FIELD OF THE INVENTION

The present invention relates to the measurement of pH in petroleum reservoirs. In particular, the present invention relates to methods for making such pH measurements and tracers suitable for use in such methods.

BACKGROUND OF THE INVENTION

Institute for Energy Technology in Norway (IFE) has, since the nineteen sixties, worked with development of tracer technology for industrial applications. Since the beginning of the nineteen eighties the focus has been on the oil and gas industry. Many passive inter-well (well-to-well) tracers have been tested and qualified and in recent years some families of partitioning tracers have also been tested in laboratory and field experiments [1].

Partitioning tracers are simultaneously injected with a passive (non-partitioning) tracer as a pulse in partitioning inter-well tracer tests (PITT). Due to the solubility of the partitioning tracers in the oil phase, these tracers will move more slowly through the reservoir than the non-retained passive tracer. When the oil/water partition coefficient for the partitioning tracer is known, the residual oil saturation can be calculated when the difference in migration times for the passive and the partitioning tracers have been measured.

The Partitioning Inter-well Tracer Test (PITT) technology has potential to become a standard method for identifying enhanced oil recovery (EOR) targets, and for evaluation of performance of EOR operations. PITTs have successfully been applied to estimate nonaqueous phase liquid contamination in the context of groundwater studies, as well as in some oil fields producing at marginal oil rates. The Partitioning Inter-well Tracers Tests to determine residual oil saturation is based on chromatographic separation of tracers in the reservoir [2], [3], [4]. Passive tracers (soluble only in water) and tracers with different oil/water partition coefficients are introduced with injection water, and samples of water are collected from the production stream for analysis. The tracers will move through the reservoir at different velocities depending on the partition coefficients and the oil saturation in the volume between injection and production wells. The oil saturation for a field with negligible oil flow rates compared to the water flow rates (a field close to residual oil saturation) can be described by chromatographic theory and calculated from the following equation: ^T R + ^T R ( ^K l) (equation 1 )

Here T _R and T _R are the retention times of the partitioning and passive water tracer, respectively, S is the residual oil saturation, and K is the partition coefficient of the partitioning tracer. K is defined by the equation below:

where

P ^" r]org, eq - the concentration of the tracer compound in the oil phase at distribution

equilibrium

P ^" r]aq, eq - the concentration of the tracer compound in the aqueous phase at distribution equilibrium

If the partition coefficient (K) is known, the residual oil saturation can be calculated from the measured difference in the arrival times between a non-partitioning (passive) and a partitioning tracer. This equation is only valid as long as the tracers do not interact with the rock material. If other factors such as interactions or non-ideal behaviour are known then they can potentially be corrected for by theory and/or testing.

It is also possible to measure oil saturation using two pH independent partitioning tracers, as shown in equation 2,

C—

Ά T ₂ Τ _λ Κ ₂ + Τ ₂ Κ _λ (equation 2) - see [5]

Here ΤΊ and T ₂ are the retention times of the pH independent partitioning tracers,

respectively, S is the residual oil saturation, and K-i and K ₂ are the partition coefficient of the pH independent partitioning tracers .respectively,

Although the Partitioning Inter-well Tracer Test (PITT) is potentially the most effective method for the assessment of residual oil saturation, there has previously been no method by which other information about the conditions of a petroleum reservoir, such as pH conditions, could be conveniently assessed, either independently or as part of such a test.

Since factors such as pH provide valuable information on conditions in a petroleum reservoir, it would be of considerable value to provide a method by which pH could be assessed, either simultaneously with a PITT, or as a follow-up in reservoirs where residual oil saturation is known. Information on pH in the reservoir is very valuable in order to better select appropriate chemicals and their optimal quantity for use in the reservoir environment, as well as the type of reservoir treatment in Enhanced Oil Recovery (EOR) processes.

SUMMARY OF INVENTION

The present inventor has now established that by use of tracers with pH dependent partitioning coefficients (K values) in combination with knowledge of residual oil saturation, a useful measurement of the pH of a subterranean petroleum reservoir can be made.

Furthermore, the inventor has established a family of tracer molecules which provide many of the desirable characteristics of petroleum reservoir tracers but have partitioning coefficients which are dependent upon pH in an appropriate range.

