Patent Publication Number: US-11028690-B2

Title: System and methodology for chemical constituent sensing and analysis

Description:
RELATED APPLICATIONS 
     This application is the Divisional of U.S. Non-Provisional application Ser. No. 14/922,182, filed Oct. 25, 2015, which claims the benefit of a related U.S. Provisional Application Ser. No. 62/067,983 filed Oct. 24, 2014, the disclosures of which are incorporated by reference herein in their entirety. 
    
    
     BACKGROUND 
     The following descriptions and examples are not admitted to be prior art by virtue of their inclusion in this section. 
     Formation fluid compositions can vary greatly, and understanding such formation fluid compositions can be helpful in assessing well completion and production strategies. A variety of technologies have been employed to facilitate fluid characterization and evaluation of hydrocarbon reserves. For example, various fluid mixing techniques have been used to detect specific chemicals located in the formation fluid. Additionally, various multiphase microreactor techniques, mass transfer techniques, and/or other techniques have been employed in an attempt to better understand formation fluid compositions. However, such technologies have limited usefulness in a variety of environments, including downhole environments. 
     SUMMARY 
     This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter. 
     In general, a system and methodology are provided to facilitate detection and analysis of constituents, e.g. chemicals, which may be found in formation fluids and/or other types of fluids. The technique comprises introducing a first fluid and a second fluid into a channel in a manner which forms slugs of the first fluid separated by the second fluid. The fluids are flowed through the channel to create a mixing action which mixes the fluid within the slugs. The mixing increases the exchange, e.g. transfer, of the chemical constituent between the second fluid and the first fluid. As a result, the amount of the chemical constituent or constituents can be determined and the fluids may be better analyzed. In many applications, the introduction, mixing, and measuring can be performed in a subterranean environment, e.g. in a wellbore environment. 
     Other or alternative features will become apparent from the following description, from the drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Certain embodiments 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 drawings illustrate only the various implementations described herein and are not meant to limit the scope of various technologies described herein. The drawings are as follows: 
         FIG. 1  is a schematic illustration of an example of a fluidic testing system deployed downhole in a well system, according to an embodiment of the disclosure; 
         FIG. 2  is a schematic illustration of an example of a channel through which slugs of a first fluid are flowed while separated by a second fluid, according to an embodiment of the disclosure; 
         FIG. 3  is a schematic illustration of an example of a slug of the first fluid undergoing a mixing action by creating, for example, a vortex, according to an embodiment of the disclosure; 
         FIG. 4  is a cross-sectional view of an example of a channel for fluid flow, according to an embodiment of the disclosure; 
         FIG. 5  is a schematic illustration of an example of a serpentine channel having slugs of a first fluid separated by a second fluid, according to an embodiment of the disclosure; 
         FIG. 6  is a schematic illustration of an example of a fluidic sensing and analysis system, according to an embodiment of the disclosure; 
         FIG. 7  is a graphical representation of an example of measurement of a chemical constituent following microfluidic slug flow mixing, according to an embodiment of the disclosure; and 
         FIG. 8  is a graphical representation of another example of measurement of a chemical constituent following slug mixing, according to an embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     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 disclosure. 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.” 
     As used throughout the specification and claims, the term “downhole” refers to a subterranean environment, particularly in a wellbore. “Downhole tool” is used broadly to mean any tool used in a subterranean environment including, but not limited to, a logging tool, an imaging tool, an acoustic tool, a permanent monitoring tool, and a combination tool. 
     The disclosure herein generally involves a system and methodology which facilitate the sensing and analysis of constituents, e.g. chemicals, which may be found in formation fluids and/or other types of fluids. The technique comprises introducing a first fluid and a second fluid into a channel in a manner which forms slugs of the first fluid separated by the second fluid. Depending on the application, the first fluid and the second fluid may comprise a liquid and a gas, respectively. In other applications, the first and second fluids may comprise two liquids, such as two immiscible liquids. 
     The intermittent fluids are flowed through the channel to create a mixing action which mixes the slugs of fluid. By way of example, the channel may comprise a capillary which is relatively long and having a comparatively small cross-sectional dimension, e.g. diameter. The mixing may be accomplished by creating a vortex in the slugs, thus increasing the exchange, e.g. transfer, of the chemical constituent between the second fluid and the first fluid. As a result, the chemical constituent or chemical constituents of interest can be sensed/detected for analysis of the fluids, e.g. formation fluids. In many applications, the introduction, mixing, and measuring can be performed in a subterranean environment, e.g. in a wellbore environment. In an embodiment, the system and methodology may be employed for enhanced component mass transfer and equilibration between immiscible fluids via slug flow mixing for downhole fluid analysis. 
     In general, embodiments described herein are related to a method and system for improving the exchange of components between two fluids. By way of example, the two fluids may comprise a gas and a liquid or two immiscible liquids. Depending on the application, the methodology may be used at a surface location, at a wellsite, at a downhole wellbore location, and/or at a test location subjected to downhole conditions of high temperature and high pressure. The embodiments improve the exchange between the two fluids by increasing a surface area between the two fluids which makes the methodology very effective for compound/constituent extraction. 
     According to an embodiment, a fluidic test system is provided with a narrow channel, e.g. capillary, having an inside diameter equal to or less than 500 μm and sometimes equal to or less than 200 μm. The narrow channel is relatively long and may be up to 1 m or more in length. In this example, the channel is arranged in a serpentine path on a substrate which may be part of a microfluidic chip. The two fluids are introduced into the channel in such a way as to flow as short slugs through the channel. Flow through the channel mixes the liquid in the slugs by creating, for example, a vortex within the liquid slugs. This mixing increases the exchange between the two fluids. As a result, an improved efficiency and a reduced operation time are enabled with respect to completion of component mixing between the two fluids. This allows downhole operations that involve chemical reaction, compound stripping, and fluid property manipulation to be enhanced for improved chemical sensing and analysis. 
     Chemical sensing and analysis often are helpful for downhole fluid characterization and, ultimately, in the evaluation of hydrocarbon reserves. Understanding wellbore fluid compositions, including concentration levels of corrosion causing compounds/constituents such as CO 2  and H 2 S, can be very helpful in assessing eventual well completion and production strategies. In some embodiments, the present technique enables the sensing and analysis of such fluids and fluid constituents to be carried out on a well string deployed downhole in a wellbore as opposed to a conventional approach of sending samples to a laboratory where they are reconstituted to reservoir conditions and then analyzed. 
     A variety of analytical methods, including colorimetric methods, can be used to facilitate analysis of certain constituents in reservoir fluids. Many of these analytical methods utilize interaction between a reagent (such as a water-based liquid for example) and a sample fluid. By way of example, such methods may include a methylene blue method and/or other methods using the reaction of metal ions with sulfide irons to form metal sulfide. Such methods utilize transfer of hydrogen sulfide from the gas or oil phase to the water phase. The present system and methodology may be used to facilitate a variety of these analytical methods by enhancing mass transfer between immiscible fluids without forming an emulsion. 
     As described herein, the flowing of intermittent fluids along a relatively long channel, e.g. microchannel, is highly efficient in increasing a mass transfer rate between the fluids, e.g. between a gas phase and a liquid phase. The high mass transfer rate is achieved by establishing a high surface-to-volume ratio in the channel combined with a short diffusion length of, for example, gas through the liquid. The short diffusion length can be obtained by operating the channel in a two-phase slug flow regime, sometimes referred to as a Taylor flow regime. In some applications, the slug flow is characterized by a train of liquid slugs and gas bubbles moving consecutively through the channel. The gas bubble length tends to be several times longer than the diameter of the channel, and the gas bubble diameter is almost equal to the channel diameter. Generally, a thin liquid film separates the gas bubble from the inside wall surface of the channel. 
     In some embodiments, the intensification of the mixing process also can be achieved in a liquid-liquid slug flow of two immiscible liquids. In liquid-liquid slug flow, the internal circulation in the liquid slugs substantially enhances the mass transfer at the interface between the two liquids. Thus, by flowing the intermittent slugs of immiscible liquids through the channel, the mass transfer rate between the immiscible fluids can be substantially increased. Embodiments described herein can be used to enhance component mass transfer between two otherwise immiscible fluids. However, the system and methodology also can be used to mix two or more miscible fluids with the aid of an immiscible fluid. 
     Referring generally to  FIG. 1 , an example of a fluidic testing system  20  is illustrated. In this embodiment, the fluidic testing system  20  is positioned along a well string  22  disposed in a wellbore  24  such that the fluidic testing system  20  is at a desired subterranean, e.g. downhole, location  26 . By way of example, the fluidic test system  20  may comprise an injection system  28  which introduces at least a first fluid and a second fluid into a mixing system  30 , in which at least one fluid is intermittent. The mixing system  30  has a channel structure  32 , e.g. a capillary structure, which may be arranged in a serpentine pattern or other suitable pattern. The fluid test system  20  further comprises a measurement system  34  which may be used to determine the rate or amount of mass transfer with respect to a given chemical constituent. For example, the measurement system  34  may be used to measure an amount of a chemical constituent transferred from the second fluid to the first fluid during the mixing process which occurs as the fluids flow along channel  32 . In some applications, the measurement system  34  comprises an optical measurement system as discussed above. 
     Two-phase flow in conduits, e.g. capillaries, may be referred to as slug flow and this type of two-phase flow has applications in monolith reactors and micro-mixing devices. Slug flows in channels, e.g. microchannels, provide thorough mixing and excellent heat transfer properties. However, mixing two liquid phases in a microchannel can be a challenging task due to a low Reynolds number associated with the liquid flow. However, by injecting gas bubbles intermittently inside the channel  32 , the two liquids can be mixed efficiently. The fluidic testing system  20  may be used to enable this efficient mixing of fluids to enhance mass transfer of a given chemical constituent between fluids. 
     Referring generally to  FIG. 2 , a schematic illustration of channel  32  is illustrated in the form of a microchannel. At least one of a first fluid  36  and a second fluid  38  are intermittently introduced into the channel  32  and flowed along the channel  32  in the direction indicated by arrow  40 . In this example, the first fluid  36  may be a liquid and the second fluid  38  may be a gas such that the first fluid  36  forms slugs  42  separated by the second fluid  38  which forms bubbles  44  between the slugs  42 . In this example, the size of the bubbles  44  increases along the channel  32  due to a gradual pressure drop. This increase in bubble size leads to an increase in void fraction, which is defined as the volume of gas over the total volume (V g /V total ). 
     In this example, the increase in void fraction causes a gradual increase in the fluid velocity along channel  32 . However, even though there is a gradual change in velocity and void fraction along the channel  32 , the mean-pressure-gradient along the channel  32  tends to remain constant. The movement through channel  32  creates a mixing action, e.g. a vortex  46 , in the slugs  42 , as illustrated in  FIG. 3 . Additionally, the liquid phase in the form of first fluid  36  wets an inside surface  48  of channel  32 , as illustrated in  FIG. 4 . In other words, a thin liquid film  50  is trapped between the gas phase of second fluid  38  and the solid inside surface  48 . The thickness of the liquid film  50  may be non-uniform. For example, if the cross-sectional configuration of channel  32  has four sides, e.g. rectangular or trapezoidal, the film thickness may be non-uniform and have thicker regions  52 . 
     However, the thickness of the liquid film  50  can be estimated for a suitable engineering analysis by an accurate semi-spherical equation for flow through a circular channel by the following equation: 
               δ     R   c       =       1.32   ⁢           ⁢     Ca     2   /   3           1   +     3.33   ⁢           ⁢   Ca   ⁢           ⁢     2   /   3                 
where δ is the film thickness, R C  is the channel hydraulic radius, and C a  is the capillary number. The capillary number is defined as the ratio of the viscous and interfacial forces (Ca=μ L U B /γ where μ L , U B , and γ are liquid viscosity, bubble velocity, and gas-liquid interfacial tension, respectively.
 
     In  FIG. 2 , the end caps  54  of the bubbles  44  are illustrated as semi-spherical which is how the bubbles  44  exist when stationary in a circular channel/capillary  32 . When the bubbles travel along the channel  32 , the end caps  54  deform slightly and the front end caps become slightly narrow and conically shaped while the rear end caps are generally flat. If four-sided channels  32  are employed, e.g. see the embodiment of  FIG. 4 , the end caps  54  can deviate substantially from the semi-spherical shape. 
     Substantial mass and heat transfer during the flow of slugs  42  is facilitated by employing relatively small channel cross-sections, low-intensity slug vortices  46 , high shear in the thin liquid film  50 , and a suitable cross-sectional shape, such as a four-sided cross-section. By way of example, a rectangular cross-sectional shape is suitable in many applications. The time for diffusion driven processes correlates with the square of the length of the channel  32 . Therefore, the channel  32  is constructed with a maximum cross-sectional dimension, e.g. diameter, which is small relative to the length of the channel  32 . By way of example, the length of the channel  32  may range from a few centimeters to a meter or more. 
     In many applications, the channel  32  is in the form of a microchannel having a maximum cross-sectional dimension equal to or less than 500 μm, and in some applications equal to or less than 200 μm, and in some applications equal to or less than 100 μm. Such construction of the channel  32  can improve the diffusion time substantially, e.g. by a factor of 10 6 . Additionally, high shear flow in the liquid film  50  and the powerful vortices  46  in the slugs  42  provide an intensive interphase mass transfer by convection. The bubbles  44  are engulfed in liquid, thus further facilitating an effective mass and heat transfer. The recirculating or mixing regions, e.g. vortices  46 , in the liquid slugs  42  further ensure effective mixing inside the liquid slugs  42 . 
     Referring generally to  FIG. 5 , an embodiment of channel  32  is illustrated as an example of a construction which substantially enhances mixing of concurrently flowing liquid and gas phases in the microfluidic system  20  while at high pressure and temperature. In this example, the channel  32  is in the form of a narrow capillary or microchannel which has a serpentine shape  56  such that the flow of first fluid  36  and second fluid  38  moves along a path which reverses in direction. In this example, the first fluid  36  is a liquid and the second fluid  38  is a gas. The liquid phase  36  and the gas phase  38  are brought into contact with each other at a junction  58 , such as a T-junction, to develop the desired slug flow between an inlet  60  of channel  32  and an exit  62 . 
     As described above, the slug flow along channel  32  provides enhanced mass transfer characteristics and substantially increased mixing. The improved mixing in the channel  32  is used to facilitate mass transfer between fluids, e.g. between H 2 S gas and a chemical reagent. The concentration of the H 2 S (or other chemical constituent of interest) is determined in the reagent before and after the mixing to establish effectiveness of slug flow mixing. The microfluidic mixing and testing system  20  is readily constructed to withstand downhole conditions which renders the slug flow mixing methodology described herein feasible for downhole measurement of specific chemical constituents, such as H 2 S, CO 2 , or other gases. 
     By way of example, channel  32  may be positioned on or in a variety of substrates  64 . In the illustrated embodiment, the serpentine microchannel structure of channel  32  is etched into substrate  64  such that the substrate itself forms the channel structure. By way of example, the channel  32  may be etched in a silicon substrate  64 . However, the channel  32  also may be made of polyetheretherketone (PEEK) and/or other materials suitable for use in downhole applications or other oil industry applications. In this example, the channel  32  again has a small diameter, e.g. less than 200 μm, and the length ranges from a few centimeters to at least a meter. The substrate  64  containing the channel  32 , e.g. serpentine structure  56 , may be bonded to glass, to another silicon substrate, or to another suitable substrate so as to form a closed microfluidic device, e.g. microfluidic chip, that may be mounted on, in, and/or along well string  22 . 
     Due to the micron scale dimensions of the channel  32 , the sample volume used in the fluid system  20  may be on the order of a few microliters. In some applications, the inside surface  48  of channel  32  may comprise a coating  66  formed of appropriate chemicals, polymers, or other materials to make channel  32  less sensitive to scavenging or corrosion. Various coatings  66  also may be used to change the wetting properties of the channel  32 , e.g. to change the hydrophilic or hydrophobic properties of the channel  32 . 
     In the embodiment illustrated in  FIG. 5 , the microfluidic system  20  has two ports  68 ,  70  at inlet  60 . By way of example, the ports  68 ,  70  may comprise a liquid inlet port and a gas inlet port, respectively. Sample fluids and chemical reagents are injected into channel  32  via the inlet ports  68 ,  70  which can be appropriately designed for liquid and/or gas flow according to the parameters of a given application. In some applications, the number of inlet ports can be increased to facilitate injection of multiple reagents in sequence or in other desired patterns. In a simple example, a sample fluid and a reagent flow through the inlet ports  68 ,  70  and come into contact at T-junction  58 . The T-junction  58  is constructed to have two flow paths which intercept each other at a desired angle, e.g. at a perpendicular angle which is effective for establishing the slug flow. 
     The formation of slug flow is readily controlled at the T-junction  58 , however other designs may be used to introduce the first and second fluids in such a way as to initiate the slug flow. In the case shown, the gas inlet port  70  may be intermittently introduced into a more continuous flow of liquid from the liquid inlet port  68 . In some other cases, the reverse may be true, the liquid may be intermittently introduced into a more continuous flow of gas. In still other cases, the two fluids may both be substantially continuous and the resulting combination formed into slugs due to the configuration and degree of cohesiveness or immiscibility of the fluids. 
     The volumetric flow rate of the test fluids is controlled so as to develop segmented flow where the fluids are distributed in the channel as discrete segments, e.g. slugs, of a first fluid separated by a second fluid. In the case of liquid and gas flow, the liquid flow rate can be controlled to effectively snap-off gas bubbles at the junction  58 , thus producing slug flow. 
     Referring generally to  FIG. 6 , a schematic illustration is provided of an embodiment of fluidic system  20 . In this example, system  20  is a microfluidic system used to evaluate the efficiency of a mixing process in a gas-liquid slug flow. The microfluidic system  20  comprises a microfluidic device  72  which includes substrate  64  and channel  32 . Channel  32  may be in the form of serpentine construction  56 . Liquid and gas phases are injected into channel  32  through ports  68  and  70 , respectively. To develop the slug flow, the gas can be injected at a constant flow rate through port  70  and the liquid flow rate can be gradually increased until well-defined slugs and bubbles are established in the channel  32 . 
     In this example, pressure sensors  74  are placed in a liquid flow line  76  and a gas flow line  78  used to deliver the liquid and gas phases to channel  32  via ports  68 ,  70 , respectively. The pressure sensors  74  may be employed to monitor pressure at the inlets  68 ,  70  of channel  32 . The volumetric ratio of the gas and liquid flow rates may be regulated by pumps  80 , such as computer-controlled positive displacement pumps. The gas-liquid slug flow at the end of channel  32  can be separated into liquid and gas via, for example, a liquid trap  82  and a gas collection device  84 , respectively. The separated phases may be passed through a suitable valve or valves  86  for further analysis. 
     However, the separated liquid and gas phases collected at the exit end of the channel  32  become well mixed during passage along channel  32  and can be analyzed in situ by a suitable measurement system  88 , such as an optical measurement system. The measurement system  88  may be used to interrogate the output streams and to obtain, for example, the constituent concentration or other characteristics of the sample fluid. For example, the measurement system  88  may be used to determine the amount of the chemical constituent of interest, e.g. H 2 S, transferred from the second fluid/gas to the first fluid/liquid. 
     It should be noted that fluidic system  20  may comprise a variety of other and/or additional components depending on the system construction and the environment in which fluidic system  20  is employed. For example, fluidic system  20  may be constructed for use in high pressure and high temperature environments, such as downhole environments. However, the system  20  also may be used in various other environments, including surface environments. In the specific embodiment illustrated in  FIG. 6 , examples of additional components comprise a controller  90  for controlling pumps  80 . In this example, the pumps  80  deliver the fluids from fluid sources  92 , such as a liquid source and a gas source, and through flowlines  76 ,  78  to channel  32 . Various valves  94  may be positioned along flowlines  76 ,  78  to control the flow of fluid along flow control line  76 ,  78 . Additionally, various other and/or additional components may be incorporated into the overall system. 
     In an operational example, the fluidic testing system  20  was used in mixing first fluid  36  in the form of a liquid reagent with second fluid  38  in the form of H 2 S gas. The liquid phase/reagent  36  injected into the channel  32  was formed of 2 mM Bi(NO 3 ) 2  in 1.75% poly (acrylic acid) in water which indicates absorption of the hydrogen sulfide (H 2 S) gas  38  in the liquid phase  36  by changing color. The H 2 S sample was prepared by mixing 5 ppm H 2 S in nitrogen (N 2 ) as the balancing gas. The ratio of the volumetric flow rates of the gas  38  and the liquid  36  is maintained at 5:1 during the microfluidic testing. The pressure at the inlet  60  of channel  32  was maintained at 500 psi. As described above, the gas  38  and the liquid  36  traveled along channel  32  in slug flow condition. The gas phase and the liquid phase discharged through exit  62  were then gravimetrically separated and analyzed via measurement system  88 . 
     It should be noted the measurement system  88  may vary depending on the application. By way of example, the measurement system  88  may comprise a spectrometer for analyzing the liquid and a colorimetric detector for analyzing the gas. In the specific example discussed above, the H 2 S served as the transport component and basis for assessing equilibration/mass transfer efficiency. In this example, the liquid phase/reagent  36  was analyzed before and after the slug flow mixing. As referenced above, the analysis of the liquid phase  36  may be performed on a spectrometer, such as a UV-VIS-NIR (e.g. a Cary 5000 spectrometer) while analysis of the gas phase  38  may be performed on a dry colorimetric sulfur detector (e.g. a C.I. Analytics 2010L). 
     In a variety of operational applications, the measurement system  88  may comprise suitable spectrometer and detector components mounted into the fluidic testing system  20  at a downhole location  26  in wellbore  24 . However, a variety of other components and techniques may be used to perform interrogation methods for assessing equilibration. