WO2018093272A1

WO2018093272A1 - Tracers

Info

Publication number: WO2018093272A1
Application number: PCT/NO2017/050297
Authority: WO
Inventors: Helge Stray; Tor BJØRNSTAD; Jan Kihle; Sira TERJE
Original assignee: Institutt For Energiteknikk
Priority date: 2016-11-18
Filing date: 2017-11-20
Publication date: 2018-05-24
Also published as: GB201813073D0; GB2562435A

Abstract

The invention provides for the use of at least one salt as a water tracer compound, wherein said salt comprises at least one organic or inorganic cation and at least one inorganic anionic metal complex. Typically the tracer is used in industrial water or in a subterranean reservoir, such as in monitoring fluid flow in a petroleum or geothermal reservoir. The anionic metal complex or the metal cation is detected to monitor fluid flow. The invention further provides a method for tracking the flow of at least one fluid, said method comprising applying at least one salt comprising at least one organic or inorganic cation and at least one inorganic anionic metal complex to at least one known position along the flow of said fluid and monitoring for the absence/presence and/or concentration of said inorganic anionic metal complex in samples of said at least one fluid. Other aspects of the invention include compositions comprising the described salts in a slurry with proppant particles and methods for monitoring the integrity of an isolation plug. Aggregates comprising consolidated crystals of the described salts are also provided.

Description

Tracers

Field of the invention

The invention relates to the use of tracers for monitoring the flow of fluids, particularly in subterranean environments. More particularly, the invention relates to thermally-robust tracers and to sources of such tracers to monitor and/or detect fluid flows in petroleum reservoirs/wells, shale reservoirs, groundwater reservoirs, geothermal reservoirs or in industrial processes.

Background of the invention

Tracer technology is used extensively in oil and gas exploration and recovery, and both radioactive and non-radioactive tracers that can be measured at low concentrations are applied. Tracer technology may also apply to geothermal reservoirs for monitoring flow paths of injected water, leakage into ground water etc.

In petroleum reservoir tracer studies the tracers may be injected as pulses in well-to-well studies for measuring flow paths, water velocities and water-swept volumes. Radioactive tracers can be measured at very low levels, but for public non-acceptance reasons these tracers have in recent years been replaced by chemical tracers that can be analysed at low concentrations using sophisticated analytical instrumentation (such as GC-MS, HPLC-MS, ICP-MS). One issue faced by such alternative tracers is that they must be naturally present in very low abundance in order to allow positive identification of the tracer at very low concentrations.

Many different organic compounds have been applied as tracers for such studies, among others fluorinated benzoic acids, naphthalene sulfonates and amino-naphthalene sulfonates (Serres-Piole et al., Guidelines. Journal of Petroleum Science and Engineering 98-99 (2012) 22-39). However, the environmental acceptability of organic tracers is not always high, particularly those comprising halogenated substituents. Furthermore, in high temperature reservoirs (typically > 150°C) many of the organic chemical tracers used both in petroleum and geothermal reservoirs will be less suitable due to limited thermal stability.

When the degradation rate of a chemical tracer at the actual reservoir temperature is known, the recovered amount of the tracer can be calculated (Adams et al, Geothermics, Vol. 20, pp 53-66, 1991). Such an approach can only be applied when the temperature is known and the decay rate is relatively low. Ideally, only compounds that are not broken down, not adsorbed and not precipitated should be applied in tracer studies where the purpose is to passively trace the flow of a single fluid phase like water.

Tracer sources may be placed in oil or gas production wells to monitor in-flow from the surrounding formation. For in-flow monitoring studies the tracers may be encapsulated in a polymer which is placed along the external surface of the production tube at different locations before completion of the well as described in US 6,645,769. The tracers can be attached to or encapsulated in polymers or in different types of particles, and the release of the tracers can be made dependent on type of fluid passing (oil or water), on chemical properties of the fluid (for instance pH or salinity) or, for example, on temperature. The tracers are released into the passing fluids and by analysing the concentration of tracers in the produced water, the in-flow rate of fluids from the different zones can be calculated if the correlation between release rate and flow rate can be estimated. One frequently applied method is to place polymer rods containing the tracer outside the production tube imbedded in the sand screen. The amount of tracer that can be mixed into the polymer is, however, limited (5-10 % by weight). If a larger amount is used, the release rate may be too high, resulting in a short lifetime of the tracer source. Independently of absolute rate, the amount of tracer released at the start in such release systems is usually proportionally high, and will decrease exponentially due to the increased diffusion resistance through the micro pores formed in the polymer matrix. The release rate of the tracers from such a system may be difficult to estimate since the rate will depend on physical parameters of the polymer and the fluids as well as conditions such as temperature and pressure. The water tracers are usually mixed into the polymer as salts with high solubility in water. The amount of tracer that is released by the passing water will not increase significantly if the flow rate is increased, but the release rate will be relatively constant over a large flow rate range. The rate of water inflow from different water-producing zones can therefore not be calculated directly from the concentration ratios of the different tracers found in the produced water. One way to achieve this information is to shut in the well for a certain period, then make very frequent sampling of the produced water after the production has been resumed. The tracer cloud that was released in the vicinity of the tracer containers during the shut-in period from a zone with large water in-flow will then reach the topside sampling point faster than the tracer released from other zones. Shutting down petroleum wells even for a few days is, however, not recommended from an economic point of view.

Hydraulic fracturing is frequently used for stimulating gas or oil production from petroleum reservoirs, and is particularly applied in connection with gas production from shale formations. A viscous fluid is pumped at high pressure into defined sections of the well, usually followed by ceramic spherical particles (proppants) to prevent the fractures from collapsing when production is resumed. The placement of tracer release particles in fractures located at different zones and with different tracers at each location in order to make identification of flow paths possible is described in several patents/publications (e.g., SPE 36675, US9290689, US 3,987,850, WO 2010/140032 and WO 2014/207000).

The disadvantages of polymer-encapsulated or polymer-adsorbed tracer particles are also seen in the area of hydraulic fracturing. One disadvantage is that the particles may be crushed during the process of pumping the fracturing fluid containing proppants into the fractures. This will lead to premature release of tracer and reduction of the tracer release period available for tracer monitoring. The release of tracer from a solid polymer particle will be high at the start and decrease exponentially if the release is based on dissolution of the tracer from the matrix and also if the tracer release is based on diffusion of volatile compounds through a polymer matrix.

In patent WO2014207000 the tracer particles consist of mineral crystals with inclusions of a tracer compound. The tracer is released by dissolution of the crystals over time, or in the case of inclusion of gas tracers, the tracers may also diffuse out from the crystals at high temperatures. These materials are highly effective and offer many properties which are superior to previously utilised tracer sources. The amount of tracer that can be

encapsulated into the crystals as described in WO2014207000 is however limited. In view of the above, it would be of considerable advantage to provide tracers for monitoring fluid flow in many situations, including subterranean reservoirs, which are more

environmentally acceptable and/or more thermally stable than currently used tracers. It would be an advantage if such tracers were readily detectable at low concentration and naturally present at very low levels in the systems of interest. It would be a further advantage to provide tracers with a predictable and controlled rate of release, preferably with a non- exponential decrease in release rate over time. It would be a further advantage if the tracers could be used in high pressure and/or high temperature situations such as fracturing and/or in geothermal reservoirs. It would also be advantageous if the tracer release rate from tracer sources placed at different zones for water in-flow monitoring from petroleum wells can be truly dependent on the flow of water coming in from each zone. If the correlation between release rate of the tracer and water in-flow from each zone can be established, the water inflow rate from different zones can be calculated after analysing the concentration of each tracer in the produced water.

Summary of the invention

The present inventors have now established that certain salts comprising inorganic anionic metal complexes can address some or all of the above issues.

In a first aspect, the invention therefore provides the use of at least one salt as a water tracer compound, wherein said salt comprises at least one organic or inorganic cation and at least one inorganic anionic metal complex.

In a particular embodiment, the invention provides the use in monitoring fluid flow in a subterranean reservoir, such as a petroleum reservoir or geothermal reservoir. Preferably, the invention provides the use in at least one tracer method selected from:

i) well-to-well tracer studies of petroleum reservoirs;

ii) in-flow monitoring of petroleum wells;

iii) tracer studies in fractures of shale reservoirs;

iv) tracer studies of groundwater flow; and/or

v) tracer studies of geothermal reservoirs.

In a further aspect, the invention provides a method for tracking the flow of at least one fluid (e.g. within a subterranean reservoir), said method comprising applying at least one salt comprising at least one organic or inorganic cation and at least one inorganic anionic metal complex to at least one known position along the flow of said fluid and monitoring for the absence/presence and/or concentration of said inorganic anionic metal complex in samples of said at least one fluid.

In a further aspect, the invention provides a method for monitoring the integrity of an isolation plug in a petroleum reservoir or geothermal reservoir, said method comprising applying at least one salt comprising at least one organic or inorganic cation and at least one inorganic anionic metal complex upstream of said isolation plug and monitoring for the presence of said inorganic anionic metal complex in produced fluid from said reservoir. In a further aspect, the invention provides a composition comprising a slurry of at least one salt and proppant particles in water, wherein said salt comprises at least one cation and at least one inorganic anionic metal complex.

In a further aspect, the invention provides a container containing a salt, wherein the container is closed and porous, and wherein the pores in the walls of the container are such that dissolved ions can diffuse in and out but solid particles cannot, wherein said salt comprises at least one cation and at least one inorganic anionic metal complex.

In a further aspect, the invention provides a process for producing the salt by means of mixing solutions of two or more salt precursors, wherein said salt comprises at least one organic or inorganic cation and at least one inorganic anionic metal complex.

In a further aspect, the invention provides an aggregate comprising a plurality of

consolidated salt crystals, wherein in at least one dimension said aggregate has a size of at least 5 cm, and wherein said salt comprises at least one organic or inorganic cation and at least one inorganic anionic metal complex.

In an embodiment applicable to all aspects of the invention, the inorganic anionic metal complex is preferably not [Co(CN)₆]^3".

In a further embodiment applicable to all aspects of the invention, the salt is in a semi- crystalline or crystalline form, preferably crystalline form.

In a further embodiment applicable to all aspects of the invention, the solubility of the salt in water is of 10^"1 to 10^"8 g/100ml_ H₂0.

The features of the aspects and/or embodiments indicated herein are usable individually and in combination in all aspects and embodiments of the invention where technically viable, unless otherwise indicated.

Detailed description of the invention

In the present invention, salts comprising at least one organic or inorganic cation and at least one inorganic anionic metal complex are used as water tracer compounds. Broadly, the invention can be used in many applications where a predictable release of tracer is required. The invention is based on the solubility and/or dissolution of the salts in water and the monitoring and/or detection of the inorganic anionic metal complex down to low

concentrations using analytical techniques.

Solubility and background concentration

The salts of the present invention (which are applicable to all aspects of the invention) may be applied in solution or as solids. Where the tracers are applied as solids, they are preferably selected such that they are not released into the fluid instantaneously, but rather are released over a period of time, depending upon the flow and/or conditions of the reservoir. This may be achieved by any appropriate method including several known in the art, such as polymer encapsulation. In the context of the present invention, however, it is preferable that the salts are themselves selected to be only sparingly soluble (e.g. as described herein). By relying on solubility rather than encapsulation to limit tracer release, the present invention may provide release of tracer that is predictably dependent upon the flow-rate of passing fluid. By correlating conditions of the well and observed tracer concentrations, the flow of fluid (e.g. formation water or injected water) at the site of the tracer may thus be estimated qualitatively, semi-quantitatively or quantitatively. This may be used to provide an estimate of actual flow rate at the site of a single tracer and/or may be used to provide absolute and/or relative flow rates at the sites of a plurality of different (and distinguishable) tracers.

In one preferable embodiment, 50% of the tracer may be released over a period of 1 hour to 10 years, more commonly 1 hour to 5 years, preferably over a period of 1 day to 24 months (such as 1 day to 3 weeks, 1 week to 3 or 6 months or 1 month to 24 months), following (and especially during) contact with fluids within the reservoir. Contact with fluids within the reservoir may occur immediately upon placement of the compositions in the reservoir environment. However, that contact may also occur later if the salt is contained, for example within a device that can open the composition to the fluid environment of the reservoir at a later date or time upon for example meeting the targeted conditions of pH, temperature, or salinity. Similarly, if a composition is within a device that can intermittently place the composition in contact with fluids in the reservoir environment, then this time will be taken to be the duration for which that contact was maintained.

In the case of salts used to monitor the integrity of valves, caps or similar blockages of flow, the duration of contact may be longer because the compositions may be placed in a position where fluid is present but not flowing. In one embodiment, the compositions therefore release 50% of their tracer in the above time periods of contact with flowing fluid within the reservoir. In some appropriate uses and methods, however, the contacting may be for longer periods, such as 1 month to 10 years, preferably 3 months to 5 years. Such methods may in one embodiment include methods for monitoring the integrity of caps, valves or similar items. Conversely, where the composition is used directly in continuous contact with the fluids of the reservoir then typical periods are preferably shorter, such as 1 day to 3 years, preferably 1 week to 24 months, or 1 week to 12 months.

Many anionic metal complexes can form sparingly soluble salts with cations of other metals and/or with organic cations. By selecting different combinations of cations and anions the solubility of the salts can be modified to suit a particular application. Generally, for any single application of tracer, a single anionic metal complex will be used and will thus identify that application point and/or time. A single cation or a mixture of cations may be used in order to provide the appropriate solubility and/or release profile.

It can also be envisaged that soluble salts are used for 'pulse'-like monitoring of fluid flow, such as by administering a "bolus" or pulse of tracer in solution to a fluid flow and monitoring the transport and/or distribution of that fluid by monitoring the presence, absence and/or concentration of the anionic metal complex in samples from the flow or possible flow of that fluid.

Preferably, the salts of the present invention are sparingly soluble salts. A sparingly soluble salt is here defined as a salt with a solubility in the range of 10^"1 to 10^"8 g/100ml_ H₂0 at 20°C and atmospheric pressure. Preferably, the salt has a solubility in the rangelO^"1 to 10^"5 g/100ml_ H₂0, such as 10^"2 to 10^"5 g/100ml_ H₂0. Typical solubilities for example compounds useful in all aspects of the invention are, for example, SrMo0₄ with a solubility of 0.011 g/100ml_ H₂0 at 20°C and atmospheric pressure, and SrW0₄ with a solubility of 0.0004g/100ml_ H₂0 at 20°C and atmospheric pressure. A range of 0.0001 to

0.025g/100ml_ is thus useful. Rare earth molybdates may have a solubility around 10^"2 to 10 ^"3 which thus forms a useful range, especially for these compounds.

It is preferable if the background concentration of the tracer (particularly the anionic metal complex part of the tracer) in the fluid to be monitored is low. This improves signal-to-noise ratio and allows positive identification of the tracer at very low concentrations. Background concentration is defined as the concentration of tracer compound (or the anionic metal complex) in the fluid to be measured prior to the addition of tracer. A low background concentration is here defined as a concentration of less than 10 μg/L (e.g. 0.001 to 10 μg/L, 0.001 to 10 ppb) of the anionic metal complex, such as less than 5 μg/L (5 ppb). Preferably, the background concentration is less than 1 μg/L (1 ppb) of the anionic metal complex. For some compounds, the background level may be even lower than 0.1 μg/l (0.1 ppb).

A balance between background concentration and solubility must be struck to ensure good signal-to-noise ratio and appropriate release time for a feasible amount of tracer compound applied. For example, the use of a salt which has a high background concentration and a low solubility may give analytical data with poor signal-to-noise ratio unless an onerously large amount of tracer is used. A salt with high solubility and low concentration will give a favourable signal-to-noise ratio, but the high solubility will result in the tracer salt being depleted rapidly.

Salts

The inorganic tracers, of all aspects of the present invention, have potential for being more thermally stable than their organic tracer counterparts. It is preferable, however, to monitor the anions and not cations since cations may be adsorbed to clay minerals in the reservoir rock. The salts of the present invention comprise at least one cation and an anionic metal complex. It is correspondingly the anionic metal complex which is preferably measured and/or monitored. As used herein, references to a "salt", "tracer" or "tracer salt" refer to the described salts of at least one cation and an anionic metal complex (or simply to the anionic metal complex in solution where context allows) unless specified otherwise.

The term 'salt' as used herein also encompasses solvates and hydrates of salts. For example, the solubility of a salt may potentially be tuned and/or affected by choosing certain hydrates or solvates to meet the specific requirements of the present invention. All suitable hydrates and solvates form embodiments applicable to any compatible aspect of the invention.

The salts of all aspects of the present invention comprise at least one organic or inorganic cation (i.e. at least one organic cation, at least one inorganic cation, or at least one organic cation and at least one inorganic cation). In the case of two or more cations, each cation may be selected independently from any suitable cations, such as those indicated herein. The cation may be, but is not limited to, an alkaline earth metal cation, such as a cation of Mg, Ca, Sr, Ba (and mixtures thereof), particularly Sr. Other cations, whether organic or inorganic, may be used providing the resulting salts meet the requirements of the present invention in terms of solubility and stability. Alkali metal cations, such as cations of Li, Na and/or K may be suitable, as may organic cations such as ammonium, alkylammonium, pyridine and/or substituted pyridine cations. Cations of other metals such as Pb, Zn and/or Ag may also be used and all cations may be used individually or in combination in order to achieve the desired solubility profile.

The salts of all aspects of the present invention comprise an inorganic anionic metal complex. Preferably, the metal of said inorganic anionic complex is a metal from group 4, 5, 6, 7, 9 or 16 (Via) of the periodic table. The metal is more preferably selected from the group consisting of Se, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Tc, Re, Co, Rh and Ir. Even more preferably, the metal is a group VI or a group VII metal, such as Mo, W or Re. The use of crystal salts of two mutually soluble endmember anions (whilst sharing one common crystal structure), as in the SrMo0₄-SrW0₄ series exhibiting different endmember solubilities, may also be applied for fine tuning of solubility behaviour, thus resulting in the release of two tracer anions, generally in known proportions.

In one embodiment applicable to all aspects of the invention, the inorganic anionic metal complex is a complex of a metal as hereinbefore defined with at least one X-type ligand.

In a further embodiment, applicable to all aspects of the invention, the inorganic anionic metal complex is a complex of a metal as hereinbefore defined with at least one anionic ligand selected from oxide, hydroxide, halide (e.g. fluoride, chloride, bromide or iodide), thiocyanate, and/or cyanide, preferably oxide. The complex may comprise one or more ligands of the same sort, or two or more different ligands (e.g. more than one halide or mixed oxide and cyanide ligands). It is a feature of the invention that the ligands in the anionic metal complex are not organic. However, carbon containing 'inorganic' ligands (e.g.

cyanides, and/or thiocyanates) may be used. Some metal oxygen anions tend to be more stable than their corresponding cyanide, thiocyanate or halogen complexes.

In one preferable embodiment, the complex is not a metal cyanide (i.e. does not contain solely cyanide ligands and preferably does not contain any cyanide groups). More preferably the inorganic anionic metal complex is an anionic metal oxide or halide, preferably an anionic metal oxide. The risk that the anions can be precipitated with cations in produced water or react in other ways with chemical compounds present (H₂S in particular) must also be taken into account when selecting the particular combinations of metals and ligands.

In a preferred embodiment applicable to all aspects of the invention, the anionic inorganic metal salt is of the formula M0₃ ^n" or M0₄ ^n" (M = group VI or group VII metal, n = 1 or 2). Of these two formulae, M0₄ ^n" is in general preferred due its higher stability in water solutions compared to M0₃ ^n". In particular, the complexes Mo0₄ ^2", W0₄ ^2" and Re0₄ ^" are particularly suitable for the present invention. Any other suitable inorganic anionic metal oxide may be used in any aspect of the invention providing it satisfies the requirements of the present invention in terms of, for example, solubility, stability, ease of detection and low background concentration. The choice of cation is important for the control of solubility. For instance, alkaline earth metals will form sparingly soluble salts with molybdate (Mo0₄ ^2") and tungstate (W0₄ ^2"), while perrhenate (Re0₄ ^") may form sparingly soluble salts with organic cations such as tetraphenylpyridine or Nitron (1 ,4-Diphenyl-3-phenylamino-1 ,2,4-triazolium hydroxide inner salt).

The salts of the invention may be used as amorphous, semi-crystalline or crystalline solids. Preferably, the salts are used in semi-crystalline or crystalline form, more preferably crystalline form. By a semi-crystalline salt is meant a structure which comprises crystalline and amorphous regions. Even for completely crystalline solids, the degree of structural perfection may vary. Therefore, in the present invention, 'crystals' are here understood to encompass polycrystalline structures, 'twinned' structures and/or structures containing any other crystallographic defects. The crystalline material can be crushed and sieved to desired grain sizes to ensure predictable release properties. Preferably, the salts used in the present invention may be used as crystalline particles having a size of no less than 10 μηι (e.g.

10μηι to 200mm), preferably no less than 100μηι (e.g. 200μηι or greater) and more preferably no less than 1 mm in smallest dimension.

In any appropriate aspect of the present invention, such as when applied to in-flow tracer studies, the salts of the invention as described herein can be consolidated into aggregates, such as rods or bars, that can be placed in the reservoir, for example under the sand screens. Consolidation of crystals can be performed by filling a suitable mould with the crystals and slowly pumping through a saturated solution of the same salt that the crystals consist of. Precipitation from this saturated fluid will act as a bonding agent making the tracer crystals stick together. The resulting rod, bar or other solid shape will resemble the crystals in a lump of sugar, thus producing large bars or pieces of crystal, which may be formed in a shape appropriate for application and/or may be easily manipulated into place. Variations of temperature may be used to aid such a process, such as by applying hot saturated solutions and cooling these in a mould containing crystalline material to accelerate precipitation of further salt material. Evidently, this "binder" salt may be the same salt as the tracer, or may be another salt used to control strength and/or solubility of the final product or of the tracer material. The aggregate may have any size suitable for the intended use and its shape will be determined by the mould used. For example, the aggregate may be a block, rod, bar or any other shape. Preferably, in the smallest dimension the aggregate will have a size of at least 0.5mm, such as at least 1 mm. Preferably, in one of the dimensions, the aggregate will have a size of at least 5 cm, such as at least 10 cm, at least 50 cm or even at least 1 m. Rods are a particular embodiment of this invention and may have a length of at least 1 cm, such as at least 10 cm, preferably at least 50 cm, more preferably at least 1 m. For example, the rods may have the dimensions of around 2mmx7mmx 1 m.

The salts of the present invention are attractive for use in subterranean reservoir studies because of their thermal stability compared to other tracers, such as organic tracers. It is preferable if the salt is stable as a solid or in solution (e.g. in the fluid to be monitored) at a temperature of 100°C or more, preferably 150°C or more, more preferably 200°C or more, and even more preferably 250°C or more. By 'stable' is meant a degradation of less than 10% (e.g. 0.01 % to 10%), preferably less than 5% over a period sufficient for use as a tracer. Such a period may be, for example at least 1 day (e.g. 1 day to 10 years).

Preferably the salts will be stable (as described above) for at least 1 month, preferably at least 3 months, or at least 6 months, more preferably at least 1 year or at least 2 years. Where salts (especially sparingly soluble salts) are applied for long-term monitoring (e.g. of fluid flow) it will be preferable that those salts are stable for at least 2 years, preferably at least 5 years or at least 7 years. The required stability of the salt will depend on the intended use.

In one aspect, the present invention provides a composition comprising a slurry of at least one salt and proppant particles in water, wherein said salt comprises at least one cation and at least one inorganic anionic metal complex as hereinbefore defined. Said proppant particles may be selected from sand, resin-coated sand, bauxite, ceramics or mixtures thereof. The composition may further comprise at least one chemical agent commonly used in fracturing fluids, such as a gel, foam, compressed gas, cleaning agent (e.g. acids), sodium chloride, a friction reducer (e.g. polyacrylamide), a scale deposit preventing agent, a viscosity modifiers/stabilizer, a compound for maintaining effectiveness of crosslinkers (e.g. carbonates), a disinfectant, a gelling agent, an anticorrosion agent and/or a winterizing agent.

In US5246861 , potassium cobalt hexa-cyanide (K₃[Co(CN)₆]) is applied as a non- encapsulated tracer. However, the use of such a compound as a tracer has drawbacks. The tracer was found to have limited thermal stability under petroleum reservoir conditions (> about 100°C). The solubility properties of this salt may also limit its usefulness, particularly for methods utilising solid tracers (e.g. in crystalline form).

Containers

Crystals or amorphous particles of the sparingly soluble salts can be filled into microporous tubes or containers (the tubes or containers may suitably be formed from polymer, ceramic or sintered metal). The walls of the tubes or containers have pores communicating between the inside and the outside of the container such that dissolved ions can diffuse in and out while the solid particles remain inside. The pore size can be in the range 1-200 μηι, preferably 5-100 μηι, such as 10-50 μηι or 30-70 μηι in average largest diameter. The choice of porous wall material can be adapted depending on the size of the salt particles (whether crystalline, semi-crystalline or amorphous). The containers will typically be "closed" in the sense that no opening much greater than the pore size (e.g. no opening more than 10x , preferably no opening more than 2x the average pore size) is present.

The tubes or containers can be placed at specific locations in petroleum wells, for instance along the outer surface of the production tubing imbedded in the sand screens as described in US 6,645,769 B2 or in plug leak detection tools and deployment valves as described in WO2014207000 A1. Such containers with walls of microporous polymer, containing particles of sparingly soluble tracer crystals, may also be applied as tracer sources in a variety of other industrial processes, including those described herein.

Grains of sparingly soluble salts of the mineral tracer crystals can also be mixed into gravel packs and be used as tracer sources in a similar way as when placed in microporous tubes, bags or other type of containers.

Since the tracers may be sparingly soluble salts, such salts can provide a high concentration of tracer within a container etc. In one embodiment, therefore, a container of tracer may comprise at least 10% by weight tracer (salt as defined herein) material, preferably at least 25%, and more preferably at least 50% or at least 70% tracer salt by weight. The remaining weight will comprise containing materials and membranes of suitably robust material to contain the salt. Evidently, a high proportion of tracer material provides an advantage in detection and duration.

Preparation of salts

The salts of the present invention may be prepared by any conventional means in the art. Preferably, the process for preparing the salt will be such that a semi-crystalline or crystalline, preferably crystalline, material is formed. In one embodiment the desired tracer salt is formed by means of mixing solutions of two or more salt precursors and forming the product by thermodynamically optimized reactions. The desired salt may be formed by a salt metathesis reaction (e.g. of the form A-B + C-D→ A-D + C-B). If the reaction is performed in a water solution, it is preferable that the process for forming the salt is such that the desired product precipitates, or preferably crystallizes, out of the water solution upon mixing of the reagents due to the low solubility of the product. Slow mixing of dilute solutions is preferred to ensure good crystal growth. By slowly mixing dilute solutions of, for instance, SrCI₂ and Na₂Mo0₄ together, crystals of SrMo0₄ can be formed.

As well as the slow mixing of dilute water solutions of salt precursors, crystal growth can also be achieved after heating salts containing the desired elements with a fusing agent (Borax, LiCI , Na₂C0₃.). For instance large crystals of SrMo0₄ can be formed after heating SrC0₃, Na₂C0₃ and Mo0₃ to 1 100°C followed by slow cooling to 500°C, and large crystals of SrW0₄ may be produced after heating solid SrCI₂ and solid Na₂W0₄2H₂0 to above the melting point using LiCI as a flux agent followed by slow cooling from 950°C to 600°C over a period of 48 hours (see Figure 2).

The crystalline material can be crushed and sieved to desired grain sizes. If the tracers are placed along the production tube in the form of crystals that have a known solubility at the actual temperature, the release rate will be easier to estimate.

Detect ion/monitoring

The metal anionic complexes can be analysed/detected in water samples (from produced water, water from geothermal reservoirs, ground water extracted for drinking purposes etc.) using any suitable analytical method. Suitable analytical methods for the various

embodiments of the invention include spectrometric methods and/or chromatographic methods, e.g. HPLC (high performance liquid chromatography) and/or MS (mass

spectrometry), such as hyphenated high performance liquid chromatography and inductively coupled plasma mass spectrometry (HPLC-ICP-MS). If the metallic element is not present in other ionic forms in the water samples, ICP-MS may be used directly. However, any other form or combination of spectrometry, chromatography and/or other technique (e.g.

spectroscopy) may be used.

In a preferred embodiment of the invention, the tracers are not radioactive tracers due to environmental and public non-acceptance reasons and are hence not detected by radioactive methods. However, such radioactive compounds and detection methods are not outside the technical scope of the invention. If appropriate, the inorganic anionic metal complexes may be detected by radiation detection, e.g. gamma-ray spectroscopy.

It is also possible to apply metal anionic complexes where the said metal is enriched in one or more of the natural stable isotopes. The isotopic composition of the tracer metal element in the samples can be analysed using high resolution mass spectrometry, and the presence of the tracer can be verified and the concentration calculated. The application of special isotopes of the metal elements may result in a larger number of available tracers and allows for more than one tracer salt where each salt is identifiable but all have the same solubility characteristics, allowing for more direct comparison of fluid flow at multiple sites.

Molybdenum for instance has 7 stable isotopes and tungsten has 5. In a corresponding embodiment, therefore, any of the salts indicated herein employed in any of the

embodiments described may be enriched or depleted above or below natural abundance of at least one metal isotope. Such an enhancement or depletion may be by, for example, by at least 10% (e.g. 10 to 90%), at least 20%, at least 50% or at least 70% of the total isotopic content of that element in the salt.

In one embodiment, detection of the salt may be by means of the anion, or a constituent thereof (such as the metallic element of the complex anion). In an alternative embodiment, the detection of the salt may be by means of the cation. In such cases, the cation will be selected to be a suitable tracer cation. Tracer cations will typically be naturally present in the sample at low level, will be readily detected at suitable concentrations and will form salts as described herein. At least one cation of a rare earth metal, Scandium and/or Yttrium are particularly suitable in this respect and may form all or part of the cationic component of the salt in combination with complex anions as indicated herein. These are particularly suitable for single-well studies, as described herein.

The sparing solubility that forms one embodiment of the present invention (as discussed herein) allows for correlation of the detected level of tracer with the flow of fluid (e.g.

formation or injected water) at the site of the tracer. The uses and methods of the present invention may thus include the step of correlating the detected tracer concentration with the flow of fluid (especially water) at the tracer site. Such correlation may also correlate the flow with the known solubility properties of the corresponding tracer and/or with the conditions (e.g. temperature) at the site of the tracer(s). Measurement of the flow of formation water past a tracer to at least one production well forms one embodiment. Measurement of injected and/or formation water between two points, such as two wells in an inter-well study being another. Typically in an inter-well study, at least one flow will be of injected water. Furthermore, since multiple tracers of the invention may be distinguished by means of their anions and/or cations, tracers at various sites may be distinguished. Correspondingly, at least two different (and distinguishable) tracers (e.g. 2, 3, 4 or more) may be placed at a corresponding number of different sites (e.g. at different points or zones of a formation or reservoir). Such tracers may then be detected at the same and/or different production points and the flows at (or from) each point may be detected. The correlation of each tracer, detected concentration and optionally conditions and/or solubility properties may provide an absolute estimate or measure of the flow at each point. Alternatively, a relative estimate or measure of the flow at one point in comparison with the flow at one or more other points may be made. Typically, a relative and/or absolute measure would be provided between each point or zone. Applications

The tracers of the invention are useful and utilised in various applications, particularly when applied to subterranean (which may also be taken as under the sea bed) reservoirs. Such reservoirs typically produce oil and/or gas, but may equally be geothermal reservoirs producing hot water and/or steam. The various aspects of the invention may be

implemented in one or more of many ways. Thus, for example, in all aspects of the invention the tracers / compositions may be applied within completion equipment such as sand filter sleeve, may be placed into fractures or in the annulus and/or may be placed on the surface of tubulars. In one embodiment, the tracers may be put in place during the process of hydraulic fracturing, for example in the fracturing of shale deposits. This may be, for example, by including the tracers of the invention in the fracturing fluid and/or with the proppants (e.g. in the form of compositions containing tracers and proppants as described herein).

Well-to-well tracer studies of petroleum reservoirs

In one embodiment applicable to all aspects of the invention, water soluble salts of anionic metal complexes are used as tracers for well-to-well studies in petroleum reservoirs. Tests have shown that the background concentration of for instance the anions Mo0₄ ^2", W0₄ ^2" and Re0₄ ^" in produced water from petroleum wells is low (usually <1 C^g/l), and it is anticipated that the same will be the case for a number of other metal oxygen or halide anionic complexes.

Tracer studies of geothermal reservoirs

In geothermal reservoir management it is in many cases essential to re-inject the produced fluids in order to maintain the reservoir pressure and to have a means of disposing of fluids that may potentially be hazardous to the environment. The location of the injection well is crucial for a successful exploitation of the geothermal energy, and tracer technology can be utilised to reveal the flow patterns of the injected fluids for optimal and sustainable heat extraction. Solutions of water soluble salts of at least one organic or inorganic cation and at least one inorganic anionic metal complex as described herein (e.g. salts of metal oxygen anionic complexes) can be applied for such studies. These tracers may have a better thermal stability than the organic based tracers applied today, and may be applied in high temperature (typically > 250°C) reservoirs where organic tracers may have limitations due to lack of thermal stability. In one embodiment, the salts of at least one organic or inorganic cation and at least one inorganic anionic metal complex are utilised in subterranean reservoirs at temperatures greater than 150°C, (e.g. greater than 200°C or greater than 250°C).

In-flow monitoring of petroleum wells

When the tracer is applied in the form of a crystal, where the crystal itself is formed largely or exclusively of the tracer compound, the amount of tracer available will be high compared to the situation where the tracer is mixed into a polymer or other type of matrix. The release rate will depend on the solubility of the salt applied at the actual temperature and will also to some extent depend on concentration of other salts and of the flow rate of the passing fluids. If the tracers are placed along the production tube in the form of mineral crystals (e.g. of the salts described herein) that have a known solubility at the actual temperature (i.e. the temperature of the fluid at the point of monitoring, such as within the subterranean reservoir), the release rate will be easier to estimate. The release rate will not decrease exponentially with time, but will only decline slowly as the surface area of the particles gradually becomes smaller. Since in one embodiment the salts are sparingly soluble (e.g. as described herein), the solubility of the salt is limited and the dissolution of the crystals will largely depend on the flow rate of the passing fluids. Stationary fluids becoming saturated with tracer salt before a large proportion of that salt has been dissolved.

Crystals or amorphous particles of the sparingly soluble salts can be filled into microporous polymer tubes or containers. The tubes or containers may also be ceramic or metal-sintered containers. The walls of the tubes or containers have pores where dissolved ions can diffuse in and out while the solid particles remain inside and such tubes or containers will typically have pore sizes as described herein. The tubes or containers can be placed at specific locations in petroleum wells, for instance along the outer surface of the production tubing imbedded in the sand screens as described in US 6,645,769 B2 or in plug leak detection tools and deployment valves as described in WO2014207000 A1. Grains of sparingly soluble salt tracers can also be mixed into gravel packs and be used as tracer sources in a similar way as when placed in microporous tubes, bags or other type of containers. Such containers with walls of microporous polymer containing particles of sparingly soluble tracer crystals may also be applied as tracer sources in a variety of other subterranean and industrial processes. The tracer salt will slowly be dissolved into the passing water flowing from the formation into the well.

In the case of two or more gravel packs containers, these are located at different zones during the completion of the well and may contain different types of sparingly soluble tracer salts (particularly salts formed of different anionic metal complexes). When the particles are slowly dissolved in the passing fluids and the tracer anions are transported out and analysed in the produced water, valuable information concerning water production from different zones can be obtained. Analysis of the concentration of the anionic metal tracer complexes in the produced water will make calculation of in-flow from different zones possible.

As can be seen from Experiment 5, crystals of SrMo0₄ and SrW0₄ can be placed in different zones along the production tube of petroleum wells for water in-flow monitoring, and the amount of each released tracer that will correlate to the flow rate in the different zones can be used to calculate the in-flow ratios. The number of such tracer salts with a metal containing anion is, however, limited in some applications.

For well-to-well tracer studies and other applications where the fluids are moving through a reservoir, it is important that the tracers are anions so that adsorption on negatively charged minerals can be avoided. Tracers released in water in-flow studies will be much less exposed to such minerals because such studies are single well tests where the tracers do not have to flow a great distance or over a large volume of rock. As a result, tracers in the form of cations may therefore work equally well in such water in-flow applications. By making, for instance, crystals of the molybdate salts of rare earth elements and Yttrium , 15 different tracers can be made available for in-flow monitoring. Salts like Pr₂(Mo0₄)₃, Nd₂(Mo0₄)₃ and La₂(Mo0₄)₃ have solubility values ranging from 0.0015 to 0.0025 g/100g which will be suitable for this type of application. The tracers of the invention can thus be used not only for in-flow monitoring, but also in-flow flow measurement for each of a plurality of zones, where each zone is provided with a tracer. Generally, each zone will have a tracer comprising a different rare earth metal cation. Each tracer may have the same anion, such as the molybdate anion.

In one particular embodiment, therefore, the salt of the invention comprises a rare earth metal or Scandium or Yttrium as a cation. Typically, these cations are of atomic number 57 to 71. Preferably, the metal(s) chosen for use as a rare earth element cation will naturally be present only at a low level in produced water (e.g. less than 20 ng ¹ in the relevant produced water). Typically suitable rare earth metal cations include Pr, Nd and La. In a particular embodiment, the salt may comprise a rare earth element, Scandium and/or Yttrium as a cation and a molybdate as an anion. The anion may also consist of hydroxide, or fluoride. YF₃ for instance has a reported solubility value of 0.006 g/100g at 20°C, which will be suitable for such applications. Most rare earth element hydroxides have solubility values ranging from 1 - 2*10^"5 (100 - 200 μg/L), which, considering the low background level of these elements usually found in produced water, also should be suitable.

Since rare earth metals and Scandium and Yttrium (the tracer metals) are rare, valuable and can be detected at low concentration, in one further embodiment, the salt of the invention may comprise at least two cations, where at least one is a rare earth cation, Sc or Y (e.g. as a tracer) and at least one other is the cation (organic or inorganic) such as the cation of another metal which is not a rare earth metal, Sc or Y. Suitable cations include those discussed herein such as alkali metal cation(s) and/or alkaline earth metal cation(s). In such mixed-cation salts of tracer cation(s), the rare earth cation, Y or Sc may form 1 to 90% by mole of the cations present, preferably 5 to 50%, such as around 10%, 20%, 30% or 50% by mole of the total cations present. The remaining cations will be other cations such as any of those indicated herein including, for example including alkali metal cation(s) and/or alkaline earth metal cation(s).

Application of tracer crystals in fractures of shale reservoirs

Hydraulic fracturing is used for stimulating gas or oil production from petroleum reservoirs especially in shale formations. A viscous fluid is pumped at high pressure into defined sections of the well, usually followed by particles of robust materials such as ceramic spherical particles (proppants) to prevent the fractures from collapsing when production is resumed. The placement of tracer release particles in fractures located at different zones and with different tracers at each location in order to make identification of flow paths possible is described in several patents (e.g. SPE 36675, US9290689, US 3,987,850, WO 2010/140032 and WO 2014/207000). In patent WO2014207000 A1 the tracer particles consist of mineral crystal with inclusions of a tracer compound. The tracer is released by dissolution of the crystals over time, or in the case of inclusion of gas tracers, the tracers may also diffuse out from the crystals at high temperatures. The amount of tracer that can be encapsulated into the crystals as described in WO2014207000 A1 is however limited. When the tracer salt is in the form of crystals consisting of or consisting essentially of the tracer salt itself, the amount of tracer available in the fractures will be much higher. For injection into fractures in shale reservoirs with proppants a grain/crystal size similar to the particle size of the proppants will provide good results. If the size of the crystals of tracer is too large then these may be excluded from the fractures by their size. If, however, the sizes of the particles of tracer are too small then there is the possibility that the grains will be washed out from the fractures with the produced water. A size of 0.2 to 5 times the size of the proppant particles may be appropriate, preferably 0.5 to 2 times that size. When crystals containing different metal anionic complexes are injected into different zones of the well, important information concerning the amount of water production from the zones can be obtained. Injection of such sparingly soluble tracer crystals can also be used to detect leakage of fracturing fluids into surrounding water reservoirs (e.g. into ground water for other uses, such as drinking water).

Further applications

Other applications of the salts of the present invention will be evident to the skilled worker and include, for example, a method for monitoring and/or validating the integrity of an isolation plug in a petroleum reservoir or geothermal reservoir, said method comprising applying at least one salt as defined in the description or the claims herein on the downhole side (e.g. upstream) of said isolation plug and monitoring for the tracer complex in the produced fluid from said reservoir. Thus, this and each application of the salts form a further embodiment of the invention.

Other applications of the salts comprising a cation and anionic metal complex as tracers also form aspects of the present invention. For example, other applications include monitoring and/or validating reservoir layers separation in a multi-layer reservoir, for example after an operation of stimulation or injection of chemicals to provide a flow barrier, such as in

US20110277996. Monitoring geometry of a fracture, such as described in WO2010011402 is another. Monitoring and tracking backflow of proppants, such as described in

WO2005103446 is another. Assessing recovery of material injected in a subterranean formation such as in WO2002095159 is another. Assessing the volume of fluid injection in a reservoir stimulation is yet another, such as described in WO201 1141875.

Other uses for the soluble salts of the water tracers can be hydrological studies or various industrial processes where tracer technology can be applied. Suitable industrial processes include all processes where water is used, and in particular where water is stored, recovered or recycled such that the tracers of the present invention may be used to monitor flow or particularly leakage of water from the process. Processes such as power generation (solar, geothermal, fossil-fuel or nuclear powered) where heat transfer fluids are used are typical examples. Other relevant industrial processes such as those using water to cool cutting or milling operations (e.g. in metal working operations), where water acts as a carrier (e.g. paper making) or as a general coolant (e.g. in chemical industries or in moulding, casting or glassmaking operations) are also suitable for applying the various aspects of the present invention. Any of these fluids and those used in similar applications may be monitored in the various aspects of the present invention. In particular, the tracers may be used to detect leakage of industrial water and/or to monitor or detect contamination of land and/or groundwater with industrial water. Monitoring the fate of water discharged to the environment and/or of run-off water from farms, industrial land or through drains forms a further embodiment. Brief description of drawings

Figure 1: Microscope photograph of SrMo0₄ crystals formed at 80°C by mixing solutions of 0.05M SrCI₂ and 0.05M Na₂Mo0₄ over a period of 48 hours.

Figure 2: Microscope photograph of SrW0₄ crystals formed by precipitation after fusion with LiCI to 950°C and controlled cooling to 600°C over a period of 48 hours.

Figure 3: Diagram showing concentrations of Mo0₄ ^2" and W0₄ ^2" in the collected fractions after leaching with 4%NaCI solution from microporous polyethylene containers filled with SrMo0₄ and SrW0₄ crystals.

Figure 4: Overlaid chromatogram from HPLC-ICP-MS of Re0₄ ^" in a reference standard and 8 produced water samples from different petroleum fields.

Figure 5: Overlaid chromatogram from HPLC-ICP-MS of Mo0₄ ^2" in a reference standard and 6 produced water samples from different petroleum fields.

Figure 6: Overlaid chromatogram from HPLC-ICP-MS of W0₄ ^2" in a reference standard and 6 produced water samples from different petroleum fields.

Figure 7: Plot showing release of rhenium into salt water from glass beads doped with 1 % (W/W) Re₂0₇.

Figure 8: Response curves for HTO and Re0₄ ^" in dynamic flow experiment with column filled with Basalt rock eluted with water at 300°C and 200 bars back pressure.

Figure 9: Plot showing the amount of tracer salt dissolved per hour versus time at different flow rates.

Figure 10: Plot showing correlation between dissolved tracer salt per hour versus flow rate. Examples of preparation Example 1

Two polyethylene filter cylinders of length 46mm, outer diameter 6.45mm and internal diameter 3.20mm were filled with crystals of SrMo0₄ and SrW0₄ respectively. The cylinders had a pore size of 30-70 μηι. The cylinders were filled with 4% NaCI-solution before the open end of the tubing was plugged with a silicon rubber stopper. The cylinders were placed in a glass column and the void volume of the column was filled up with 3mm glass beads. The column was placed in a heating cabinet adjusted to a temperature of 80°C, and artificial formation water containing 4% NaCI was pumped through the column at a flow rate of 0.1 mL/min. Fractions of the eluted NaCI-solution were collected on a daily basis for a period of 14 days, and the collected fractions were analysed by ICP-MS for the content of Mo and W. After about 5 days the concentration in the eluted fractions stabilized at about half of the initial value (Figure 4). After 14 days 0.98% of the total amount of SrMo0₄ had been dissolved, while the corresponding value for SrW0₄ was 0.91 %. Calculated from the mean dissolution rate during this experiment, the live-time of the crystals can be estimated to about 200 weeks. It is anticipated that similar tracer crystals where Ba substitutes Sr (as BaMo0₄) will exhibit even longer lifetimes.

Example 2

Some produced water samples from different petroleum reservoirs around the world were analysed using HPLC-ICP-MS for the concentration of the metal oxygen anions Mo0₄ ^2", W0₄ ^2" and Re0₄ ^". The content of Re0₄ ^" was found to be less than 0.1 ppb for all samples tested. Overlaid chromatograms from HPLC-ICP-MS analysis of Re0₄ ^" in the 8 produced water samples and a standard solution are shown in Figure 4. Mo0₄ ^2" was detected in one sample at 9 ppb, while in the other 5 samples tested the concentration was below 1 ppb (See Figure 6). For W0₄ ^2" the concentration was found to be 13, 6 and 2 ppb respectively for three of the samples and <1 ppb for the three other samples (See Figure 7).

Example 3

For introduction of the tracer Re0₄ ^" in fractures of shale formations the tracer should be injected as rigid particles that can withstand the high pressure applied. In order to make such particles that can release perrhenate, Re₂0₇ was mixed into Ultra Low-melting glass beads and the mixture was heated above the melting point in a closed container. After cooling, the glass was crushed and sieved to particle size of between 125 μηι and 850μηι. The glass particles were filled into a column and a solution of 4% NaCI in water was pumped through at a flow rate of 0.05 ml/min and at a temperature of 70°C. Fractions of the water passing through the column were collected on a daily basis and analysed for the concentration of rhenium using ICP-MS for analysis. The results are presented in Figure 7. The release of Re0₄ ^" was high at the start due to release of rhenium from Re₂0₇ particles located near the surface of the glass particles. The release rate gradually decreased and reached a near constant level after about ten days. Based on the assumption that the release rate continued at the same level, the lifetime of the tracer source was estimated to 112 days.

Example 4

To test the suitability of Re0₄ ^" as a tracer for geothermal reservoirs, a dynamic flow experiment at a temperature of 300°C with a back pressure of 200 bars was performed. A solution of Re0₄ and tritiated water (HTO) was injected into a column containing Basalt particles of grain size 125 to 250 μηι. The column was 42 cm long with an inner diameter of 8 mm. Deionized water was pumped through the column at a flow rate of 0.05 ml/min.

Fractions eluted from the column were collected every 5 minutes (250 μΙ fractions), and the fractions were analysed for the concentration of HTO and Re. A plot of the response curves is shown in Figure 8. The recovery of Re was calculated to 91 % and the Re was eluted in nearly the same volume as HTO. It may therefore be concluded that Re0₄ ^" behaved similarly as the ideal water tracer HTO in this experiment. The slightly faster elution of the Re0₄ ^" was due to ion exclusion effects. The neutral HTO molecules are able to diffuse into pores of the Basalt rock particles, while the negative perrhenate ions will be excluded from entering the finest pores of the rock particles having a negative surface charge and will move somewhat faster through the column.

Example 5

The following experiment was performed to test the release rate of Mo0₄ ^2" ions and W0₄ ^2" ions from crystals of SrMo0₄ and SrW0₄ at different flow rates of water. Two PEEK polymer columns of 50 mm length and 4 mm internal diameter were filled with SrMo0₄ and SrW0₄ crystals respectively. The crystals had been sieved to a grain size between 250 and 850 μηι and had been washed in water before drying. A water solution containing 1 % NaCI was pumped through each of the columns at different flow rates ranging from 0.05 to 0.2 ml/min. The columns were kept in a heating cabinet at 80°C during the experiment. The fractions eluted at different flow rates were collected on a daily basis and analysed by ICP-MS for the concentration of Mo and W. The results are presented graphically in Figure 9. A correlation curve for the dissolved tracer salt SrMo0₄ and SrW0₄ versus flow rate is shown in Figure 10.

Claims

1. The use of at least one salt as a water tracer compound, wherein said salt comprises at least one organic or inorganic cation and at least one inorganic anionic metal complex.

2. The use of claim 1 wherein the inorganic anionic metal complex is not Co(CN)₆ ^3".

3 The use of any preceding claim in monitoring fluid flow in a subterranean reservoir, such as a petroleum reservoir or geothermal reservoir.

4. The use of any preceding claim in at least one tracer method selected from

i) well-to-well tracer studies of petroleum reservoirs;

ii) in-flow monitoring of petroleum wells;

iii) tracer studies in fractures of shale reservoirs;

iv) tracer studies of groundwater flow; and/or

v) tracer studies of geothermal reservoirs.

5. The use of any preceding claim wherein the salt is in a semi-crystalline or crystalline form, preferably crystalline form.

6. The use of any preceding claim wherein the salt has a solubility in water of 10^"1 to 10^" ⁸ g/100ml_ H₂0, preferably of 10^"2 to 10^"5 g/100ml_ H₂0, at 20°C and atmospheric pressure.

7. The use of any preceding claim wherein the metal of said inorganic anionic metal complex is a metal from group 4, 5, 6, 7, 9 or 16 (Via) of the periodic table.

8. The use of any preceding claim wherein the metal of said inorganic anionic metal complex is a metal selected from Se, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Tc, Re, Co, Rh and Ir.

9. The use of any preceding claim wherein the metal of said inorganic anionic metal complex is a group VI or group VII metal, preferably Mo, W or Re.

10. The use of any preceding claim wherein the anionic metal complex is a complex of a metal with at least one X-type ligand, and/or a ligand selected from oxide, halide (e.g.

fluoride, chloride, bromide or iodide), thiocyanate, and/or cyanide, preferably oxide.

receding claim wherein the inorganic anionic metal complex is

12. The use of any preceding claim wherein said cation is at least one inorganic and/or organic cation, preferably comprising at least one alkaline earth cation such as Mg, Ca, Sr, Ba, particularly Sr.

13. The use according to any preceding claim, wherein said cation comprises at least one cation selected from Scandium, Yttrium and a rare earth element of atomic number 57 to 71 , such as Pr, Nd and La.

14. The use of any preceding claim wherein the presence of the anionic metal complex is monitored by qualitative, quantitative or semi-quantitative detection of said anionic metal complex by suitable analytical techniques, such as chromatographic and/or spectrometric techniques, e.g. HPLC and/or MS, especially HPLC and/or ICP-MS.

15. The use of any preceding claim wherein said inorganic anionic metal complex is stable as a solid or in solution (e.g. in the fluid to be monitored) at a temperature of at least 100°C, preferably at least 150°C, more preferably at least 200°C, and even more preferably at least 250°C.

16. The use of any preceding claim wherein the background concentration of inorganic anionic metal complex in the fluid to be monitored is of less than <1 C^g/L.

17. The use according to any preceding claim wherein the metal salts are consolidated into rods or bars.

18. The use according to any preceding claim wherein the at least one metal salt is contained within one or more closed porous containers, such as a bag or closed tube, and wherein the pores in the walls of the container are such that dissolved ions can diffuse in and out but solid particles cannot.

19. The use according to any preceding claim wherein at least two different salts are applied to a reservoir wherein the at least two salts comprise different inorganic anionic metal complexes.

20 The use according to claim 19 wherein each of the two or more salts are placed at a different location, (e.g. within a reservoir).

21. The use according to any preceding claim further comprising relating the absence, presence and/or concentration of the tracer or each tracer to the flow of at least one fluid (e.g. within a reservoir).

22. The use according to any preceding claim for monitoring fluid flow within a fracture in a reservoir, preferably a petroleum reservoir.

23. The use according to any preceding claim wherein the salt is injected into a fracture as a solution, suspension and/or slurry.

24. The use of claim 23 wherein the salt is injected into a fracture as a slurry with proppant particles.

25. The use according to any preceding claim wherein the release rate of the tracer and the flow rate of the water are correlated such that the detection of the tracer enables the flow rate of the water to be estimated.

26 The use according to claim 25 for use in in-flow monitoring, wherein the said water is formation water.

27 The use according to claim 25 for use in well-to-well tracer studies, wherein the said water is formation water and/or injected water.

28 The use according to claim 25 or claim 26 wherein at least two different tracers are used respectively in at least two zones and detection of said at least two tracers is used to determine the relative and/or absolute in-flow of water at each zone.

29. A method for tracking the flow of at least one fluid, said method comprising applying at least one salt comprising at least one organic or inorganic cation and at least one inorganic anionic metal complex to at least one known position along the flow of said fluid and monitoring for the absence/presence and/or concentration of said inorganic anionic metal complex in samples of said at least one fluid.

30 The method of claim 29 wherein said flow of at least one fluid is within ground water, within a geothermal reservoir and/or within a reservoir for petroleum and/or gas production.

31 The method of claims 29 or 30 wherein said metal salt is as defined in any of claims 1 to 25.

32. A method for monitoring the integrity of an isolation plug in a petroleum reservoir or geothermal reservoir, said method comprising applying at least one salt comprising at least one organic or inorganic cation and at least one inorganic anionic metal complex upstream of said isolation plug and monitoring for the presence of said inorganic anionic metal complex in produced fluid from said reservoir.

33. The method of claim 32 wherein the presence of the tracer in the produced fluids is taken to indicate a leakage from said plug.

34. A composition comprising a slurry of at least one salt and proppant particles in water, wherein said salt comprises at least one organic or inorganic cation and at least one inorganic anionic metal complex

35 The composition of claim 29 wherein said proppant particles are selected from sand, resin-coated sand, bauxite, ceramics or mixtures thereof.

36 The composition of claim 29 or 30 further comprising at least one chemical commonly used in fracturing fluids, such as a gel, foam, compressed gas, cleaning agent (e.g. acids), sodium chloride, a friction reducer (e.g. polyacrylamide), a scale deposit preventing agent, a viscosity modifiers/stabilizer, a compound for maintaining effectiveness of crosslinkers (e.g. carbonates), a disinfectant, a gelling agent, and anticorrosion agent and/or a winterizing agent.

37. A container containing a salt, wherein the container is closed and porous, and wherein the pores in the walls of the container are such that dissolved ions can diffuse in and out but solid particles cannot, wherein said salt comprises at least one organic or inorganic cation and at least one inorganic anionic metal complex.

38. The container of claim 37 which is a polymeric, ceramic or metal-sintered container.

39. A process for producing a salt by means of mixing solutions of two salt precursors and forming the product by crystallisation from solution, wherein said salt comprises at least one organic or inorganic cation and at least one inorganic anionic metal complex.

40. An aggregate comprising a plurality of consolidated salt crystals, wherein in at least one dimension said aggregate has a size of at least 5 cm, and wherein said salt comprises at least one organic or inorganic cation and at least one inorganic anionic metal complex.