OF A WELL

Field

The present specification relates to formulations containing tracer compounds, to their use in monitoring the efficacy of acid stimulation in a subterranean reservoir or formation, to methods of fabricating such formulations, and to methods of monitoring a flow of fluid using such formulations during or after acid stimulation of a subterranean reservoir or formation.

Background

The use of tracers to monitor aspects of the performance of hydrocarbon wells is an established technique. The tracers may be water tracers, in that they are predominantly soluble or dispersible in water, oil tracers, in that they are predominantly soluble or dispersible in the hydrocarbons, or partitioning tracers, in that they are soluble or dispersible between both the water and hydrocarbon phases.

Some tracing methods will employ more than one type of tracer and use the difference in behaviour to deduce properties of the hydrocarbon formation. For example, partitioning and water tracers may be injected into a production well along with injected water and then monitored as they are subsequently produced from the well. The time difference between the production of the water tracers, which are produced with the returning injected water, and the partitioning tracers, whose production is delayed by their interaction with the hydrocarbons in the formation, can be used to deduce parameters relating to the local remaining hydrocarbon content of the formation. Alternatively, applications may use only water tracers. For example, water tracers may be introduced in an injection well and their presence monitored at adjacent production wells in order to obtain information about the flux of water from the injection well to the production well.

In addition to injection techniques, it is also known to introduce tracers into a well by including them in articles placed into the well. By detecting the rate of tracer production over time, information can be deduced about the performance of the hydrocarbon well.

Tracers should be detectable in small to very small quantities, for example at levels below 100 parts per billion (ppb), preferably at levels of 50 ppb or lower, more preferably at levels of 10 ppb or lower, and most preferably in the parts per trillion (ppt) range (that is, at levels less than 1 ppb). The levels are typically determined on a mass/mass basis. The tracers should also be environmentally acceptable with low toxicity for insertion into the ground and usage, for example, in reservoir applications, but they must also be species that are not naturally present in the ground in such quantities as to contaminate the results of a tracer study.

Typical detection methods include gas chromatography - mass spectrometry (GC-MS), gas chromatography - mass spectrometry - mass spectrometry (GC-MS-MS), liquid chromatography - mass spectrometry (LC-MS), liquid chromatography - mass spectrometry - mass spectrometry (LC-MS-MS) and high-performance liquid chromatography (HPLC), which can typically detect very low concentrations of the tracers in the produced fluids. It is desirable that tracers should be detectable in low quantities and also that they can be reliably distinguished from other tracers and species which are naturally present in reservoir fluids.

Acid stimulation is a known technique aimed at improving oil recovery from a subterranean reservoir of formation. Typically, an acid such as hydrochloric acid is injected via a wellbore into a region of the surrounding reservoir or formation in order to increase the permeability of the reservoir or formation. Depending on the nature of the geological formation, the acid may react with soluble substances in the formation matrix to enlarge the pore spaces, or may dissolve the entire formation matrix. In either case, the acid treatment helps to increase the permeability of the formation in order to enhance recovery of hydrocarbons during subsequent production.

One way the acid can be delivered to a desired region of the reservoir or formation is by lowering into a well sections of pipework containing holes or valves that can be opened and closed at selected intervals along its length in order to allow the acid to be injected into the formation or reservoir.

A problem with current acid stimulation techniques is the lack of understanding and assessability of how effective an acid stimulation treatment is over the whole well length and whether the formation or reservoir has been stimulated effectively in all intended sections of the well. The costs associated with drilling wells, particularly the horizontal sections thereof, are very high. Therefore, there is a need for a solution that would provide information regarding the effectiveness of acid stimulation, potentially indicating how the well is producing and whether a partial or full horizontal length of adjacent well(s) needs to be drilled.

It is an object of the present invention to address or mitigate one or more problems of the prior art.

It is an object of the present invention to provide a suitable approach allowing a user to assess the effectiveness of an acid stimulation procedure. Summary

The present inventors have found that current acid stimulation techniques for hydrocarbon reservoir systems do not provide information regarding how effective an acid stimulation treatment is over a predetermined well section and/or whether the formation or reservoir has been stimulated effectively in all intended sections of the well.

