Patent Publication Number: US-2023144199-A1

Title: Multifunctional fluorescent polymer-clay composite tracers

Description:
TECHNICAL FIELD 
     This document relates to methods and compositions used in tagging and tracing subterranean cuttings produced during drilling. 
     BACKGROUND 
     Subterranean cuttings that are produced during drilling operations can provide critical information, for example, the lithology and mineral composition of the subterranean formation. However, cuttings produced at the drill head travel to the surface via the annulus, and it is difficult to accurately determine or even estimate lag time during this upward trip. This makes analyzing the depth at which these cutting originated difficult. 
     Mud tracers can be used to determine mud cycle time, for example, the circulation time, however, the estimating the depth of cuttings based on circulation time is inaccurate, especially if the wellbore includes long horizontal sections or the return trip time is lengthy. For example, when the return trip is longer than half an hour, it is common to have depth uncertainties of more than 6 meters (20 feet). This, in turn, compounds errors in characterizing the formation according to the depth of the cuttings. More efficient mud tracer materials and rapid detection techniques for these tracers are highly desirable. 
     SUMMARY 
     This disclosure describes compositions and methods that can be used to determine the origin depth of a wellbore cutting. 
     In some implementations, a polymer-clay composite tag includes a nanoclay. The nanoclay includes a plurality of layers, and a polymer intercalated between the layers of the nanoclay. The polymer is functionalized with a fluorescent dye. 
     In some implementations, a method of synthesizing a polymer-clay composite includes functionalizing a polymer with a fluorescent dye to yield a fluorescent polymer and intercalating the fluorescent polymer in a nanoclay. 
     In some implementations, a method of synthesizing a polymer-clay composite includes intercalating styrene-based monomers or acrylate-based monomers and a fluorescent dye between the layers of a nanoclay and inducing radical polymerization to polymerize the styrene-based monomers and fluorescent dye or acrylate-based monomers and fluorescent dye to yield a fluorescent polymer-clay composite. 
     In some implementations, a method of determining the origin location of a subterranean sample includes mixing a polymer-clay composite tag into a fluid. The polymer-clay composite tag includes a nanoclay. The nanoclay includes layers and a fluorescent polymer intercalated between the layers of the nanoclay. The method includes flowing the fluid through a work string into a subterranean formation, recovering subterranean samples from the subterranean formation, and determining the origin location of the subterranean sample by detecting the presence of the polymer-clay composite tag. 
     The details of one or more implementations of the disclosure are set forth in the accompanying drawings and the description that follows. Other features, objects, and advantages of the disclosure will be apparent from the description and drawings, and from the claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG.  1    shows an example schematic of a drilling operation  10  for a well  12 . 
         FIG.  2    shows an example schematic of the effect of pH on cationic adsorption on the surface of clay layers. 
         FIG.  3    shows an example reaction schematic where a fluorescent dye and a polymer structure are conjugated to yield a fluorescent polymer. 
         FIG.  4    shows an example reaction schematic where the polymer is first adsorbed to the clay, followed by a reaction between a fluorescent dye and the polymer. 
         FIG.  5    shows an example schematic where monomeric units of the polymer structure and the fluorescent dye are intercalated in between layers of nanoclay structure, followed by polymerization to yield a fluorescent polymer-clay composite. 
         FIG.  6    shows an example reaction of isothiocyanate with a primary or secondary amine to yield a substituted thiourea. 
         FIG.  7    shows an example reaction between an NHS ester and a primary or secondary amine. 
         FIG.  8    shows an example reaction between an isothiocyanate containing dye and a reactant that includes a primary amine and an allyl group to yield an allyl-containing fluorescent dye. 
         FIG.  9    shows an example reaction between an isothiocyanate containing fluorescent dye and 3-buten-1-amine to yield an allyl-containing fluorescent dye. 
         FIG.  10    shows an example reaction of an allyl-containing fluorescent dye with a styrene monomer, where peroxide-induced polymerization yields a fluorescent styrene-based polymer. 
         FIG.  11    shows an example schematic where the fluorescent polymer is dissociated from the complex by adjusting the pH or introducing a solvent. 
         FIG.  12    shows a flowchart of an example method of determining the origin location of a subterranean sample. 
         FIG.  13    shows an example fluorescence spectrum of an FITC-PEI-bentonite functionalized polymer-clay composite. 
         FIG.  14    shows an example fluorescence spectrum of an RBITC-PS-bentonite functionalized polymer-clay composite. 
         FIG.  15 A  shows an example SEM image of bentonite nanopowder before functionalization. 
         FIG.  15 B  shows an example SEM image of bentonite nanopowder before functionalization. 
         FIG.  15 C  shows an example SEM image of bentonite nanopowder functionalized with FITC-PEI. 
