NO333249B1 - A method for preventing the build-up of deposits on the inner wall of a structure which feeds a hydrocarbon fluid stream, a system for treating and transporting a hydrocarbon fluid stream, and using said method and system for reducing the deposition of precipitating materials. - Google Patents

Abstract

The present invention relates to a method of preventing the build-up of deposits on the inner wall of a structure which carries a first hydrocarbon fluid stream at a first temperature containing depositing material, the first temperature being above a Deposit Appearance Temperature of the precipitating material. wherein the first stream is mixed with a cooler second seed stream having a second temperature comprising seed particles to precipitate the precipitating material, and the resulting mixing stream is passed further downstream, where the first stream is cooled in a controlled manner to a third temperature closer to the second temperature of the second stream before mixing. The invention also relates to a system for the process.

Description

The present invention relates to a method and device for preventing or reducing deposits, such as from hydrate and wax particles on the inner walls of pipelines that transport a hydrocarbon fluid, where seed particles are used to precipitate such particles in the main stream.

Background for the invention

Wax and hydrate deposits on the inner wall of oil pipes are a serious problem in the infrastructure of today's oil production: when hot oil flows through a pipeline with cold walls, wax and hydrate will precipitate out and stick to the walls. This will in turn reduce the pipeline's cross-sectional area which, without proper countermeasures, will lead to pressure loss, and eventually a complete blockage of the pipeline.

Existing technologies that deal with the problem include removing the deposits by: Plug driving (pigging): mechanical scraping of the deposits from the pipe wall by

regular intervals.

Chemical inhibition: addition of chemicals that prevent deposits.

Direct Electrical Heating (DEH): electrical heating keeps the pipeline warm (above the deposit appearance temperature).

Plug driving is a complex and expensive operation. If no loop is available, a plug must be inserted underwater using remote-controlled tools. If more wax is deposited than the plug diameter is designed for, the plug can become stuck in the pipeline, resulting in costly operations and stoppages in production to remove the plug.

Chemical inhibition is also expensive and there are currently no chemicals available that completely reduce wax deposition. The results of such inhibition are uncertain and the intervals and amounts of chemicals used are therefore often unnecessarily high. Furthermore, the chemicals used are classified as highly problematic in terms of the environment and the dosage of such chemicals should be kept to a minimum.

Electrical heating above the temperature for deposit appearance is very expensive due to both high installation and operating costs. Consequently, electric heating is not feasible for long-distance transport.

Other known methods are described in the prior art where:

US 6,070,417 B1 describes a method for making a slurry in which solids are deposited and removed mechanically from the surface on which they are deposited, by a runner or plug that circulates in a lumen or loop.

US 6,656,366 B1 describes a method for reducing the build-up of solids in hydrocarbon streams produced from wells. The described method is based on deposition by cooling and mechanical removal of the deposit, by using a runner as above or a spiral coil that mechanically removes deposits.

EP 334 578 describes the injection of a cold dewaxing solvent into scraper coolers to remove deposits.

WO 2004/059178 describes a seeding technique based on recycling a cold drop stream of hydrocarbon fluids with hydrate/wax particles in the hot well stream. Dry hydrate/wax particles are formed by "crash cooling" as slurry particles form in the main flow of a reaction zone rather than depositing on the wall. The fluids are cooled to ambient temperature near the reaction zone. The slurry particles in the main stream are transported further with the gas and oil over long distances. However, the mixing of the hot well flow and the cold discharge flow will result in a mixture with a temperature above sea temperature, which mixture will be cooled down to ambient sea temperature and thereby generate wax deposits in the cooling zone.

CA 1289497 C describes a method for inhibiting the formation of immobile wax deposits on the inside of a pipeline that transports waxy petroleum oils. A small amount of an oil containing a large number of small wax particles or suspending crystals is added to the waxy oil. The wax in the waxy oil collects around the small wax particles or crystals to form larger wax particles that remain in suspension in the oil.

With current technology, long-distance multiphase transport of waxed fluids is significantly limited due to wax control. Plug driving is not possible over such large distances and electric heating is limited by costs. Transporting wax as solid particles in a cold flow is a well-known idea (cold flow or slurry flow) which is under investigation by many groups. Cold flow is considered to be one of the promising candidates to circumvent this problem. The problem with cold flow is how deposits should be handled in the cooling zone. The solution according to the present invention provides a way to avoid such build-ups in the cooling zone.

The intention of the present invention is to provide an improved method for removing deposits which is cost-effective both to install and operate, which is applicable for long-distance transport and which can be adapted to different situations.

Consequently, there is a need for a method and device to improve the present technique for preventing deposits on pipelines by nucleation.

