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CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present document is based on and claims priority to U.S. Provisional Application Ser. No. 62/073,025 filed Oct. 31, 2014, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     A wellbore may be drilled to locate and produce hydrocarbon-based fluids. The wellbore is drilled by a downhole drilling tool having a drill bit which is advanced into the formation to form a wellbore. As the drilling tool is advanced, drilling mud is pumped through the drilling tool and out through the drill bit to cool the drilling tool and to carry away cuttings. The fluid exits the drill bit and circulates back up to the surface before being recirculated back down to the drilling tool. The drilling mud also may be used to form a mud cake lining the wellbore. 
     During the drilling operation, various downhole evaluations may be performed to determine characteristics of the wellbore and/or surrounding formation. Depending on the application, the downhole evaluations may be conducted with devices contained in the drilling tool. However, the devices also may be deployed downhole via a wireline after the drilling tool has been removed. Examples of devices employed in performing downhole evaluations may include probes, packers, fluid analyzers, and/or sensors to obtain and measure downhole characteristics which may indicate the presence of hydrocarbons. 
     SUMMARY 
     In general, a methodology and system are described for providing improved formation evaluation with a downhole tool having a fluid analyzer. According to an embodiment, the fluid analyzer of the downhole tool comprises an atomic absorption spectroscopy (AAS) system and/or other evaluation systems and may be positioned in a wellbore penetrating a subterranean formation. The downhole tool further comprises a downhole flowline for receiving a sample fluid from the subterranean formation and delivering the sample fluid to the fluid analyzer. The atomic absorption spectroscopy system has a light source to generate light and to excite atoms of a substance, if present, in the sample fluid. The atomic absorption spectroscopy system also comprises a detector to measure how much light has been absorbed by the substance. 
     However, many modifications are possible without materially departing from the teachings of this disclosure. Accordingly, such modifications are intended to be included within the scope of this disclosure as defined in the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Certain embodiments of the disclosure will hereafter be described with reference to the accompanying drawings, wherein like reference numerals denote like elements. It should be understood, however, that the accompanying figures illustrate the various implementations described herein and are not meant to limit the scope of various technologies described herein, and: 
         FIG. 1  is a schematic illustration of an example of a downhole drilling tool deployed in a wellbore and including a formation evaluation system for performing a downhole evaluation, according to an embodiment of the disclosure; 
         FIG. 2  is a schematic illustration of an example of a downhole wireline tool deployed in a wellbore and including a formation evaluation system for performing a downhole evaluation, according to an embodiment of the disclosure; 
         FIG. 3  is a schematic illustration of a portion of a downhole tool having an example of a formation evaluation system including a fluid analyzer with an atomic absorption spectroscopy system, according to an embodiment of the disclosure; 
         FIG. 4  is a schematic illustration of a portion of a downhole tool having another example of a formation evaluation system including a fluid analyzer with an atomic absorption spectroscopy system, according to an embodiment of the disclosure; 
         FIG. 5  is a schematic illustration of a portion of a downhole tool having an example of a formation evaluation system including a fluid analyzer with an atomic absorption spectroscopy system and a vaporization system, according to an embodiment of the disclosure; 
         FIG. 6  is a schematic illustration of a portion of a downhole tool having another example of a formation evaluation system including a fluid analyzer with an atomic absorption spectroscopy system and a vaporization system, according to an embodiment of the disclosure; 
         FIG. 7  is a schematic illustration of a portion of a downhole tool having another example of a formation evaluation system including a fluid analyzer with an atomic absorption spectroscopy system and a vaporization system, according to an embodiment of the disclosure; 
         FIG. 8  is a schematic illustration of a portion of a downhole tool having another example of a formation evaluation system including a fluid analyzer with an atomic absorption spectroscopy system and a vaporization system, according to an embodiment of the disclosure; 
         FIG. 9  is a schematic illustration of a portion of a downhole tool having another example of a formation evaluation system including a fluid analyzer with an atomic absorption spectroscopy system, a vaporization system, and a system for inducing flow into the atomic absorption spectroscopy system, according to an embodiment of the disclosure; and 
         FIG. 10  illustrates an example of a procedure for utilizing an atomic absorption spectroscopy system under a micro-flow condition, according to an embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous details are set forth to provide an understanding of some embodiments of the present disclosure. However, it will be understood by those of ordinary skill in the art that the system and/or methodology may be practiced without these details and that numerous variations or modifications from the described embodiments may be possible. 
     Embodiments described herein facilitate formation evaluation involving fluid analysis. For example, systems, devices, and methods are described facilitate performance of fluid analysis downhole using atomic absorption spectroscopy. In an example, a fluid analyzer is positioned in a downhole tool which is deployed into a wellbore for measuring properties of downhole fluid drawn into the wellbore and into the downhole tool. In a specific example, the fluid analyzer comprises a flowline, e.g. a primary flowline and/or at least one micro-flowline, combined with an atomic absorption spectroscopy system. A photodetector may be provided in the atomic absorption spectroscopy system to measure atomic absorption at wavelengths used to identify a specific substance or substances in the downhole fluid. 
     Formation evaluation as used herein relates to the measurement, testing, sampling, and/or other analysis of well site materials such as gases, liquids, and/or solids. Such formation evaluation may be performed at the downhole location and/or at a surface location to provide desired data, e.g. data related to downhole parameters or material properties. Examples of downhole parameters include temperature, pressure, permeability, porosity, and/or other desired parameters. Material properties may comprise properties of the sampled fluid such as viscosity, composition, density, and/or other desired properties. Various downhole parameters and properties may be measured in combination with the atomic absorption spectroscopy analysis. 
     Fluid analysis as used herein relates to a type of formation evaluation of downhole fluids which may be fluids from the wellbore, formation, reservoir, and/or other fluids located at a well site. Fluid analysis may be performed by a fluid analyzer to detect/measure a substance and sometimes to measure other fluid properties, e.g. viscosity, composition, density, temperature, pressure, flow rate, optical parameters, and/or other desired properties. Fluid analysis may be performed using a variety of systems and devices, as described in greater detail below. 
     Referring generally to  FIG. 1 , an example of a well system  20  is illustrated. In this example, well system  20  comprises a drill string  22  having a downhole tool  24 , e.g. a drilling tool, and a drill bit  26  operated to form a borehole  28 , e.g. a wellbore. In some applications, the downhole tool  24  may comprise or may be combined with a while-drilling tool  30 , e.g. a measurement-while-drilling (MWD) tool, logging-well-drilling (LWD), and/or other while-drilling tools. The downhole tool  24  may be conveyed downhole via a suitable conveyance  32 , e.g. drill pipe, coiled tubing, wireline, supported by surface equipment  34 , e.g. a drilling rig. 
     The downhole tool  24  also comprises a testing system  36  for testing fluids downhole to enable analysis of a surrounding formation  38 , e.g. to determine the potential for hydrocarbon production from a reservoir located in formation  38 . By way of example, the testing system  36  may comprise a probe  40  adapted to seal with a wall  42  of the wellbore  28  so as to enable drawing of a fluid sample from the surrounding formation  38  and into the downhole tool  24  as represented by arrows  44 . 
     In this example, the testing system  36  comprises a formation evaluation tool  46  having a fluid analyzer  48  for analyzing formation fluid drawn into the downhole tool  24 . The fluid analyzer  48  may comprise an atomic absorption spectroscopy system  50  capable of generating and detecting atomic absorption in downhole fluids, as described in greater detail below. The formation evaluation tool  46  also may comprise a flowline  52  for receiving the formation fluid sample from probe  40 . The flowline  52  also passes the fluid sample to the fluid analyzer  48  to enable fluid analysis. 
     A surface control system  54 , e.g. a computer-based processing system, may be used to communicate with the downhole tool  24 . For example, power signals, command signals, data signals, and/or other types of signals may be communicated between surface unit  54  and downhole tool  24 . In some applications, the surface unit  54  may be used to provide power downhole for powering the fluid analyzer  48 . 
     Referring generally to  FIG. 2 , another example of well system  20  is illustrated. In this example, the downhole tool  24  is a wireline tool that may be used for performing formation/fluid evaluation. As illustrated, the downhole wireline tool  24  similarly comprises testing system  36  for testing fluids downhole to enable analysis of a surrounding formation  38 , e.g. to determine the potential for hydrocarbon production from a reservoir within formation  38 . The testing system  36  may again comprise probe  40  oriented to seal with wall  42  of wellbore  28  so as to enable drawing of a fluid sample from the surrounding formation  38  and into the downhole tool  24  as represented by arrows  44 . The conveyance  32  may be in the form of wireline used to lower the downhole wireline tool  24  to a desired position in wellbore  28 . In some applications, a backup piston or pistons  56  may be used for pushing the downhole tool  24  and probe  40  against the wellbore wall  42  adjacent formation  38 . 
     In  FIGS. 1 and 2 , examples of downhole tool  24  are illustrated but other configurations and types of downhole tools may be used to perform formation evaluation. Additionally, various configurations of the fluid analyzer  48  may be combined with the downhole tool  24  to enable testing of various fluid samples and/or formation characteristics. In some applications, the fluid analyzer  48  may be positioned in whole or in part at other suitable locations. For example, portions of the fluid analyzer  48  may be located at the surface, at other downhole locations, and/or at off-site facility locations. 
     By positioning the testing system  36  and fluid analyzer  48  in the downhole tool  24 , real-time data may be collected in situ at downhole conditions. For example, real-time data on temperatures, pressures, sample content, density, flow rate, optical parameters, and/or other data may be collected at downhole conditions by positioning the probe  40  and fluid analyzer  48  where the downhole fluids are located and/or where the fluid sample calibrations are performed. In some applications, fluid samples also may be retrieved and taken to the surface and/or to off-site locations for analysis, e.g. additional analysis. Furthermore, data and test results collected from various locations, e.g. from various wellbores, may be analyzed and compared to further enhance the formation evaluation. 
     In various analysis procedures, atomic absorption spectroscopy analysis is performed in borehole  28  at downhole tool  24 . Atomic absorption spectroscopy is a method for identifying specific substances, e.g. Hg, Pb, Cd, Zn, and/or other substances. Each atom of a given substance has a unique condition of electron potential although the electron normally stays at a ground state. When a light is emitted to the atom, some electrons can be excited by the light if the wavelength of the light is equivalent with a potential difference between the electron ground state and the electron excited state. Hence, a specific substance can be identified and measured by measuring the intensity of the specific wavelength light because the specific wavelength light is absorbed by electron excitement resulting from the unique potential difference of the specific type of atom. In many applications, the substance can be identified and measured in parts per million or even parts per billion using atomic absorption spectroscopy. 
     To measure atomic absorption, a light source or radiation source with a narrow spectral width is utilized. For example, the light source or radiation source for atomic absorption spectroscopy may have a very narrow spectral width of atomic absorption on the order of, for example, about 0.01 nm. In some applications, the atomic absorption spectroscopy system  50  may utilize an atomic absorption photometer in which a hollow cathode lamp (HCL) can be used. The spectral width of an emission line of a hollow cathode lamp may be even narrower than a line in an atomic absorption spectrum. The atomic absorption spectroscopy system  50  may be implemented in downhole tool  24  to provide a downhole fluid analysis system for identifying and measuring a specific substance received as a fluid sample in a flowline. The atomic absorption spectroscopy system  50  may include or may be combined with a variety of system elements, such as a photodetector which may be connected with a lens, filter, amplifier, and/or other features to facilitate the fluid analysis for a given application. 
     Referring generally to  FIG. 3 , for example, an embodiment of atomic absorption spectroscopy system  50  is illustrated as incorporated into fluid analyzer  48  of downhole tool  24 . In this embodiment, the atomic absorption spectroscopy system  50  of fluid analyzer  48  is positioned for cooperation with flowline  52  which receives a fluid sample from formation  38  via probe  40 . The fluid sample is represented by arrow  58  and may comprise liquid and/or gas phases. 
     The atomic absorption spectroscopy system  50  may comprise a radiation source  60 , e.g. a light source, which emits light waves or other suitable radiation. The light waves may be directed through a lens  62  which, in turn, focuses the light waves to an area  64  for atomic absorption. In the illustrated example, the area for atomic absorption  64  is located within flowline  52 . As described above, the energy of the light waves focused at area  64  can be used to excite atoms of a specific substance to enable determination of the presence of the specific substance and analysis of its content according to atomic absorption spectroscopy. 
     As illustrated, the light waves may be directed into flowline  52  through a window  66 , e.g. a glass window with a pressure seal. The light energy flows through the area for atomic absorption  64  and out of flowline  52  through a corresponding window  68  having an appropriate pressure seal. The light waves continue to travel through a receiving lens  70  of a photodetector  72 . The photodetector  72  may be used to measure atomic absorption, and the atomic absorption data can be used to determine the presence and content of the specific substance or substances. In other words, the photodetector  72  enables atomic absorption spectroscopy analysis by measuring an intensity of a wavelength associated with the substance, thus enabling detection and quantitative analysis of the substance in the fluid sample  58  received in flowline  52 . In some applications, the photodetector  72  may work in cooperation with surface system  54  to process the atomic absorption data. 
     Various arrangements of flowline  52  and atomic absorption spectroscopy system  50  may be constructed to facilitate formation/fluid analysis for a given application. In the embodiment illustrated in  FIG. 3 , for example, the flowline  52  is routed along a generally linear path and the light energy emitted from radiation source  60  is directed laterally through the flowline  52 . In the embodiment illustrated in  FIG. 4 , however, the flowline  52  extends through an offset portion  74  and the atomic absorption area  64  is disposed along the offset portion  74 . This allows the light energy from radiation source  60  to be directed longitudinally, e.g. axially, along the offset portion  74  of flowline  52  and thus over a longer path. The longer path may enhance measurement sensitivity due to an increased chance of the substance absorbing the light during the longer light passage. In other words, the longer length of the passage through which the light passes increases the chances for light absorption by the substance and may induce higher performance of the system due to an improved signal-to-noise ratio. 
     Referring generally to  FIG. 5 , another embodiment of testing system  36  and fluid analyzer  48  is illustrated. In this embodiment, the testing system  36  further comprises a vaporization system  76  which vaporizes and/or atomizes sample  58  and works in cooperation with atomic absorption spectroscopy system  50 . The vaporization system  76  may be used to change the fluid sample  58  flowing along flowline  52  by, for example, changing a liquid phase to a gas phase. By changing a liquid phase sample to a gas phase sample in some applications, the vaporization system  76  can further help the atomic absorption spectroscopy system  50  determine the presence and amount of a desired substance by enhancing the atomic absorption testing. The vaporization system  76  also may perform as an atomization system or work in cooperation with an atomization system. As further illustrated in  FIG. 6 , the vaporization system  76  also may be combined with the configuration of atomic absorption spectroscopy system  50  in which the light energy is directed along offset portion  74 . 
     With respect to the embodiments illustrated in  FIGS. 5 and 6 , the vaporization/atomization system  76  is located along flowline  52  at a position upstream of the atomic absorption spectroscopy system  50 . However, the vaporization system  76  also may be combined with the atomic absorption spectroscopy system  50 . As illustrated in  FIG. 7 , for example, the vaporization system  76  is combined with the atomic absorption spectroscopy system  50  along a linear portion of the flowline  52 . In the embodiment illustrated in  FIG. 8 , another embodiment is provided in which the vaporization system  76  is combined with the atomic absorption spectroscopy system  50  along the offset portion  74  of flowline  52 . Depending on the specifics of a given application, the vaporization system  76  may comprise various vaporization devices, such as heating devices, depressurizing devices, and/or other suitable devices to facilitate transition of a liquid phase sample to a gas phase sample. As described above, the vaporization system  76  may function as a vaporization system and/or atomization system. 
     Referring generally to  FIG. 9 , another embodiment of testing system  36  and fluid analyzer  48  is illustrated. In this embodiment, the testing system  36  comprises a plurality of flowlines, e.g. a primary flowline and at least one micro-flowline. By way of example, the testing system  36  comprises a micro-flowline  78  and sometimes a plurality of micro-flowlines  78 ,  80  which work in cooperation with the primary flowline  52 . The atomic absorption spectroscopy system  50 , alone or in combination with the vaporization system  76 , may be positioned along one of the micro-flowlines. In the specific example illustrated, the atomic absorption spectroscopy system  50  and the vaporization system  76  are positioned along micro-flowline  80 . In some applications, the fluid volume for sampling can be very small, e.g. a microliter, so the overall testing system  36  may be constructed with a microfluidics system  82  having micro-flowlines  78 ,  80 . 
     In operation, sample probe  40  is used to introduce fluid sample  58  along primary flowline  52 . A portion of the fluid sample  58  is diverted to the micro-flowlines  78 ,  80  by, for example, a flow resistance device  84 , e.g. a check valve, which provides flow resistance along primary flowline  52 . The flow of sample fluid separated from the primary flowline  52  can be controlled as it passes through microfluidics system  82 . For example, a controllable first valve  86  disposed along micro-flowline  78  and a controllable second valve  88  disposed along micro-flowline  80  may be selectively actuated to control flow along the micro-flowlines  78 ,  80 . In some applications, a second flow resistance device  90  may be disposed along micro-flowline  80  downstream of atomic absorption spectroscopy system  50 . 
     During testing, the sample fluid  58  may be controllably directed into micro-flowline  80  and through atomic absorption spectroscopy system  50  by opening valve  88  and closing valve  86 . In some applications, the fluid sample flowing along micro-flowline  80  also may be subjected to vaporization system  76 . A more detailed example of an operational procedure for testing fluid samples is described below and illustrated in the table of  FIG. 10 . 
     It should be noted that another sensor system or systems  92  may be disposed along at least one of the flowline  52 , micro-flowline  78 , and/or micro-flowline  80 . By way of example, the sensor system  92  may comprise a gas chromatography system having a small volume sampling and vaporization function. The methodology also may involve identifying different substances within the fluid sample  58  via different types of sensors. In some applications, sensors may be used to detect other optical parameters of light emitted through the downhole fluid sample. 
     Referring again to  FIG. 10 , the illustrated table provides a sequence for testing a fluid sample with atomic absorption spectroscopy system  50  and vaporization system  76 . In this example, the valve # 1  listed in the table corresponds with valve  86  of  FIG. 9  and the valve # 2  listed in the table corresponds with valve  88  of  FIG. 9 . During an initial normal condition, valve  86  is open, valve  88  is closed, and vaporization system  76  is off. The fluid sample  58  is then injected into micro-flowline  80  by closing valve  86  and opening valve  88  while the vaporization system  76  remains off. Subsequently, valve  86  is opened and valve  88  is closed to prepare for sample testing, e.g. substance detection and measurement. 
     During fluid sample testing, valve  86  remains open, valve  88  remains closed, and vaporization and/or atomization system  76  is turned on to, for example, convert liquid sample phase to gas sample phase. After testing, the sample may be cleaned from micro-flowline  80  by closing valve  86  and opening valve  88 . While cleaning out the fluid sample, the vaporization and/or atomization system  76  may be shut off. The microfluidic system  82  is then returned to the normal operating condition. 
     Depending on the specifics of a given application, the downhole tool  24  may comprise various other and/or additional components arranged in desired configurations. Additionally, the sampling procedures may be performed during drilling operations or during various other downhole operations. The sampling probe and related components for obtaining the fluid sample from the surrounding formation may be adjusted according to the structure of the downhole tool and/or environmental parameters. Similarly, the size, components, and configuration of the testing system  36  may be adjusted according to the configuration of the downhole tool  24  and to accommodate various environmental constraints or other parameters. Depending on the application, the atomic absorption spectroscopy system  50  may be used to measure absorption of the light and/or intensity, e.g. fluorescence, of the light. 
     Although a few embodiments of the disclosure have been described in detail above, those of ordinary skill in the art will readily appreciate that many modifications are possible without materially departing from the teachings of this disclosure. Accordingly, such modifications are intended to be included within the scope of this disclosure as defined in the claims.

Summary:
A technique facilitates formation evaluation with downhole devices which may include fluid analyzers having atomic absorption spectroscopy (AAS) systems. According to an embodiment, a fluid analyzer of a downhole tool may be positioned in a wellbore penetrating a subterranean formation. The downhole tool comprises a downhole flowline for receiving a sample fluid. Additionally, the fluid analyzer comprises a flowline positioned to receive the sample fluid for analysis by the atomic absorption spectroscopy system. The atomic absorption spectroscopy system has a light source to generate light and to excite atoms of a substance in the sample fluid. The atomic absorption spectroscopy system also comprises a detector to measure how much light has been absorbed by the substance, thus enabling the atomic absorption spectroscopy analysis.