         FIG.  15 D  shows an example SEM image of bentonite nanopowder functionalized with FITC-PEI. 
         FIG.  15 E  shows an example SEM image of bentonite nanopowder functionalized with RBITC-polystyrene. 
         FIG.  15 F  shows an example SEM image of bentonite nanopowder functionalized with RBITC-polystyrene. 
     
    
    
     Like reference symbols in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     Reference will now be made in detail to certain embodiments of the disclosed subject matter, examples of which are illustrated in part in the accompanying drawings. While the disclosed subject matter will be described in conjunction with the enumerated claims, it will be understood that the exemplified subject matter is not intended to limit the claims to the disclosed subject matter. 
     Provided in this disclosure, in part, are methods, compositions, and systems for accurately determining the origin depth or location of cuttings produced during drilling.  FIG.  1    illustrates an example of a drilling operation  10  for a well  12 . The well  12  can be in a wellbore  20  formed in a subterranean zone  14  of a geological formation in the Earth&#39;s crust. The subterranean zone  14  can include, for example, a formation, a portion of a formation, or multiple formations in a hydrocarbon-bearing reservoir from which recovery operations can be practiced to recover trapped hydrocarbons. Examples of unconventional reservoirs include tight-gas sands, gas and oil shales, coalbed methane, heavy oil and tar sands, gas-hydrate deposits, to name a few. In some implementations, the subterranean zone  14  includes an underground formation including natural fractures  60  in rock formations containing hydrocarbons (for example, oil, gas, or both). For example, the subterranean zone  14  can include a fractured shale. In some implementations, the well  12  can intersect other suitable types of formations, including reservoirs that are not naturally fractured in any significant amount. 
     The well  12  can include a casing  22  and well head  24 . The wellbore  20  can be a vertical, horizontal, deviated, or multilateral bore. The casing  22  can be cemented or otherwise suitably secured in the wellbore  20 . Perforations  26  can be formed in the casing  22  at the level of the subterranean zone  14  to allow oil, gas, and by-products to flow into the well  12  and be produced to the surface  25 . Perforations  26  can be formed using shape charges, a perforating gun, or otherwise. 
     For a drilling treatment  10 , a work string  30  can be disposed in the wellbore  20 . The work string  30  can be coiled tubing, sectioned pipe, or other suitable tubing. A drilling tool or drill bit  32  can be coupled to an end of the work string  30 . Packers  36  can seal an annulus  38  of the wellbore  20  uphole of and downhole of the subterranean zone  14 . Packers  36  can be mechanical, fluid inflatable, or other suitable packers. 
     One or more pump trucks  40  can be coupled to the work string  30  at the surface  25 . The pump trucks  40  pump drilling mud  58  down the work string  30  to lubricate and cool the drilling tool or drill bit  32 , maintain hydrostatic pressure in the wellbore, and carry subterranean cuttings to the surface. The drilling mud  58  can include a fluid pad, proppants, flush fluid, or a combination of these components. The pump trucks  40  can include mobile vehicles, equipment such as skids, or other suitable structures. 
     One or more instrument trucks  44  can also be provided at the surface  25 . The instrument truck  44  can include a drilling control system  46  and a drilling simulator  47 . The drilling control system  46  monitors and controls the drilling treatment  10 . The drilling control system  46  can control the pump trucks  40  and fluid valves to stop and start the drilling treatment  10 . The drilling control system  46  communicates with surface and subsurface instruments to monitor and control the drilling treatment  10 . In some implementations, the surface and subsurface instruments may comprise surface sensors  48 , down-hole sensors  50 , and pump controls  52 . 
     Additives  81  can be mixed with drilling mud  58  and flowed through the reservoir. In some implementations, the additives are tags that can embed into or decorate the surface of cuttings produced by the drill bit. For example, the additives can have high affinity non-specific binding onto cutting surfaces as a result of physico-chemical and/or ionic forces, for example, Van der Waals, hydrophobic/hydrophilic interactions, and oppositely charged surfaces. When drilling mud is introduced into the subterranean formation via the drill bit, tags that are included in the mud will contact the subterranean formation for the first time at the drill head. If the depth or relative position of the drill head and the lag time of the mud in the drill string are known, cuttings that are tagged with a specific tag can be accurately assigned an origin depth or position. Accordingly, the origin location of the cutting can be accurately determined. 
     In some implementations, more than one tag can be used. The tags can be uniquely identifiable. Accordingly, cuttings that include or are decorated with a first tag can be assigned to a first depth or position, and cuttings that include or are decorated with a second tag can be assigned to a second depth or position. The number of tags is not limited to two, and the tags can uniquely identify a third, fourth, fifth, etc. depth or position. 
     The tags described herein are multi-modal tags, meaning that each tag includes a unique combination of features that can be orthogonally detected. Accordingly, the variations in the features of the tags can act as a uniquely identifiable “barcode” or “fingerprint.” In addition, the combination of orthogonally detectable features expands the number of uniquely identifiable tags that can be produced. 
     The tags described herein include nanoclays. The nanoclays are nanoparticles or micro-sized particles with nanoscale layered mineral silicates, for example, bentonite, illite, and kaolinite. The nanoclays have similar density and surface properties as the clays typically used in drilling mud formulations. Accordingly, the tags described herein can travel well with the clays in the drilling mud, and contact subterranean cuttings at the drill bit. The tags are small, with a total size less than about 25 μm. Therefore, the tags can embed into the pores of cuttings at real time during drilling, accurately tagging the cuttings, and allowing the origin location of the cuttings to be determined. 
     In addition, the tags described herein can be rapidly detected with high sensitivity. In some implementations, the tags can be detected at the drilling site, making detection more rapid and allowing the drill operators to make drilling decisions based on real-time data. 
     Another advantage of the tags described herein is that they can be identified in two stages. In a rapid stage, a first analysis can be conducted quickly at the drilling site using a first procedure. This allows for the easy separation of tagged materials from untagged materials. After the rapid stage, the tagged materials can be subjected to a second analysis, either off-site or on-site. Accordingly, the multi-stage approach includes a first analysis that acts as a screening process. The screening process reduces the number of cuttings that need to be transported and subsequently analyzed. This approach saves time, labor, and laboratory costs. In addition, this multi-modal identification allows for rapid identification and real-time analysis that can aid in drilling operations. For example, the knowledge that a certain type of cutting is produced at a certain location can influence subsequent drilling decisions. 
     The tags described herein include clay. Clay is a major component of water-based drilling muds. The most commonly used drilling mud clays include bentonite, illite, and kaolinite. These clays have layered structures and can swell in water. The layered structures give the clays a very large total surface area, and the surfaces of the layers in the clays are highly charged and can serve as super-absorbent surfaces for guest molecules under specific conditions. For example, as a swelling clay, sodium montmorillonite, a predominant component of bentonite, has strong swelling capacity and can expand its original volume by up to a factor of eight. Within the layered structure of sodium montmorillonite, the negatively charged surfaces of the montmorillonite sheets are charged-balanced by exchangeable ionic species. For example, cationic compounds can be adsorbed onto montmorillonite via electrostatic interactions. 
     Organic compounds may also adsorb onto the clay through other interactions, including hydrogen bonding and van der Waals forces. This adsorptive property is pH dependent. The electric surface properties of clay minerals vary with the pH of the surrounding solution. In more detail, the surface of these clays contains ionizable aluminol groups (AlOH) that have amphoteric behavior, i.e., can take up either a proton or a hydroxyl (OH − ) group depending on the isoelectric point (IEP) of the clay and the pH of the surrounding solution. This property is illustrated by Equations 1-3. 
       ≡Clay-AlOH+H + →≡Clay-Al—OH 2   + (at pH&lt;IEP)  Eq. 1
 
       ≡Clay-AlOH 2   + +2OH − →≡Clay-AlO − +2H 2 O (at pH=IEP)  Eq. 2
 
       ≡Clay-AlOH+2OH − →≡Clay-AlO − +H 2 O (at pH&gt;IEP)  Eq. 3
 
     When the pH is less than the isoelectric point of the clay, the surface charge of the clay is positive. When the pH is greater than the isoelectric point of the clay, the surface charge is negative. Accordingly, the pH of a surrounding solution can be manipulated to selectively adsorb or desorb guest cationic species.  FIG.  2    shows an example schematic of the effect of pH on cationic adsorption on the surface of clay layers  202 , where H +  represents a proton in aqueous solution and A +  represents a guest cationic species. When the pH is equal to the isoelectric point, the net charge is zero. At low pH conditions, i.e., below the isoelectric point, there is electrostatic repulsion between the positively charged clay surfaces and the anionic species in solution. As the pH increases, the surfaces of the clay become negatively charged and there is competitive adsorption of the protons and guest cationic species. At intermediate pH values, the guest cationic species is stably adsorbed to the clay surfaces. At high pH values, the cationic guest species disassociates from the clay surfaces. Accordingly, these clays can serve as controlled release vehicles. Presented herein are methods to incorporate polymers and cationic polymers into clay nanoparticles, for example, bentonite nanoparticles, to yield functionalized nanoclays. 
     In some implementations, the functionalized nanoclays are a fluorescent polymer-clay composite that can be used in subterranean applications. The composite includes a fluorescent moiety and a polymer network intercalated into a nanoclay. The fluorescent and polymer components offer fingerprint-like identification and can be detected by multiple orthogonal analytical methods. This means that composites that include unique combinations of fluorescent and polymer components can be uniquely identified and differentiated from one another. 
