Patent Publication Number: US-2023141819-A1

Title: Multifunctional magnetic tags for mud logging

Description:
TECHNICAL FIELD 
     This document relates to compositions and methods to determine an origin location of a subterranean cutting or sample. 
     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 nanoparticle tag includes a superparamagnetic iron oxide core, an intermediate layer comprising a fluorescent dye, and a polymer shell. 
     In some implementations, a method of making a nanoparticle tag includes providing a superparamagnetic iron oxide nanoparticle core, functionalizing the surface of the superparamagnetic iron oxide nanoparticle core to yield a functionalized nanoparticle core, and covalently bonding a fluorescent dye to the functionalized nanoparticle core. 
     In some implementations, a method of determining the origin location of a subterranean sample includes mixing a nanoparticle tag into a fluid, wherein the nanoparticle tag includes a superparamagnetic iron oxide core, an intermediate layer comprising a fluorescent dye, and a polymer shell. The method includes flowing the fluid through a work string into a subterranean formation, recovering subterranean samples from the subterranean formation, separating tagged samples from untagged samples using a magnet, and determining an origin location of the subterranean sample by analyzing the fluorescent signal of the nanoparticle 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 of a magnetic nanoparticle tag. 
         FIG.  3    shows an example reaction of an Fe 3 O 4  nanoparticle (NP) surface, trimethoxysilylpropyl modified polyethylenimine (silane-PEI), and allyltrimethoxysilane (ATMOS). 
         FIG.  4    shows an example reaction of an Fe 3 O 4  nanoparticle (NP) surface, 3-glycidoxypropyltrimethoxysilane (GPTMS), and allyltrimethoxysilane (ATMOS). 
         FIG.  5    shows an example reaction between an amine and an isothiocyanate to yield a thiourea. 
         FIG.  6    shows an example reaction of a PEI group and the isothiocyanate-containing dye FITC. 
         FIG.  7    shows an example reaction between a primary amine and an epoxy. 
         FIG.  8    shows an example reaction between a secondary amine and an epoxy resin. 
         FIG.  9    shows an example of an amine-containing dye, Rhodamine 123, binding to an epoxy-containing functionalized nanoparticle. 
         FIG.  10    shows a flowchart of an example method of determining the origin location of a subterranean cutting produced during drilling. 
         FIG.  11    shows an example TEM image of synthesized Fe 3 O 4  nanoparticles. 
         FIG.  12    shows an example SEM image of polystyrene coated, functionalized nanoparticles. 
         FIG.  13    shows an example fluorescence analysis of FITC-functionalized Fe 3 O 4  nanoparticles coated in polystyrene at an excitation wavelength of 495 nm. 
         FIG.  14    shows an example fluorescence analysis of RBITC-functionalized Fe 3 O 4  nanoparticles coated in polystyrene at an excitation wavelength of 530 nm. 
     
    
    
     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 tags, methods, and systems for tagging cuttings produced during a drilling operation. These tags, methods, and systems can be used to determine the origin location of a cutting or subterranean sample. The tags can absorb to, or decorate cuttings or subterranean samples. For example, the tags can have high affinity non-specific binding onto cuttings or subterranean samples as a result of physico-chemical and/or ionic forces, for example, Van der Waals, hydrophobic/hydrophilic interactions, and oppositely charged surfaces. The tags can be identified by multiple orthogonal techniques, and therefore the various combinations of orthogonally detectable features create a library of uniquely identifiable tags. 
       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. 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 magnetic nanoparticle tags (MNIPs) that can be used to tag subterranean cuttings or samples.  FIG.  2    shows an example of a magnetic nanoparticle tag  200 . The tag includes a magnetic core  202 , a fluorescent dye intermediate layer  204 , and a polymer shell  206 . The magnetic core can be either a single superparamagnetic nanoparticle or a cluster of aggregated superparamagnetic nanoparticles. The magnetic core allows samples tagged with the magnetic nanoparticle tags to be separated and collected by a magnetic field. The fluorescent dye can yield a fluorescent signal. The fluorescent signal can be detected and used to identify the magnetic tag. Further, the polymer shell  206  can be analyzed with mass spectrometry to produce a unique signal based on its constituent monomers. The polymer shell can be either a continuous coating layer or discrete patch decoration on the fluorescent magnetic cores. Unique combinations of fluorescence and mass spectrometry signals can be used to create a library of uniquely identifiable magnetic nanoparticle tags. Advantageously, all of the components of the MNPs are inexpensive and non-toxic. Therefore, these tags can be readily used in large quantities and in subterranean applications. 
     The tags are synthesized with covalent and/or physico-chemical bonding between the magnetic core, dye molecules, and polymer. The resulting composite nanoparticles are chemically stable in an aqueous environment. Accordingly, the tags are suitable for use in oil-based or water-based drilling muds and subterranean applications. 
     The magnetic core  202  can include inorganic iron oxides, for example magnetite Fe 3 O 4 , maghemite gamma-Fe 2 O 3 , cobalt ferrite CoFe 2 O 4 , nickel ferrite NiFe 2 O 4 , or manganese ferrite MnFe 2 O 4 . The size of the primary nanoparticles of iron oxide are controlled to be less than 15 nm, so that a single or cluster of iron oxide nanoparticles are superparamagnetic. Superparamagnetism is a form of magnetism that appears in small ferromagnetic or ferrimagnetic nanoparticles. In sufficiently small nanoparticles, magnetization can randomly flip direction under the influence of temperature, and thus their magnetization appears to be in average zero, i.e., in the superparamagnetic state. In this state, an external magnetic field is able to magnetize the nanoparticles, similarly to a paramagnet. Superparamagnetic iron oxide particles (SPIONs) can be magnetized by an external magnetic field, however, SPIONs do not show magnetic interactions after the external magnetic field is removed. Accordingly, tags that include a superparamagnetic core can be separated, collected, and preconcentrated by an applied magnetic field, for example by using a magnet. In some implementations, the magnetic properties of the core can be used to separate unbound tags from drilling mud. Accordingly, these tags can be removed from the mud and the mud can be used again, either without tags or with a new tag. This prevents residual tags from contributing to background signals or interfering with subsequent drilling operations. 
     The magnetic core  202  can be synthesized by a precipitation reaction of Fe 3+  and Fe ions at a 2:1 molar ratio in a basic solution at room temperature. The basic solution can include NaOH or NH 3 . H 2 O in solution. Equation 1 shows the precipitation of these ions to iron oxide Fe 3 O 4 . The superparamagnetic Fe 3 O 4  nanoparticles as the magnetic cores in these syntheses. In the presence of oxygen, the precipitated Fe 3 O 4  can slowly react with O 2  to yield gamma-Fe 2 O 3 , as shown by the Equation 2. The resulting gamma-Fe 2 O 3  nanoparticles are also superparamagnetic, and therefore does not interfere in the function of the resulting nanoparticle. 
     
       
         
           
             
               
                 
                   
                     
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     In some implementations, the surfaces of the iron oxide nanoparticles are functionalized by silane coupling agents. The silane coupling agents can contain different functional groups. For example, polyethylenimine (PEI) groups can be grafted onto the hydroxyl groups on the iron oxide surfaces through a hydrolysis reaction of trimethoxysilylpropyl modified polyethylenimine (silane-PEI). In some implementations, the PEI group is a linear polyethylenimine that contains secondary amines. In some implementations, the PEI group is a branched PEI group that contains primary, secondary, and tertiary amino groups. 
     In another example, allyl groups can be bound to the surface of the iron oxide nanoparticle by a reaction between the hydroxyl groups on the iron oxide surface and allyltrimethoxysilane (ATMOS) or 3-(trimethoxysilyl)propyl methacrylate (TMSPMA).  FIG.  3    shows an example reaction of an Fe 3 O 4  nanoparticle (NP) surface, silane-PEI with n and 4n repeating units giving a molecular weight 1500-1800 g/mol, and ATMOS to yield a functionalized nanoparticle. In this example, the functionalized nanoparticle contains PEI functional groups as well as allyl functional groups.  FIG.  4    shows an example reaction of an Fe 3 O 4  nanoparticle (NP) surface, 3-glycidoxypropyltrimethoxysilane (GPTMS), and ATMOS to yield a functionalized nanoparticle. In this example, the functionalized nanoparticle contains PEI function groups as well as epoxy functional groups.  FIGS.  3 - 4    illustrate reactions with an Fe 3 O 4  iron oxide core, however, it is understood that these reactions can also occur on the surface of a γ-Fe 2 O 3  nanoparticle, a CoFe 2 O 4  nanoparticle, a NiFe 2 O 4  nanoparticle, or a MnFe 2 O 4  nanoparticle. Therefore, the functionalized nanoparticle can contain either an Fe 3 O 4 , a γ-Fe 2 O 3 , a CoFe 2 O 4 , a NiFe 2 O 4 , or a MnFe 2 O 4  core. Further, the functionalized nanoparticle can be functionalized with a single functional group or with any combination of the functional groups described herein. For example, the functionalized nanoparticle can contain only PEI, epoxy, or allyl, or any combination of these groups. In some implementations, the functional groups on the functionalized nanoparticles are used to incorporate fluorescent dyes to yield the fluorescent dye layer. 
     The fluorescent dye layer  204  can include fluorescent dyes, for example fluorescein isothiocyanate (FITC), tetramethylrhodamine isothiocyanate (TRITC), and Rhodamine B isothiocyanate (RBITC). Other suitable dyes include Rhodamine 123, Congo Red, Evans Blue, and NIR-797, etc. The individual fluorescent dyes have unique fluorescence properties, i.e., different emission spectra under different excitation wavelengths. Therefore, in addition to being detectable by fluorescence imaging or spectroscopic methods, the fluorescent dyes can be used to differentiate unique tags from one another. 
     In some implementations, the fluorescent dyes are covalently bonded to a functionalized iron oxide nanoparticle by a reaction between a PEI group and an isothiocyanate-containing dye. The PEI group can contain primary, secondary, or tertiary amines. Isothiocyanate groups will react with primary and secondary amines to yield substituted thioureas.  FIG.  5    shows a general reaction between an amine and an isothiocyanate to yield a thiourea. The functional groups R I  and R II  can be H, alkyl, or aryl groups. Accordingly, fluorescent dye molecules that contain isothiocyanate can be covalently bonded to a functionalized nanoparticle that contains PEI groups. Suitable isothiocyanate-containing fluorescent dyes include fluorescein isothiocyanate (FITC), Rhodamine B isothiocyanate (RBITC), tetramethylrhodamine isothiocyanate (TRITC), and NIR-797 isothiocyanate. These dyes have demonstrated stability at subterranean and reservoir conditions, such as high temperatures and pressures, at least for several days. Accordingly, these dyes are suitable for use in drilling and wellbore operations.  FIG.  6    shows an example reaction of a PEI group and the isothiocyanate-containing dye FITC. 
     In some implementations, a fluorescent dye can be covalently bound to a functionalized nanoparticle that contains epoxy groups. For example, the reaction between an epoxy and an amine functional group yields a cured epoxy resin.  FIG.  7    shows the general reaction between a primary amine and an epoxy.  FIG.  8    shows the general reaction between a secondary amine and an epoxy resin. For nanoparticles that are functionalized to include epoxy groups, dye molecules with amine groups can covalently bind to the nanoparticles. For example, Rhodamine 123, Congo Red, and Evans Blue containing amino group can react with functionalized nanoparticles that contain epoxy groups.  FIG.  9    shows an example of an amine-containing dye, Rhodamine 123, binding to an epoxy-containing functionalized nanoparticle. 
     Each of the fluorescent dyes described herein have a unique emission spectrum under different excitation wavelengths. Therefore, each of these dyes, and the tags that incorporate these dyes, can be uniquely identified by fluorescence analysis. Table 1 lists the excitation and emission wavelengths of the dyes described herein. 
                     TABLE 1                  Maximum Excitation and Emission       Wavelengths of Fluorescent Dyes                                     Excitation   Emission           Dye   Wavelength (nm)   Wavelength (nm)                       FITC   495   519           Rhodamine 123   507   529           TRITC   544   570           RBITC   570   595           Congo Red   497   614           Evans Blue   540   680           NIR-797   795   817                        
In some implementations, the tags described herein can include a polymer shell. The polymer shell  206  can include a polymer that contains styrene-based, methacrylate-based, or amine-based monomers. This polymer can be a thermally depolymerizable or degradable polymer. Accordingly, the polymer can be decomposed into its constituent monomers, for example by pyrolysis-gas chromatography-mass spectrometry (pyrolysis-GC-MS). Polymers that contain unique monomers and/or unique amounts of monomers can be “fingerprinted” by pyrolysis-GC-MS analysis. In other words, tags with unique combinations of monomers can be differentiated from one another using mass spectrometry. Suitable monomers include styrene, p-methylstyrene, p-methoxystyrene, 2,4-dimethylstyrene, 2,4,6-trimethylstyrene, 4-chlorostyrene, 3-chlorostyrene, 2-chlorostyrene, 4-bromostyrene, 3-bromo styrene, 2-bromostyrene, 4-fluorostyrene, 3-fluorostyrene, 2-fluorostyrene, 4(trifluoromethyl)styrene, 3-(trifluoromethyl)styrene, 2-(trifluoromethyl)styrene, 2,3,4,5,6-pentafluorostyrene, allylbenzene, allylpentafluorobenzene, phenyl methacrylate, hexyl methacrylate, butyl methacrylate, isobutyl methacrylate, propyl methacrylate, vinyl methacrylate, methyl methacrylate, 2-hydroxyethyl methacrylate, pentafluorophenyl methacrylate, allylamine, 3-buten-1-amine, N-allylmethylamine, allylmethylamine, N-vinylformamide, 2-methyl-2-propen-1-amine, and 2-methylallylamine.
 
     In a general procedure for the polymer coating, 100 mL of fluorescence dye-labeled Fe 3 O 4  nanoparticle suspension at 1-5 wt % in DI water in a round-bottom flask was degassed with N 2  for 15 min, and then 2.5 g sodium dodecyl sulfate (SDS) was dissolved in the DI water. Next, 0.2 g of potassium persulfate (K 2 S 2 O 8 ) was added with stirring under an N 2  purge. After dissolution of the K 2 S 2 O 8  initiator and the solution was heated to 90° C., 1-5 mL of styrenic monomer was injected at 0.02 mL/min by a syringe using a programmable syringe pump. After addition of the monomer, the reaction was allowed to proceed for 30 min and the reaction was then cooled to room temperature. The weight ratio of nanoparticle core and polymer shell in the synthesized composite particles can be adjusted by relative concentration of the nanoparticle suspension and monomer solution. 
     In some implementations, allyl groups that are present in the functionalized iron oxide nanoparticles can be used to react with polymers to create the polymer coating. These allyl groups can react with monomers during radical induced polymerization by persulfate ions, which chemically anchors the polymer layer onto the nanoparticles. Free-radical polymerization is a type of chain-growth polymerization in polymer synthesis. 
     The fluorescent dyes and polymer shells describe in this application can be used in any combination to result in a library of uniquely identifiable tags. These tags can be used as mud logging tracers. For example, 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 bind onto surface of reservoir rocks through physical adsorption as a result of physico-chemical and/or ionic forces, for example, Van der Waals, hydrophobic/hydrophilic interactions, and oppositely charged surfaces. Thus the tags can embed into or decorate the subterranean formation and the cuttings produced by the drill. The drilling mud carries the cuttings with the tags to the surface of the wellbore, where they can be recovered and analyzed. In some implementations, the cuttings recovered at the surface are cursorily washed with water before subsequent separation and analysis to remove any unbound tags on the cutting surface. 
     In some implementations, the tagged cuttings or samples are separated from the untagged samples using a magnet. Advantageously, this also pre-concentrates the tagged samples, and reduces the number of samples that need to be subsequently analyzed. The identity of the tags can be determined by a number of techniques, including fluorescent analysis and mass spectrometry. Fluorescent analysis can be used to determine the identity of the fluorescent dye present in the tag. Typical fluorescence analysis includes fluorescence imaging or fluorescence spectroscopy. The fluorescence images can be taken by a camera system or fluorescence microscopy under UV or visible light excitation, while the fluorescence spectra can be recorded by a portable spectroscopic system on site. In some implementations, the fluorescent analysis can occur at the wellbore or drilling site. The fluorescent analysis can provide a first set of real-time data about the tags and cuttings. This data can be used to make subsequent decisions about drilling operations. Alternatively or in combination, the tags can be analyzed with mass spectrometry, for example pyrolysis-gas chromatography-mass spectrometry, as described herein. In some implementations, the mass spectrometry analysis can be done at the wellbore or drilling site. In other implementations, the mass spectrometry analysis occurs off-site, for example in a laboratory. 
     The tags described herein can be engineered with unique fluorescence and mass spectrometry signals, as described in detail above. Accordingly, different combinations of different fluorescence and mass spectrometry signals can create a library of uniquely identifiable tags. In some implementations, a first tag can be introduced to a subterranean formation at a first time point, and a second tag can be introduced to a subterranean formation at a second time point. Therefore, when the position of the drill bit and the lag time of the tags as they travel through the work string is known, cuttings or subterranean samples tagged with a first tag can be assigned a first origin location, and subterranean samples tagged with a second tag can be assigned to a second origin location. The number of tags is not limited to two, and a plurality of tags can be used to assign a plurality of origin locations. Further, unbound tags can be separated from drilling mud using a magnet. Accordingly, the mud can be recycled and reused for subsequent tags, without background interference from previous tags. 
       FIG.  10    shows a flowchart of an example method  1000  of determining the origin location of a subterranean cutting produced during drilling. At  1002 , a magnetic nanoparticle tag is mixed into a fluid. At  1004 , the fluid is flowed through a work string into a subterranean formation. At  1006 , subterranean samples are recovered from the subterranean formation. At  1008 , a magnet is used to separate tagged samples from untagged samples. At  1010 , the origin location of the subterranean sample is determined by analyzing the magnetic nanoparticle tag. 
     Example 1: Synthesis of Fe 3 O 4  Nanoparticles 
     A 500 mL solution of 0.1 M Fe 3+  and 0.05 M Fe 2+  was prepared by dissolving 27.0 g FeCl 3 .6H 2 O and 13.9 g FeSO 4 .7H 2 O in deionized (DI) water in a conical flask, then 25 mL of 29.5 wt % ammonia solution was added dropwise at a rate of 2.5 mL/min into the solution under vigorous stirring. With the addition of ammonia solution, iron hydroxide nanoparticles formed immediately, and the solution turns to black and viscous and then to deep brown and becomes more fluid. The formed colloidal suspension was continuously stirred for &gt;12 hours at room temperature, which ages iron hydroxide to iron oxide in water suspension. According to the stoichiometric ratio, nominal 5.84 g Fe 3 O 4  was produced in the batch of synthesis.  FIG.  11    shows an example TEM image of synthesized Fe 3 O 4  nanoparticles. 
     Example 2: Surface Functionalization of Fe 3 O 4  Iron Oxide Nanoparticles with FITC 
     In a typical functionalization of the synthesized Fe 3 O 4  NPs, a mixture of silane agents, 1 g trimethoxysilylpropyl modified polyethylenimine (Silane-PEI) and 0.25 g allyltrimethoxysilane (ATMS) in 25 mL ethanol, were added to the above iron oxide suspension, and the reaction was allowed to complete under stirring for another 12 hours. The functionalized Fe 3 O 4  NPs were collected by a magnet and redispersed in 500 mL water. To label the magnetic NPs by fluorescent dye, 0.05 g FITC in 25 mL water, was added to the functionalized Fe 3 O 4  suspension and stirring for 6 hours, allowing the dye molecules covalently bonding to amine groups. 
     Example 3: Polystyrene Coating of FITC-Functionalized Fe 3 O 4  Nanoparticles 
     To coat the magnetic FITC labeled Fe 3 O 4  NPs with a polymer, 2 mL of tetramethylammonium hydroxide (TMAOH) was added to the Fe 3 O 4  suspension under mechanical stirring. After 15 min of deoxygenating with N 2  bubbling, 6.0 mL styrene was added into the reaction mixture. The reaction mixture was heated to 75° C. and 1.0 g ammonium persulfate was added to initiate polymerization, which reacted for 2 h. The resulting polystyrene (PS) coated magnetic fluorescent NPs were harvested by a magnet. With this example synthesis procedure, the weight ratio of magnetic NP to the polymer is about 1:1 in the resulting composite particles. Depending on the amount of monomer used during the coating process, the weight ratio of magnetic NP to the polymer can be tuned from 9:1 to 1:9. 
       FIG.  12    shows an example SEM image of polystyrene coated, functionalized nanoparticles. 
     Example 4: Fluorescence Analysis of FITC-Functionalized Fe 3 O 4  Coated Nanoparticles 
       FIG.  13    shows an example fluorescence analysis of FITC-functionalized Fe 3 O 4  nanoparticles coated in polystyrene at an excitation wavelength of 495 nm. These functionalized, coated nanoparticles were analyzed at a concentration of 1 wt %, 5 wt % and 25 wt % in mud. A tagged mud was prepared by mixing the synthesized FITC-Fe 3 O 4 —PS nanoparticles and clay powder (bentonite) at 1:99, 5:95 and 25:75 weight ratios, and then 1 g of the mixture was mixed with 10 mL water. The slurry with fluorescent magnetic tags was applied to wet the core when cutting a core of dolomite rock (diameter 3.8 cm) by a Buehler Isomet low speed saw. A piece of the sliced rock was collected and sonicated in 5 mL of water, and then the water suspension was transferred to a quartz cell. For the fluorescence measurement, a neodymium magnet was placed against a wall of the quartz cell to collect the magnet tagged particles for 10 mins, and then the spectra were recorded on the collected particles by a Horiba NanoLog-3 fluorescence spectrometer in a front face reflectance mode at excitation wavelength of 495 nm. 
     Example 5: Surface Functionalization of Fe 3 O 4  Iron Oxide Nanoparticles with RBITC 
     To functionalize the surface of the synthesized Fe 3 O 4  NPs, a mixture of silane agents, 1 g trimethoxysilylpropyl modified polyethylenimine (Silane-PEI) and 0.25 g allyltrimethoxysilane (ATMS) in 25 mL ethanol, was added to the iron oxide suspension as described in Example 1, and the reaction was allowed to complete under stirring for another 12 hours. The functionalized Fe 3 O 4  NPs were collected by a magnet and redispersed in 500 mL water. To label the magnetic NPs with the dye RBITC, 0.05 g RBITC in 25 mL water was added to the functionalized Fe 3 O 4  suspension and stirred for 6 hours, allowing the dye molecules to covalently bond to amine groups. 
     Example 6: Polystyrene Coating of RBITC-Functionalized Fe 3 O 4  Nanoparticles 
     To coat the magnetic RBITC labeled Fe 3 O 4  NPs with a polymer, 2 mL of tetramethylammonium hydroxide (TMAOH) was added to the Fe 3 O 4  suspension under mechanical stirring. After 15 min of deoxygenating with N 2  bubbling, 6.0 mL methyl methacrylate was added into the reaction mixture. The reaction mixture was heated to 75° C. and 1.0 g ammonium persulfate was added to initiate polymerization, which reacted for 2 h. The resulting polymethyl methacrylate (PMMA) coated magnetic fluorescent NPs were harvested by a magnet. With this example synthesis procedure, the weight ratio of magnetic NP/polymer is about 1:1 in the resulting composite particles. Depending on the amount of monomer used during the coating process, the weight ratio of magnetic NP/polymer could be tuned from 9:1 to 1:9. 
     Example 7: Fluorescence Analysis of RBITC-Functionalized Fe 3 O 4  Coated Nanoparticles 
       FIG.  14    shows an example fluorescence analysis of RBITC-functionalized Fe 3 O 4  nanoparticles coated in polystyrene at an excitation wavelength of 530 nm. These functionalized, coated nanoparticles were analyzed at a concentration of 1 wt %, 5 wt % and 25 wt % in mud. A tagged mud was prepared by mixing the synthesized RBITC-Fe 3 O 4 —PS nanoparticles and clay powder (bentonite) at 1:99, 5:95 and 25:75 weight ratios, and then 1 g of the mixture was mixed with 10 mL water. The slurry with fluorescent magnetic tags was applied to wet the core when cutting a core of dolomite rock (diameter 3.8 cm) by a Buehler Isomet low speed saw. A piece of the sliced rock was collected and sonicated in 5 mL of water, and then the water suspension was transferred to a quartz cell. For the fluorescence measurement, a neodymium magnet was placed against a wall of the quartz cell to collect the magnet tagged particles for 10 mins, and then the spectra were recorded on the collected particles by a Horiba NanoLog-3 fluorescence spectrometer in a front face reflectance mode at excitation wavelength of 495 nm. 
     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: 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                 Unit of Measure 
                 Full form 
               
               
                   
                   
               
             
            
               
                   
                 g 
                 gram 
               
               
                   
                 mL 
                 milliliter 
               
               
                   
                 M 
                 molar 
               
               
                   
                 cm 
                 centimeter 
               
               
                   
                 nm 
                 nanometer 
               
               
                   
                 wt % 
                 weight percent 
               
               
                   
                 ° C. 
                 degrees Celsius 
               
               
                   
                 min 
                 minute 
               
               
                   
                 h 
                 hour 
               
               
                   
                   
               
            
           
         
       
     
     In some implementations, a nanoparticle tag includes a superparamagnetic iron oxide core, an intermediate layer comprising a fluorescent dye, and a polymer shell. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The superparamagnetic iron oxide core includes Fe 3 O 4 . 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The superparamagnetic iron oxide core includes γ-Fe 2 O 3 . 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The superparamagnetic iron oxide core includes CoFe 2 O 4 , NiFe 2 O 4 , or MnFe 2 O 4 . 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The fluorescent dye includes an isothiocyanate functional group. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The fluorescent dye includes fluorescein isothiocyanate, Rhodamine B isothiocyanate, tetramethylrhodamine isothiocyanate, or NIR-797, or any combination thereof. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The fluorescent dye includes an amine functional group. The fluorescent dye includes Rhodamine 123, Congo Red, or Evans Blue, or any combination thereof. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The polymer shell includes styrene-based monomers, methacrylate-based monomers, or amine-based monomers, or any combination thereof. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The polymer shell includes monomers selected from the group consisting of styrene, p-methylstyrene, p-methoxystyrene, 2,4-dimethylstyrene, 2,4,6-trimethylstyrene, 4-chlorostyrene, 3-chlorostyrene, 2-chlorostyrene, 4-bromostyrene, 3-bromostyrene, 2-bromostyrene, 4-fluorostyrene, 3-fluorostyrene, 2-fluorostyrene, 4-(trifluoromethyl)styrene, 3-(trifluoromethyl)styrene, 2-(trifluoromethyl)styrene, 2,3,4,5,6-pentafluorostyrene, allylbenzene, allylpentafluorobenzene, phenyl methacrylate, hexyl methacrylate, butyl methacrylate, isobutyl methacrylate, propyl methacrylate, vinyl methacrylate, methyl methacrylate, 2-hydroxyethyl methacrylate, pentafluorophenyl methacrylate, allylamine, 3-buten-1-amine, N-allylmethylamine, allylmethylamine, N-vinylformamide, 2-methyl-2-propen-1-amine, and 2-methylallylamine, or any combination thereof. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The weight ratio of the superparamagnetic iron oxide core to the polymer is from 9:1 to 1:9. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The weight ratio of the superparamagnetic iron oxide core to the polymer is 1:1. 
     In some implementations, a method of making a nanoparticle tag includes providing a superparamagnetic iron oxide nanoparticle core, functionalizing the surface of the superparamagnetic iron oxide nanoparticle core to yield a functionalized nanoparticle core, and covalently bonding a fluorescent dye to the functionalized nanoparticle core. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The method includes covalently bonding a polymer to the functionalized nanoparticle core. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The polymer includes styrene-based monomers, methacrylate-based monomers, or amine-based monomers, or any combination thereof. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The polymer includes monomers selected from the group consisting of styrene, p-methylstyrene, p-methoxystyrene, 2,4-dimethylstyrene, 2,4,6-trimethylstyrene, 4-chlorostyrene, 3-chlorostyrene, 2-chlorostyrene, 4-bromostyrene, 3-bromostyrene, 2-bromostyrene, 4-fluorostyrene, 3-fluorostyrene, 2-fluorostyrene, 4-(trifluoromethyl)styrene, 3-(trifluoromethyl)styrene, 2-(trifluoromethyl)styrene, 2,3,4,5,6-pentafluorostyrene, allylbenzene, allylpentafluorobenzene, phenyl methacrylate, hexyl methacrylate, butyl methacrylate, isobutyl methacrylate, propyl methacrylate, vinyl methacrylate, methyl methacrylate, 2-hydroxyethyl methacrylate, pentafluorophenyl methacrylate, allylamine, 3-buten-1-amine, N-allylmethylamine, allylmethylamine, N-vinylformamide, 2-methyl-2-propen-1-amine, and 2-methylallylamine, or any combination thereof. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The weight ratio of the superparamagnetic iron oxide core to the polymer is from 9:1 to 1:9. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The weight ratio of the superparamagnetic iron oxide core to the polymer is 1:1. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The superparamagnetic iron oxide nanoparticle core includes Fe 3 O 4 . 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The superparamagnetic iron oxide nanoparticle core includes γ-Fe 2 O 3 . 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The superparamagnetic iron oxide nanoparticle core includes CoFe 2 O 4 , NiFe 2 O 4 , or MnFe 2 O 4 . 
     This aspect, taken alone or combinable with any other aspect, can include the following features. Functionalizing the surface of the superparamagnetic iron oxide nanoparticle core includes functionalizing the surface of the superparamagnetic iron oxide core with polyethylenimine groups, allyl groups, or epoxy groups, or any combination thereof. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. Functionalizing the surface of the superparamagnetic iron oxide core with polyethylenimine groups, allyl groups, or epoxy groups, or any combination thereof includes functionalizing the surface of the superparamagnetic iron oxide core with trimethoxysilylpropyl modified polyethylenimine, 3-(trimethoxysilyl)propyl methacrylate, or glycidoxypropyltrimethoxysilane, 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 functional group. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The fluorescent dye includes fluorescein isothiocyanate, Rhodamine B isothiocyanate, tetramethylrhodamine isothiocyanate, or NIR-797, or any combination thereof. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The fluorescent dye includes an amine functional group. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The fluorescent dye comprises Rhodamine 123, Congo Red, or Evans Blue, or any combination thereof. 
     In some implementations, a method of determining the origin location of a subterranean sample includes mixing a nanoparticle tag into a fluid, wherein the nanoparticle tag includes a superparamagnetic iron oxide core, an intermediate layer comprising a fluorescent dye, and a polymer shell. The method includes flowing the fluid through a work string into a subterranean formation, recovering subterranean samples from the subterranean formation, separating tagged samples from untagged samples using a magnet, and determining an origin location of the subterranean sample by analyzing the fluorescent signal of the nanoparticle tag. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The method includes analyzing the polymer shell of the nanoparticle tag with mass spectroscopy. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. Analyzing the polymer shell with mass spectroscopy includes analyzing the polymer shell with pyrolysis-gas chromatography-mass spectrometry. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The method includes washing the subterranean sample with water before determining the origin location of the subterranean sample. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The method includes removing unbound tags from the fluid using a magnet. 
     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 “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.