In a first aspect, the present invention therefore provides a method for assessing the pH of a subterranean petroleum reservoir having an injection well and a production well, said method comprising: a) determining the residual oil saturation (S) of said petroleum reservoir; b) injecting at least a first tracer having a first, pH dependent, partition coefficient and a second tracer having a second, pH independent, partition coefficient into said injection well; c) measuring the presence and/or concentration over time of said first tracer and said

second tracer in produced water from said production well; d) determining the retention times for each of said first tracer and said second tracer; e) relating the residual oil saturation and the retention times and partition coefficients of each of said first and second tracers to the pH of the petroleum reservoir; wherein step a) is conducted prior step e). The residual oil saturation is required for

determining pH in the method of the invention but may be calculated at any stage. For example, it may be pre-known or pre-measured, or it may be determined as part of the method of the invention. In the latter case, S may be determined simultaneously with any of steps b) to d).

In particular, it is preferable that the "second" tracer will be a "passive" water tracer having a very low partition coefficient, such as 0.1 or below. Use of such a passive tracer as the second tracer allows for the effective partition coefficient of the first (pH dependent) tracer under the conditions of the reservoir to be calculated. Such a calculation can be made, for example, by means of equation 3 below: (equation 3) wherein S is the residual oil saturation, T _R ^P and T _R are the retention times of the pH dependent partitioning and passive tracer, respectively and K _pH is the partition coefficient of the pH dependent tracer.

In analogy with equation 2 above, K _pH can also be determined using one pH-dependent tracer and one pH-independent tracer even if the latter is not a "passive" water tracer with very low K value. A generalisation of equation 3 to account for the partitioning of the pH- independent tracer will then be derived, in the way that equation 2 above is a generalisation of equation 1 .

Knowledge of the K _pH value then allows establishment of the pH under reservoir conditions by known methods, such as laboratory testing of K _pH variation with pH. Thus, by additionally determining the variation of K _pH with pH, the pH conditions corresponding to the K _pH calculated from equation 3 can be established. In general, the pH determined in the various aspects of the present invention will be the average pH to which the tracers are exposed. Thus, where pH of a petroleum reservoir is referred to herein, this will be the average pH of the volume to which a tracer is exposed. This may be the average pH over the flow between an injection point and a production point.

It is also possible to determine the pH in the reservoir using two pH-dependent tracers (providing these have pKa values which are different by at least 0.5 pH units). For this method, S should be known and the variation of K-value ratio between the two pH dependant tracers with varying pH should be known.

The methods of the invention are applied primarily to a petroleum reservoir having an injection well and a production well, although certain other embodiments are also possible (see below). It is preferred that the injection well and the production well are separate wells, although it is possible to conduct the method of the invention where the injection well and the production well are a single well, running with opposite flow directions for the two functions. In such a situation, the well will be run to inject the tracers (preferably in a form that generates a pH dependent tracer in-situ ) there may then be a pause to allow for generation of a pH-dependent tracer (e.g. from a pH-independent form or precursor such as a esterified form). The well would then be run in production mode and the various tracers measured in the produced fluid. An "injection well" as used herein will be any well or bore into the reservoir environment which is capable of functioning as an injection well. Thus a "injection well" as indicated herein could be a dedicated injection well or could be a test well, production well or other bore functioning to inject the tracers utilised in any of the

embodiments of the invention.

The present inventors have further established that certain substituted phenols are highly effective as pH dependent tracers, having a pKa around the typical pH of a petroleum reservoir. In a further aspect, the present invention therefore provides the use of at least one phenol of formula i) as a pH dependent partitioning tracer in a petroleum reservoir.

wherein each of Ri to R ₅ is independently selected from H, F, CI, Br, I, CF ₃ CF ₂ CI, CFCI ₂ , CCI ₃ , OCH ₃ , OC ₂ H ₅ , CHO, CN and N0 ₂ (particularly H, F, CI, Br, I, CF ₃ CF ₂ CI, CFCI ₂ and CCI ₃ ) and wherein at least one of Ri to R ₅ is not H. Thus, at least one of groups R-i, R ₂ , R ₃ , R ₄ and/or R ₅ is a functional (e.g. halogenated) group such as F, CI, Br, I, CF ₃ CF ₂ CI, CFCI ₂ , CCI ₃ , OCH ₃ , OC ₂ H ₅ , CHO, CN and N0 ₂ . More than one of groups of Ri to R ₅ may be a functional (e.g. halogenated) group, particularly from the listed groups, and any two may be the same or different. Preferred groups Ri to R ₅ include H, CI, F, Br, CF ₃ CF ₂ CI, CFCI ₂ ,CCI ₃ , OCH ₃ and CN (e.g. H, CI, F, Br, CF ₃ CF ₂ CI, CFCI ₂ and CCI ₃ ). Particularly preferred groups Ri to R ₅ include H, F and CI. It is preferred that at least one of groups Ri to R ₅ is hydrogen, preferably at least two of groups Ri to R ₅ are hydrogen.

