GB2530305A

GB2530305A - Tracers

Info

Publication number: GB2530305A
Application number: GB1416526.0A
Authority: GB
Inventors: Martin Foss; Tor Bjã Rnstad; Terje Sira
Original assignee: Institutt for Energiteknikk IFE
Current assignee: Institutt for Energiteknikk IFE
Priority date: 2014-09-18
Filing date: 2014-09-18
Publication date: 2016-03-23
Anticipated expiration: 2034-09-18
Also published as: GB201416526D0; GB2530305B

Abstract

Method for monitoring precipitation of at least one wax component from a hydrocarbon-containing fluid stream (4) during the flow of said fluid stream through a fluid transport system (2) having at least one in-flow point (3) and at least one out-flow point (7). The method comprises: introducing at least one labelled wax into said hydrocarbon-containing fluid stream at at least one in-flow point; and measuring the relative or absolute concentration of said labelled wax in at least one sample taken at at least one out-flow point. The method may comprise sampling and analysing wax components from the hydrocarbon-containing fluid, identifying suitable wax components and generating labelled waxed based upon such components. Specific molecular weight or isotopes may be used to label the wax. Monitoring may be carried out periodically.

Description

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Tracers

Field of the Invention

The present invention relates to the monitoring of wax deposition and precipitation in pipelines and other equipment used in the handling and transport of fluids produced from a subterranean reservoir. In particular, the invention relates to the monitoring of wax precipitation in process equipment where such wax is deposited or potentially deposited from a hydrocarbon-containing fluid produced from a subterranean reservoir.

Background to the Invention

Development and transportation of oil and gas fields in increasingly difficult areas pose significant challenges. This includes a significant portion of the subsea fields under development today. One of the most challenging issues is wax deposition inside the transportation and production tubing. This issue is regarded as one of the main flow assurance subjects in subsea and cold climate production where the produced fluids are cooled to below the wax appearance temperature for the fluids [1]. The wax appearance temperature is a property of the oil and can be as high as 65°C. in recent years there has been a significant research effort on the understanding of the deposition and the mitigation of waxing [1-4]. The composition and properties of the wax itself has been quite well understood but a remaining challenge is the monitoring of the wax initiation and growth in the production tubing and equipment. The use of pigs and pressure loss is typically used to monitor pipelines [2, 5]. The problem with pigging inspections is the significant amount of work required to run a pigging operation which might interrupt or possible stop the production temporarily. Pressure loss measurements are typically not sensitive enough to pick up deposition until the deposition has formed a significantly thick layer on the wails of the pipeline.

It would evdently be of considerable value to provide a method by whch waxing initiation and/or the extent of waxing can he determined andior monitored before waxing has reached the severity required to cause a measurable drop in pressure or flow. In particular, if waxing initiation can be detected before a significant layer has built up on the transport system, such as the pipeline or valves, chokes etc thereof then mitigation measures may be used prior to waxing becoming a significant problem. Equafly, where waxing might be an issue then mitigation measures can be avoided unless or until initiation of waxing takes place. in such situations the cost and complexity of mitigation measures can be avoided or postponed.

The present inventors have now estabhshed that by use of selected labelled waxes or wax-like molecules, a method can be provided to significantly improve the sensitivity and ease by which the onset and growth of wax can be monitored. The authors thus herein disclose a novel method for wax tracing.

In addition to the abovementioned novel wax tracing method, the same monitoring technique can be used for monitoring of precipitation of asphaltenes.

Summary of the Invention

In a first aspect the invention provides a method for monitoring precipitation of at least one wax component from a hydrocarbon-containing fluid stream during the flow o said fluid stream through a fluid transport system having at least one in-flow point and at least one out-flow point, said method comprising: i) introducing at least one labelled wax into said hydrocarbon-containing fluid stream at at least one in-flow point; and ii) measuring the relative or absolute concentration of said labelled wax in at least one samp;e taken at at least one out-flow point Key optional steps in such a method include additionally: a) taking a sample of said hydrocarbon-containing fluid stream at the in-flow point; b) analysing the sample of said hydrocarbon--containing fluid; c) identifying the structure and/or molecular weight of at east one wax component in said sample of said hydrocarbon-containing fluid; d) selecting or generating at least one abefled wax based upon at least one wax component identified in step C); e) utlising the at east one labelled wax selected or generated in step d) as the at least on labelled wax in step i) wherein steps a) to d), if included, are carried out prior to steps i) and ii).

As used herein the labelled wax is used as a tracer in measuring the precipitation of wax in the fluid transport system. The term tracer is therefore used herein to indicate a labelled wax.

Evidently, the in-flow and out-flow points referred to herein will be points on the fluid transport system where material can be introduced and extracted respectively, in both cases, these may be at an end of the fluid transport system, for example at the beginning (e.g. wellhead) or end of a pipeline respectively. Equally, such in-flow and out-flow points may be positioned at any appropriate points along the fluid Lransport system. At least one in-flow point will be up-stream of at least one out-flow point but many combinations of in-and out-flow points may be usefully employed. Where more than one in-flow point is utilised, the same tracer may be used at each or preferably a unique tracer will be introduced at each in--flow point.