The present specification addresses this problem by providing a method of stimulating a well, the method comprising: providing a well structure, the well structure comprising at least one solid tracer composition comprising at least one tracer compound and at least one reactive substrate which is configured to dissolve, degrade, disintegrate or break down to release the at least one tracer compound when exposed to an acid stimulation fluid but which does not release the at least one tracer compound when exposed to a neutral aqueous fluid; injecting an acid stimulation fluid into the well structure; producing fluid from the well structure, and analysing the produced fluid for the at least one tracer compound.

The present specification also provides a solid tracer composition for use in the aforementioned method, the solid tracer composition comprising at least one tracer compound and at least one reactive substrate which is configured to dissolve, degrade, disintegrate or break down to release the at least one tracer compound when exposed to an acid stimulation fluid but which does not dissolve, degrade, disintegrate or break down to release the at least one tracer compound when exposed to a neutral aqueous fluid.

A person of skill in the art will appreciate that the specific chemical nature of the acid stimulation fluid may depend on the type of geological formation or reservoir. However, typically the acid stimulation fluid may comprise or may consist of an inorganic acid, e.g. hydrochloric acid or hydrofluoric acid, or may comprise or may consist of an organic acid., e.g. acetic acid or formic acid.

The at least one reactive substrate comprises at least one substance configured to dissolve, degrade, disintegrate or break down when exposed to an aqueous acid and/or to an aqueous acidic solution. Typically, the reactive substrate may comprise at least one substance configured to dissolve, degrade, disintegrate or break down when exposed to an aqueous solution having a pH lower than about 3, e.g. lower than about 2, e.g. lower than about 1, typically between 0 and 1. Conversely, the solid tracer composition is configured to not release the at least one tracer compound when exposed to an aqueous solution having a pH above 1, 2, 3, or 4. For example, the reactive substrate may comprise at least one substance configured to dissolve, degrade, disintegrate or break down when exposed to a aqueous acidic solution of a strong acid, e.g. HCI, having a concentration (by volume) of at least 5%, e.g. least 10%, e.g. at least 15%, typically between 10% and 15%. Typically, the composition may be configured to remain substantially intact and/or is configured to not dissolve, degrade, disintegrate or break down when exposed to an aqueous solution, e.g. when exposed to an aqueous solution having a pH above about 4, e.g. above about 3, e.g. above about 2, e.g. above about 1. The formulation may be water-stable at a pH above about 4, e.g. above about 3, e.g. above about 2, e.g. above about 1.

The reactive substrate may be configured to remain substantially intact and/or may be configured to not dissolve, degrade, disintegrate or break down when exposed to an aqueous solution, e.g. when exposed to an aqueous solution having a pH above about 4, e.g. above about 3, e.g. above about 2, e.g. above about 1. The formulation may be water-stable at a pH above about 4, e.g. above about 3, e.g. above about 2, e.g. above about 1.

By such provision, the composition may be deployed to a desired location, e.g. to a desired portion of the well or well structure, for example during or after deployment or positioning of well equipment such as pipework, and/or may remain at that location under normal conditions, whilst remaining substantially intact.

Advantageously, the at least one tracer compound may be water-stable and/or may not dissolve, degrade, disintegrate or break down upon exposure to a/the acid. By such provision, upon exposure to an acid such as a stimulation fluid, the solid composition may dissolve, degrade, disintegrate or break down, releasing the tracer compound(s) into the stimulation fluid.

The at least one reactive substrate may comprise or may consist of an inorganic substrate which is soluble in acidic aqueous solutions but not in neutral aqueous solutions. For example, the solid tracer composition may comprise at least 50%, 60%, 70%, or 80% by weight of the inorganic substrate. By providing a majority of the solid tracer composition as an acid soluble inorganic material, this ensures that the solid tracer composition degrades to release the tracer on contact with an acid stimulation fluid but does not release the tracer when exposed to a neutral aqueous solution.

The inorganic substrate may comprise or may consist of one or more carbonate salts, e.g. calcium carbonate and/or magnesium carbonate. Upon exposure to an acid, e.g. to the acid stimulation fluid, the carbonate salt may react to generate water, carbon dioxide, and a soluble salt such as calcium chloride or magnesium chloride.

The inorganic substrate may comprise or may consist of one or more sulphate salts, e.g. calcium sulphate and/or magnesium sulphate. Upon exposure to an acid, e.g. to the acid stimulation fluid, the sulphate salt may react to generate sulphuric acid and a soluble salt such as calcium chloride or magnesium chloride. The at least one reactive substrate may comprise a polymer. The polymer may be susceptible to degradation, e.g. by hydrolysis, upon exposure to an acid, e.g. to the acid stimulation fluid. The polymer may comprise or may consist of a polyester, a polyorthoester, a polyimine, a polyacetal, a polyhydrazone, or the like. An advantage of using a polymer as a reactive substrate is that the polymer may also act as a binder for the composition.

Advantageously, the solid tracer composition may be provided as a solid object or article, such as a disc, a ball, a cube, or any other suitable shape. It will be understood that the particular form or shape of the article may be selected to best suit the intended deployment location, e.g. the specific location within oil well equipment or pipework.

The solid tracer composition may further comprise at least one binder. By such provision, the solid tracer composition may be capable of retaining its shape during and/or after deployment to a desired location, for example until exposure to an acid. However, the solid tracer composition should not contain so much binder that it prevents dissolution/degradation of the acid soluble/degradable (e.g. inorganic) component of the composition to release the tracer. According, the solid tracer composition may comprise no more than 30%, 20%, 15%, or 10% by weight of the polymeric binder.

Thus, in an embodiment, there may be provided a solid tracer composition, wherein the composition comprises at least one tracer compound, at least one reactive substrate, and at least one binder, and wherein the composition is configured to dissolve, degrade, disintegrate or break down when exposed to an acid.

Advantageously, the binder may be water-stable. The binder may be capable of allowing the solid tracer composition to retain its shape during and/or after deployment to a desired location, whilst allowing the at least one reactive substrate to react, e.g. dissolve, degrade, disintegrate or break down, upon exposure to an acid. Thus, the binder may act as a water stabiliser, protecting the composition from water degradation but allowing the at least one reactive substrate to react, e.g. dissolve, degrade, disintegrate or break down, upon exposure to an acid.

The binder may further act as a controlled release agent. The binder may control the rate of reaction of the composition, e.g. of the reactive substrate, with a/the acid. The binder may control the rate at which the composition, e.g. the reactive substrate, reacts, e.g. dissolves, degrades, disintegrates or breaks down, upon exposure to a/the acid. Typically, the presence of a binder may slow down the rate of release of the at last one tracer, compared to a composition free of binder. The presence of a binder may slow down the rate of dissolution, degradation, or disintegration of the composition, compared to a composition free of binder. Typically, the binder may comprise or may consist of an organic binder, e.g. a polymeric material, such as polyvinylpyrrolidone (PVP).

Brief Description of the Drawings

For a better understanding of the present invention and to show how the same may be carried into effect, certain embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:

Figure 1 shows an apparatus used in the preparation of a solid tracer composition according to an embodiment; and

Figure 2 shows a solid tracer composition according to an embodiment.

Detailed Description

As described in the summary section, the present disclosure provides a solid tracer composition which is configured to dissolve, degrade, disintegrate or break down when exposed to an acid such as an acid stimulation fluid. The solid tracer composition comprises at least one tracer compound and at least one reactive substrate which is configured to dissolve, degrade, disintegrate or break down when exposed to an acid, e.g. to an acid stimulation fluid. The composition may further comprise a binder.

Advantageously, the at least one tracer compound may be water-stable and/or may not dissolve, degrade, disintegrate or break down upon exposure to a/the acid. By such provision, upon exposure to an acid such as a stimulating fluid, the solid composition may dissolve, degrade, disintegrate or break down, releasing the at least one tracer compound into the stimulating fluid. The at least one tracer compound may flow with the acid stimulation fluid into the formation and/or reservoir and/or may remain in the formation and/or reservoir until production is started.

At least one tracer compound may comprise a water tracer, e.g. a naphthalene sulfonic acid salt such as sodium 2-nathphalene sulfonate. By such provision upon release from the solid composition after exposure to an acid, e.g. acid stimulation fluid, the water tracer may remain in the aqueous phase. The at least one tracer compound, e.g. water tracer, may be adapted to flow with a water phase, e.g. from the formation or reservoir to surface, for example during backflow of the water phase subsequent to acid stimulation and/or during production of formation water. Examples of water tracers include, but are not limited to, fluorinated benzoic acids such as 2-fluorobenzoic acid; 3-fluorobenzoic acid; 4-fluorobenzoic acid; 3,5- difluorobenzoic acid; 3,4-difluorobenzoic acid; 2,6-difluorobenzoic acid; 2,5-difluorobenzoic acid; 2,3-difluorobenzoic acid; 2,4-difluorobenzoic acid; pentafluorobenzoic acid; 2, 3, 4, 5- tetrafluorobenzoic acid; 4-(trifluoro-methyl)benzoic acid; 2-(trifluoromethyl)benzoic acid; 3- (trifluoro-methyl)benzoic acid; 3,4,5-trifluorobenzoic acid; 2,4,5-trifluorobenzoic acid; 2,3,4- trifluorobenzoic acid; 2,3,5-trifluorobenzoic acid; 2,3,6-trifluorobenzoic acid; and 2,4,6- trifluorobenzoic acid and the brominated, chlorinated and iodinated analogs thereof, as disclosed for example in US10927292B2.

At least one tracer compound may comprise an oil tracer. By such provision upon release from the solid composition after exposure to an acid, e.g. acid stimulation fluid, the oil tracer may typically migrate or move from the aqueous phase (acid stimulation fluid) to an oil phase, e.g. to a hydrocarbon phase such as hydrocarbons present in the formation or reservoir. The least one tracer compound, e.g. oil tracer, may be adapted to flow with an oil phase, e.g. from the formation or reservoir to surface, for example during production or recovery of hydrocarbons from the formation or reservoir. Examples of oil tracers include, but are not limited to, 4-iodotoluene, 1,4-dibromobenzene, l-chloro-4-iodobenzene, 5-iodo-m-xylene, 4- iodo-o-xylene, 3,5-dibromotoluene, 1,4-diiodobenzene, 1,2-diiodobenzene, 2,4- dibromomesitylene, 2,4,6-tribromotoluene, 1-iodonaphthalene, 2-iodobiphenyl, 9- bromophenanthrene, 2-bromonaphthalene, bromocyclohexane, 1,2-dichlorobenzene, 1,3- dichlorobenzene, 1,4-dichlorobenzene, 1-bromododecane, bromooctane, l-bromo-4- chlorobenzene, bromobenzene, 1,2,3-trichlorobenzene, 4-chlorobenzylchloride, l-bromo-4- fluorobenzene, perfluoromethylcyclopentane (PMCP), perfluoromethylcyclohexane (PMCH), perfluorodimethylcyclobutane (PDMCB), m-perfluorodimethylcyclohexane (m-PDMCH), o- perfluorodimethylcyclohexane (o-PDMCH), p-perfluorodimethylcyclohexane (p-PDMCH), perfluorotrimethylcyclohexane (PTMCH), perfluoroethylcyclohexane (PECH), and perfluoroisopropylcyclohexane (IPPCH), as disclosed for example in US10927292B2.

The present specification also provides a well structure comprising at least one solid tracer composition as described herein. The well structure may comprise at least one solid object or article, each object or article comprising a solid tracer composition according to the first aspect. The solid object(s) or article(s) may be provided in the shape of a disc, a ball, a cube, or any other suitable shape.

The well structure may comprise a plurality of solid objects or articles, each object or article comprising a solid tracer composition according to the first aspect. Advantageously, two or more, e.g. each, of the plurality of solid objects, may comprise a solid tracer composition each having a different tracer compound. Thus, solid objects having different tracer compounds may be provided within the well structure, e.g. pipework, at different locations. By such provision, detection of a given tracer may be associated with acid stimulation having occurred at a particular location, e.g. of the well structure or pipework. This may allow information to be obtained regarding section(s) of the well structure that are functioning either effectively or not effectively. For example, this may allow identification of section(s) of the well structure that are functioning effectively and of section(s) of the well structure that are not functioning effectively.

