide range of useful temperatures.

Another advantage of the present invention is that the dispersion mixture which is formed is sufficiently stable to allow transportation of the crude oil, and to allow for short periods during which there is cessation of movement within the system. Therefore, the method of transportation of the present invention does not require heating or thermal insulation for the crude oil to be effectively and economically transported.

In addition, if the additive does separate from the dispersion mixture due to excess addition of chemical or prolonged cessation of movement of the crude oil mixture, the dispersion mixture can be reformed by mixing, even under low shear conditions.

A further advantage of the present invention is that the exude oil that is recovered from the method has a water content which is sufficiently low that the crude oil can be refined to generate valuable petrochemicals.

Various modifications may be made to the invention herein described without departing from the scope thereof.