In one preferred embodiment, at least one of groups Ri to R ₅ is F, CF ₂ CI, CFCI ₂ or CF ₃ and in particular, in one embodiment, 1 , 2 or 3 of groups Ri to R ₅ are F. It is believed that compounds of formula i) wherein at least one of R-i, R ₃ and/or R ₅ is F are highly effective.

In an alternative embodiment, at least one of one of groups Ri to R ₅ may be OCH ₃ , OC ₂ H ₅ , CHO, CN or N0 ₂ and particularly at least one (e.g. 1 , 2 or 3) of groups Ri to R ₅ may be a CN. This may be with or without at least one (e.g. 1 , 2 or 3) F or CI groups at R-i to R ₅ . The total number of non-H substituents at R-i to R ₅ will evidently be not more than 5 and will generally be not more than 4 (i.e. 1 , 2, 3 or 4).

The tracers of the present invention may advantageously be used in combination with other tracers to assess the pH in a petroleum reservoir and described herein. It is preferred that the tracers of the invention are used with "passive" tracers. They may also be used in combination with both passive tracers and non-pH dependent partitioning tracers in order to assess both pH and residual oil saturation in a single test.

BRIEF DESCRIPTION OF THE DRAWINGS AND TABLES shows examples of chemical structures of compounds from the groups:

monofluoro, difluoro, trifluoro, trifluoromethyl, monochloro, dichloro, monomethoxy, mononitrile and mononitro phenols. shows how K-values (measured in North Sea oil and a synthetic formation water) varies with pH for two fluorinated phenols. shows how K-values (measured in North Sea oil and a synthetic formation water) varies with pH for 2-methoxyphenol and 2,6-dimethoxyphenol

Shows the protonated (right) and de-protonated (left) forms of 2-fluorophenol.

Shows the calculated octanol/water Log K of several compounds against pH.

Shows a chromatographic separation and detection of several fluorinated phenols using LC-HRMS. The peaks shown are as follows, 3.23 mins is 2- fluorophenol, 3.42 mins is 2,6-difluorophenol, 3,57 mins is 4-fluorophenol, 3.76 min is 3-fluorophenol, 3.91 mins is 2,4,6-trifluorophenol, 3.99 mins is 2,3,6- trifluorophenol, 4.54 mins is 3,5-difluorophenol and 5.54 mins is 4- (trifluoromethyl)phenol.

Shows examples of pKa values (calculated ) for phenols and benzyl alcohols.

Shows the calculated partition coefficients from static experiments (North sea oil and a synthetic formation water) varies with pH for 9 phenol derivatives.

DETAILED DESCRIPTION Alcohols have been used as partitioning tracers to estimate amounts of non-aqueous phase liquid in porous media and remaining oil in the swept area between wells (e.g. McClesky sandstone field test, Landmark method, Leduc test, Ranger field test [5], [6]). However, all previous uses of partitioning tracers have been with pH independent compounds. Some highly effective benzyl alcohol tracers are disclosed, for example, in WO2014/096459.

These are based upon benzyl alcohols, which are pH independent under the conditions of a typical petroleum reservoir.

The use of passive water tracers and pH dependent tracers to determine the pH in the swept volume between wells (with or without simultaneous use of pH independent oil/water partitioning tracers to determine residual oil saturation) is not known. Furthermore there have been no specific proposals of classes of compounds suitable for such a method. The use of phenols, especially halogenated phenols as pH dependent tracers is thus also unique.

Phenols are suitable choice pH dependent tracers since they generally have pKa's within two pH units of the expected reservoir conditions. Adding a moiety such as a halogen (e.g.

chlorine or especially fluorine) or a CN-group to the phenol ring will reduce the pKa of the phenol tracer. This will generally bring the pKa closer to pH of the reservoir. Adding further halogenated or similar (e.g. fluorine or CN) moieties will further reduced the pKa - see Table 1 . This variation will give a range of tracers with different pKa's suitable for different petroleum reservoirs conditions, e.g. sandstone or limestone. The K values of several of these compounds are also suitable for inter-well tracer tests - Table 2.

In one embodiment applicable to all aspects of the present invention, the pH dependent tracers (the first tracer in the method described above) will show K-values in the system to be investigated of between 0.05 and 15, preferably 0.05 and 10 and mostly preferably 0.1 and 8. In one embodiment, the pH dependent tracers show these ranges of K values at their respective pKa. In an alternative embodiment, the pH dependent tracers show these ranges of K values at pH 7.

In general the pH in a sandstone reservoir will typically be 6.0 +/- 0.5 and for a limestone reservoir around 7.3 +/- 0.5.

Preferably, pH-dependent partitioning tracers should have a pKa within 3 pH units, more preferably within 2 pH units, of the reservoir in which they are to be used. This will result in K-values that will vary significantly with pH in the range of the reservoir. It is the nature of the pH scale that a compound will be mostly protonated (to around 99%) at two pH units under their pKa and at a pH of two pH units above their pKa will be largely de-protonated (again to around 99%). Thus the partitioning into the oil phase, which will be dependent upon charge, will change most markedly at a pH around the pKa of the tracer. Several substituted phenols are ideal for either sandstone or limestone reservoirs or both. Table 1 shows pKa values for the hydroxyl group in a range of substituted phenols, as well as phenol, benzyl alcohol, methoxyphenol, hydroxybenzonitrile and a substituted benzyl alcohol for comparison. Table 2 shows some K-values of halogenated phenols at representative pH conditions.

Table 1. Examples of pKa values (calculated) for phenols and benzyl alcohols.

It can be seen that both fluorinated and chlorinated phenols and hydroxybenzonitriles have pKas in a useful range. In particularly, phenols with one or two fluorine or chlorine moieties are highly appropriate. Substitution with at least one chlorine and/or fluorine at one or more of the 2, 4, and/or 6, -positions provides tracers with highly appropriate pKa values. Thus, the compounds used in all aspects of the invention may have these properties. 2,6- Difluorophenol is especially useful for both sandstone or limestone reservoirs as it has both a relatively low pKa and K-value.

Table 2. Examples of measured partition coefficients, K-values, between a North sea oil and a synthetic formation water at 70°C, and how they vary with pH

pH independent tracers, which may be used in the PITT method to determine residual oil saturation either prior to the methods of the present invention or as a part of such methods, are compounds that do not change their oil/water partitioning properties significantly over the pH region of interest. A change of more than 5% in the oil/water distribution coefficients (k- value) over 0.5 pH unit would be considered significant. The range of interest in petroleum reservoirs is between pH 5.5 and pH 7.8. Figure 5 shows how the Log K-values (calculated in an octanol/water system) vary with pH. It shows that 2,6-difluorobenzyl alcohol (pH independent oil/water partitioning tracer[1 ]) has a stable log K over the pH range of interest, whereas the log K values of 2-fluorophenol and 2,6-difluorophenol vary with pH in the pH range of interest (pH dependent oil/water partitioning tracers). Studies of this type, in combination with equation 2, can be used to determine the average pH conditions of the petroleum reservoir by correlating the K _pH value calculated from equation 2 with pH. This is illustrated in Figure 5.

The oil/water distribution coefficients (K-values) of pH dependent tracers should be between 0.05 and 15 when the pH of the oil/water system is the same as the tracer's pKa, preferably, 0.05 and 10 and ideally 0.1 and 8. The pKa of the pH dependent tracers should be within 3 pH units of the system to be investigated, preferably 2 units and ideally 1 pH unit. The pH ranges of systems of interest are between pH 4 and pH 10, preferably 5 and 9, ideally 5.5 and 7.8.

Correspondingly, the pKa of the pH dependent tracers will generally be between 1 and 13 or between 2 and 12, preferably between 3 and 7 and especially between 2.5 and 10.8, or between 3.5 and 9.8.

It is a considerable advantage if the tracers used in inter-well tracer tests of all types

(including all methods and uses of the invention) are detectable at low concentrations and can be distinguished from compounds naturally present in the petroleum reservoir. However, many phenols are naturally present in oil reservoirs. Furthermore, radiolabeled compounds (which are easy to detect and might historically have been used) should be avoided due to regulatory restrictions in many areas. Halogenated phenols are both unique in the reservoir environment and more chemically and biologically stable than corresponding molecules without halogen atoms. They can, furthermore, be detected to a high sensitivity in the produced fluids (such as produced water) from the reservoir.

Structural formulas of examples of compounds from four groups of fluorinated phenols tested are shown in Figure 1 . The compounds could be analyzed using liquid chromatography with mass spectrometric detection (LC-MS) in synthetic produced water, Figure 6. Detection limits of 50 ng/l (ppt) could be obtained depending on the level of interferences from the sample matrix. Some results from the laboratory tests of these compounds are shown in Table 2.

In one embodiment, the tracers used in all embodiments of the present invention are detectable down to a level of 100ng/l or less, preferably 50ng/l or less and more preferably 20ng/l or less.

Isomers from the two groups of fluorinated phenols have been tested successfully as pH dependent partitioning tracers. Two types of chlorinated phenols have also been tested but the combinations of chlorinated and fluorinated benzyl alcohols may function equally well due to similar chemical properties.

In one aspect, the present invention relates to the use of certain substituted phenols of formula i) as pH-dependent partitioning tracers in a petroleum reservoir, as well as to the corresponding compounds for that use. Compounds of formula i) have the general formula: wherein each of Ri to R ₅ is independently selected from H, F, CI, Br, I, CF ₃ CF ₂ CI, CFCI ₂ , CCI ₃ , OCH ₃ , OC ₂ H ₅ , OCH, CN and N0 ₂ (particularly H, F, CI, Br, I, CF ₃ CF ₂ CI, CFCI ₂ and CCI ₃ ) and wherein at least one of Ri to R ₅ is not H. Preferred R groups include those indicated herein.

Particular examples of compounds of formula i) which are suitable for use in all aspects of the present invention include at least one fluorinated phenol of formulae F1 to F26.

wherein each R group is independently selected from H, CI, Br, I, CF ₂ CI, CFCI ₂ and CCI ₃ . Preferably each R group is independently selected from H and CI. In one embodiment, all R groups in formulae F1 to F26, are hydrogen. In one embodiment 1 , 2 or 3 R groups of any of formulae F1 to F26, are CI. The remaining R groups may be any specified herein but will preferably be H.

Further particular examples of compounds of formula i) which are suitable for use in all aspects of the present invention include at least one chlorinated phenol of formulae CM to CI26:

wherein each R group is independently selected from H, F, Br, I, CF ₂ CI, CFCI ₂ and CCI ₃ . Preferably each R group is independently selected from H and F. In one embodiment, all R groups in formulae CM to CI26, are hydrogen. In another embodiment 1 , 2 or 3 R groups of any of formulae CM to CI26, are F. The remaining R groups may be any specified herein but will preferably be H.

Further particular examples of compounds of formula i) which are suitable for use in all aspects of the present invention include at least one of the following chlorinated fluorinated phenols;

In one preferred embodiment of the invention, the compounds of formula i) which are suitable for use in all aspects of the present invention are the compounds shown in Figure 1.

In a further, highly effective embodiment compatible with all aspects of the invention, the phenols is at least one selected 2-Fluorophenol, 3-Fluorophenol, 4-Fluorophenol, 2,4- Difluorophenol, 2,6-Difluorophenol, 3,5-Difluorophenol, 2,4,6-Trifluorophenol, 2- Chlorophenol, 3- Chlorophenol, 4- Chlorophenol, 2,4-Dichlorophenol, 2,6- Dichlorophenol, 3,5- Dichlorophenol and 2,4,6-Trichlorophenol.

The halogenated phenols for use in the various aspects of the present invention are typically highly stable in aqueous solution and such stability is a considerable advantage since degradation reduces the concentration of tracer available for detection.

Preferably, the compounds of formula i) (and the preferred compounds as indicated herein) are stable in water at concentration levels typical in water samples from oil reservoirs (typical concentration level is 50 ppt to 100 ppb) for at least 4 weeks at reservoir temperatures.

Preferably such compounds are stable for at least 6 weeks, preferably at least 8 weeks under such conditions. Preferably, this stability will be exhibited at temperatures of at least 80°C, more preferably at least 100°C, most preferably at temperatures of at least 150°C. "Stable" in this context may be taken as having a concentration of tracer compound within 20% of the starting concentration as measured by GC-MS, more preferably within 10%.

A further feature of the compounds used in the various aspects of the present invention is their high detectability. Specifically, the compounds of formula i) (and the preferred compounds as indicated herein) are preferably detectable by GC-MS down to a

concentration of 500 ppt (parts per trillion) or lower. Preferably this detection limit will be 100 ppt or lower, more preferably 50 ppt or lower. It is possible for the detection limit to be still lower, such as 1 ppt or 100 ppq.

A still further important feature of the compounds used in the various aspects of the present invention is their relatively low environmental impact. Specifically, the compounds of formula i) (and the preferred compounds as indicated herein) may be classified as "red" or better (e.g. "red" or "yellow") according to the HOCNF (Harmonized Offshore Chemical Notification Format for chemicals released to the North Sea) testing criteria. Those tracers of the invention not containing halogens (e.g. those containing only substituents (Ri to R ₅ ) selected from H, OCH ₃ , OC2H ₅ , CHO, CN and N0 ₂ ) may be preferred. This is particularly where a low environmental impact is desirable since such compounds are expected to have a particularly low impact. A yet further feature of the compounds used in the various aspects of the present invention is their low reaction with and sorption onto materials typically found in oil fields such as rock, particularly limestone and/or sandstone. Specifically, the compounds of formula i) (and the preferred compounds as indicated herein) will typically be stable in the presence of sandstone and/or limestone for at least a month, more preferably at least two months under aqueous conditions at temperatures corresponding to oil reservoir temperatures. Preferably, this stability will be exhibited at temperatures of at least 80°C, more preferably at least 100°C, most preferably at temperatures of at least 150°C. "Stable" in this context may be taken as having a concentration of tracer compound within 20% of the starting concentration as measured by GC-MS, more preferably within 10%.

In the various methods of the invention, as disclosed herein, any tracer molecule that satisfies the requirements of a tracer and has a pKa/pKb around the pH of the reservoir may be used. Thus, any molecule which is detectable (preferably as discussed herein) and has a suitable ionisable group (such as a protonatable or de-protonatable group) with a suitable pKa/pKb (as discussed herein) may be used. Alcohols, particularly phenols and other aromatic alcohols are highly suitable, as may be certain organic acids or amines.

Preferable pH-dependent tracers for use in any of the method embodiments of the invention will be compounds of formula i) as described herein, particularly any of the preferred compounds.

In the various methods, first tracer will be a "pH-dependent partitioning tracer". This will typically have any of the features described herein for such tracers and preferred tracers. In particular, this may be a tracer of formula i) or any of the preferred disclosures herein. The second tracer will be a non-pH dependent tracer. This can have any known K-value but will preferably be effectively a "passive" tracer, having a low K value. For example, a passive tracer may have a K value below 0.01 , such as 10 ^"8 to 0.005.

In one embodiment a "passive" tracer may have a partition coefficient of less than 10 ^"2 , e.g. less than 10 ^"3 , such as less than 10 ^"4 , less than 10 ^"5 or less than 10 ^"6 (between seawater and oil at 80°C)\

Where the residual oil saturation, S, is known, only the first and second tracers are required for the functioning of the method of the present invention. Conveniently, however, this saturation may be measured simultaneously with the pH measurement. This has the advantage of determining two parameters by injection of only three tracers, optionally as a single injection. Furthermore, it means that the various tracers are exposed to the same sweep volume between the injection well and the production well, thereby providing a more reliable pH measurement.

Where a partitioning tracer (non-pH dependent) is also used or generally where the residual oil saturation is measured as a part of the method of the present invention, a third tracer will be used. This is compared with the second tracer to derive the saturation (S). Suitable methods for this are disclosed, for example in WO2014/096459, the disclosure of which is hereby incorporated by reference.

Methods of the invention where residual oil saturation are determined as part of the method may be carried out as follows: i) injecting at least a first tracer having a, pH dependent, first partition coefficient, a second tracer having a, pH independent, second partition coefficient (typically of less than 0.1 ) and a third tracer having a pH independent, third partition coefficient (typically of at least 0.25) into said injection well; ii) measuring the presence and/or concentration over time of said first tracer, said second tracer and said third tracer in produced water from said production well; iii) determining the retention times for each of said first tracer, said second tracer and said third tracer; iv) relating the retention times and partition coefficients of each of said second and third tracers to oil saturation of reservoir; v) relating the residual oil saturation and the retention times and partition coefficients of each of said first and second tracers to the pH of the petroleum reservoir.

Where the second tracer is a passive tracer and the third tracer is a partitioning tracer then equation 1 above may be used in step iv) to relate the retention times and partition coefficients of each of said second and third tracers to oil saturation of reservoir.

Where the second and third tracers are partitioning tracers (independent of pH) then equation 2 above may be used in step iv) to relate the retention times and partition coefficients of each of said second and third tracers to oil saturation of reservoir

Where the second tracer is a passive tracer and the residual oil saturation has been determined in step iv) then equation 3 above may be used in step v) to relate the residual oil saturation and the retention times and partition coefficients of each of said first and second tracers to the pH of the petroleum reservoir. Where one passive tracer and more than one partitioning tracer and/or more than one pH- dependent tracer is used then equations 1 and 2 may be used two or more times as appropriate, or a general equation developed.

Where a non-pH dependent partitioning tracer (e.g. third tracer) is used in any aspect or embodiment of the present invention then this may be a benzyl alcohol of formula B1 :

wherein each of Ri to R ₅ is independently selected from H, F, CI, Br, I, CF ₃ CF ₂ CI, CFCI ₂ , CCI ₃ , OCH ₃ , OC ₂ H ₅ , CHO, CN and N0 ₂ (particularly H, F, CI, Br, I, CF ₃ CF ₂ CI, CFCI ₂ and CCI ₃ ) and wherein at least one of Ri to R ₅ is not H.

Although described herein as "first", "second" and in some embodiments "third" tracers (as well as subsequent tracers) for linguistic clarity, the order of injection of the tracers need not be according to this nomenclature. For example, in one embodiment, all tracers may be injected simultaneously. Alternatively, the first, second and optionally third tracers may be injected sequentially in any order or combination. Where more than two tracers are used, any two or more may be injected simultaneously. In one embodiment, the first tracer injected is the tracer with lowest partition coefficient (e.g. a "passive" tracer).

Although referred to herein as separate tracers, any two or more tracers described herein may be injected simultaneously. Furthermore, any two or more such tracers may be formulated as a single molecule that may be cleaved into separate tracers under the conditions of the injection or reservoir. It is preferred that each tracer is formulated as a separate chemical entity but such generation "in situ" may take place and could be rendered more effectively by halting production from the reservoir for a period after injection of such a tracer precursor to allow the cleavage reaction(s) to occur.

The methods and uses of the present invention have been presented in the context of a petroleum reservoir, and this forms the primary use of such methods, uses and compounds. However, such a method may also be used in any situation where two phases, particularly a mobile aqueous phase and a comparatively stationary organic phase exist. Groundwater reservoirs contaminated with hydrocarbons, for example, form a further situation where the methods of the invention could be used analogously. Similarly, the methods and uses may be applicable to the flow of water through coal-bearing rocks and deposits. Such methods and uses evidently form further aspects of the present invention and all of the disclosures and preferred disclosures made herein apply equally to these.

REFERENCES:

1 . Dugstad 0yvind et al., TRACERS, NO20121558, 2014-06-23 and WO2014/096459

2. Cooke, C.E.J., Method of Determining Fluid Saturation in Reservoirs, 1971 .

3. Jin, M., et al, Partitioning tracer test for detection, estimation and remediation performance assessment of subsurface nonaqueous phase liquids, Water resources research, 1995. 31 (5): p. 1201 -121 1 .

4. Deans, H.H., Using chemical tracers to measure fractional flow and saturation in-situ, in Fitlh Symposium on Improved Methods for Oil Recovery of the Sociely of Petroleum Engineers of AIME held In Tulsa, Oklahoma, April 16-19, 1978.1978, SPE: Tulsa,

Oklahoma,.

5. Lichtenberger, G.J., Field Applications of Interwell Tracers for Reservoir

Characterization of Enhanced Oil Recovery Pilot Areas, in SPE Production Operations Symposium1991 , Society of Petroleum Engineers: Oklahoma City, Oklahoma.

6. Zemel, B., Tracers in Oil Field 1994, New York: Elsevier.