Abstract:
Some principles described herein contemplate implementation of downhole imaging for the characterization of formation fluid samples in situ, as well as during flow through production tubing, including subsea flow lines, for short term investigation, permanent, and/or long term installations. Various methods and apparatus described herein may facilitate downhole testing. For example, some embodiments facilitate multi-dimensional fluorescence spectrum measurement testing downhole and correlating the fluorescence with other oil properties.

Description:
FIELD 
   The present disclosure relates generally to methods and systems for investigating subterranean formation fluids. More particularly, some aspects of this disclosure are directed to methods and systems for spectral imaging to characterize downhole fluids. 
   BACKGROUND 
   Fluid characterization is very important to the assessment of economic viability for a hydrocarbon-bearing reservoir formation. Some wireline tools such as Schlumberger&#39;s MDT (Modular Dynamic Tester) are used to sample formation fluids, store it in a set of bottles, and retrieve it to surface while keeping the fluid pressurized. Such samples are known as live fluids. These live fluids are then sent to an appropriate laboratory to be characterized. Characterization of the fluids may include composition analysis, fluid properties and phase behavior. 
   Understanding reservoir fluid phase behavior is key to proper planning and development of the respective fields and design of the production system. Understanding reservoir fluid phase behavior involves conducting a number of very important measurements on the fluid at realistic reservoir and production conditions. In most cases, changes in temperature (T) and pressure (P) of the formation fluid lead to phase changes, including phase separation (e.g., liquid-vapor, liquid-solid, liquid-liquid, vapor-liquid etc.), and phase recombination. For example, while most hydrocarbons exist as a single phase at initial reservoir conditions (i.e., composition, pressure, and temperature), they often undergo reversible (and possibly some irreversible) multi-phase changes due to pressure, composition and/or temperature reduction during production and flow to the surface facilities. 
   Liquid-Solid-Vapor phase boundaries are typically measured at a laboratory using state-of-the-art-technologies, such as Schlumberger&#39;s pressure-volume-temperature (PVT) unit coupled to Schlumberger&#39;s laser-based Solids Detection System (SDS) and Schlumberger&#39;s high-pressure microscope (HPM). Detailed descriptions of these state-of-the art technologies and their applications for the study of phase behavior and flow assurance of petroleum fluids have been published and are known to those of skill in the art. 
   However, the one trend in the industry is to perform more and more analysis of the formation and the formation fluid properties directly downhole to avoid the difficulties associated with sample preservation when lifted uphole and delays associated with sample transportation and analysis in a remote laboratory. Tools like Schlumberger&#39;s MDT can, for example, be retrofitted with a spectrometer module such as a Live Fluid Analyser or Gas Condensate Analyser in order to provide basic information on the fluid composition (Gas-to-oil ratio (GOR), water content, basic crackdown of hydrocarbon fractions (C 1 , C 2 -C 5 , C 6 +)). These measurements are performed by infrared (IR) absorption spectroscopy. 
   Nevertheless, current measurements of certain downhole characteristics do not facilitate full analysis of the formation and fluids, especially in situ. Fluorescence measurements downhole as discussed herein may be used to more fully characterize formations and formation fluids. In addition, U.S. Patent Application Publication Number 2004/0000636 assigned to Schlumberger Technology Corporation and invented by Oliver Mullins et al. discusses determining dew precipitation onset pressure in a sample located downhole in an oilfield reservoir, which may include measuring 1D fluorescence. 
   Further, while there has been some use of video imaging downhole in wireline tools, current technology is generally limited to applications related to production logging. Most current downhole imaging is dedicated to borehole wall imaging and has low spatial resolution (although commonly-owned U.S. patent application Ser. No. 11/204,134 discusses additional imaging capability). DHV International, for example, provides downhole video services to the oil and gas industry for diagnosis of borehole problems such as fishing out lost tools, mechanical inspection, and fluid entry surveys. There is room to improve methods and systems to more fully characterize formation fluids downhole. 
   SUMMARY 
   The present specification may meet the above-described needs and others. In one embodiment, the present disclosure provides a method comprising providing a downhole testing tool, deploying the downhole testing tool into a borehole, and performing a multi-dimensional fluorescence spectrum measurement downhole. In this, the disclosure herein contemplates applications in wireline tools, drilling and measuring tools, permanent monitoring, production logging, among others, with deployment modes that include conventional wireline and drilling systems, and slickline, coiled tubing, clamping devices, etc. 
   In one embodiment, two of the multi-dimensional fluorescence spectrum measurements comprise wavelength of excitation light and fluorescence spectrum. In one embodiment, two of the multi-dimensional fluorescence spectrum measurements comprise fluorescence relaxation time and fluorescence spectrum. In one embodiment, performing the multi-dimensional fluorescence spectrum measurement comprises two dimensional fluorescence imaging with a charged-coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) camera. The method may include communicating the multi-dimensional fluorescence spectrum measurement uphole. One embodiment of the method comprises performing the multi-dimensional fluorescence spectrum measurement downhole at multiple boreholes, comparing the multi-dimensional fluorescence spectrum measurements at the multiple boreholes, and determining connectivity between the multiple boreholes based on the comparing of the multi-dimensional fluorescence spectrum measurements. In one embodiment, the downhole testing tool further comprises a pressure-volume control unit. Some embodiments of the downhole testing tool comprise a portion of a wireline tool. In one embodiment, the downhole testing tool is permanently installed downhole and in fluid communication with a production line. Some embodiments further comprise performing a multi-dimensional fluorescence spectrum measurement in a lab on a same fluid measured downhole, and comparing the multi-dimensional fluorescence spectrum measurement of the lab with multi-dimensional fluorescence spectrum measurement performed downhole. The comparison may be used to establish a clean chain of custody. 
   One aspect provides a method of identifying subterranean fluids. The subterranean or downhole fluids may be formation fluids, drilling muds, or other fluids. The method comprises characterizing a formation fluid sample downhole according to multi-dimensional fluorescence spectrum measurements. In one embodiment, two of the multi-dimensional fluorescence spectrum measurements comprise wavelength of excitation light and fluorescence spectrum. In another embodiment, two of the multi-dimensional fluorescence spectrum measurements comprise fluorescence relaxation time and fluorescence spectrum. In another embodiment, performing the multi-dimensional fluorescence spectrum measurement comprises two dimensional fluorescence imaging with a charged-coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) camera. In one embodiment, a light source and the camera comprise a transmission imaging configuration. In one embodiment, a light source, a reflector, and a camera comprise a back-scattered imaging configuration. 
   One aspect provides a method of identifying subterranean formation fluids, comprising providing a downhole testing tool having an optical fluid analyzer, deploying the downhole testing tool into a borehole, exciting an energy state of the formation fluids adjacent to the optical fluid analyzer above a ground state, measuring fluorescence light emitted by the formation fluids in a relaxation process from an excited state to the ground state, and plotting fluorescence spectra as a function of time. One embodiment further comprises comparing plots of fluorescence spectra as a function of time with samples from various boreholes. One embodiment further comprises comparing plots of fluorescence spectra as a function of time with samples of known properties. One embodiment further comprises comparing plots of fluorescence spectra as a function of time with samples from various boreholes, and determining similarities between the plots of the samples from the various boreholes to anticipate well connectivity. 
   One embodiment provides a downhole apparatus. The downhole apparatus comprises a downhole lab module. The downhole lab module comprises a sample flow line, a sample cell in fluid communication with the sample flow line, the sample cell comprising at least one transparent window, a light source adjacent to the sample cell, a spectrometer for detecting fluorescence, and a set of instructions, that, when executed, perform multi-dimensional fluorescence spectrum measurements downhole. One embodiment further comprises a set of instructions that, when executed, excite an energy state of the formation fluids adjacent to the optical fluid analyzer above a ground state, measure fluorescence light emitted by the formation fluids in a relaxation process from an excited state to the ground state, and plot fluorescence spectra as a function of time. One embodiment further comprises a set of instructions that, when executed, excite an energy state of the formation fluids adjacent to the optical fluid analyzer above a ground state, measure fluorescence light emitted by the formation fluids in a relaxation process from an excited state to the ground state, and plot wavelength of excitation light versus fluorescence spectrum. One embodiment further comprises a camera capable of 2D fluorescence imaging of formations downhole. 
   One embodiment provides a downhole apparatus comprising a downhole lab module, the downhole lab module comprising a cell having an optical window in contact with a downhole formation, a light source adjacent to the cell, a spectrometer for detecting fluorescence emitted from the formation, and a set of instructions, that, when executed, perform multi-dimensional fluorescence spectrum measurements downhole of the formation. 
   One aspect provides a method comprising downhole fluid mapping. The downhole fluid mapping comprises providing a downhole testing tool, deploying the testing tool into a borehole, measuring fluorescence downhole, and correlating fluorescence measurements with oil properties. In one embodiment, downhole fluid mapping occurs in the presence of emulsions. The correlating may comprise plotting wave-number versus fluorescence intensity. In one embodiment, the method comprises generating a correlation function based on the plot. In one embodiment, the method comprises matching the correlation function to a correlation function for a known oil. In one embodiment, the fluorescence measurements comprise 2D fluorescence measurements. The fluorescence measurements may comprise a 2D fluorescence map, and the method may further comprise slicing the 2D fluorescence map at different energy levels to find a correlation between fluorescence and oil properties. 
   According to one embodiment, the correlating comprises plotting asphaltenes and/or resins weight fraction versus fluorescence intensity. The method may comprise generating a correlation function based on the plot. The method may further comprise matching the correlation function to a known oil. 
   In one embodiment, the correlating comprises plotting C36+ content versus fluorescence intensity, generating a correlation function based on the plot, and matching the correlation function to a known oil. 
   Another aspect provides a method of downhole fluid mapping. The downhole fluid mapping comprises providing a downhole testing tool, deploying the testing tool into a borehole, measuring 2D fluorescence downhole, plotting wave-number versus fluorescence intensity, plotting asphaltenes and/or resins weight fraction versus fluorescence intensity, plotting C36+ content versus fluorescence intensity, generating correlation functions based on the plots, and matching the correlation functions to a known oil. In one embodiment, the 2D fluorescence measurements comprise a 2D fluorescence map, and the method further comprises slicing the 2D fluorescence map at different energy levels to find stronger correlation functions. 
   One method comprises fingerprinting oils from a particular basin. The fingerprinting comprises taking 2D fluorescence measurements downhole, plotting the 2D fluorescence measurements versus at least one oil property, and generating a correlation function between the 2D fluorescence measurements and at least one oil property. The method may further comprise generating 2D fluorescence contour plots. The at least one oil property may comprise a plurality of oil properties. In one embodiment, the plurality of oil properties comprises wave number, asphaltenes and/or resin weight fraction, and C36+ content. 
   One embodiment provides a downhole apparatus. The downhole apparatus comprises a downhole lab module, the downhole lab module comprising a sample flow line, a sample cell in fluid communication with the sample flow line, the sample cell comprising at least one optical or transparent window, a light source adjacent to the sample cell, a spectrometer for detecting fluorescence, and a set of instructions, that, when executed, perform multi-dimensional fluorescence spectrum measurements downhole and correlate fluorescence measurements with oil properties. In one embodiment, the light source illuminates an emulsion. In one embodiment, the downhole apparatus further comprises a set of instructions that, when executed, excite an energy state of the formation fluids adjacent to the optical fluid analyzer above a ground state, measure the fluorescence emitted by the formation fluids in a relaxation process from an excited state to the ground state, and plot fluorescence spectra as a function of time. 
   Additional advantages and novel features will be set forth in the description which follows or may be learned by those skilled in the art through reading these materials or practicing the principles described herein. Some of the advantages described herein may be achieved through the means recited in the attached claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings illustrate certain embodiments and are a part of the specification. Together with the following description, the drawings demonstrate and explain some of the principles of the present invention. 
       FIG. 1  is a fluorescence spectrum of various crude oils. In the diagram of  FIG. 1 , a 470 nm LED is used for excitation light. 
       FIG. 2  is a cut-away cross-sectional view of a composition fluid analyzer (CFA) fluorescence detector according to one embodiment. 
       FIG. 3A  is a fluorescence spectrum of oil at different excitations of light according to one embodiment. 
       FIG. 3B  is a contour plot of the fluorescence spectra shown in  FIG. 3A . 
       FIG. 4A  illustrates a contour plot of an actual oil sample fluorescence spectra for a dead crude oil generated in a lab according to the prior art. 
       FIG. 4B  illustrates a contour plot of an actual oil sample fluorescence spectra for another dead crude oil generated in a lab according to the prior art. 
       FIGS. 5A-5C  illustrate a 2D fluorescence measurement concept of spectrum vs. relaxation time according to one embodiment. 
       FIG. 6A  is a schematic diagram showing an optical layout of a downhole flow line imaging apparatus.  FIG. 6A  illustrates a transmission configuration. 
       FIG. 6B  is another schematic diagram showing an optical layout of a downhole flow line imaging apparatus.  FIG. 6B  illustrates a backscatter configuration. 
       FIG. 7A  is a schematic optical layout for optical fluorescent measurement in a flow line according to one embodiment.  FIG. 7A  illustrates a transmission layout. 
       FIG. 7B  is a schematic optical layout for optical fluorescent measurement of a formation fluid according to another embodiment.  FIG. 7B  illustrates a reflection layout. 
       FIG. 7C  is a schematic optical layout for optical fluorescent measurement of a formation fluid according to another embodiment.  FIG. 7C  illustrates a fiber bundle layout. 
       FIG. 8A  is a schematic of an apparatus for downhole optical fluorescent measurement at a formation surface according to one embodiment. 
       FIG. 8B  is a magnified detail schematic of the apparatus of  FIG. 8A . 
       FIG. 9A  is another schematic diagram showing an optical layout of a downhole flow line fluorescence detection apparatus utilizing a tunable or pulsed light source. 
       FIG. 9B  illustrates a plot of fluorescence intensity or relaxation as a function of time according to use of the apparatus of  FIG. 9A . 
       FIG. 9C  illustrates a plot of light source intensity as a function of time when an optical filter is used with the apparatus of  FIG. 9A . 
       FIG. 9D  illustrates a light pulse for the light source of  FIG. 9A  according to one embodiment. 
       FIG. 10  is a chart illustrating a correlation between absorption cut-off (wave-number) and fluorescence intensity (area) for twenty nine dead oils. 
       FIG. 11  is a chart illustrating a correlation of absorption and fluorescence with asphaltenes and/or resin weight fraction (for twenty nine dead oils). 
       FIG. 12  is a chart illustrating a correlation of absorption and fluorescence with C36+ content (for twenty nine dead oils). 
       FIG. 13  illustrates 2D fluorescence of twenty-nine dead oils from many different basins ranked (left to right, top to bottom) according to their asphaltenes+resin weight fraction from SARA analysis. 
       FIG. 14  illustrates a correlation between fluorescence and asphaltenes+resin weight fraction for six oils from a similar region in the Middle East. Slicing 2D fluorescence map at different energies reveals that correlation is higher for 325 nm excitation than 470 nm. 
   

   Throughout the drawings, identical reference numbers and descriptions indicate similar, but not necessarily identical elements. While the principles described herein are susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. However, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention includes all modifications, equivalents and alternatives falling within the scope of the appended claims 
   DETAILED DESCRIPTION 
   Illustrative embodiments and aspects of the invention are described below. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, that will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. 
   Reference throughout the specification to “one embodiment,” “an embodiment,” “some embodiments,” “one aspect,” “an aspect,” or “some aspects” means that a particular feature, structure, method, or characteristic described in connection with the embodiment or aspect is included in at least one embodiment of the present invention. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” or “in some embodiments” in various places throughout the specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, methods, or characteristics may be combined in any suitable manner in one or more embodiments. The words “including” and “having” shall have the same meaning as the word “comprising.” 
   Moreover, inventive aspects lie in less than all features of a single disclosed embodiment. Thus, the claims following the Detailed Description are hereby expressly incorporated into this Detailed Description, with each claim standing on its own as a separate embodiment of this invention. 
   A fluorescence (FL) spectrum reflects energy structure that is determined by bonds between atoms in a molecule. Fluorescence measurement is often used in the chemical analysis field. In order to observe a fluorescence spectrum, an energy state excitation is necessary. Fluorescence light is emitted during the relaxation process from an excitation state to a ground state. 
     FIG. 1  shows fluorescence spectra of various crude oils labeled A-E. The large, truncated peak in  FIG. 1  represents excitation light that happens to be 470 nm blue LED light. The broader peaks A-E in the 500-600 nm wavelength regions represent fluorescence signals. As illustrated in  FIG. 1 , different crude oil samples have different spectral shapes at the longer wavelength ranges (as opposed to the nearly identical peak shapes at the excitation light wavelength). However, it is currently difficult to obtain much useful or detailed information about the crude oils based on the fluorescence spectra alone because of the simplicity of the shapes associated with the fluorescence spectra that are excited by certain excitation light sources. 
   CFAs have been implemented for fluorescence measurement, but the excitation wavelength is typically a single wavelength and only two emission wavelengths are currently detected.  FIG. 2  is a schematic drawing of one fluorescence detector  100  that may be part of a downhole tool (e.g. a downhole wireline tool) that may be used to make fluorescence measurements. The fluorescence detector  100  includes a light source, for example a 470 nm LED light source  102 . The fluorescence detector  100  may also include two fluorescence detection channels FL# 0  and FL# 1  that have different cutoff optical filters  104 . The fluorescence detector  100  may also include a reflected light detection channel  106 . The channels FL# 0 , FL# 1 , and reflected light detection channel  106  may be arranged in an optical prism such as a sapphire prism  108 . Each channel (FL# 0 , FL# 1 , and reflected light detection channel  106 ) may include an optical rod such as a glass rod  110  to direct light from an optical window  112  adjacent a sample  114 . 
   In the embodiment of  FIG. 2 , fluorescence detection channels FL# 0  and FL# 1  of a downhole testing tool may have λ 1  and λ 2  cutoff wavelength optical filters  104 , respectively. However, other optical filters may also be used, including additional detection channels. The fluorescence detection channels FL# 0  and FL# 1  may be used to observe rough spectrum shapes (such as the ones shown in  FIG. 1 ). In one embodiment, the first fluorescence detection channel FL# 0  integrates intensity of fluorescence spectrum from λ 1  and longer, and the second fluorescence detection channel FL# 1  integrates intensity of fluorescence above λ 2 . The intensity of the light reflected at an interface  116  between bottom window  112  and a flow line  116  carrying the sample  114  depends on the refractive index of the sample  114  in the flow line  116 . However, approximately 100% of the light (represented by arrows  118 ) from the λ emission LED light source  102  is reflected when air flows in the flow line  116  adjacent the window  112 . Less than 100% of the light from the λ LED light source  102  is reflected if fluid flows through flow line  116 . 
   As mentioned above, fluorescence spectroscopy (1D or one dimension) has been used to get general—but not detailed—information about formation fluids and other downhole fluids (e.g. drilling muds) based on the spectral shapes measured. However, according to some embodiments, more than 1D fluorescence spectroscopy, for example at least 2D fluorescence measurements, may be taken to further characterize formations, downhole fluids, formation fluids, etc. (and such measurements and characterization may be done downhole or in situ). Mud itself does not exhibit fluorescence. Therefore, finding fluorescence in a mud fluid may indicate that oil-bearing formations have been reached. 
   In one embodiment illustrated in  FIGS. 3A-3B , a 2D measurement of spectrum vs. wavelength of excitation light is shown.  FIG. 3A  illustrates fluorescence spectra for different excitation light sources. Sharp peaks  120  in shorter wavelength locations again represent spectra of excitation light. Broader peaks  122  at the longer wavelengths represent fluorescence spectra for the sample (e.g. oil). In one aspect, many fluorescence spectra may be measured with many kinds of excitation light to generate a 2D fluorescence spectrum contour plot such as the one shown in  FIG. 3B .  FIGS. 4A-4B  represent 2D fluorescence measurement data or contour plots from actual dead crude oils that were generated in an uphole lab according to the prior art.  FIG. 4A  is a 2D spectrum of Sahara crude oil, and  FIG. 4B  is a 2D spectrum of Nigeria light crude oil. As shown in  FIGS. 4A-4B , different samples (such as different oils) exhibit different fluorescence spectrum in uphole labs. In particular,  FIGS. 4A-4B  illustrate contour plots of dead oil from a tanker spill. Fujita, M., “ Analysis and Identification of Spilled Oil in Ocean, ” J APAN  E NVIRONMENTAL  M EASUREMENT  &amp; C HEMICAL  A NALYSIS,  Vol. 19, No. 4, 1990 (rough translation of titles from Japanese). As understood by those of ordinary skill in the art having the benefit of this disclosure, a dead oil is one that is no longer pressurized but has been subjected to atmospheric pressure. Further, one of ordinary skill in the art having the benefit of this disclosure will understand that the volatile fraction of a dead oil will have evaporated from the liquid phase. However, according to principles described herein, different 2D fluorescence spectra are generated downhole, and can be compared to known measurements to identify or characterize the samples downhole. Fluorescence may be of particular interest for the characterization of the aromatic fraction of oil. Part of the aromatic compounds can be quite volatile under atmospheric pressure conditions. With the direct implementation downhole according to principles described herein, the aromatic fraction of a live oil is characterized downhole in one embodiment and would otherwise be lost if the sample was depressurized (as with a tanker spill). 
   Other at least 2D fluorescence measurements may also be used to characterize samples. For example,  FIGS. 5A-5C  illustrate 2D fluorescence measurements of a spectrum as a function of relaxation time. Generally, fluorescence “relaxes” over time and fluorescent intensity decreases exponentially with time.  FIG. 5A  illustrates a typical fluorescence intensity plot over time. Moreover, the fluorescence spectrum shape changes over time.  FIG. 5B  represents different fluorescence spectra at each of four different times (T 1 , T 2 , T 3 , and T 4 ). The changes in fluorescence spectra over time are characteristic of particular sample compositions that can be recorded and compared to the same criteria of known samples. Thus, the 2D fluorescence spectrum including fluorescence spectrum and the relaxation time axes shows the relaxation process from an excited energy state and is identifiable for different samples.  FIG. 5C  is a 2D fluorescence spectrum plot showing fluorescence spectrum as a function of relaxation time. The relaxation spectra in the 2D plot of  FIG. 5C  illustrates a relaxation process that includes features that may be unique to sample compositions (such as different crude oils). 
   In addition to or separate from using 2D fluorescence measurements to identify samples, some aspect may simply “fingerprint” samples. For example, in one aspect, 2D fluorescence measurements are taken for a first downhole sample in a first borehole. Another set of 2D fluorescence measurements may be taken for a second sample in a second borehole. The 2D fluorescence measurements or “fingerprint” of the first sample may be compared to the “fingerprint” of the second sample to evaluate formation connectivity. For example, if a 2D fluorescence spectrum of crude oil in one formation indicates the same 2D fluorescence spectrum in another formation, it is likely that the formations are connected somewhere. 
   Further, in addition to the two different 2D fluorescence measurements that may be taken, for example, downhole by a downhole tool and relayed uphole, other imaging downhole is also contemplated.  FIGS. 6A and 6B  illustrate optical layouts of downhole tools having flow line imaging capability according to some embodiments.  FIG. 6A  is a transmission configuration wherein light from a light source  130  or fluorescence from a sample in a flow line  128  is imaged by a camera  132  (which may be a CCD (charged coupled device), a CMOS (complementary metal oxide semiconductor) camera, or other camera). In the embodiment of  FIG. 6A , the light source  130  and the camera  132  are placed on opposite sides of a flow line sample cell  134  having windows  136 . The sample cell  134  is fluidly connected to the flow line  128 . The sample cell  134  comprises the one or more window  136  shown in  FIG. 6A . The windows  136  comprise a material that is at least partially transparent to light. The windows  136  may be made, for example, of sapphire. Many possible configurations of the light source  130  and camera  132  are contemplated herein. Two possible configurations are shown in  FIGS. 6A-6B . In one embodiment the camera  132  is a spectral camera. The camera  132  can provide spectral information in function of pixels. A backscatter configuration shown and described below in connection with  FIG. 6B  may provide the same measurements as those described above, but with spatial resolution instead of averaging it within sample volume. 
   In a backscatter imaging configuration as shown in  FIG. 6B , the light source  130  and the camera  132  may both be arranged on the same side of a sample cell  234 . The sample cell  234  may thus include only one window  136 . A beam splitter  138 , which is shown as a tilted plate between the sample cell  234  and the camera  132 , is used to direct light to the sample cell  234  while also allowing backscattered light to return to the camera  132 . Accordingly, direct electromagnetic radiation from the light source  130  is directed to the window  136  by the beam splitter  138 , and radiation may be reflected from the sample and detected by the camera  132 . Reflected light may also be due to light reemitted by the sample in the flow line  128  itself because of fluorescence. 
     FIG. 7A  illustrates an embodiment wherein a wavelength selectable light source  230  for excitation is arranged on one side of a sample flow line  128  and a lens  240  is arranged between the sample flow line  128  and a spectrometer  232  opposite of the light source  230 .  FIGS. 7A-7C  illustrate several possible apparatuses that may be used for the 2D FL measurement.  FIG. 7A  illustrates a measurement in a transmission configuration.  FIGS. 7B and 7C  illustrate mechanisms for measurement in reflection configurations. In  FIG. 7A , the excitation light is provided by light source  230 , transmitted through the sample  128 , and through optical windows  136 . A lens system  240  collects the light transmitted through the optical cell  134  and transmits it to the spectrometer  232 . In  FIG. 7B , the scheme is the same but the analyzed light is one reflected from the cell via window  236 . In  FIG. 7C , instead of using a lens system to guide the light, an optical fiber  242  is used. 
   According to some aspects, fluorescence imaging may be used to discriminate between oil-bearing formations and other formations. For example, limestone containing oil will emit fluorescent light following excitation, which can be viewed by a camera, while other formations that do not bear hydrocarbons will tend not to emit any fluorescence. Some of the embodiments that may be used to image fluorescence downhole and help determine which formations contain hydrocarbons are depicted in  FIGS. 7B-7C . 
     FIG. 7B  schematically illustrates an apparatus wherein the sample  128  is a formation fluid sample. An optical window  236  may be directly adjacent to or even attached to the formation of interest. A light source such as the wavelength selectable light source  230  emits light through the optical window  236  at an angle and excites the formation fluid  128  and/or any oil in the formation fluid  128 . If there is oil in the formation fluid  128 , fluorescent light is emitted as the oil relaxes from the excited state. Fluorescence (if any) is directed back through the optical window toward a camera or spectrometer  232  where it is detected or imaged. A lens  240  may focus fluorescence to the spectrometer  232 . Accordingly, fluorescence may be viewed or detected uphole via the downhole system illustrated in  FIG. 7B  to help determine whether formations of interest are hydrocarbon-bearing. In some aspects the downhole system illustrated in  FIG. 7B  may be used to also determine the composition of any hydrocarbons. 
     FIG. 7C  schematically illustrates another downhole apparatus wherein the sample  128  is a formation fluid. According to the embodiment of  FIG. 7C , the optical window  136  is adjacent to or attached to the formation and also coupled to fiber optic bundles  242  and  244 . The first fiber optic bundle  242  is optically coupled between the wavelength selectable light source  230  and the optical window  136 . The light source  230  emits light through the optical window  136  via the first fiber optic bundle  242  and excites the formation fluid  128  and/or any oil in the formation fluid  128 . If there is oil in the formation fluid  128 , fluorescent light is emitted as the oil relaxes from the excited state. Fluorescence (if any) is directed back through the optical window and to the camera or spectrometer  232  via the second fiber optic bundle  244  where it is detected or imaged. Again, fluorescence may be viewed or detected uphole via the downhole system illustrated in  FIG. 7C  to help determine whether formations of interest are hydrocarbon-bearing. Other configurations including a downhole excitation source and a downhole detector may also be used. 
   Accordingly, in one aspect a method may be implemented which includes providing a downhole testing tool, deploying the downhole testing tool into a borehole, and performing a multi-dimensional fluorescence spectrum measurement downhole. In one aspect two of the multi-dimensional fluorescence spectrum measurements comprise wavelength of excitation light and fluorescence spectrum. Performing the multi-dimensional fluorescence spectrum measurement may comprise plotting wavelength of excitation light versus fluorescence spectrum. 
   In one aspect, two of the multi-dimensional fluorescence spectrum measurements comprise fluorescence relaxation time and fluorescence spectrum. Performing a multi-dimensional fluorescence spectrum measurement may comprise plotting fluorescence relaxation time versus fluorescence spectrum. In one aspect, performing the multi-dimensional fluorescence spectrum measurement comprises two dimensional fluorescence imaging with a charged-coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) camera. 
   In one aspect, the method includes communicating the multi-dimensional fluorescence spectrum measurements uphole. The methods may include performing multi-dimensional fluorescence spectrum measurements downhole at multiple boreholes, comparing the multi-dimensional fluorescence spectrum measurements at the multiple boreholes, and determining connectivity between the multiple boreholes based on the comparing of the multi-dimensional fluorescence spectrum measurements. Moreover, in one embodiment the downhole testing tool (which may be a wireline tool) shown in part in FIGS.  2  and  6 A- 7 A further comprises a pressure-volume control unit. Further, in one embodiment the downhole testing tool is permanently installed downhole and in fluid communication with a production line. 
   Turning now to  FIGS. 8A-8B , another configuration that may be used for 2D fluorescent imaging is shown. According to the embodiment of  FIGS. 8A-8B , a downhole tool  800  may include an extendable pad  802  enabling characterization of fluids  806  at a formation surface  804 . The pad  802  may be pushed or pumped out to remove the mud cake and establish optical contact with the surface  804  of the formation  808 . A window  810  interfaces between the formation  808  and the inside of the downhole tool  800  (which may be fluorescence measurement capable as described above). Light sources  812  excite the formation  808  and any fluids or oils  806  at the surface  804 . A spectral camera  814  behind the window  810  performs the imaging, such as fluorescence imaging, and facilitates fluid characterization in accordance with principles described above. 
     FIGS. 9A-9D  illustrate apparatus and plots associated with some embodiments. Referring to  FIG. 9A , one apparatus that may be used to measure fluorescence downhole is shown. Similar to the embodiment of  FIGS. 6A-6B ,  FIG. 9A  illustrates a schematic optical layout of a downhole tool having flow line imaging capability.  FIG. 9A  fluorescence acquisition is collected by a spectrometer  332 . Excitation light  335  from a light source  330  excites a sample  328  in a flow line  334  through an optical window  336 . In the imaging configuration shown in  FIG. 9A , the light source  330  and the spectrometer  332  may both be arranged on the same side of the optical window  336 . Fluorescent light  337  resulting from the excitation light  335  is emitted from the sample  328  and detected by the spectrometer  332 . 
   The light source  330  may comprise a tunable light source, and it may also comprise a pulsed light source. A pulsed light source may have an intensity vs. time plot shown in  FIG. 9B  wherein light intensity  339  drops off when the light is pulsed at a cutoff time τ cutoff . τ cutoff  may be less than 1 μs.  FIG. 9D  illustrates light pluses  343  from the source  330  according to one embodiment. 
   In some embodiments, the apparatus of  FIG. 9A  may include an optical filter between the optical window  336  and the spectrometer  332 . For example, as shown in  FIG. 9A , a long pass optical filter  333  (e.g. λ cutoff &lt;1 μm) may be arranged in front of the spectrometer  332 . Operation of the apparatus arrangement of  FIG. 9A  may result in a plot like the one shown in  FIG. 9C . As shown in  FIG. 9C , activation of the light source  330  results in an excitation light spike  341 . The excitation light is turned off or pulsed, and fluorescent light from the sample  328  ( FIG. 9A ) continues to be detected by the spectrometer  332  ( FIG. 9A ), but fluorescence transmission to the spectrometer  332  ( FIG. 9A ) may be limited by the long pass filter  333 . The apparatus of  FIG. 9A  may be used according to principles described herein to generate fluorescence measurements and help characterize or identify downhole samples. 
   In some cases, 2D fluorescence measurements may be important to facilitate downhole fluids characterization when transmission measurements fail. Transmission measurements generally do not work, for example, with emulsions. In heavy oil reservoirs drilled with water based muds, fluid mapping is problematic because of the formation of stable water-in-oil (W/O) emulsions. Stable W/O emulsions add significant complexity to sample acquisition and may preclude standard downhole fluid analysis measurements. However, the inventors discovered that fluorescence measurements yield signals that are dependent on oil type but independent of the state of emulsion (even at very high water fractions). Thus, downhole fluorescence measurements can be used to perform hydrocarbon fluid mapping in the reservoir according to principles described herein. According to one aspect, one can perform 2D fluorescence mapping to emulsions. 
   A long wavelength absorption edge for most crude oils results from polycyclic aromatic hydrocarbons (PAH). The coloration is linearly dependent on the concentration of these chromophores in accord with Beers law: 
                 A   =         -   log     ⁢     I     I   O         =       ∑   i     ⁢           ⁢       ɛ   i     ⁢     c   i     ⁢   1                 (   1   )               
where: A is absorption,
     Io the incident light intensity,   I the transmitted light intensity,   ε i  is the molar extinction coefficient for component i, and   c i  is the concentration of component i and l is the path length.   
   The equation for the fluorescence intensity for solutions undergoing diffusional fluorescence quenching is obtained from analysis of the excited state decay rate:
 
 k   F   =k   Fo   +[Q]k   Q   (2)
 
where: k F  is the excited state decay rate and the measured fluorescence decay rate,
     k Fo  is the intrinsic fluorescence decay rate in the absence of quenchers,   [Q] is the quencher concentration, and   k Q  is the diffusional quenching rate constant which is diffusion limited.   

   The Stern-Volmer equation is obtained from Eq. 2: 
                       I   Fo       I   F       -   1     =         k   Q       k   Fo       ⁡     [   Q   ]               (   3   )               
Equation 3 shows that, for I Fo &gt;&gt;I F  (which applies for crude oils), the fluorescence intensity for a concentrated sample is proportional to the quencher concentration. The quenchers are the large PAHs that have red shifted electronic transitions, i.e., the same molecular fractions that give rise to crude oil coloration.
 
   It can be shown that to the zeroth order, both crude oil coloration (Eq. 1) and crude oil fluorescence intensity (Eq. 3) are linearly dependent on the population of large PAH chromophores. Thus, for a given crude oil, one can quantitatively relate coloration and fluorescence intensity ( FIG. 10 ). 
   For the same reasons described above, one discovers correlations between the absorption cutoff and or fluorescence intensity and fractions which include large PAH molecules.  FIG. 11  shows an example of such correlations with asphaltenes+resin fraction.  FIG. 12  shows another example of a correlation between absorption cutoff (fluorescence intensity) and C36+ weight fraction, respectively. 
   The plots comprising  FIGS. 10-12  include oils from many different geographic regions. It is expected that these correlations will strengthen when the data are restricted to a single basin. It is also expected that these correlations will further improve when the data are restricted to a single zone where compositional gradients are due to processes such as biodegradation. Other correlations may be discovered by those of ordinary skill in the art having the benefit of this disclosure with routine experimentation following the principles described herein, such as between absorption cutoff (fluorescence intensity) and density or viscosity. 
     FIG. 13  displays 2D fluorescence contour plots for twenty-nine different dead oils from many different geographic regions, and Table 1 provides associated “SARA” analysis. As known to those of ordinary skill in the art having the benefit of this disclosure, SARA analysis is a method for characterization of heavy oils based on fractionation, whereby a heavy oil sample is separated into smaller quantities or fractions, with each fraction having a different composition. Fractionation is based on the solubility of hydrocarbon components in various solvents used in this test. Each fraction comprises a solubility class containing a range of different molecular-weight species. In this method, the crude oil is fractionated to four solubility classes, referred to collectively as SARA: saturates, aromatics, resins, and asphaltenes. Saturates are generally iso- and cyclo-paraffins, while aromatics, resins, and asphaltenes form a continuum of molecules with increasing molecular weight, aromaticity, and heteroatom contents. Asphaltenes may also contain metals such as nickel and vanadium. The SARA method is sometimes referred to as Asphaltene/Wax/Hydrate Deposition analysis. 
   
     
       
             
             
             
             
             
           
             
             
             
             
             
           
         
             
               TABLE 1 
             
             
                 
             
             
               sample 
               saturate 
               aromatic 
               resin 
               asphaltene 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
               24 
               96.2 
               3.7 
               0.1 
               0 
             
             
               21 
               86.1 
               13 
               0.9 
               0 
             
             
               2 
               61.3 
               34.5 
               4.2 
               0 
             
             
               14 
               72.8 
               21.8 
               5.1 
               0.3 
             
             
               17 
               72.7 
               20.8 
               6.4 
               0 
             
             
               10 
               67.1 
               24.9 
               7.9 
               0 
             
             
               3 
               67.1 
               24.6 
               8.3 
               0 
             
             
               20 
               51.2 
               38.7 
               10 
               0.1 
             
             
               1 
               60.8 
               28.9 
               9.8 
               0.4 
             
             
               19 
               63.1 
               26.7 
               10.1 
               0.1 
             
             
               6 
               60.8 
               28.6 
               10.5 
               0 
             
             
               8 
               50.1 
               38 
               11.7 
               0.2 
             
             
               15 
               63.6 
               24 
               12 
               0.4 
             
             
               18 
               59.4 
               26.9 
               13.6 
               0.1 
             
             
               22 
               59.1 
               26.8 
               13.1 
               1 
             
             
               32 
               59.6 
               26.1 
               13.7 
               0.7 
             
             
               28 
               60.8 
               24.5 
               14.2 
               0.5 
             
             
               7 
               61.2 
               22.7 
               15.4 
               0.6 
             
             
               12 
               52.6 
               31.3 
               15.2 
               0.9 
             
             
               9 
               55.5 
               27.1 
               15 
               2.5 
             
             
               13 
               45.9 
               33.9 
               17.3 
               3 
             
             
               5 
               52.1 
               27.2 
               16.1 
               4.9 
             
             
               27 
               48.7 
               29.4 
               19 
               3.1 
             
             
               25 
               46.7 
               30.2 
               18.8 
               4.5 
             
             
               29 
               45.6 
               31 
               19.1 
               4.6 
             
             
               16 
               53.4 
               22.8 
               17.7 
               6.6 
             
             
               30 
               45.1 
               30.4 
               19.6 
               5.1 
             
             
               31 
               47 
               27.3 
               18.7 
               7.6 
             
             
               23 
               45.6 
               28.5 
               19.8 
               6.5 
             
             
               26 
               43.2 
               30.8 
               20.2 
               6.2 
             
             
               4 
               38.3 
               35.3 
               18.5 
               8.6 
             
             
               11 
               37.9 
               32.4 
               18.3 
               12.9 
             
             
                 
             
           
        
       
     
   
   There appears to be no simple linear correlation with SARA analysis because of the distortion that self-absorption induces in the distributions. However, when fluorescence maps of oils from a single region are carefully examined, they may provide a sensitive fingerprint that can be correlated empirically with oil properties. Also, a 2D fluorescence map can be advantageously sliced at different energies and the strength of any correlations (e.g. density, C36+, asphaltene/resin, weight fraction) may depend on a specific excitation/emission combination, as shown in  FIG. 14 . 
   The preceding description has been presented only to illustrate and describe certain embodiment and aspects. It is not intended to be exhaustive or to limit the invention to any precise form disclosed. Many modifications and variations are possible in light of the above teaching. Moreover, the principles described herein are applicable to drilling and measurement operations, production logging, permanent monitoring, well services for injected fluid, etc. 
   The embodiments and aspects were chosen and described in order to best explain the principles of the invention and its practical application. The preceding description is intended to enable others skilled in the art to best utilize the principles described herein in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims.