Patent Publication Number: US-2023141596-A1

Title: Multifunctional fluorescent tags for subterranean applications

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 nanoparticle tag includes a fluorescent core and a polymer shell. The fluorescent core includes an up-converting material. The polymer shell includes styrene-based monomers or methacrylate-based monomers, or any combination thereof. 
     In some implementations, a nanoparticle tag includes a fluorescent core and a polymer shell. The fluorescent core includes a long-persistent fluorescent material. The long-persistent fluorescent material consists essentially of CaTiO 3 :Pr 3+ , CaAl 2 O 4 :Eu 2+ /Nd 3+ , Ca 3 (PO 4 ) 2 :Sm 3+ /Mn 2+ , SrAl 2 O 4 :Eu 2+ /Dy 3+ , SrMgSi 2 O 7 :Eu/Dy, Sr 4 Al 14 O 25 :Eu/Dy, ZnGa 2 O 4 :Cr 3+ , LiGa 5 O 8 :Cr 3+ , Zn 3 Ga 2 Ge 2 O 10 :Cr 3+ /Pr 3+ , or Zn 3 Ga 2 Ge 2 O 10 :Cr 3+ /Eu 3+ , or any combinations thereof. The polymer shell includes styrene-based monomers or methacrylate based monomers, or any combinations thereof. 
     In some implementations, a nanoparticle tag includes a fluorescent core and a polymer shell. The fluorescent core includes an encapsulated lanthanide complex where the encapsulated lanthanide complex includes a lanthanide ion where all metal binding sites of the lanthanide ion are occupied by ligands. The polymer shell includes styrene-based monomers or methacrylate-based monomers, or any combination thereof. 
     In some implementations, a method of determining the origin location of a subterranean rock sample includes mixing a nanoparticle tag into a fluid, flowing the fluid through a work string into a subterranean formation, recovering subterranean rock samples from the subterranean formation, and determining an origin location of the subterranean rock sample by detecting the presence of the nanoparticle tag on the subterranean rock sample. 
     In some implementations, a method of characterizing the origin depth of a cut rock sample includes using a barcoded tracer to determine the origin location based on the travel time of the produced rock cuttings from a drill head to the surface via an annulus. The barcoded tracer includes a nanoparticle tag with a fluorescent core and a polymer shell. The method includes using fluorescence detection to detect the presence of the nanoparticle tags on the cut rock sample and analyzing the polymer shell to yield fingerprint monomer information of the nanoparticle tags on the cut rock sample. 
     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 of a drilling treatment for a well. 
         FIG.  2    shows an example schematic of a nanoparticle tag. 
         FIG.  3    shows an SEM image of an example up-converting NaYF 4 :Yb/Er nanoparticle core. 
         FIG.  4    shows an emission spectrum of a NaYF 4 :Yb/Er nanoparticle. 
         FIG.  5    shows an example SEM image of Cr 3+ -doped lithium gallate nanoparticles. 
         FIG.  6    shows an example of a long-persistent spectrum of LiGa 5 O 8 :Cr 3+  that was recorded after excitation by a UV-lamp (at 254 nm) for 5 minutes. 
         FIG.  7    shows an SEM image of an example of synthesized polystyrene-coated up-converting NaYF 4 :Yb/Er nanoparticles. 
         FIG.  8    shows an example SEM image of a Eu(TTA) 3 phenanthroline complex coated with polystyrene. 
         FIG.  9    shows an example SEM image of a Dy(TTA) 3 phenanthroline complex. 
         FIG.  10    shows an example of a time-resolved fluorescence spectrum of a polystyrene-coated SLE nanoparticle. 
         FIG.  11    shows an example of the visible fluorescence of up-converting nanoparticles which exhibit blue color by NaYF 4 :Yb/Tm (i), green color by NaYF 4 :Yb/Er (ii), and orange color by NaYF 4 :Yb/Ho (iii). 
         FIG.  12    shows an example time-gate fluorescence image of a long-persistent nanoparticle in bentonite mud. 
         FIG.  13    shows an example spectrum of NaYF 4 :Yb/Er powder with bentonite powder. 
         FIG.  14    shows a flowchart of an example method of determining the origin location of a subterranean rock sample. 
     
    
    
     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, permanently, or semi-permanently 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 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.” In addition, the combination of orthogonally detectable features expands the number of uniquely identifiable tags that can be produced and uniquely identified. 
     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 first 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 first analysis, 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. 
     The tags can include multi-functional composite nanoparticles. As used herein, in some embodiments, a nanoparticle is less than 1000 nm in diameter. In some embodiments, the nanoparticle is less than 5000 nm in diameter. In some embodiments, the nanoparticle is less than 10 μm in diameter. 
       FIG.  2    shows an example schematic of a multi-modal tag  200 . In some implementations, the tags include an inorganic fluorescent core  202  and a thermally depolymerizable or degradable shell  204 . The core  202  and the shell  204  can be determined by orthogonal detection means. For example, the fluorescent core can be detected by fluorescence spectroscopy and the shell can be detected by mass spectrometry. Accordingly, a library of tags can be created where each tag includes a unique combination of fluorescence and mass spectrometry signals. The tags can therefore include “barcodes” and can be uniquely identified. 
     As described herein, the multi-modal nature of these tags allows for a two-stage analysis. In a first stage, a rapid, fluorescence measurement can be made. Cuttings that fluoresce (i.e., contain a fluorescent tag), can be manually or automatically separated and can be designated for further analysis. In a second stage, the depolymerizable or degradable shell can be analyzed with mass spectrometry. 
     For example,  FIG.  14    shows a flowchart of an example method  1400  of determining the origin location of a subterranean rock sample. At  1402 , the nanoparticle tag is mixed into a fluid. At  1404 , the fluid is flowed through a work string into a subterranean formation. At  1406 , subterranean rock samples are recovered from the subterranean formation. At  1408 , the origin location of the subterranean rock sample is determined by detecting the presence of the nanoparticle tag on the subterranean rock sample. 
     In some implementations, the inorganic fluorescent core  202  can include an up-conversion nanoparticle. Up-conversion refers to a phenomenon where a material absorbs two or more incident photons with relatively low energy and emits a single photon with higher energy. Generally, absorption occurs in the near-infrared range, and emission occurs in the visible, near-infrared (at shorter wavelengths) or ultraviolet regions of the electromagnetic spectrum. Up-conversion can occur in rare earth element doped oxides. In contrast, most of the materials that are typically encountered during drilling, for example rock, mud, and crude oil, do not display up-conversion properties. Therefore, a tag that includes up-conversion properties can be easily identified in mud or subterranean materials, without interference from background up-conversion signals. Further, the lack of up-conversion properties in the drilling materials and naturally occurring subterranean materials negates the need for complex sample preparation, for example, separation or purification steps. 
     The up-converting nanoparticles in the core  202  can include rare-earth element doped oxides. In addition to displaying up-conversion properties, these materials are environmentally friendly and can be used in subterranean formations. The up-converting nanoparticles can include a crystal structure that is doped with a rare earth element. The rare earth elements can include ytterbium (Yb), erbium (Er), holmium (Ho), thulium (Th), and combinations thereof. The crystal structure can include sodium salts of yttrium fluoride (NaYF 4 ), sodium salts of gadolinium fluoride (NaGdF 4 ), calcium titanate (CaTiO 3 ) and ytterbium phosphate (YPO 4 ). Examples of the nanoparticles including these elements and salts are shown in Table 1. The fluorescent core can consist essentially of the nanoparticles listed in Table 1, or any combination thereof. In some implementations, the fluorescent core and include a binder or other inactive material. 
     
       
         
           
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Examples of Up-Converting Nanoparticle Cores 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                   
                 NaYF 4 :Yb/Er 
               
               
                   
                 NaYF 4 :Yb/Ho 
               
               
                   
                 NaYF 4 :Yb/Tm 
               
               
                   
                 NaGdF 4 :Yb/Er 
               
               
                   
                 NaGdF 4 :Yb/Ho 
               
               
                   
                 NaGdF 4 :Yb/Tm 
               
               
                   
                 YPO 4 :Yb/Er 
               
               
                   
                 YPO 4 :Yb/Ho 
               
               
                   
                 YPO 4 :Yb/Tm 
               
               
                   
                 CaTiO 3 :Yb/Er 
               
               
                   
                 CaTiO 3 :Yb/Ho 
               
               
                   
                 CaTiO 3 :Yb/Tm 
               
               
                   
                   
               
            
           
         
       
     
     The nanoparticle cores as shown in Table 1 emit light in different colors at a visible range under near infrared (NIR) excitation. The visible spectrum is between 400 nm and 700 nm. For example, nanoparticles doped with Yb/Tm can emit blue light, nanoparticles doped with Yb/Er can emit green light, and nanoparticles doped with Yb/Ho can emit orange light. Accordingly, any cut rock samples that are recovered at the surface can be excited with a near infrared source. Any rock samples that emit visible light can be collected and allocated for subsequent mass spectrometry analysis. 
     In some implementations, the inorganic fluorescent core  202  can include a long-persistent nanoparticle. Long-persistent or persistent luminescence is a type of photoluminescence where the emission persists for seconds, minutes, hours, or even days after ceasing the excitation irradiation. This is also referred to as phosphorescence or “afterglow.” Most of the materials encountered during drilling, for example, rock, mud, and crude oil, do not exhibit persistent luminescence. Therefore, a tag that includes a long-persistent material can be uniquely identified without background luminescence signals from surrounding materials. In addition, long-persistent luminescent nanoparticles can be detected without constant external illumination, for example, in the dark, to avoid background interference from drilling materials, such as naturally occurring auto-fluorescence or light scattering. 
     The long-persistent nanoparticles can include crystals structures doped with praseodymium ions (Pr 3+ ), europium ions (Eu 2+ ), neodymium ions (Nd 3+ ), samarium ions (Sm 3+ ), manganese ions (Mn 2+ ), dysprosium ions (Dy 3+ ), chromium ions (Cr 3+ ), or combinations thereof. The crystal structures can include calcium titanium oxide (CaTiO 3 ), calcium aluminum oxide (CaAl 2 O 4 ), calcium phosphate (Ca 3 PO 4 ) 2 , SrAl 2 O 4 , Sr 4 Al 14 O 25 , SrMgSi 2 O 7 , zinc gallium oxide (ZnGa 2 O 4 ), zinc gallogermanates (Zn 3 Ga 2 Ge 2 O 10 ), or lithium gallates (LiGa 5 O 8 ). Examples of long-persistent cores including these elements are shown in Table 2. 
     
       
         
           
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Examples of Long-Persistent Nanoparticle Cores 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                   
                 CaTiO 3 :Pr 3+   
               
               
                   
                 CaAl 2 O 4 :Eu 2+ /Nd 3+   
               
               
                   
                 Ca 3 (PO 4 ) 2 :Sm 3+ /Mn 2+   
               
               
                   
                 SrAl 2 O 4 :Eu 2+ /Dy 3+   
               
               
                   
                 SrMgSi 2 O 7 :Eu 2+ /Dy 3+   
               
               
                   
                 Sr 4 Al 14 O 25 :Eu 2+ /Dy 3+   
               
               
                   
                 ZnGa 2 O 4 :Cr 3+   
               
               
                   
                 Zn 3 Ga 2 Ge 2 O 10 :Cr 3+ /Pr 3+   
               
               
                   
                 Zn 3 Ga 2 Ge 2 O 10 :Cr 3+ /Eu 3+   
               
               
                   
                 LiGa 5 O 8 :Cr 3+   
               
               
                   
                   
               
            
           
         
       