In one key embodiment, he absolute or relative concentration measured at step ii) may be compared to one or more threshold values. Where wax is depositing within the transport system then the labelled wax component concentration will fall during transport and thus by comparing to one or more threshold values a decision can be made regarding the degree of wax precipitation occurring. Thus the method of the invention may additionally comprise undertaking at least one wax mitigation measure when th.e relative or absolute concentration measured in step ii) falls below a threshold value. Evidently more than one threshold may be used and in such a case each may indicate a need for one or more wax mitigation measures. 4 -

By monitoring waxing usng any of the methods of the present invention, an ongoing measure of the degree of waxing may he estabhshed. This may be Formed, for example, by sampling periodicafly or continuously and rnuftiplying the concentration of labefled wax by the time between samples or measurements to give an approximate integral or area-under-curve for wax precipitation against time. The method of the invention may thus comprise additionafly undertaking at least one wax mitigation measure when the area under a curve (integral) of relative or absolute concentration measured in step ii) against time for a predetermined period thUs below a threshold value. Again it will be evident that more than one threshold may be used and more than one mitigation measure may be deployed depending upon which threshold is crossed.

Suitable wax mitigation measures appropriate for any aspect of the present invention comprise at least one of: starting or increasing the addition of at least one dc-waxing component to said hydrocarbon-containing fluid stream; mechanically dc-waxing (for example pigging) at east a part oF said fluid transport system; or thermally de-waxng at leasL a part of said fluid-transport system -An important contribution of the present invention is the facility to generate labelled waxes that may co-precipitate with wax components in a hydrocarbon-containing fluid stream. In a further aspect the invention therefore provides a method For the generation of at least one labelled wax comprising: a) taking a sample of a hydrocarbon-containing fluid stream at an in-flow point of a fluid transport system; h) analysing the sample of said hydrocarbon--containing fluid; c) identifying the structure and/or molecular weight of at least one wax component in said sample of said hydrocarbon-contain;ng fluid; d) selecting a wax based upon at least one wax component identified in step c); -is-e) covalenfly or isotopicafly modifying the structure of the selected wax with at east one label In a stifl further aspect the invention additionaUy provides th.e use of at east one labelled wax to monitor precipitation of at east one wax component from a hydrocarbon-containing fluid stream during the flow of said fluid stream through a iluid transport system having at least one n-flow point and at least one out-flow point. Such a labelled wax may be a wax generated by any of the methods described herein. Such a use may be according to any of the methods described herein. Such generation may be by modification of a purified or commercial wax component or by synthesis of the labelled wax(s) from an appropriate starting material.

Detailed Description of the Invention

The invention relates to the monitoring of wax deposition and precipitation in pipelines and other equipment. The technique is simple and efficient and can be used for early detection of the onset 01 deposition. The technique can be utihzed on existing fields where an njection hne to the wellhead is installed (MEG injection lines, inhibitor injection lines etc.).The basis for the invention is to inject a selected labelled wax (which may be a true labelled wax or a labelled wax-like molecule, both of which are referred to as a labelled wax herein where context allows) with properties specificafly chosen to enable the product to precipitate alongside the wax component precipitahng from the solution. By measuring the residual concentration of the product, information regarding onset and growth of wax can be monitored by periodic, frequent or continuous measurements. In one embodiment the wax component may be tailored to the specific produced m;xture and the waxes present therein By appropriate choice of one or more wax components, the method will he able to detect deposition of a wide range of waxes with different chemical compositions. This method where tracers are used is especially suited for fields where sarnpl;ng at the wellhead is difficuit, such as subsea fields. Oil samples from the well are typically only taken before production is started.

During field development at a hydrocarbon reservoir, such as an oil field, oil is sampled from the reservoir. By careful characterization of relevant samples a description of the oil components can be obtained. This knowledge can then be used to determine the quantity and type of components forming wax in the production systems.

Waxes are alkanes (saturated hydrocarbons) typically heavier than C18 (such as C20-C100 [7]) that are produced as part of the hydrocarbon phase in oil and gas-condensate production.

These components of the oil are subject to precipitation due to changes in solubility (temperature, pressure, compatibility issues) and will, if deposited on the interior of pipelines, separators, chokes or other types of process equipment, cause significant production upsets (5).

Being able to monitor the amount of wax travelling from the wellhead throughout the system is therefore a significant advantage since deposition and risk of damage or blockage of the production system can be detected. Knowing when wax deposition is initiated would greatly improve the cost efficiency of the mitigation methods deployed to handle the wax. Detection of wax onset allows countermeasures to be set in place before the wax deposition becomes unmanageable. Typical spots where wax might precipitate include areas with changes in turbulence (separators etc.), temperature (cold spots etc.), pressure (choke etc.), chemistry (injection points etc.) and surface properties (process equipment).

Ability to measure onset of wax deposition or changes in wax deposition rate in detail will be critical for several situations in pipelines or process equipment. Examples of situations where monitoring is useful are: When system changes are made with respect to changes in conditions such as pressure and temperature. This includes shut-down, start-up, choke changes, pigging etc. Pressure and temperature change may lead to precipitation also in the near-well area. The solubility of wax is a strong function of pressure.