     Presented herein are multiple synthetic routes for synthesizing fluorescent polymer-clay composites. In some implementations, a fluorescent dye is chemically grafted to a polymer network to yield a fluorescent polymer network. The fluorescent polymer network is inserted into the layers of the clay to form a stable sandwich structure. 
     In some implementations, the reaction between the fluorescent dye and the polymer structure can occur before the polymer is adsorbed onto a nanoclay structure.  FIG.  3    shows an example reaction schematic where a fluorescent dye  306  and a polymer structure  304  are conjugated to yield a fluorescent polymer  308 . The fluorescent polymer  308  is intercalated in nanoclay layers  202  to yield a fluorescent polymer-clay composite  310 . 
     In some implementations, the polymer structure is intercalated into the nanoclay structure before the dye is reacted with the polymer structure to yield a fluorescent polymer-clay composite.  FIG.  4    shows an example reaction schematic where the polymer  304  is first adsorbed to the clay, followed by a reaction between a fluorescent dye  306  and the polymer. 
     In some implementations, a mixture of monomers and fluorescent dyes are polymerized in the presence of a nanoclay to yield a fluorescent polymer-clay composite.  FIG.  5    shows an example schematic where monomeric units  303  of the polymer structure and the fluorescent dye  306  are intercalated in between layers  202  of nanoclay structure, followed by polymerization to yield a fluorescent polymer-clay composite  310 . 
     Each of the components of the fluorescent polymer-clay composite provide advantageous effects. For example, the fluorescent dyes provide multiple avenues for disambiguation between different tags containing different fluorescent moieties. The fluorescent dyes can be distinguished from one another by their fluorescent properties. Alternatively or in addition, thermal treatment at certain temperatures can cause the dye molecules to be released from the polymer structure, and the polymer can further degrade into segments which are related to their original monomers. The released fluorescent dyes or polymer segments can be further identified by mass spectrometry, for example by pyrolysis-gas chromatography—mass spectrometry (Pyrolysis-GC-MS). 
     The fluorescent polymer-clay composites also contain polymers. Different types of polymers in different composites can be differentiated from one another. In some implementations, the polymer is a water soluble, linear, or branched polyamine. Polyamines and alkylpolyamines have been shown to have strong and stable adsorption to various clays. The polyamine structure includes primary and secondary amines. In some implementations, the polyamine structure can be engineered to degrade with predictable degradation products. For example, the polymers can be degraded at high temperature through pyrolysis process. These degradation products can be measured chromatographically, for example with pyrolysis gas chromatography-mass spectroscopy (Pyrolysis-GC-MS). Accordingly, polyamine polymers with different degradation products can be differentiated from one another, and can be used to create a library of uniquely identifiable fluorescent polymer-clay composites. 
     A fluorescent dye can be covalently bound to the polyamine structure to yield a fluorescent polyamine structure. In some implementations, the fluorescent dye can be attached by a reaction between primary and secondary amines of the polyamine structure with isothiocyanate containing fluorescent dyes. Suitable isothiocyanate dyes include fluorescein isothiocyanate (FITC) and Rhodamine B isothiocyanate (RBITC). For example, poly(allylamine hydrochloride) and FITC can react to form poly(fluorescein isothiocyanate allylamine hydrochloride).  FIG.  6    shows an example reaction of isothiocyanate with a primary or secondary amine to yield a substituted thiourea, where R is a fluorescent dye and where R I  and R H  can be H, alkyl, or aryl groups. 
     In some implementations, the fluorescent dye can be covalently bound to the polyamine structure with another type of amine-reactive functional group. For example, a fluorescent dye that contains an N-hydroxysuccinimide ester (NHS ester) will react to form a covalent bond with a primary or secondary amine.  FIG.  7    shows an example reaction between an NHS ester and a primary or secondary amine, where R is a fluorescent dye and where R I  and R II  can be H, alkyl, or aryl groups. 
     In some implementations, fluorescent polymer-clay composites can by synthesized with polyacrylic acid (PAA), polymethyl methacrylate (PMMA), polyvinylpyrrolidone (PVP), or polyvinyl alcohol (PVA). In some implementations, fluorescent polymer-clay composites can be synthesized with polystyrene-based polymers. Polystyrene (PS) based polymers have strong interactions with clays. The polystyrene-based polymers can be conjugated with a fluorescent dye. In some implementations, the dye is functionalized before conjugation with the polymer. For example, an isothiocyanate containing dye can be functionalized to introduce a reactive allyl group. Suitable isothiocyanate dyes include FITC and RBITC.  FIG.  8    shows an example reaction between an isothiocyanate containing dye and a reactant that includes a primary amine and an allyl group to yield an allyl-containing fluorescent dye. The reactant can be a monomer with allyl and amino groups. Suitable monomers with allyl and amino groups include allylamine, N-allylmethylamine, N-vinylformamide, 2-methyl-2-propen-1-amine, and 2-methylallylamine, 3-buten-1-amine, 4-vinylaniline, or 3-vinylaniline. In more detail,  FIG.  9    shows an example reaction between an isothiocyanate containing fluorescent dye and 3-buten-1-amine to yield an allyl-containing fluorescent dye. 
     Next, the allyl-containing fluorescent dye can be conjugated to a styrene-based monomer or an acrylate-based monomer by radical polymerization.  FIG.  10    shows an example reaction of an allyl-containing fluorescent dye with a styrene monomer, where free radical-induced polymerization yields a fluorescent styrene-based polymer with n number of repeating units. Depending on the numbers of n, the polymers can have molecular weights from hundreds to tens of thousands of Daltons. This polymerization reaction can take place in the presence of a nanoclay to yield a fluorescent polymer-clay composite.  FIG.  10    shows an example reaction with a styrene monomer, however, other styrene-based monomers or acrylate-based monomers can also be used. Suitable monomers include, but are not limited to, styrene, p-methyl styrene, p-methoxystyrene, 2,4-dimethyl styrene, 2,4,6-trimethyl styrene, 4-fluorostyrene, 3-fluorostyrene, 2-fluorostyrene, 4-chlorostyrene, 3-chlorostyrene, 2-chlorostyren, 4-bromostyrene, 3-bromostyrene, 2-bromostyrene, 4-vinylbenzyl chloride, allylbenzene, allylpentafluorobenzene, acrylate, benzyl acrylate, phenyl methacrylate, hexyl methacrylate, butyl methacrylate, isobutyl methacrylate, propyl methacrylate, vinyl methacrylate, methyl methacrylate, 2-hydroxyethyl methacrylate, pentafluorophenyl methacrylate, N-allylbenzylamine, (vinylbenzyl)trimethylammonium chloride, or N,N-dimethylvinylbenzylamine, or any combination thereof. Accordingly, the polymers that are formed can have different fingerprint-like molecular signatures and can be differentiated from one another using mass spectrometry, for example pyrolysis-gas chromatography-mass spectrometry. The different polymers can therefore be used to create a library of uniquely identifiable fluorescent polymer-clay composites. 
     As described herein, the fluorescent polymer-clay composites can be used in subterranean applications and subsequently analyzed by fluorescence spectroscopy and mass spectrometry. In some implementations, the fluorescent polymer-clay composite can be analyzed by releasing the fluorescent polymer from the surfaces of the nanoclay. For example, controlled release of the fluorescent polymer can be achieved by specific solvents or by adjusting the pH of a solution containing the fluorescent polymer-clay composite.  FIG.  11    shows an example schematic of this process, where the fluorescent polymer is dissociated from the complex by adjusting the pH or introducing a solvent. In some implementations, adjusting the pH to approximately 10 results in the release of a fluorescent polyamine polymer from the composite. The released fluorescent polyamine polymer can then be analyzed, for example with fluorescence analysis or mass spectroscopy as described herein. Alternatively, an organic solvent can be used to release the fluorescent polystyrene-based polymer from the composite. Suitable organic solvents include alcohol, acetone, acetonitrile, tetrahydrofuran, chloroform, and toluene. The released fluorescent polystyrene-based polymer can then be analyzed, for example with fluorescence analysis or mass spectroscopy as described herein. 
     The fluorescent polymer-clay composites described herein can be used as tags in subterranean applications, for example as an additive in drilling mud. The fluorescent polymer-clay composites described herein readily mix with water-based or oil-based drilling mud, for example by simple mechanical stirring. 
     The tags can be mixed with the drilling mud and flowed or pumped down a work string into a subterranean formation. The mud and tag mixture exits the work string at the drill bit. Accordingly, the tags come into contact with the subterranean formation for the first time as they exit the drill bit. The tags can embed into or decorate the subterranean formation and the cuttings produced by the drill. The drilling mud carries the cuttings to the surface of the wellbore, where they can be recovered and analyzed. 
     As described herein, the multi-modal nature of these tags allows for a two-stage analysis. In a first stage, for the collected cuttings, a rapid fluorescence measurement can be made in situ on the cuttings by a portable fluorescence imaging system or a portable reflectance fluorescence spectrometer. For the cutting samples exhibiting fluorescence, a second stage lab analysis can be applied. In the second stage, the cuttings with adsorbed nanoclay tags can be rinsed and dispersed into water or organic solvents (for example, methanol, ethanol, acetone, acetonitrile, tetrahydrofuran, chloroform, or toluene, etc.) or a mixture of water-organic solvent for example, a water-alcohol mixture). Sonication and pH adjustment may be applied to promote the fluorescent polymer release from the nanoclay to solution, and the filtered solution can be used for further precise analysis, for example, fluorescence spectroscopy or Pyrolysis-GC-MS. 
     For example,  FIG.  12    shows a flowchart of an example method  1200  of determining the origin location of a subterranean sample. At  1202 , the nanoparticle tag is mixed into a fluid. At  1204 , the fluid is flowed through a work string into a subterranean formation. At  1206 , subterranean samples are recovered from the subterranean formation. At  1208 , the origin location of the subterranean sample is determined by detecting the presence of the nanoparticle tag on the subterranean sample. 
     In some implementations, the fluorescent polymer is dissociated from the nanoclay, for example by adjusting the pH or with an organic solvent. Accordingly, the identity of the fluorescent polymer-clay composite can be determined by subsequent fluorescence or mass spectrometry analyses. 
     Example 1: Synthesis of Fluorescent Polyethylenimine—Clay Composites 
     0.1 grams of fluorescein isothiocyanate (FITC) or Rhodamine B isothiocyanate (RBITC) were dissolved in 20 mL of 95% ethanol in water. Next, 1 g of 50% polyethylenimine (PEI) was added to the solution under magnetic stirring. The mixture was allowed to react overnight, resulting in dye molecules that were grafted to PEI via stable covalent bonds. The solution was further diluted to 100 mL with water. 10 g of bentonite nanopowder was added with stirring. The bentonite and fluorescent PEI was allowed to react overnight to yield a fluorescent polymer-clay composite where the fluorescent PEI was intercalated into the bentonite nanoparticles.  FIG.  13    shows an example fluorescence spectrum of an FITC-PEI-bentonite functionalized polymer-clay composite, as well as the fluorescence spectrum of bentonite without functionalization. The excitation wavelength was 495 nm. As shown in  FIG.  13   , the functionalized polymer-clay composite shows a strong fluorescence signal whereas the bentonite itself is not fluorescent. 
     Example 2: Synthesis of Fluorescent Polystyrene—Clay Composites 
     1 g of 50% PEI in water was diluted in 80 mL of water, followed by the addition of 10 g of dry powder of bentonite nanoclays. The mixture was stirred for 24 hours to yield bentonite nanoclays with intercalated PEI. Next, 20 mL of dye solution (0.1 g of FITC or RBITC in 95% ethanol) was added to react with PEI overnight to yield the fluorescent polymer-clay composite.  FIG.  14    shows an example fluorescence spectrum of an RBITC-PS-bentonite functionalized polymer-clay composite, as well as the fluorescence spectra of bentonite without functionalization. The excitation wavelength was 540 nm. As shown in  FIG.  14   , the functionalized polymer-clay composite shows a strong fluorescence signal whereas the bentonite itself is not fluorescent. 
       FIGS.  15 A- 15 F  show SEM images of bentonite nanopowder before and after functionalization with FITC-PEI and RBITC-PS. 
       FIG.  15 A  shows an example SEM image of bentonite nanopowder before functionalization at 20,000× magnification.  FIG.  15 B  shows an example SEM image of bentonite nanopowder before functionalization at 50,000× magnification. 
       FIG.  15 C  shows an example SEM image of bentonite nanopowder functionalized with FITC-PEI at 20,000× magnification.  FIG.  15 D  shows an example SEM image of bentonite nanopowder functionalized with FITC-PEI at 50,000× magnification. 
       FIG.  15 E  shows an example SEM image of bentonite nanopowder functionalized with RBITC-polystyrene (PS) at 20,000× magnification.  FIG.  15 F  shows an example SEM image of bentonite nanopowder functionalized with RBITC-PS at 50,000× magnification. 
     The term “about” as used in this disclosure can allow for a degree of variability in a value or range, for example, within 10%, within 5%, or within 1% of a stated value or of a stated limit of a range. 
     The term “substantially” as used in this disclosure refers to a majority of, or mostly, as in at least about 50%, 60%, 70%, 80%, 90%, 95%, 96%, 97%, 98%, 99%, 99.5%, 99.9%, 99.99%, or at least about 99.999% or more. 
     The following units of measure have been mentioned in this disclosure: 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
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     In some implementations, a polymer-clay composite tag includes a nanoclay. The nanoclay includes a plurality of layers, and a polymer intercalated between the layers of the nanoclay. The polymer is functionalized with a fluorescent dye. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The polymer includes polyethylenimine or polyallylamine. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The polymer includes a polystyrene-based polymer. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The polystyrene-based or polyacrylate-based polymer is synthesized from monomers selected from styrene, p-methylstyrene, p-methoxystyrene, 2,4-dimethyl styrene, 2,4,6-trimethylstyrene, 4-fluorostyrene, 3-fluorostyrene, 2-fluorostyrene, 4-chlorostyrene, 3-chlorostyrene, 2-chlorostyrene, 4-bromostyrene, 3-bromostyrene, 2-bromostyrene, 4-vinylbenzyl chloride, allylbenzene, allylpentafluorobenzene, acrylate, benzyl acrylate, phenyl methacrylate, hexyl methacrylate, butyl methacrylate, isobutyl methacrylate, propyl methacrylate, vinyl methacrylate, methyl methacrylate, 2-hydroxyethyl methacrylate, pentafluorophenyl methacrylate, N-allylbenzylamine, (vinylbenzyl)trimethylammonium chloride, and N,N dimethylvinylbenzylamine, or any combination thereof. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The fluorescent dye is an isothiocyanate-containing dye. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The isothiocyanate-containing dye includes fluorescein isothiocyanate or Rhodamine B isothiocyanate. 
     In some implementations, a method of synthesizing a polymer-clay composite includes functionalizing a polymer with a fluorescent dye to yield a fluorescent polymer and intercalating the fluorescent polymer in a nanoclay. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The polymer includes polyethylenimine or polyallylamine. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. Functionalizing the polymer with a fluorescent dye includes reacting polyethylenimine or polyallylamine with an isothiocyanate-containing dye. 
     In some implementations, a method of synthesizing a polymer-clay composite includes intercalating styrene-based monomers or acrylate-based monomers and a fluorescent dye between the layers of a nanoclay and inducing radical polymerization to polymerize the styrene-based monomers and fluorescent dye to yield a fluorescent polymer-clay composite. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The styrene-based monomers or acrylate-based monomers include monomers selected from styrene, p-methylstyrene, p-methoxystyrene, 2,4-dimethyl styrene, 2,4,6-trimethylstyrene, 4-fluorostyrene, 3-fluorostyrene, 2-fluorostyrene, 4-chlorostyrene, 3-chlorostyrene, 2-chlorostyrene, 4-bromostyrene, 3-bromostyrene, 2-bromostyrene, 4-vinylbenzyl chloride, allylbenzene, allylpentafluorobenzene, acrylate, benzyl acrylate, phenyl methacrylate, hexyl methacrylate, butyl methacrylate, isobutyl methacrylate, propyl methacrylate, vinyl methacrylate, methyl methacrylate, 2-hydroxyethyl methacrylate, pentafluorophenyl methacrylate, N-allylbenzylamine, (vinylbenzyl)trimethylammonium chloride, and N,N dimethylvinylbenzylamine, or any combination thereof. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The fluorescent dye includes an isothiocyanate-containing dye. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The method includes functionalizing the isothiocyanate-containing dye to include an allyl group. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. Functionalizing the isothiocyanate-containing dye to include an allyl group includes functionalizing the isothiocyanate dye with a monomer containing an allyl group, wherein the allyl group-containing monomers include monomers selected from the group consisting of allylamine, N-allylmethylamine, N-vinylformamide, 2-methyl-2-propen-1-amine, and 2-methylallylamine, 3-buten-1-amine, and 4(or 3)-vinylaniline, or any combination thereof. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. Inducing radical polymerization includes inducing radical polymerization with a peroxide. 
     In some implementations, a method of determining the origin location of a subterranean sample includes mixing a polymer-clay composite tag into a fluid. The polymer-clay composite tag includes a nanoclay. The nanoclay includes layers and a fluorescent polymer intercalated between the layers of the nanoclay. The method includes flowing the fluid through a work string into a subterranean formation, recovering subterranean samples from the subterranean formation, and determining the origin location of the subterranean sample by detecting the presence of the polymer-clay composite tag. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. Detecting the presence of the polymer-clay composite tag includes analyzing the subterranean sample for a fluorescence signal. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The method includes analyzing the fluorescent polymer with mass spectrometry. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. Analyzing the fluorescent polymer with mass spectrometry further includes dissociating the fluorescent polymer from the nanoclay before analyzing with mass spectrometry. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. Analyzing the fluorescent polymer with mass spectrometry includes degrading the polymers via a pyrolysis process, and identifying the degradation products through a pyrolysis gas chromatography-mass spectroscopy (Pyrolysis-GC-MS) process. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The fluorescent polymer includes polyethylenimine or polyallylamine. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. Dissociating the fluorescent polymer from the nanoclay includes raising the pH of the polymer-clay composite tag. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The fluorescent polymer includes a polystyrene-based or polyacrylate-based polymer. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The polystyrene-based or polyacrylate-based polymer is synthesized from monomers selected from styrene, p-methylstyrene, p-methoxystyrene, 2,4-dimethyl styrene, 2,4,6-trimethyl styrene, 4-fluorostyrene, 3-fluorostyrene, 2-fluorostyrene, 4-chlorostyrene, 3-chlorostyrene, 2-chlorostyrene, 4-bromostyrene, 3-bromostyrene, 2-bromostyrene, 4-vinylbenzyl chloride, allylbenzene, allylpentafluorobenzene, acrylate, benzyl acrylate, phenyl methacrylate, hexyl methacrylate, butyl methacrylate, isobutyl methacrylate, propyl methacrylate, vinyl methacrylate, methyl methacrylate, 2-hydroxyethyl methacrylate, pentafluorophenyl methacrylate, N-allylbenzylamine, (vinylbenzyl)trimethylammonium chloride, and N,N dimethylvinylbenzylamine, or any combination thereof. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. Dissociating the fluorescent polymer from the nanoclay includes introducing an organic solvent to the nanoclay. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The organic solvent comprises alcohol, acetone, acetonitrile, tetrahydrofuran, chloroform, or toluene. 
     The term “solvent” as used in this disclosure refers to a liquid that can dissolve a solid, another liquid, or a gas to form a solution. Non-limiting examples of solvents are silicones, organic compounds, water, alcohols, ionic liquids, and supercritical fluids. 
     The term “room temperature” as used in this disclosure refers to a temperature of about 15 degrees Celsius (° C.) to about 28° C. 
     The term “downhole” as used in this disclosure refers to under the surface of the earth, such as a location within or fluidly connected to a wellbore. 
     As used in this disclosure, the term “drilling fluid” refers to fluids, slurries, or muds used in drilling operations downhole, such as during the formation of the wellbore. 
     As used in this disclosure, the term “stimulation fluid” refers to fluids or slurries used downhole during stimulation activities of the well that can increase the production of a well, including perforation activities. In some examples, a stimulation fluid can include a fracturing fluid or an acidizing fluid. 
     As used in this disclosure, the term “fracturing fluid” refers to fluids or slurries used downhole during fracturing operations. 
     As used in this disclosure, the term “fluid” refers to liquids and gels, unless otherwise indicated. 
     As used in this disclosure, the term “subterranean material” or “subterranean zone” refers to any material under the surface of the earth, including under the surface of the bottom of the ocean. For example, a subterranean zone or material can be any section of a wellbore and any section of a subterranean petroleum- or water-producing formation or region in fluid contact with the wellbore. Placing a material in a subterranean zone can include contacting the material with any section of a wellbore or with any subterranean region in fluid contact the material. Subterranean materials can include any materials placed into the wellbore such as cement, drill shafts, liners, tubing, casing, or screens; placing a material in a subterranean zone can include contacting with such subterranean materials. In some examples, a subterranean zone or material can be any downhole region that can produce liquid or gaseous petroleum materials, water, or any downhole section in fluid contact with liquid or gaseous petroleum materials, or water. For example, a subterranean zone or material can be at least one of an area desired to be fractured, a fracture or an area surrounding a fracture, and a flow pathway or an area surrounding a flow pathway, in which a fracture or a flow pathway can be optionally fluidly connected to a subterranean petroleum- or water-producing region, directly or through one or more fractures or flow pathways. 
     As used in this disclosure, “treatment of a subterranean zone” can include any activity directed to extraction of water or petroleum materials from a subterranean petroleum- or water-producing formation or region, for example, including drilling, stimulation, hydraulic fracturing, clean-up, acidizing, completion, cementing, remedial treatment, abandonment, aquifer remediation, identifying oil rich regions via imaging techniques, and the like. 
     As used in this disclosure, a “flow pathway” downhole can include any suitable subterranean flow pathway through which two subterranean locations are in fluid connection. The flow pathway can be sufficient for petroleum or water to flow from one subterranean location to the wellbore or vice-versa. A flow pathway can include at least one of a hydraulic fracture, and a fluid connection across a screen, across gravel pack, across proppant, including across resin-bonded proppant or proppant deposited in a fracture, and across sand. A flow pathway can include a natural subterranean passageway through which fluids can flow. In some implementations, a flow pathway can be a water source and can include water. In some implementations, a flow pathway can be a petroleum source and can include petroleum. In some implementations, a flow pathway can be sufficient to divert water, a downhole fluid, or a produced hydrocarbon from a wellbore, fracture, or flow pathway connected to the pathway. 
     As used in this disclosure, “weight percent” (wt %) can be considered a mass fraction or a mass ratio of a substance to the total mixture or composition. Weight percent can be a weight-to-weight ratio or mass-to-mass ratio, unless indicated otherwise. 
     A number of implementations of the disclosure have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosure.