Title: Chemical tracers for use in high salinity and/or high temperature environments

Technical Field

[0001] This disclosure pertains to the petrochemical field and more particularly to the use of chemical tracers to evaluate properties of hydrocarbon production sites.

Background Art

[0002] Hydrocarbon reservoirs are rock formations containing fluid media including oil and water. Wells are drilled to have access to the reservoir and extract oil. It is important to evaluate the condition of the reservoir, for example to obtain information about the amount and the distribution of the oil in the reservoir or about physical properties of the rock formations that are relevant to the extraction of the oil.

[0003] Chemical tracers are used for this kind of evaluation. There exist different applications of such tracers.

[0004] One such application is known as the Single Well Tracer Test (SWTT). It aims to measure the quantity of oil in a hydrocarbon production reservoir. It is particularly used in cases where the oil is almost stationary, i.e. the well produces a lot of water. For example, the production flow consists of at least 90% water. This indicates that there is not much mobile oil around the well. The objective, in such conditions, of the SWTT method is to find out how much immobile oil is trapped near a producing well.

[0005] In the SWTT application, a tracer is injected in the hydrocarbon production well. The injected tracer travels into the rock formations to a certain distance from the hydrocarbon production well while this tracer partitions between oil and water. These partitioning properties will depend on the quality of the oil and the quality of the water, more particularly the salinity of the water and the general conditions of temperature of the environment.

[0006] After a while, the injection of the tracer is stopped, and a dedicated waiting time is allocated. This tracer has the property to hydrolyze in an aqueous phase. The control of the hydrolysis rate of the tracer is important. This hydrolysis rate depends on salinity and temperature. This tracer only hydrolyzes in the aqueous phase. The waiting time is designed to yield a partial hydrolysis of the tracer. [0007] Then, the hydrocarbon well is switched to production. A secondary tracer, i.e. the one resulting from hydrolysis, arrives first at the outlet because this secondary tracer does not partition with the oil. This secondary tracer returns directly at the same time as the water. The primary tracer comes in a second stage due to its partitioning with oil.

[0008] The test is similar to a chromatography. In chromatography theory, the retention time depends on the chromatography column quality which is controlled through the type of substrate. For instance this stationary phase can be made of silica, and its quantity. In the SWTT, the difference in retention time between the two tracers is measured. The partition coefficient with oil is also known. In comparison to the chromatography method, the oil acts as the stationary phase. The quantity of stationary phase, i.e. the quantity of oil, is unknown.

[0009] In an extreme case, if there was no oil in the hydrocarbon production well, both tracers, the primary and secondary one, would arrive at the same time. On the other hand, if there was a lot of oil, the primary tracer would partition with oil for a longer time and this primary tracer would be even more delayed. Thus, from the difference of retention time between the primary and secondary tracers, information on the quantity of oil in the hydrocarbon production well can be obtained.

[0010] These tests are conventionally done with esters as tracers, the most commonly used being ethyl acetate. Ethyl acetate is injected and partially hydrolyzed into ethanol. When the hydrocarbon well is put into production, ethanol, which is totally miscible with water, does not partition to the oil phase and arrives ahead of the ester. The latter is slowed down by oil partitioning.

[0011] In practice, the tracers need to be injected away from the hydrocarbon production well. Indeed, the oil saturation close to the hydrocarbon production well is usually less representative of the overall reservoir saturation. For the accuracy of this method, it is preferred that the tracer hydrolyses as little as possible during transit times. More precisely, most of the hydrolysis should occur during the shut-in phase of the well.

[0012] However, when the temperature is high, typically above 80 to 100°C, the hydrolysis rate of ethyl acetate becomes significative. When ethyl acetate is injected, it hydrolyzes rapidly even during the injection phase. Thus, a significant part of the secondary tracer will have hydrolyzed before it has even reached the targeted region. When the hydrocarbon well is put into production, it will have no more primary tracer.

[0013] On top of that, the partition coefficient between water and oil of the primary tracer should be within a given range. Indeed, if the primary tracer has very little affinity with the oil, the oil content will have no effect. On the other hand, if the primary tracer has a large affinity with the oil, the primary tracer will remain stuck close to the hydrocarbon production well and will not be able to travel in the reservoir. There is an optimum on the partition coefficient value. The partition coefficient depends on temperature and salinity. When temperature and salinity increase, the partition coefficient increases dramatically. Therefore, the tracers are not able to investigate very far from the well. A balance is needed between where the tracer must be placed and the time of the measurement, taking into account the hydrolysis constants. Considering the existing method using esters, as soon as the temperature exceeds 80°C and the salinity exceeds 100,000 ppm, the method becomes inefficient. The quality of the result will be dramatically lowered due to the under-optimal tracer parameters.

[0014] In a report from the US Department of Energy (DOE), “The single well chemical tracer method for measuring residual oil saturation”, October 1980, H.A. Deans, et al. discuss how to reach an optimal design and gives ranges of partition coefficients.