* When the chemistry is changed in the system. This might be due to operations such as chemical injection to mitigate other production issues (emulsions, flow improvers, inhibitors etc.), new tie-ins, EOR operations.

* When the composition of the produced fluids change. This includes changes in the fluid composition due to late life pressure decrease in the reservoir and changes to the reservoir temperature.

Thus, for example, the method of the invention may be used continuously or periodically (as described herein) for at least I month, preferably 6 months or 1 year following an event which changes the conditions of the fluid or fluid transport system. Such events include those described herein, such as a change or partial change in source of the hydrocarbon-containing fluid stream; the addition, removal or replacement of at least one element of the fluid transport system; the addition, removal or change in concentration of at least one additive introduced into the hydrocarbon-containing fluid stream; or a change in conditions of temperature and/or pressure at at least one point on the fluid transport system.

References herein to a "fluid transport system" relate to any system of one or more components which may be used to transport a hydrocarbon-containing fluid (particularly a fluid comprising oil). Typical components comprised in such a fluid transport system include pipes (e.g. a pipeline), valves, chokes, filters, mixers, separators (test, sand or phase separators), joints and/or thermal expansion joints.

One alternative to the use of tracer would be to use a chemical analysis technique such as GC-MS/MS to monitor the actual wax produced from the well. In order to use GO monitoring regular sampling at the wellhead would be required. Sampling from the wellhead is, however, not feasible for most subsea fields, due to high cost, and the technique would therefore be difficult and expensive to implement. The operation of a GO-MS analyzing technique for single components En the oH is also not trivial and would probably not he feasible offshore. The use of this technique is further complicated by mixing of fluids from different wefis which would require even more sampling points at the weflheads.

In the present invention various "tags or labels' (the terms are used herein equivalently) are employed to aUow the wax component to be detected. Such labels may be detected by any suitable method but preferred examples include: * one or more existing atoms of l.he wax or wax-like molecule is exchanged with another stable or radioactive isotope of the same chemical element, * exchange of an atom (especially a hydrogen atom) with a radioactive isotope of another foreign chemical element without changing the main properties of the molecule or * covalently attach a chemical group with measurable properties, for instance, special fluorescent properties.

Examples of these types of labels include, without limitation, the following: * Tritium (T or 3H) to substitute one or more 1H-atoms in the molecule, 1t to substitute one or more stable 1'C-atoms in the molecule, if the wax molecule contains sulphur or 32P if the molecule contains phosphorous.

These are all beta emitters and are best analysed with liquid santillation counting (LSC) after sampling. On-site analysis is possible.

Different element radioactive substitution: * Halogene isotopes where the most preferable are 1251 and 1311. In some cases one may regard also 82Br, hut iL half-life of 35 hours may, in some cases, he too shod.

These are all beta emitters with associated and specific gamma radiation. Analysis can be performed with gamma spectroscopy after sampling, but the gamma radiation also offers the possibility of analyzing the tracers continuously, non-destructively and on-line directly in the gas stream through the non-transparent tubing wall. On-site analysis is possible.

Stable isotopes of hydrogen and/or carbon, and these are deuterium (D or 2H) and 13C.

Possible, but less preferred, is the labeling with 15N if the molecule originally contains nitrogen, if the molecule originally contains oxygen or MS if the molecule contains sulphur.

In the case of isotopic substitution with isotopes of elements other than C or H, a majority of the stable atoms in the selected wax molecule tracer will have to be exchanged with their corresponding mentioned isotopes. In the case of Hydrogen or carbon, it remains preferable to have multiple substitutions such as at least 10% preferably at least 25%, more preferably at least 50% of the relevant atoms. For instance, it may be necessary to substitute more than half of all hydrogen atoms with deuterium in order to achieve the needed analytical sensitivity. Such molecules are analysed, after collection and combustion, with isotope mass spectrometry in a laboratory environment.

Optically detectable labels: * A molecular group with fluorescent properties. This most often involves double bonds and Pi-orbitals (u-electrons). Fig.37. below shows an example of such a fluorescent group. The compounds can be analysed with high sensitivity by laser-induced fluorescence, both after sampling and possibly also in situ (in-stream). If molecular separation is required, it may be accomplished by the use of LC with reversed phase or size exclusion columns. Other fluorescent labels are well known, for example from: Table of fluorophores: http:llpingu.salk.edulflow/fluo.html Database of fluorophores: http:/Iwww.fluorophores.tugraz.atlsubstance/ Candidate wax molecules for covalent labelling: The waxy molecules to be covalently labelled should either be halogenated or contain functional group(s) like -OH, -COOH, -NH2. The waxy molecules may also be esters which can be hydrolysed to create -OH or -COCH-functional groups for subsequent labelling.

Typical examples from various wax molecules carrying appropriate functional groups are given in Fig. I below.

In one aspect the present invention relates to a method of generating at least one labelled wax.

This may be achieved by the methods disclosed herein utilising chemistry known in the art.

One particularly suitable method is by the modification of fatty alcohols.

Fatty alcohols are especially versatile for further labelling. Table I indicates briefly the conversion reactions which may be used to convert an alcohol into a set of other waxy components. The type of reagent is listed in the middle column and the resulting functional group on the right Labelling may take place through the reactants which may contain the tracer tag (e.g. an heavy or radioactive isotope) or may carried out by means of a further reaction (e.g. covalent bonding of a fluorophore to an add group): Thble 1: Genera/ reactions w/th fafly a/coho/s + Oxygen aldehydes and/or carboxyhc acids + Alkak m&t:n carboxyUc adds + Alkyne =z> Vinyl ether + Carboxyc add =-Ester + Hydrogen hahde = Alkyl hahde Fatty aohol + ----------------------------------------- + Ammona/Amuie = arnnes + Aldehyde/Ketone:r> acetals + Sulfide Thiols + Alkoholate/H2S = Xanthates + Metals/Metal halides _ Metal alkoxides It is important to ensure that the resulting labefled waxy molecule is representative for a defined wax molecular range with respect to deposition temperature. Also, thermal and/or microbial stability and possible interactions (or lack of such) with surrounding material must be examined in dedicated laboratory experiments. Thermal stability is checked in static batch experiments extended over time where sealed glass ampoule samples of the labelled molecules are subjected to various temperatures ranging from a few °O to 50-60 DC under gentle shaking.

Samples are extracted as a function of time Lor analysis of the remaining concentration of tracers. in this tern perature range, th,e biodegradation is the most probable degradation mechanism. If analysis is carried out with. GC, the molecules have to be sufficiently stable at temperatures > 300 °C in order to conduct the analysis. However, GO analysis is most practical for the precursor wax molecule while LC at moderate temperatures wiil be the analytical method of choice for the tracer molecules labelled with a non-radioactive fluorescent label.

As discussed above, three primary methods are considered as examples of suitable moiecular labels/tags: The primary examples are radioactive nuclides, stable (less common, especially heavy) isotopes and optically detectable (fluorescent) groups. Further description of the properties of these tags is provided below.

The radioactive labels The relevant nuclear properties of the most practical radionuclides for this kind of labelling are given in table 2 below. 1'

Table 2: Nudear data for the candidate radionuclide labels Radiation type Radio-Half-and energy nuclid (absolute Production method Comments life e intensity in brackets) Thermal n-reactions: Soft i -1: 3He(n,o3H Some hydrogen (H) atoms, as 3H 1 12 emission: .. a natural component in aD = 18 key 2. Li(na) H hydrocarbons, can be (100%) Commercially substituted with T (H) avadabie Reactor neutron Relatively soft B r r+nn' ---r A carbon ( C), as a natural 5730 y:emb01 Eumax N(n;p)14C component in aD hydrocarbons, key Commercially can be substituted with 14C available The relevance of the 31P-label depends on the molecular content of phosphorus, either Hiqher-enerqy fi -. .*-naturaflv ct rnn or 2 14.26 -emission B. Reactor rraaation: . - --- pm. 35 introduced. For nstance d -1fi0. key A(np) S phosphate (-O-PO3H.2) or (lOOio, phosphonate (-P03H2) groups.

Highly suitable for mdecules naturafly containing phosphorus The relevance of 35 label in a wax molecules depends on its possible content of S-atoms, Reactor irradiation: either naturally occurring or Relatively soft ii 1 35Cn of5S introduced. For instance in d -emission: ELm 34 attached sulphate (-O-SO3H) or = 167.3 key: S(n.y) sulphonic acid (-SO3H) groups.

(100%) Commercially Highly suited for molecules avaflable naturafly containing sulphur.

for oxygen (e.g thiol in place of alcohol) ft-emission: ERmax = 444 key The relevance of 82Br-label in a (98.6%) wax molecule depends on its y-radiation possible content of halogen °4Br 3534 main energi-es Reactor irradiation: atoms either naturally occurring h in key: 776.5 Br(n,y)82Br or (more likely) where a (83.5%), 554.3 substitution can be made (70.8%), 619.1 without affecting waxing (43.4%), 1044.0 propeilies.

(27.2%) The relevance of 24l_ label in a 59.41 Electron capture Examples of wax molecule depends on its 1-' d (EC = 100%) reactions: possible content of halogen v-radiation in 1: 123Sb.2nY25 atoms either naturally occurring key: 35.5;1261 or (more likely) where a Rj.i.0' sLro. Y 124 126 --converted 3: Xe(n,y)Xe rr> without affecting waxing conversion [3 decay to properties.

electrons Commercially available 13emission two main El. Tne reevance of 131p_ label n a energs. pm Reactor rracaton: wax molecule depends on its (sq 9%' E2 -1. Fission of U possible content of halogen 1311 8.02 d = 333.8 key 2: °Te(ny31Te c atoms either naturally occurring (7 3% decay La 1311 or (more likely) where a .ommercis y rrauian available without affecting waxing main energy n properties.

key: 364,5 _______ _______ (81.7%) ___________________ ___________________________ The stable isotooe label Table 3: Some information on stable isotopic labels

Stable

Comments nuclide The fraction of deuterium in natural hydrogen is 0.015 %. This means that! in average, out of one million hydrogen atoms 150 will be deuLerium.

2H (0) Exchanging hydrogen with deuterium to a degree 10% (e.g. 6 to 90%, preferably 10 to 80%, such as 10 to 30%) in wax molecules will be sufficient for a good wax tracer without essentially changing the molecular properties.

The fraction of 130 in natural carbon is 1.10%. A good wax tracer based on this carbon isotope should contain as many 3C-atoms as possible, preferably at least 20% (e.g.20 to 99%), more preferably at least 50% or at least 70%.

The fraction of 15N in natural nitrogen is 0.366 91 The relevance of 1N- 15N label in a wax molecule depends on its possible content of N-atoms, for instance in attached amine (-NH2) groups.

The fraction of 180 in natural oxygen is 0.200 %. The relevance of 18O label in a wax molecule depends on its possible content of 0-atoms, for instance in attached acidic (-000H) groups, on alcohol groups (-OH)! ketones or others.

The fraction of 34S in natural sulphur is 4.21 %. Th.e relevance of 34S-label in a wax molecule depends on its possible content of S-atoms, for instance in attached sulphate (-O-SO3H) or sulphonic acid (-SO3H) groups.

The fluorescent label -14 fluorescent abefling is accomplished using a chemically reactive derivative of a fluorophore.

Many types of fluorophores exist. It is not practical to present such a list here. Therefore, see reference list below for appropriate web addresses. A few examples of common reactive groups nd u de: sothiocyanate derivatives such as FITC and TRITC (derivatives of fluorescein and rhodamine), see Formulae I and II. They are reactive towards pnrnary arnines in a waxy molecule to form a thioureido linkage between the wax molecule and the fluorescent dye.

H H H2 Formula 1: Fluoresce/n isothiocyanate Formula Ii: Rhodamine isothiocyanate dat/vat/va FlTC derivative (TRiTc) Succinimidyl esters such as NHS-fluorescein (Formula Ill). They are reactive towards amino groups to form an amido bond. -15

HOH

OH

Formula Ill: NHS-Fluorescein Reaction of such reactive dyes with another (especiauy a wax) molecule results in a stable covalent bond formed between a fiuorophore and a labelled molecule.

FoUowing a fluorescent labelUng reaction, it is often necessary to remove any norireacted fluorophore from the labeiled target molecule. This may he accomplished by size exclusion chromatography, taking advantage of the size difference between fluorophore and labefled wax fli olecu le Reactive fluorescent dyes are commercially available. They can be obtained with different reactive groups for attachment to various functional groups within a wax molecule. They are also available in labelling kits that contain all th.e components to carry out a labelhng reaction.

In the present invention, the concentration of labelled wax is measured at at east one out-flow point. Such a concentration measurement may be made by any of the techniques described herein, such as GC-MS, radiation detection or fluorescence detection. The concentration of wax may be measured as an absolute' value, by measurement of the labelled wax. This may be compared with the amount injected and with standard curves or using a known dilution factor to provide information on how much labelled wax has precipitated in the transport system. -16

Alternatively, a "relative" concentration measurement may be made by including a second, non-precipitating component. Such a non-precipitating component may optionafly be added simultaneously with the abefled wax and at a known concentration relative to that labeHed wax.

By taking a "relative" measurement of labefled wax concentration in comparison with the concentration of non-precipitating component. the degree of precipitation of the labefled wax may be determined. This assessment can be made without exact knowledge of the level of dflution or the precise amount injected because the non-precipitating component wifl act as an internal standard against which the labefled wax is measured.

A key aspect of the present invention is the irection of the labelled wax "tracers' into the weh-stream. Injection points include weflhead, upstream chokes, tie-inns or other areas where the system sees pressure or temperature changes. Injection of the tracer can be done using existing infrastructure such as service lines, MEG injection lines or other injection lines available in the system.

By choosing the correct molecule based on pre-studies of the wax forming for each specific system the tracer molecule should precipitate with the wax. Identification of the wax component to be used can be done by chromatography such as GO or GC--MS, or similar methods [8]. This analysis will identify all relevant wax components. This includes linear, branched and cyclic aikanes that might be part of the precipitating wax. The wax to be characterized may he lound through precipitation experiments.

Relevant wax for characterization can be identified through wax appearance and wax deposition testing. Actual crude/condensate from the field where the tracer shall be used or oil with comparable properties should be used in the precipitation testing. The wax component needs to be chosen so that it will precipitate to an extent to where the difference between corn. ponents injected and lost is measurable. Several labeled wax molecules might be used in combination, each specifically chosen to co-precipitate with the relevant wax molecules of the natural sample, Care will be taken to make sure the wax tracers used will not precipitate by itself. Compatibility -17 testing of the oil, wax tracer and any other chemicals added to the system may therefore be necessary. The detection 01 the amount of wax deposited can be done either at the sample point or by careful samphng foflowed by exsitu monitoring.

It is foreseeable that an online measurement unt can be constructed in order to enable contnLious monitoring. This might he desired in extreme situations where the time from onset of wax deposition and blocking of the pipeline/equipment is short In addition to the generation and use of precipitating labelled wax tracers it is suggested that a reference molecule (labelled non-precipitating component) is injected alongside the wax tracer.

This might be a regular oil tracer or a custom molecule created during the wax tracer production. Any molecule that can be traced can be used as long as no precipitation of the molecule is expected. Reference molecules might also be used where precipitation rate and characteristics are known but some precipitation does take place.

A sketch showing the basic principle of the injection and analysis scheme is seen in Figure 4.

The sketch is provided as an example of a simplified system and many other designs and implementations will be evident to the skilled worker. The system is applicable to both floating production systems and systems with onshore processing of the produced fluids.

The following are three example tracer injection methods: Pulse injection: A well-known amount of labelled wax (tracer) is dissolved in a known small (a few litres, such as ito 10 litres) liquid (LI) volume with relatively high tracer concentration. The liquid Li is chemically compatible with the fluid transported in the pipeline.

This traceNlabelled volume is injected by high-pressure displacement pumps as a slug in a short period of time (e.g. 10 seconds to 30 minutes). The tracer slug is optionally followed by a few litres (e.g. ito 10 litres) of non-labelled liquid LI to rinse out any remaining traces of the tracer liquid from the injection tubing to make ready for another tracer pulse injection at a later time. The pumps needed here are ordinary hgh-pressure tracer injection or chemical injection pumps. The injection principle is illustrated in Figure 5a.

Continuous njection: Lahefled wax (tracer) molecules are dissolved to a known but dfluted concentration in a hquid which is compatible with the fluid in the pipeline. The labelled volume may be several hundred litres (e.g. 100 to 1000 iitres) For steady state flow in the pipeline, injection is carried out at constant volumetric rate with a small-volume high-pressure metering pump (for instance an HPLC pump). Thus, tracer is injected in a constant concentration In cases where there are time differences in the pipeline volumetric flow rate, the injection rate should be controfled by feedback from the log of the pipeline flow rate in order to maintain a constant concentration of the tracer at the injection point. A variation of this injection regime is the so-called square-pulse injection where a constant concentration is maintained for some minutes, hours or days. Such a squarepulse injection may be repeated at intervals. The injection principle is illustrated in Figure 5b.

iii. Contnuous dissolution and injection: This procedure requires that the tracer wax molecules exist in the solid (precipitated) form. The solid is encapsulated in such a way that part of its surface is exposed to a liquid L2 which upon contact with the solid may dissolve wax, and thereby wax tracer molecules, from the solid surface. The liquid L2 is also chemically compatible with the fluid in the pipeline. Dissolution is slow, but sufficient to create a well delectable tracer concentration in the fluid in the liquid [.2 which flows over the exposed solid surface. The same type of pump as described in section ii. above may be used here. The concentration of the tracer in the injection fluid will not be exactly constant in time. The concentration depends on the shape of the exposed surface. However, for fluorescent or radiolabeiled tracers, this concentration can be continuously logged in specially designed measurement cells on the injection tubing. The fluorescent tracers will be analysed with light and/or laser-induced fluorescence and the radiotracers with either gamma spectrometry or solid scintillation counting where the solid scintillator consists of a detector cell filled with scintillating glass or plastic pellets in contact with the continuously flovng tracerconLaining injection Iluid.

The injection principle and attached monitoring system is illustrated in Figure bc.

In each of the cases i, ii and Ui, the solvent (or dissolving liquid) may contain a component iiich can operate as a passive tracer for the pipeline flow.

Interpretation of measurements Interpretation of results from pulse injection experiments are ri praxis performed by so<aHed moment analysis normaily used in Residence-Time--Distribution (RTD) experiments. Here, the total integral under the recorded tracer production curve and the shape of the curve are parameters to be considered. This method is regarded as weD-known, and will not be further detailed here. Comparison with a possible coinjected passive tracer may also indicate the degree of chromalographic behavior of wax molecules throughout the transportation process in the pipehne.

Interpretation of results from the continuous injection method is straight foard: Providing steady state in the transportation pipeDne, the tracer concentration in the exit fluid reaches a constant concentration level which is ower than the injected concentration. The difference has been irreversibly precipitated in the pipeline. In cases where a square-pulse injection has been performed, additional information on the precipitation and flow structure may be found in the front and tail of the production profile.

In case Hi above where wax tracer is continuously dissolved from a preformed wax precipitate and subsequently injected, the interpretation process is somewhat more complicated. Here, one needs to take into account the actually injected concentrations as measured on-line by the nuclear or fluorescent detection equipment. However, this calculation will be within the capacity of the skilled worker.

For all cases, the simultaneous application of two or more wax tracer molecules with different precipitation properties (or deposition temperature) will further indicate real deposition temperatures in the pipeline and also give information on the total wax deposition kinetics. Such information is important to decide about the need for a pigging operationS n addition to the abovementioned nov& wax tracng method, a corresponding monitonng technique can be used for monitoring of precipitation of asphaltenes. Such materias are potentiafly also a threat to the stabe operation of a fluid transport system since they can precipitate, particuariy in vicinity of the well head. By utilizing a similar methoddogy for identification of tracer and ahefling of the identified specie the technique will be able to detect onset and precipitation rate of asphatenes using ahelling and analyticS techniques corresponding to those disclosed herein for wax. The chemical structure of asphatenes differs from that of wax due to muftipe fused aromatic rings and the presence of heteratomic species such. as oxygen, nitrogen and metal atoms in the structure [6]. However, the procedure for identification of a suitable tracer molecule or population of tracers may be readily based on those disclosed herein for waxes to enable labelling of appropriate asphaltenes This evidently forms further aspects of the invention, corresponding to those aspects disclosed herein for wax.

Citations [1] S. Suppiah. A. Ahmad. C. Alderson, K. Akbarzadeh, J. Gao. J. Shorthouse, l.A. Khan, C. Forde, A. Jamaluddin, Oil and Gas Facilities, 2 (2012).

[2] HA. Craddock, K. Mutch, K. Sowerby, SW. McGregor, J. Cook, C. Strachan, A Case Study in the Removal of Deposited Wax From a Major Subsea Flowline System in the Gannet Field, in: International Symposium on Oiffield Chemistry, Society of Petroleum Engineers, Houston, Texas, U.S.A., 2007.

[3] K. Akharzadeh., J. Ratuiowski, D Eskin, T. Davies, SPE Projects, Facilities & Construction, 5 (2010) pp. 49.57 [4] P. Manfield, W, Nisbet, J. Balius, G. Broze, L. Vreenegoor. ViOn, \Nax-Off': Understanding and Mitigating Wax Deposition n a Deepwater Subsea Gas/Condensate Fbwhne, in: Offshore Technology Conference, Houston, Texas, 2007.

[5] D.J. Bilyeu, T.X. Chen, Cearing Hydrate and Wax Blockages in a Subsea Flowline, in: Offshore Technology Conference, Houston, Texas, 2005.

[6] Diaflo MS. Cagin T. Faulon JL, Goddard fl WA. 2000. Thermodynamic properties of asphaltenes: a predictive approach based on computer assisted structure elucidation and atornistic simulations. Elsevier Science. 40:103-127 [7] H. Alboudwarej, Z. Huo, E.C. Kempton, Row-Assurance Aspects of Subsea Systems Design for Production of Waxy Crude Oils, in: SPE Annual Technical Conference and Exhibition, Society of Petroleum Engineers, San Antonio, Texas, USA, 2006.

[3] P. Marriott, Journal of Separation Science, 27(2004) 1360-1360.

Brief Summary of the Figures

Figure 1 shows examples of molecular classes which can be used as precursors for labelUng operations.

Figure 2 shows an outline of a test apparatus suitable for testing and validation of potential labelled waxes (tracers).

Fig.3: shows various possible methods for labelling an example wax molecule. 1. Is the unlabeled wax molecule. 2.to 7. represent the molecule with vanous labels: 2. tritium-laheled, 3.

4C-!ahe!ed, 4. 11 i-labeled, 5. 13C*-substituted, 6. deuteriuni-substituted and 7. Functional group attached.

Figure 4: shows one potential layout of a production system where it is suggested that wax tracers are injected through a chemicals injection line to the wellhead or tie-in junction. By monitoring the traces produced to onshore facilities or to another offshore installation the initiation and magnitude of the waxing can then be monitored. in addition to the transport lines wax tracers can be used to monitor deposition in process equipment.

Figure ba. shows a sketch of wax tracer pulse njection system.

Figure 5b. shows a sketch of wax tracer irection system for continuous constant concentration injection.

Figure 5c. shows a sketch of wax tracer injection system for continuous dissolution of preformed wax precipitate containing wax tracer molecules with detectors for continuous logging of injected tracer concentration.

Examples

Example 1 -study of wax deposition Deposition of waxy components is studied in dynamic flooding experiments in the laboratory where wax-containing condensates are pumped through steel tubes packed with steel wool to increase the contact surface. Deposition of gamma emitting wax tracers are detected directly on the outside of the steel tubing with a gamma detector. Wax molecules labeled with beta-emitting tracers or with fluorescent labels are analysed in the collected fluid at the exit of the flow rig.

Two procedures are applied: 1. The liquid is pumped through the equipment only once, i.e. the internal wails of the flow loop are continuously contacted with fresh wax-containing condensate at constant concentration. 2. The condensate is recycled without any solute addition. In these experiments parameters are temperature, wax type and concentration and tracer type. A principle sketch. is shown in Fig. 2.

Legends to Figures: Figure 1: Examples of molecular classes which can be used as precursors for labelling operations.

Figure 2: Principle sketch of flow loop for study of wax deposition and qualification of wax tracer molecules.

Figure 3 -some example labelling methods. 1. is unlabelled. 2-7 show various labelling methods.

Figure 4:The figure shows one potential layout of a production system where it is suggested that wax tracers are injected through a chemicals injection line to the wellhead or tie-in junction. By monitoring the traces produced to onshore facilities or to another offshore installation the initiation and magnitude of the waxing can then be monitored. In addition to the transport lines wax tracers can be used to monitor deposition in process equipment.

1: Riser, IC) 2: transport pipeline, 3: tracer injection line, C"J 4: production wells, 5: well/head, 6: multiphase separator, 7: analysis point, 8: reservoir section 1, 9: reservoir section 2, 10: sealing fault.

Figure 5a. Sketch of wax tracer pulse injection system.

Figure 5b. Sketch of wax tracer injection system for continuous constant concentration injection.

Figure Sc. Sketch of wax tracer injection system for continuous dissolution of preformed wax precipitate containing wax tracer molecules with detectors for continuous logging of injected tracer concentration.

Claims

Claims: 1) A method for monitoring precipitation of at least one wax component from a hydrocarbon-containing fluid stream during the flow of said fluid stream through a fluid transport system having at least one in-flow point and at least one out-flow point, said method comprising: i) introducing at least one labelled wax into said hydrocarbon-containing fluid stream at at least one in-flow point; and ii) measuring the relative or absolute concentration of said labelled wax in at least one sample taken at at least one out-flow point.
2) The method of claim I additionally comprising: a) taking a sample of said hydrocarbon-containing fluid stream at the in-flow point b) analysing the sample of said hydrocarbon-containing fluid; c) identifying the structure and/or molecular weight of at least one wax component in said sample of said hydrocarbon-containing fluid; d) selecting or generating at least one labelled wax based upon at least one wax component identified in step C); e) utilising the at least one labelled wax selected or generated in step d) as the at least on labelled wax in step i); wherein steps a) to d) are canied out prior to steps I) and ii).
3) The method of claim 2 wherein a distribution of molecular weights of wax components is identified in step c) and wherein said at least one labelled wax is selected or generated in step d) to have a molecular weight in the lightest quartile of said distribution of molecular weights.
4) The method of claim 2 or claim 3 wherein the structure of at least one wax component is identified in step c) and said at least one labelled wax is selected or generated in step d) to have a structure comprising the structure of at least one wax component identified in step c) (optionally with at least one element removed or substituted) and at least one label.
5) The method of any preceding claim additionally compflsing undertaking at least one wax mitigation measure when the relative or absolute concentration measured in step ii) falls below a threshold value.
6) The method of any preceding claim additionally comprising undertaking at least one wax mitigation measure when the area under a curve (integraD of relative or absolute concentration measured in step ii) against time for a predetermined period falls below a threshold value.
7) The method of claims or claim 6 wherein said at least one wax mitigation measure comprises at least one ot starting or increasing the addition of at least one de-waxing component to said hydrocarbon-containing fluid stream; mechanically de-waxing (pigging) at least a part of said fluid transport system; or thermally de-wSng at least a part of said fluid-transport system.
8) The method of any preceding claim wherein step i) is carried out periodically (e.g. once a week, once a month, or once every three months) or continuously.
9) The method of any preceding claim wherein step ii) is carried out periodically (e.g. once a week, once a month, or once every three months) or continuously.
10) The method of any preceding claim wherein step i) is carried out periodicafly for at east 1 year foflowing an event seiected from; a change or partial change in source of the hydrocarbon-containing fluid stream; the addition, removal or replacement of at east one element of the fluid transport system; the addition, removal or change in concentration of at east one additive introduced into the hydrocarbon-containing fluid stream; or a change in conditions of temperatt re and/or pressure at at least one point on the fluid transport system.
11) The method of any preceding claim wherein said fluid transport system comprises at least one component selected from a pipeline, a valve, a choke, a separator or similar fluid systems.
12) The method of any preceding claim wherein at east one labelled wax and at least one labelled non-precipitating component are introduced in step i) at a known relative concentration and step ii) comprises measuring the relative concentration of sad at least one labeHed wax and at least one labehed non-precipitating component in said at least one sample.
13) The method of any preceding claim wherein said at least one labelled wax is labelled by means of a radioactive isotope, a non-radioactive heavy isotope or a covalently attached fluorophore.
14) The method of claim 13 wherein said labelled wax is labelled by means of a radioactive isotope selected from 3H, 14C, S, 3P, i25 l or 82Br.
15) The method of claim 13 wherein said labelled wax is labelled by means of a heavy isotope selected from 2H, 13C, 15N, ISO or MS.
16) A method for the generation of at east one labelled wax comprising: a) taking a sample of a hydrocarbon-containing fluid stream at an in-flow point of a fluid transport system; b) analysing the sample of said hydrocarbon-containing fluid; c) identifying the structure and/or molecular weight of at least one wax component in said sample of said hydrocarbon-containing fluid; d) selecting a wax based upon at least one w component identified in step c); e) covalenfly or isotopically modify!ng the selected wax with at east one label.
17) Use of at least one labelled wax to monitor precipitation of at east one wax component from a hydrocarbon-containing fluid stream during the flow of said fluid stream through a fluid transport system having at east one in-flow point and at least one out-flow point.
18) The use of claim 17 comprising i) introducing at least one labelled wax into said hydrocarbon-containing fluid stream at at least one in-flow point, and ii) measuring the relative or absolute concentration of said abeHed wax in at least one sample taken at at east one out-flow point.