It will be understood that each location within the well structure may be provided with one or more solid articles, so long as the tracer compounds at each location are the same. For example, there may be provided one article at each location. Alternatively, there may be provided a plurality of articles at each location, and the tracer compounds in the articles at that location may be the same. Flowever, the tracer compound(s) in the article(s) at other locations within the well structure may be different. Thus, the tracer compound(s) in the solid article(s) provided at a first location of the well structure may be different from the tracer compound(s) in the solid article(s) provided at a second (different) location of the well structure.

Alternatively, or additionally, one or more locations within the well structure may be provided with one or more solid articles, the one or more articles at at least one location including different tracer compounds. For example, one or more articles at a first location may include a first tracer, e.g. a water tracer, and one or more articles at the first location may include a second tracer, e.g. an oil tracer. Alternatively, one or more articles at a first location may include two or more different tracers, e.g. a first tracer such as a water tracer, and a second tracer such as an oil tracer.

When the well structure comprises two or more solid objects or articles, or sets of solid objects or articles, at different locations of the well structure, each article or each set of articles comprising different water tracer compounds, information may be obtained regarding the flow of fluid, e.g. acid stimulation fluid into the formation, and/or regarding the flow of fluid, e.g. water phase, from the formation or reservoir during flushing or production.

When the well structure comprises two or more solid objects or articles, or sets of solid objects or articles, at different locations of the well structure, each article or set of articles comprising different oil tracer compounds, information may be obtained regarding the flow of fluid, e.g. hydrocarbons, from the formation or reservoir during production.

The information obtained may be qualitative, e.g. identification of regions of the formation or reservoir producing water or hydrocarbons.

The information obtained may be quantitative, e.g. determination of the amount or rate of water or hydrocarbons being produced by an associated region of the formation or reservoir.

The features described above in relation to the first aspect may equally apply in relation to the second aspect and, merely for brevity, are not repeated here.

The present specification also provides a method of stimulating a well, the method comprising: providing a well structure, the well structure comprising at least one solid tracer composition according to the first aspect; and injecting an acid stimulation fluid into the well structure.

The method may comprise providing a well structure, the well structure comprising at least one solid object or article, each object or article comprising a solid tracer composition according to the first aspect. The solid object(s) may be provided in the shape of a disc, a ball, a cube, or any other suitable shape.

The method may comprise providing a well structure having a plurality of solid objects or articles, each object or article comprising a solid tracer composition according to the first aspect. Advantageously, two or more, e.g. each, of the plurality of solid objects, may comprise a solid tracer composition each having a different tracer compound. Thus, the method may comprise providing solid objects having different tracer compounds at different locations within the well structure, e.g. pipework.

The method may comprise monitoring an inflow of hydrocarbons from a/the reservoir or formation.

The method may comprise producing a region of the formation or reservoir, e.g. a region of the reservoir or formation adjacent or near the well structure.

The method may comprise analysing the produced fluid, e.g. the produced water phase and/or the produced hydrocarbons phase.

The method may comprise detecting and/or measuring one or more tracer compounds within the produced fluid.

Using the present method, detection of a given tracer may be associated with acid stimulation having occurred at a particular location, e.g. of the well structure or pipework. This may allow information to be obtained regarding the section(s) of the well structure that are functioning either effectively or not effectively. For example, this may allow identification of the section(s) of the well structure that are functioning effectively and of the section(s) of the well structure that are not functioning effectively.

When the well structure comprises two or more solid objects, each comprising a solid tracer composition having a different water tracer compounds, the method may allow information to be obtained regarding the flow of fluid, e.g. acid stimulation fluid into the formation, and/or regarding the flow of fluid, e.g. water phase, from the formation or reservoir during flushing or production.

When the well structure comprises two or more solid objects, each comprising a solid tracer composition having a different oil tracer compounds, the method may allow information to be obtained regarding the flow of fluid, e.g. hydrocarbons, from the formation or reservoir during production.

The method may comprise generating qualitative information, e.g. identifying regions of the formation or reservoir producing water or hydrocarbons.

The method may comprise generating quantitative information, e.g. determining an amount or rate or water or hydrocarbons being produced by an associated region of the formation or reservoir.

The present specification also provides a method of preparing a solid tracer composition, the method comprising: mixing at least one tracer compound and at least one reactive substrate, wherein the at least one reactive substrate is configured to dissolve, degrade, disintegrate or break down when exposed to an acid.

The method may comprise shaping the composition into a solid object or article, e.g. a disc, a ball, a cube, or any other suitable shape.

The method may comprise forming the composition, e.g. casting or pressing the composition, into a solid article.

The method may comprise drying the composition, e.g. object.

The method may comprise mixing the at least one tracer compound and the at least one reactive substrate with at least one binder. This may be advantageous to permit the composition, e.g. object, to retain its shape during and/or after deployment to a desired location, for example until exposure to an acid.

A generic method of preparing a solid tracer composition free of a binder is as follows:

A reactive substrate is mixed with a tracer compound in demineralised water to form a paste.

The paste is cast into a mould or placed into a press, and subsequently cured to produce a solid article.

A generic method of preparing a solid tracer composition containing a binder is as follows:

A reactive substrate is mixed with a tracer compound and a binder in demineralised water to form a mixture. The mixture is transferred into a press and is compressed to form a solid article, and is subsequently dried.

A number of more detailed examples are set out below to illustrate different embodiments of the present invention. Examples

Example 1 - Preparation of a solid tracer composition including a binder

Materials

Reactive substrate: Calcium carbonate

Tracer compound: Sodium 2-nathphalene sulfonate

Binder: 20% PVP10 [Polyvinylpyrrolidone, average MW 10, 000 at 20% wt/wt solution in demineralised water]

Preparation

Calcium carbonate powder was blended with Sodium 2-nathphalene sulfonate powder and 20% PVP10 solution was added to form a damp solid.

The relative amounts of each component were as follows:

85.4 wt % CaCC>3;

5.0 wt % PVP10 (as a 20% solution); and 9.6 wt % Sodium 2-nathphalene sulfonate.

The mixture was transferred into a tablet press 10 containing a die set, as illustrated in Figure 1, and was compressed to form a solid tablet 20.

The tablet 20 was left to dry for 24 hours, and is shown in Figure 2.

The final product contained approximately 10% by weight of the tracer compound (Sodium 2-nathphalene sulfonate).

Example 2 - Preparation of a solid tracer composition without a binder

Materials

Reactive substrate: Calcium carbonate

Tracer compound: Sodium 2-nathphalene sulfonate

Preparation

The reactive substrate (calcium carbonate powder) was mixed with the acid-stable water tracer (sodium 2-naphthalene sulfonate) and demineralised water to form a paste. The paste was cast into a mould and cured for four days at 85°C, to produce a solid article. Example 3 - Preparation of a solid tracer composition without a binder

Materials

Reactive substrate: Calcium sulfate (gypsum - CaSC .O.ShhO)

Tracer compound: Sodium 2-nathphalene sulfonate

Preparation

The reactive substrate (calcium sulfate powder) was mixed with the acid-stable water tracer (sodium 2-naphthalene sulfonate) and demineralised water to form a paste. The paste was cast into a mould and cured for four days at 85°C, to produce a solid article.

Example 4 - Preparation of a solid tracer composition without a binder

Materials

Reactive substrate: Calcium carbonate

Tracer compound: Sodium 2-nathphalene sulfonate

Preparation

The acid-stable water tracer (sodium 2-naphthalene sulfonate) was pre-blended with demineralised water to form an aqueous solution. The solution was then mixed with the reactive substrate (calcium carbonate powder) to form a paste. The paste was cast into a mould and cured for four days at 85°C, to produce a solid article.

Example 5 - Effect of binder

Materials

2g tablet of a solid tracer composition with a binder as per example 1 (Tablet 1)

2g tablet of a solid tracer composition without a binder as per example 2 (Tablet 2)

Experiment

Each tablet was placed in a respective beaker containing approximately 25mL of an aqueous acid solution of 10% HCI (by volume).

Tablet 1 was fully dissolved in approximately 40 minutes.

Tablet 2 was fully dissolved in approximately 40 seconds. The above demonstrates the potential for the binder to act as a controlled release agent for the tracer upon exposure to an aqueous acid solution.

While this invention has been described with reference to certain examples and embodiments, it will be understood to those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the appended claims.