Summary of the invention

It is an object of the present invention to at least partially overcome the above-mentioned problems, and to provide an improved nucleation method. This object and other objects which will emerge from the following description are achieved by a method and a system according to the appended independent claims. Advantageous embodiments are set forth in the appended dependent claims.

In a first aspect, the invention relates to a method for preventing the build-up of deposits on the inner wall of a structure which carries a first hydrocarbon fluid flow at a first temperature containing precipitating material, the first temperature being above a temperature for deposit appearance (Deposit Appearance Temperature) (DAT) of the precipitating material, wherein the first stream is mixed with a cooler second seed stream of a second temperature comprising seed particles to precipitate the precipitating material, and the resulting mixed stream is further transported downstream, where the first stream is cooled in a controlled manner to a third temperature closer to the second temperature of the second stream before mixing, in the range of 0-20° C above (DAT).

In one embodiment, the invention also relates to a method where the precipitating materials are dissolved or molten material, preferably waxes and/or hydrates, more preferably selected from one or more of asphaltenes, higher paraffins, hydrates and inorganic and organic salts.

In addition, controlled cooling can be controlled by a heating device, selected from heating elements, Direct Electrical Heating (DEH) or induced heat, or a heat exchanger around the first flow, where the heat is continuous or periodic, preferably in pulses.

In a further embodiment, the second seed stream is a recycled discharge stream from the mixing stream.

In one embodiment, the third temperature is above DAT or in the range 1-H^C, more preferably within 2-15<>C, optionally 2-1 (TC or optionally 2-5<0>C above DAT. Alternatively, the third temperature is equal to DAT In yet another alternative, the third temperature T^id is below DAT.

In one embodiment, the DAT is the Wax Appearance Temperature (WAT) or the Hydrate Dissociation Temperature (DAT). In another embodiment, the third temperature is in the range of 30-40°C, preferably 30-35°C, and more preferably about 30<*>0.

In another embodiment, the third temperature is T^kj:

Tkaid<>>WAT; and

Tkaic (2 to 15)°C + WAT when WAT > HDT; or

Tkaid (2 to 15)°C + HDT when WAT < HDT.

or

Tkaid<<>WAT; and

Tkaid>(2 to 15)°C + HDT

In one embodiment, the heating device periodically heats the pipe walls to remove deposited wax, preferably by heat pulses.

In another aspect, the invention also provides a system for treating and transporting a hydrocarbon fluid stream containing precipitating solids at a temperature above a deposition appearance temperature (DAT) of the precipitating solids, the system comprising a first line for the stream and a second line for a seed stream at a temperature below the DAT containing seed particles, the first and second conduits are connected at a mixing point or at a mixer, where precipitable solids are precipitated, to a third conduit to transport the mixed stream downstream in a conduit, the first conduit comprising a controlled cooling section before the mixer or mixing point comprising heating means to bring the hydrocarbon fluid stream to a temperature closer to the second temperature of the second stream before the mixing point or mixer in the range of 0-20°C above (DAT).

In one embodiment, the heating device is selected from heating elements, Direct Electrical Heating (DEH) or induced heating, or a heat exchanger around the first stream.

In another embodiment, the second line is a recirculation line from the downstream line, preferably comprising a pump.

In one embodiment, the heating device can provide periodic heating of the pipe walls to remove deposited wax, preferably by heat pulses.

A third aspect of the invention relates to the use of a method or system for reducing the deposition of precipitating material in a pipeline for the transport, development or treatment of hydrocarbons.

Furthermore, the invention relates to the use of the methods and systems for cleaning the inner walls of the equipment described.

The present invention will now be described in further detail by means of exemplary embodiments and with reference to the attached drawings, none of which should be interpreted as limiting the scope of the invention.

Drawings

Figure 1 shows an embodiment of an addition point in a pipeline according to known technology. Figure 2 shows an embodiment of a recycling loop on a pipeline according to known technology. Figure 3 is an example of an x-y diagram, which shows the ratio of cold:hot fluid to mixture.

Figure 4 shows an embodiment of the present invention in a pipeline.

Figure 5 shows an embodiment of the present invention in a Saturn Flow application.

Figure 6 shows an embodiment in a pipeline according to Figure 5.

Definitions

The fluid flow to which the present invention can be applied can be a single-phase or multi-phase flow comprising hydrocarbons and optionally H2O and/or gases such as CO2, H2S etc. and/or salts and/or additives such as various inhibitors. Advantageously, the present invention can be applied to any equipment that transports hydrocarbons.

The equipment can be any type of process equipment used to transport hydrocarbons such as the well itself, the wellhead, a reactor, and any pipeline and topside equipment used in the development or processing of hydrocarbons.

The "precipitating material" referred to within this document, and also referred to as "wax" or "hydrate", refers to solids that precipitate from fluids due to thermo-dynamic changes. These solids include typical dissolved solids in crude oil at wellbore conditions such as asphaltenes, higher paraffins, hydrates, and inorganic and organic salts. The composition of the wax will depend on the origin of the fluid flow.

The "deposit appearance temperature" (DAT) is the highest temperature at which deposition is observed, for wax this is the "wax appearance temperature" (WAT) and for hydrate it is the "hydrate dissociation temperature" ( hydrate dissociation temperature)

(HDT). The exact temperature will depend on the fluid composition and pressure. However, a person skilled in the art can easily obtain this value, for example, through simple experimentation.

The "bulk flow temperature" is the temperature of the fluid flow.

Description of the invention

The present invention will be described in more detail with reference to the attached figures.

Figure 1 shows, according to known technology, a pipeline 1 in an environment 10, such as seawater at temperature TSj0, which conducts a hot hydrocarbon fluid drilling stream 100 at temperature Twell, such as from a wellhead. A conduit 2 supplies a cooler addition stream 200 at temperature Tdm to the main stream 100, the additional stream containing seed particles 3 to a mixing point 20 on the pipeline 1. The addition stream 200 is mixed with the well stream 100 to a precipitating stream 101, with a mixing temperature Tbianding-1 the precipitating stream 101 , the seed particles 3 promote the precipitation or crystallization of particles 4 in the main stream, of precipitating material present in the well stream 100 which otherwise tends to be deposited on the inner walls of the pipeline. Further down the pipeline, the temperature of the precipitating stream 101 cools down to the ambient temperature and the precipitation stops, resulting in a transport stream 102. However, during the cooling of the precipitating stream, some deposition of the precipitating material in the stream will also occur on the internal the walls of the pipeline 1.

Figure 2 shows, according to known technology, a pipeline as in Figure 1, but where the addition flow 200 is actually a recycled discharge flow fira a downstream outlet in the pipeline, according to "Saturn technology", which concept has been described previously.

In short, the Saturn technology is based on the idea that dry hydrate and wax particles can be transportable and non-agglomerating under flow and shut-off conditions. By recycling a cold drop stream 201 of hydrocarbon fluids, with a temperature Tkim approximately equal to TSj0, with hydrate/wax particles 3 into the hot well stream 100 with temperature Tbrann, dry hydrate/wax particles 4 can be formed in the precipitating main stream 101, not only on the pipe walls. All hydrate and wax are intended to form as slurry particles 4 in the main part in a reaction zone, and the fluids are cooled to ambient temperature TSj0 in the vicinity of the reaction zone. In this way, no deposits on the pipe walls and blockage will occur when the slurry particles are transported further with gas and oil over long distances, i.e. after the splitter in Figure 2.

However, the recycled cold flow 201 at Tkim« TSj0 must be mixed with the hot well flow 100 at Twell as close to the production distributor as possible to avoid wax deposition and hydrate formation during shut-downs. The temperature near the mixing point will be very high, and may be well above the hydrate equilibrium temperature, unless the velocity of the recycled 201 stream is much higher than the well stream 100. At lower cuts, i.e. lower hydrate concentrations in the recycled stream, for example <5% , the hydrate present in the cold recycle stream 201 is likely to melt rather than serve as seed particles for further hydrate growth, if the temperature Twanding near the mixing point is much higher than DAT, such as the hydrate dissociation temperature. Thereby, the Saturn power technology can only be operated either at high recycling rates or at high water cuts. The former is not intended, and the latter is often not possible until the field has been in production for a certain time.

Figure 3 shows the mixing temperature, such as Tbiandmg, when a cold and hot fluid is mixed at different mixing conditions. It can be seen that the temperature of the hot fluid has a large effect on the mixture temperature.

Depending on the mixing ratio between the recirculating cold fluids and the incoming hot fluids and the temperatures of the two fluids, the temperature at the mixing point can be very different. Figure 3 shows the approximate mixing temperature when a cold and hot fluid is mixed as a function of the mixing ratio. The heat capacity per unit volume of the hot and cold fluids is assumed to be identical in the estimate. For real applications, the heat capacity per unit volume of the incoming well flow will be somewhat lower than that of the recirculating cold fluid due to hydrate particles in the latter. Thereby, the mixture temperature profiles in a field application where the recirculating cold fluid contains hydrate slurry particles will be lower than shown in Figure 3.

In addition, during the cooling of the precipitating streams 101, some deposit 5 of the non-precipitating material in the stream will also occur on the inner walls of the pipeline 1 in the cooling zone as mentioned above.

It would therefore be preferred if the temperature of the well stream 100 was at a temperature close to DAT, such as the wax appearance temperature (WAT), before mixing the cold drop stream 201 with the hot well stream 100 with the precipitating stream 101.

However, to only provide cooling, for example by a long uninsulated pipe installed before the mixing point to avoid the above problem, would result in wax deposition in the uninsulated pipe section, which could cause serious operational problems. Furthermore, this part of the pipeline will be exposed to hydrate plugging and/or gel formation during production shutdowns.

Figure 4 shows an embodiment according to the invention that allows the main well stream 100 to be cooled in a controlled manner to a temperature Tkaid, which temperatures are close to but above the DAT, such as the wax appearance temperature (WAT), in a cooling pipe

or cooling section 12 of a pipe before a mixing point where the well stream mixes with the drop stream 201, or any other seed stream for that matter. Alternatively, a mixer can be used for such mixing. The temperature Tkaid can be understood as the wall temperature of the pipe at the end or outlet of the cooling section 12 and/or the temperature of the fluid that exits the cooling section 12.

The cooling pipe 12, in this embodiment positioned between the wellhead and the point of addition of the seed flow 201, can be any type of pipe or other equipment adapted to the task of a temperature-controlled cooling of the well flow 100.1 present embodiment an uninsulated pipe equipped with heat capacity in the form of heating elements 40, as in direct electrical heating (DEH) or induced heating, which is used to perform the cooling of the well stream. Alternatively, a heat exchanger can be used with a water jacket surrounding the cooling pipe 12. Hot fluid, such as hot water from other process units can be used to control the cooling of the pipe 12, with the hot fluid running against or co-current with the pipe flow.

By cooling the well stream to a temperature just above DAT, in the case where WAT is used as DAT, WAT is generally higher than the temperature at which hydrates are stable, this should be safe in terms of hydrate formation as well. In cases where the hydrate dissociation temperature (HDT) is higher than the WAT, the mixing should take place when the well stream has a temperature slightly higher than the HDT.

The inserted length of the cooling pipe 12 can be considerable (see Figure 5), and it must be protected using heat capacity to:

1) avoid blocking/gelling during production shutdowns; and

2) avoid wax deposition/blocking during production by:

a. keep the internal pipe temperature above DAT; or b. periodically heat the pipe walls to remove deposited wax.

1) Prevention of hydrate blocking and wax gel formation during shutdowns

During production shutdowns, gel formation and blocking by waxes and hydrates can occur as the process conditions will change and the fluid mixing due to, for example, turbulence will cease. Phase separation and settling can also cause such gelation and blocking. Keeping the temperature of the inner walls of the cooling pipe 12 and the fluid within above DAT, such as HDT and/or WAT, by continuous heating or heating to such temperatures during start-up may be necessary, when using the heating devices 4.

2) Prevention of wax blocking/deposition and multiplication of wax deposition during production a. In order to keep the inner pipe wall temperature above DAT (HDT and/or WAT), the cooling pipe 12 can be insulated sufficiently to keep the inner pipe wall temperature above or close to DAT for the oil . This may require heavier insulation near the mixing point than further upstream.

b. If the inner pipe wall temperature of cooling pipe 12 is allowed to cool down to DAT (HDT and/or WAT) or below, deposits such as wax or hydrates will be deposited on the inside walls of cooling pipe 12. The cooling pipe does not need to be insulated in such operating mode, unless a longer cooling distance is desirable, for example to extend the deposition area to keep the deposition thickness low, or to increase the deposition capacity of the cooling pipe before countermeasures are taken.

Figure 6 shows an embodiment in which the well stream 100 at Twell is cooled down to a temperature Tkaidunder DAT which causes deposit 6 to form on the inside walls of the cooling pipe 12, which results in a partial awaxing stream 103 with a temperature Tkaidunder DAT. The partially dewaxed stream 103 is mixed with the release stream 201 in a mixing point and essentially all remaining wax in the stream is precipitated as agglomerates 4 by the seed particles 3, which leave the reaction zone clean.

Preferably, the temperature of the well stream is only cooled to a temperature Tkai below WAT, but not below HDT when WAT > HDT, to precipitate only the wax components on the inside walls of the cooling section.

By obtaining a cooled awoken stream 103 with a temperature Tkaidmye closer to that of the recycled stream 201 of Twjm than in the previous examples, such as for example below 30°C, the degree of recycling can be reduced, with reference to Figure 3. Alternatively, or in in addition, the water cuts of the recycled stream can be lowered.

The countermeasures would be to: i) periodically heat the pipe walls to remove deposited wax, or ii) perform a limited heat pulse to only loosen/remove the deposit without melting it.

i) Melting the wax will redissolve it in the stream and further downstream

deposit it again on the wall which is undesirable.

ii) However, it is possible to loosen already deposited wax from a pipe wall by

quickly increase the wall temperature in a heat pulse that does not dissolve the wax,

but it only loosens, which thereby enables the transport of the wax as particles. The wax is torn off the wall by the fluid flow as solid particles and can be transported downstream and has no or very little tendency to be deposited on walls or other surfaces.

Any of the wax removal methods above will preferably be initiated based on wax thickness measurements, such as a predetermined thickness to activate the measurement to be made, or based on production events such as shutdowns.

Claims (17)

1. A method for preventing the build-up of deposits on the inner wall of a structure carrying a first hydrocarbon fluid stream 100 at a first temperature Twell containing precipitating material, the first temperature Twells above a deposit appearance temperature (DAT) of the precipitating the material, where the first stream 100 is mixed with a cooler second seed stream 201 of a second temperature Tchime-comprising seed particles 3 to precipitate the precipitating material, and the resulting mixed stream 104 is transported further downstream, characterized in that the first stream is cooled in a controlled manner to a third temperature Tkaidcloser to the second temperature Tidmav the second flow before mixing, in the range of 0-2 O^C above DAT. 2. Method according to claim 1, where the precipitating materials are dissolved or molten material, preferably waxes and/or hydrates, more preferably selected from one or more of asphaltenes, higher paraffins, hydrates and inorganic and organic salts. 3. Method according to claim 1 or 2, where the controlled cooling is controlled by a heating device 40, selected from four heating elements, direct electrical heating (Direct Electrical Heating) (DEH) or induced heat, or a heat exchanger around the first stream, where the heat is continuous or intermittent , preferably in pulses. 4. Method according to any one of claims 1-3, where the second seed stream 201 is a recycled drop stream from the mixture stream 104. 5. Method according to any one of claims 1-4, wherein the third temperature T^id is above DAT or in the range 1-H^C, more preferably within 2-15<>C, optionally 2-10^C or optionally 2-5°C above DAT. 6. Method according to any one of claims 1-5, wherein the third temperature T^id is equal to DAT. 7. Method according to any one of claims 1-5, wherein the third temperature Tkaider below DAT. 8. A method according to any one of claims 1-5, wherein DAT is the wax appearance temperature (WAT) or the hydrate dissociation temperature (HDT). 9. Method according to any one of claims 1-8, where T is in the range 30-40 "C, preferably SO-SS^C, and more preferably about SO^C. 10. A method according to any one of claims 1-9, wherein the third temperature is as follows: Tkaid<>>WAT; and Tkaid (2 to 15)°C + WAT when WAT > HDT; or Tkaid (2 to 15)°C + HDT when WAT < HDT. 11. A method according to any one of claims 1-10, wherein the third temperature is as follows: Tkaid<<>WAT; and<T>kaid<>>(2 to 15)°C + HDT 12. Method according to any one of claims 3-11, where the heating device 40 periodically heats the pipe walls to remove deposited wax, preferably by heat pulses. 13. System for treating and transporting a hydrocarbon fluid stream 100 containing precipitating solids at a temperature T well above a temperature for deposition appearance (DAT) of the precipitating solids, the system comprises a first line 1 for the stream 100 and a second line 22 for a seed stream 201 at a temperature Tkimunder DAT containing seed particles 3, the first and second lines are connected at a mixing point or at a mixer, where precipitating solids precipitate out, to a third line 11 to transport the mixed stream 104 downstream in a line 11, characterized in that the first line comprises a controlled cooling section 12 before the mixer or mixing point comprising heating devices 40 to bring the hydrocarbon fluid stream 100 to a temperature Tkaid which is closer to the second temperature T of the second stream before the mixing point or mixer, in the range of 0-2 O^C above DAT. 14. System according to claim 13, wherein the heating device 40 is selected from heating elements, direct electrical heating (Direct Electrical Heating) (DEH) or induced heating, or a heat exchanger around the first stream. 15. System according to claim 13 or 14, where the second line is a recirculation line from the downstream line 11, preferably comprising a pump 23. 16. System according to any one of claims 13-15, where the heating device 40 can provide periodic heating of the pipe walls to remove deposited wax, preferably by heat pulses. 17. The use of a method according to any one of claims 1-12, or a system according to any one of claims 13-16 for reducing the deposition of precipitating materials in a pipeline for transporting, developing or treating hydrocarbons.