[0015] Another application of chemical tracers in oil production is referred to herein as the Single Well Acquisition of Relative Mobilities (SWARM) method. It aims to measure relative permeabilities of oil and water from a hydrocarbon production reservoir, at the scale of this hydrocarbon production reservoir.

[0016] Flows in porous media are governed by Darcy's law: at a given permeability, by imposing a pressure drop, the flow of water is determined. Depending on the saturation in a rock formation of oil and water, the mobility of water and oil will not be the same because the fluids are organized in a particular way in a porous medium. Obtaining relative permeabilities is useful since it allows to calculate the quantity of oil that will be recovered from a field as a function of time, by injecting water.

[0017] Usually, relative permeabilities are evaluated in a petrophysical laboratory using core flooding tests. In the SWARM application, tracers are used to make these measurements directly on the field. This method gives a representative result at the scale of the operations. With cores, a microscopic scale is reached whereas with tracers a macroscopic scale is obtained.

[0018] In SWARM applications, unlike in SWTT applications, the hydrocarbon wells concerned are generally those that produce mostly oil. When water is injected into a hydrocarbon production well, the oil spreads but not uniformly. Thus, near the hydrocarbon well, there is some residual oil that remains trapped. The further away from the well, the more the oil saturation is found. This phenomenon is the result of relative permeabilities. If this profile is known, the relative permeabilities will be known. [0019] In the SWARM application, a plurality of tracers is chosen with different partition coefficients, for example three tracers with, respectively:

- a high partition coefficient: the tracer partitions very strongly with the oil: the tracer will tend to remain close to the hydrocarbon well;

- a low partition coefficient: this tracer will go far into the hydrocarbon reservoir because it will only be minorly slowed down;

- an intermediate partition coefficient: the tracer will be positioned at an intermediate distance.

[0020] Thus, in the oil saturation profile having a low saturation value close to the well and gradually a larger oil saturation, the three tracers are placed in a zone of low oil saturation, a zone of medium oil saturation and a zone of large oil saturation. However, the level of these oil saturations is not known. Each tracer will generate a secondary tracer after hydrolysis. When the hydrocarbon well is put into production, the oil is pushed back on the other side and all the tracers are going to be produced back at the well.

[0021] The method for analyzing data linked to the tracers and obtaining fractional flow and relative mobilities is classically known as described in the US patent US3990298 and the article from Society of Petroleum Engineers entitled “Single well chemical tracer tests provide rapid, in-situ saturation and fractional flow measurements in the Aurora oil field, Alaska” (SPE-190122) which is a proceeding from a conference given by F. Paskvan et al. in California in April 2018.

[0022] The SWARM method works as long as the partition coefficients of the tracers are under control, i.e. with a high, medium and small partition coefficient. However, it becomes very difficult in high salinity, high temperature conditions to have a tracer with a small partition coefficient.

[0023] Similarly to the problem encountered with the SWTT application, hydrolysis will occur faster and faster when salinity and/or temperature increases. Consequently, all the tracers and particularly the one with small partition coefficient could have completely hydrolyzed too early. The SWARM method then becomes inaccurate.

[0024] There is therefore a need for efficient chemical tracers in various applications when high salinity and/or high temperature media are tested in oil reservoirs.

[0025] WO 2014/090823 A1 discloses carbonate and glycol ester compounds for use as tracers for SWTT applications with high salinity and/or temperature conditions. However, carbonates tend to hydrolyze too fast, making them unsuitable, while glycol ethers may lack reactivity (low kH). Therefore, other tracers are needed. Summary

[0026] This disclosure proposes a use of a cyclic acetal compound for injection as a tracer in a production well, to evaluate properties of a hydrocarbon reservoir in which the production well is drilled, wherein the cyclic acetal compound comprises a cycle having two oxygen atoms linked to a carbon atom which is part of a CH ₂ or alkyl group.

[0027] Thanks to this cyclic acetal compound, it is possible to operate in water salinities above 100000 ppm and at temperatures between 25°C and 150°C, preferably above 80°C.

[0028] The two oxygen atoms of the cycle may be linked to a CH ₂ group.

[0029] The cyclic acetal compound may be a dioxolane or substituted dioxolane compound.

[0030] The substituted dioxolane is stable and the hydrolysis speed is moderated.

[0031] The cyclic acetal compound may comprise at least one of 1 ,3-dioxolane and 4- methyl-1 ,3-dioxolane.

[0032] The cyclic acetal compound may be a dioxane or substituted dioxane compound.

[0033] The cyclic acetal compound may comprise at least one of 4-methyl-1 ,3-dioxane and 1 ,3-dioxane.

[0034] The cyclic acetal compound may be injected in combination with other compounds which include one or more molecules chosen in the group consisting of glycol ethers, acetals and esters.

[0035] The cyclic acetal compound may be injected in combination of at least two other additional compounds, each compound determining one of three partitioning coefficients between oil and water in at least three ranges comprised between 0.5 and 2.5, 2.5 and 6.5, 6.5 and 15.

[0036] The additional compounds may comprise at least two molecules chosen in the group consisting of tertiobutyl ether of ethylene glycol and tertiobutyl ether of propylene glycol.

[0037] The additional compounds may consist of methyl 2,2-dimethyl-3- hydroxypropionate, methyl 3-methoxyisobutyrate for a production well below 80°C and a hydrocarbon reservoir brine above lOOOOOppm.

[0038] The hydrocarbon reservoir temperature may be between 25°C and 150°C, and a hydrocarbon reservoir brine having a salinity above 100000 ppm. [0039] The hydrocarbon reservoir temperature may be above 80°C and a hydrocarbon reservoir brine having a salinity above 100000 ppm.

Description of Embodiments

[0040] In chemical tracer applications such as SWTT and SWARM, a partition coefficient between water and oil depends on salinity. As previously explained, this makes conventional esters unusable at high salinity, i.e. above 100000 ppm.

[0041] With the use of compounds of the cyclic acetal family as tracers, it is possible to operate in water salinities above 100000 ppm and at temperatures between 25°C and 150°C, preferably above 80°C.

[0042] Acetal compounds have a formula:

[0043] The compound is cyclic when the Ri and R ₂ radicals are linked together. ‘A’ is a CH ₂ or alkyl group with the constraint of having two oxygen atoms linked to the same carbon atom which is part of the CH ₂ or alkyl group.

[0044] A cyclic acetal compound thus comprises a cycle having two oxygen atoms linked to a CH ₂ or alkyl group. This cyclic acetal compound comprises a substituent R-i and a substituent R ₂ comprising alkyl groups or cyclic alkyl including at least 2 carbons, preferably between 2 to 4 carbons.

[0045] Such compounds are advantageously used in applications including SWTT, described in details in the above-cited DOE report “The single well chemical tracer method for measuring residual oil saturation”, and SWARM which is disclosed, for example, in the US patent US3990298 and the article from Society of Petroleum Engineers entitled “Single well chemical tracer tests provide rapid, in-situ saturation and fractional flow measurements in the Aurora oil field, Alaskd’ (SPE-190122), conference in California in April 2018, F. Paskvan et al.

[0046] Examples of cyclic acetal compound that are suitable for the above-mentioned applications include dioxolanes, substituted dioxolanes, dioxanes and substituted dioxanes. More particularly 1 ,3-dioxolane, 4-methyl-1 ,3-dioxolane, 4-methyl-1 ,3-dioxane and 1 ,3-dioxane are appropriate, as well as mixtures thereof.

[0047] For lower temperatures applications (below 80°C), the carbon between the two oxygens in cyclic acetal may be substituted. The substitution promotes an electrondonating inductive effect which facilitates the hydrolysis, through carboxonium intermediate stabilization. However when temperature increases, the hydrolysis speed increases substantially for the substituted derivatives. Consequently, for higher temperature applications, cyclic acetals may not be substituted in order to controls the hydrolysis rate. To summarize, for high temperature applications, the carbon between the two oxygens may be unsubstituted (presence of CH ₂ ) because the more this position is substituted, the more unstable the tracer becomes and the easier the hydrolysis occurs.

[0048] In a SWARM application, the cyclic acetal compound may be completed by using other compounds including one or more molecules chosen in the group comprising a glycol ether compound, an ester.

[0049] By modifying esters used in the prior art, a resistance against high water salinity can be given. These new esters can resist at a hydrocarbon reservoir brine having a salinity above 100000 ppm. It comprises molecules such as methyl 2,2-dimethyl-3- hydroxypropionate, methyl 3-methoxyisobutyrate for the hydrocarbon production well below 80°C and the hydrocarbon reservoir brine having a salinity above lOOOOOppm.

[0050] Without being bound by theory, a possible explanation on the reasons why these esters compounds methyl 2,2-dimethyl-3-hydroxypropionate and methyl 3- methoxyisobutyrate are suitable as chemical tracers in applications like SWTT and SWARM may be that, by adding an electro-donor group in alpha carbonyl position and by adding an oxygen atom, the electro-donor group provides stability regarding hydrolysis and a additional oxygen causes the partition coefficient between oil and water to decrease.

[0051] The esters compounds methyl 2,2-dimethyl-3-hydroxypropionate and methyl 3- methoxyisobutyrate may be used alone or in combination with cyclic acetals and/or glycol ether.

[0052] The glycol ether compound comprises tertio-butyl ether of ethylene glycol and/or tertiobutyl ether of propylene glycol.

[0053] Indeed, tertio-butyl ether of ethylene hydrolyses by breaking a C-0 bond to make tert-butanol and ethylene glycol. It is very resistant at high temperature, preferably above 80°C, because it hydrolyses slowly.

[0054] For a given crude oil, experimental results at 127°C and with a salinity of 200,000ppm are given in the following tables:

[0055] In these conditions, the partition coefficient and the hydrolysis rate are lower for 1 ,3-dioxolane and 4-methyl-1 ,3-dioxolane than ethyl acetate. It confirms the observations given above.

[0056] It will be appreciated that the embodiments described above are illustrative of the invention disclosed herein and that various modifications can be made without departing from the scope as defined in the appended claims.