     
     These long-persistent nanoparticles emit different wavelengths in visible or NIR ranges after being charged by ultraviolet (UV) or visible light, for example the UV and visible portion of sunlight or by a flashlight. Since the long-persistent cores continue to phosphoresce even after removal of the excitation source, the images of the glowing long-persistent nanoparticles can be captured in darkness. The detection sensitivity of these nanoparticles can be achieved at a pico-gram (10 −12  g) level for pure powder samples. 
     Accordingly, any cut rock samples that are recovered at the surface can be excited with a UV or visible light source. Any rock samples that emits a long-persistent signal can be collected and allocated for subsequent mass spectrometry analysis. 
     In some implementations, the nanoparticle tag can include an encapsulated lanthanide complex. Lanthanides have unique time-resolved luminescence effects. Accordingly, lanthanides can be identified with a time-gated fluorescence imaging method. For example, a rock sample can be irradiated with UV light at 254 nm, followed by a delay, followed by a fluorescence measurement. The delay can be configured to be long enough that any natural fluorescence decays, while the fluorescence of the lanthanide persists. Therefore, by making the fluorescence measurement delayed, the lanthanide ions can be detected without background interference from any naturally fluorescent material present in the rock sample. 
     In some implementations, the lanthanide ions in the tags are complexed. For example, the lanthanide ions can be complexed with LX-type ligands, or with dative (L2) type ligands. LX-type ligands include β-diketonates, for example 1,1,1-trifluoro-3-(2-theonyl)acetone (TTA) and benzyl trifluoroacetone (BTFA). These ligands can act as an antenna for energy transfer, which adsorbs energy from an excitation light and transfers the energy to a certain energy level of the rare earth ions, which subsequently enhances the emission of the rare earth ions. Dative (L2) ligands include trioctylphosphine oxide (TOPO), bipyridine, and phenanthroline. In some implementations, a combination of LX-type and dative (L2) ligands are used to coordinate a lanthanide ion. Suitable lanthanide ions include Eu 3+ , Tb 3+ , Sm 3+  and Dy 3+ . The fluorescence can be measured by time-resolved fluorescence spectroscopy. 
     In some implementations, the nanoparticle core is coated with a shell  204 , for example a thermally depolymerizable or degradable polymer. Any of the cores described above, i.e., the up-converting nanoparticles, long-persistent nanoparticles, and complexed lanthanide ions can be coated with a polymer shell. The polymer shell adds another layer of detection to an orthogonal detection scheme. For example, the polymer shell can include monomer fingerprint information. Monomer fingerprint information, as used herein, includes variations in monomer composition of the polymer shell, which can be determined by various techniques. For example, the monomer fingerprint information may be determined by spectroscopic techniques, including, for example, Fourier transform infrared spectroscopy (FTIR), among others. Further, when the polymer is uniquely identifiable, combinations of uniquely identifiable cores and uniquely identifiable polymer coatings expand the number of tags that are available and can be uniquely identified. Detection of the polymer can be orthogonal to the detection of the nanoparticle core. For example, while the nanoparticle core can be detected with fluorescence measurements, the polymer core can be detected with mass spectrometry. Accordingly, the uniquely identifiable core and uniquely identifiable polymers can be mixed and matched to create a library of uniquely identifiable tags. 
     In some implementations, the polymer can be detected by mass spectrometry based on its constituent monomers. For example, the polymer coating can decompose into constituent monomers at elevated temperatures. Therefore, the detection of these individual constituent monomers with mass spectrometry can be accomplished with a pyrolysis-gas chromatography mass spectrometry (pyrolysis-GC/MS) procedure. 
     In some implementations, the polymer coating is polystyrene or a polystyrene derivative. Styrene based polymers cleanly decompose into their constituent monomers at elevated temperatures. Accordingly, the entire polymer mass contributes to the generation of detectable units, maximizing atom economy. In addition, styrene based polymers can be coated onto nanoparticles to form a polymer shell. The styrene based polymer can include monomers that are derivatives of styrene, p-methylstyrene, p-methoxystyrene, 2,4-dimethylstyrene, 2,4,6-trimethylstyrene, 4-chlorostyrene, 4-bromostyrene, or mixtures thereof. 
     In some implementation, the polymer shell includes a methacrylate. For example, the polymer shell can include monomers that are derivatives of acrylate, benzyl methacrylate, butyl methacrylate, cyclohexyl methacrylate, dodecyl methacrylate, 2-ethoxyethyl methacrylate, ethyl methacrylate, hexyl methacrylate, 2-hydroxyethyl methacrylate, isobutyl methacrylate, isopropyl methacrylate, methyl methacrylate, octadecyl methacrylate, octyl methacrylate, phenyl methacrylate, propyl methacrylate, 2-chloroethyl methacrylate, or mixtures thereof. In some implementation, the polymer shell can include both styrene-based and methacrylate-based monomers. 
     A typical procedure for generating the polymer-coated nanoparticle includes providing a suspension of the nanoparticles in water at a concentration of 1-5 wt %, adding an initiator and heating the solution, and injecting monomers, for example, styrene monomers, into the solution. After adding the monomers, the reaction is allowed to proceed for 30 minutes and then cooled to room temperature. The weight ratio of nanoparticle core to polymer shell in the synthesized composite particles can be adjusted by adjusting the relative concentration of the nanoparticle suspension and monomer solution. 
     The synthesized composite nanoparticles are water-soluble and therefore mix readily with water-based drilling mud, for example simply by mechanically stirring the tags into a mud. For an oil-based drilling mud, the synthesized composite nanoparticles can be surface-modified by surfactant to form a microemulsion, and then mixed with the mud. The tags described herein are stable in either water-based or oil-based drilling muds. 
     Tags that are introduced into drilling mud first contact the subterranean formation at the drill bit. These tags can embed into, or permanently or semi-permanently decorate cuttings produced by the drill bit. When the drill bit location and mud lag time to drill bit is known, the cuttings recovered at the surface of the wellbore can be identified and correlated to a known subterranean location or depth. 
     Cuttings that are recovered at the surface can be rapidly screened on-site, for example near the oil well or drilling operation, by fluorescence measurement. In some implementations, only rock samples or cuttings that are exhibiting fluorescence, i.e. tagged cuttings, will be brought to a lab for further analysis. This significantly narrows down the number of rock samples for the subsequent labor and cost intensive lab analysis. A lab protocol has been developed to extract the tags from the cuttings recovered from water-based mud. Typically, collected cut rock samples are rinsed by water and ethanol to remove unattached mud on their surface, and then the rock samples are placed in 10 ml deionized (DI) water and sonicated for 1 hour. The water-soluble tracer is present in the sonicated solution and can be collected by filtration for analysis. Organic solvent such as chloroform can be used to extract organic soluble part of the tracers for analysis. Subsequent analysis can include pyrolysis GC/MS or gas chromatography—flame ionization mass spectrometry (GC-FID/MS). The detectability of the tags at as little of 1 ppm in drilling mud can be achieved. 
     Example 1: Synthesis of Up-Conversion Nanoparticles 
     For the synthesis of up-conversion NaYF 4 :Yb 3+ Er 3+  nanoparticles with 20% Yb and 2% Er dopants, yttrium(III) chloride hexahydrate (YCl 3 .6H 2 O), ytterbium(III) chloride hexahydrate (YCl 3 .6H 2 O) and erbium(III) chloride hexahydrate (ErCl 3 .6H 2 O) at stoichiometric ratio were added into mixture of oleic acid, oleylamine and 1-octadecene (1:1:1.5 volume ratio) in a three-neck round-bottom flask. The mixture was heated to 120° C. under an Ar atmosphere and vigorous stirring until a homogeneous solution was formed, and then was cooled to room temperature. A mixture of NaOH and NH 4 F in methanol was added dropwise to the solution and stirred for 30 min and then heated to 100° C. to remove methanol under vacuum. Finally, the reaction mixture was heated to 300° C. at a heating rate of 20° C./min and then maintained at this temperature for 6 hours under an Ar atmosphere. Upon completion of the reaction, the formed nanoparticles were purified by washing with ethanol, separated by centrifuge, and then dried at 70° C.  FIG.  3    shows an SEM image (JEOL 7100FT Thermal FE SEM, operated at 15 kV) of an example up-converting NaYF 4 :Yb/Er nanoparticle core.  FIG.  4    shows an example emission spectrum of a NaYF 4 :Yb/Er nanoparticle. The nanoparticle was excited with a 980 nm excitation wavelength, resulting in the emission spectra shown in  FIG.  4   . The fluorescence or time-resolved fluorescence spectra were measured by a Horiba NanoLog-3 spectrometer with a Xe lamp or a 980-nm laser beam (200 mW) as excitation source, or by an Ocean Optics QEPro portable spectrometer. As can be seen in this example, the up-converting NaYF 4 :Yb/Er nanoparticle emits light in the visible spectrum. 
     Example 2: Synthesis of Long-Persistent Nanoparticles 
     Trivalent chromium doped lithium gallate (LiGa 5 O 5 :Cr 3+ ) nanoparticles were synthesized by a sol-gel method followed by calcinations at high temperature. A solution was prepared by dissolving stoichiometric amount of lithium nitrate (LiNO 3 , 99%), gallium nitrate [Ga(NO 3 ) 3 , 99.999%], and chromium nitrate [Cr(NO 3 ) 3 , 98.5%] into 1:1 mixed solvent of water-methanol under magnetic stirring. During the stirring process, polyacrylic acid (PAA, MW ˜2000) as a chelating agent was added into the solution to form chelate complexes. Then ammonium hydroxide solution (NH 3 .H 2 O, 29.5 wt %) was added to form colloidal suspension. When a homogeneous sol was formed, the colloidal particles were separated from the suspension by centrifuge and dried at 100° C. Finally, the dried particles were calcinated in a muffle furnace at ˜1100° C. for 6 hours to form LiGa 5 O 5 :Cr 3+  nanoparticles exhibiting NIR persistent luminescence and photo-stimulated persistent luminescence (PSPL) properties.
 
 FIG.  5    shows an example SEM image of Cr 3+ -doped lithium gallate nanoparticles.  FIG.  6    shows an example of a long-persistent spectrum of LiGa 5 O 8 :Cr 3+  that was recorded after excitation by a UV-lamp (at 254 nm) for 5 minutes.
 
     Example 3: Synthesis of Polymer Coated Composite Nanoparticles 
     100 mL of NaYF 4 :Yb/Er 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. 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, the solution was heated to 90° C. and 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.  FIG.  7    shows an example of an SEM image of synthesized polystyrene-coated up-converting NaYF 4 :Yb/Er nanoparticles. 
     Example 4: Encapsulated Luminescent Lanthanide (Eu) Complexes in Polymer Nanoparticles 
       FIG.  8    shows an example SEM image of a Eu(TTA) 3 phenanthroline complex coated with polystyrene. The sample was imaged by a field-emission SEM (JEOL JSM-7100F SEM) with an accelerating voltage of 15 kV to collect the images. The nanoparticle solution is drop-cast on a silicon wafer substrate then dried in vacuum overnight for SEM imaging. 
     The nanoparticles were synthesized with an emulsion polymerization protocol. The emulsion polymerization was conducted using 3 wt % of 50 ml Sodium Dodecyl Sulfate or IGEPAL CA-897 in a three-neck round-bottom flask in an oil bath on a hotplate. The round-bottom flask was equipped with inlet for nitrogen gas and inlet of tubing to inject the monomer, for example styrene monomers. The solution was stirred and degassed for about 30 minutes under nitrogen flow, and then 1 ml of 5 mg/ml Eu(TTA) 3 phenanthroline complex in acetone was added to the solution. Next, 25 mg of ammonium persulfate (APS) or V-50 and 100 μg of 1 M NaOH solution were added. A syringe containing 3 ml of uninhibited styrene monomer was prepared by mixing styrene monomer with 10% of divinylbenzene as a crosslinker. The flask was heated to 80° C. for 10 min. About 1 ml of styrene monomer was injected dropwise into the solution at a rate of 0.02 ml/min. Heating and magnetic stirring was maintained under nitrogen for about 2 hours. An additional coating can be processed by adding 10 mg of APS or V-50 followed by 1 ml of styrene monomer at 0.2 ml/min at 80° C. Heating and magnetic stirring are maintained under nitrogen for about 2 hours. 
     Example 5: Encapsulated Luminescent Lanthanide (Dy) Complexes in Polymer Nanoparticles 
       FIG.  9    shows an example SEM image of a Dy(TTA) 3 phenanthroline complex. The nanoparticles were synthesized with an emulsion polymerization protocol. The emulsion polymerization was been conducted using 3 wt % of 50 ml Sodium Dodecyl Sulfate or IGEPAL CA-897 in a three-neck round-bottom flask in an oil bath on a hotplate. The round bottom flask was equipped with an inlet for nitrogen gas and inlet of tubing to inject the monomer, for example styrene. The solution was stirred and degassed for about 30 minutes under nitrogen flow then 1 ml of 5 mg/ml Dy(TTA) 3 phenanthroline complex in acetone was added to the solution. Next, 25 mg of ammonium persulfate (APS) or V-50 and 100 μg of 1 M NaOH was added to the solution. A syringe containing 3 ml of uninhibited styrene monomer prepared by mixing styrene monomer with 10% of divinylbenzene as crosslinker. The flask was then heated to 80° C. for 10 min. About 1 ml of styrene monomer was dripped into the solution at a rate of 0.02 ml/min. Heating and magnetic stirring were maintained under nitrogen for about 2 hours. An additional coating can be processed by adding 10 mg of APS or V-50, followed by 1 ml of styrene monomer at a rate of 0.2 ml/min at 80° C. Heating and magnetic stirring were maintained under nitrogen for about 2 hours. 
     Example 6: Encapsulated Luminescent SLE Eu Nanoparticles 
       FIG.  10    shows an example of a time-resolved fluorescence spectrum of a polystyrene-coated sheathed lanthanide emitter (SLE) Eu(TTA) 3 phen complex nanoparticle. Fluorescence was recorded with a 50 μs delay after excitation at 360 nm using a Horiba Nanolog system. The samples were transferred in a quartz cuvette for fluorescence measurements. 
     Example 7: Detection by Fluorescence Imaging—Up-Converting Nanoparticles 
     Because of the unique fluorescence properties of up-converting nanoparticles and long-persistence nanoparticles, both types of particles can be detected by direct imaging using a camera or time-gated camera.  FIG.  11    shows an example of the visible fluorescence of up-converting nanoparticles with different compositions under 980 nm excitation by a near infrared laser, which exhibits blue color by NaYF 4 :Yb/Tm (i), green color by NaYF 4 :Yb/Er (ii) and orange color by NaYF 4 :Yb/Ho (iii). 
     Example 8: Detection by Fluorescence Imaging—Long-Persistent Nanoparticles 
     The fluorescence of long-persistent nanoparticles in drilling mud was analyzed with a time-gated camera. A drop of a bentonite mud sample that includes a long-persistent nanoparticle tag was smeared and dried on to the surface of a black tape.  FIG.  12    shows an example time-gated fluorescence image of a long-persistent nanoparticle, SrAl 2 O 4 :Eu 2+ /Dy 3+ , in bentonite mud at concentrations of (a) 0.1 wt %, (b) 0.01 wt %, and (c) 0.001 wt %. Even though the samples include many interfering compounds that can fluoresce in the UV-visible range under the same excitation, the long-persistent luminescence of these tags allows for the interfering fluorescence signals to be eliminated or reduced by time-resolved gating of the detector. 
     Example 9: Detection by Fluorescence Spectroscopy 
     The fluorescence of up-converting nanoparticles and long-persistent nanoparticles can be detected by a fluorescence spectrometer, for example as a portable spectrometer. A suitable spectrometer is available from Ocean Optics. The tags described herein with different compositions have different spectra under different excitations.  FIG.  13    shows an example spectrum of NaYF 4 :Yb/Er powder with 0.0001 wt % bentonite powder, under excitation of a 980 nm laser at 200 mW. Bentonite is commonly used in mud fluids and therefore these conditions simulate the real world drilling conditions. 
     In some implementations, a nanoparticle tag includes a fluorescent core and a polymer shell. The fluorescent core includes an up-converting material. The polymer shell includes styrene-based monomers or methacrylate-based monomers, or any combination thereof. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The styrene-based monomers include monomers derived from styrene, p-methylstyrene, p-methoxystyrene, 2,4-dimethylstyrene, 2,4,6-trimethylstyrene, 4-chlorostyrene, or 4-bromostyrene, or any combination thereof. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The methacrylate-based monomers include monomers derived from acrylate, benzyl methacrylate, butyl methacrylate, cyclohexyl methacrylate, dodecyl methacrylate, 2-ethoxyethyl methacrylate, ethyl methacrylate, hexyl methacrylate, 2-hydroxyethyl methacrylate, isobutyl methacrylate, isopropyl methacrylate, methyl methacrylate, octadecyl methacrylate, octyl methacrylate, phenyl methacrylate, propyl methacrylate, or 2-chloroethyl methacrylate, or any combination thereof. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The fluorescent core includes a rare-earth element doped oxide. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The fluorescence core consists essentially of NaYF 4 :Yb/Er, NaYF 4 :Yb/Ho, NaYF 4 :Yb/Tm, NaGdF 4 :Yb/Er, NaGdF 4 :Yb/Ho, NaGdF 4 :Yb/Tm, YPO 4 :Yb/Er, YPO 4 :Yb/Ho, YPO 4 :Yb/Tm, CaTiO 3 :Yb/Er, CaTiO 3 :Yb/Ho, or CaTiO 3 :Yb/Tm, or any combinations thereof. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The polymer shell is a thermally depolymerizable or degradable shell. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The polymer shells includes monomer fingerprint information. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The nanoparticle is less than 1000 nm in diameter. 
     In some implementations, a nanoparticle tag includes a fluorescent core and a polymer shell. The fluorescent core includes a long-persistent fluorescent material. The long-persistent fluorescent material consists essentially of CaTiO 3 :Pr 3+ , CaAl 2 O 4 :Eu 2+ /Nd 3+ , Ca 3 (PO 4 ) 2 :Sm 3+ /Mn 2+ , SrAl 2 O 4 :Eu 2+ /Dy 3+ , SrMgSi 2 O 7 :Eu/Dy, Sr 4 Al 14 O 25 :Eu/Dy, ZnGa 2 O 4 :Cr 3+ , LiGa 5 O 8 :Cr 3+ , Zn 3 Ga 2 Ge 2 O 10 :Cr 3+ /Pr 3+ , or Zn 3 Ga 2 Ge 2 O 10 :Cr 3+ /Eu 3+ , or any combinations thereof. The polymer shell includes styrene-based monomers or methacrylate based monomers, or any combinations thereof. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The styrene-based monomers include monomers derived from styrene, p-methylstyrene, p-methoxystyrene, 2,4-dimethylstyrene, 2,4,6-trimethylstyrene, 4-chlorostyrene, or 4-bromostyrene, or any combination thereof. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The methacrylate-based monomers include monomers derived from acrylate, benzyl methacrylate, butyl methacrylate, cyclohexyl methacrylate, dodecyl methacrylate, 2-ethoxyethyl methacrylate, ethyl methacrylate, hexyl methacrylate, 2-hydroxyethyl methacrylate, isobutyl methacrylate, isopropyl methacrylate, methyl methacrylate, octadecyl methacrylate, octyl methacrylate, phenyl methacrylate, propyl methacrylate, or 2-chloroethyl methacrylate, or any combination thereof. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The polymer shell is a thermally depolymerizable or degradable shell. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The polymer shells includes monomer fingerprint information. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The nanoparticle is less than 1000 nm in diameter. 
     In some implementations, a nanoparticle tag includes a fluorescent core and a polymer shell. The fluorescent core includes an encapsulated lanthanide complex where the encapsulated lanthanide complex includes a lanthanide ion where all metal binding sites of the lanthanide ion are occupied by ligands. The polymer shell includes styrene-based monomers or methacrylate-based monomers, or any combination thereof. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The styrene-based monomers include monomers derived from styrene, p-methylstyrene, p-methoxystyrene, 2,4-dimethylstyrene, 2,4,6-trimethylstyrene, 4-chlorostyrene, or 4-bromostyrene, or any combination thereof. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The methacrylate-based monomers include monomers derived from acrylate, benzyl methacrylate, butyl methacrylate, cyclohexyl methacrylate, dodecyl methacrylate, 2-ethoxyethyl methacrylate, ethyl methacrylate, hexyl methacrylate, 2-hydroxyethyl methacrylate, isobutyl methacrylate, isopropyl methacrylate, methyl methacrylate, octadecyl methacrylate, octyl methacrylate, phenyl methacrylate, propyl methacrylate, or 2-chloroethyl methacrylate, or any combination thereof. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The encapsulated lanthanide complex includes a lanthanide ion complexed with LX-type ligands. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The LX-type ligands include β-diketonates. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The β-diketonates comprise 1,1,1-trifluoro-3-(2-theonyl)acetone, or benzyl trifluoroacetone, or any combination thereof. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The encapsulated lanthanide complex includes a lanthanide ion complexed with dative type ligands. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The dative type ligands include trioctylphosphine oxide, bipyridine, or phenanthroline, or any combination thereof. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The polymer shell is a thermally depolymerizable or degradable shell. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The polymer shells comprises monomer fingerprint information. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The nanoparticle is less than 1000 nm in diameter. 
     In some implementations, a method of determining the origin location of a subterranean rock sample includes mixing a nanoparticle tag into a fluid, flowing the fluid through a work string into a subterranean formation, recovering subterranean rock samples from the subterranean formation, and determining an origin location of the subterranean rock sample by detecting the presence of the nanoparticle tag on the subterranean rock sample. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The fluid includes a drilling mud. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. The nanoparticle tag includes a fluorescent core and a polymer shell. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. Detecting the presence of the nanoparticle tag on the rock sample includes analyzing the subterranean rock sample for a fluorescence signal, and analyzing the subterranean rock samples with a fluorescence signal to determine the identity of the polymer shell. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. Analyzing the subterranean rock sample for a fluorescence signal includes analyzing the subterranean rock sample for an up-conversion signal. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. Analyzing the subterranean rock sample for an up-conversion signal includes exciting the subterranean rock sample with an ultraviolet light source, and observing a resulting visible or ultraviolet fluorescence signal. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. Analyzing the subterranean rock sample for a fluorescence signal includes analyzing the subterranean rock sample with a time-gated fluorescence measurement. 
     This aspect, taken alone or combinable with any other aspect, can include the following features. Determining the identity of the polymer shell includes analyzing the polymer shell 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. Analyzing the polymer shell with mass spectroscopy comprises analyzing the polymer shell with gas chromatography—flame ionization detection/mass spectrometry. 
     In some implementations, a method of characterizing the origin depth of a cut rock sample includes using a barcoded tracer to determine the origin location based on the travel time of the produced rock cuttings from a drill head to the surface via an annulus. The barcoded tracer includes a nanoparticle tag with a fluorescent core and a polymer shell. The method includes using fluorescence detection to detect the presence of the nanoparticle tags on the cut rock sample and analyzing the polymer shell to yield fingerprint monomer information of the nanoparticle tags on the cut rock sample. 
     The following units of measure have been mentioned in this disclosure: 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                 Unit of Measure 
                 Full form 
               
               
                   
                   
               
             
            
               
                   
                 nm 
                 nanometer 
               
               
                   
                 μm 
                 micrometer 
               
               
                   
                 μs 
                 microsecond 
               
               
                   
                 wt % 
                 weight percent 
               
               
                   
                 mW 
                 milliwatt 
               
               
                   
                   
               
            
           
         
       
     
     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 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.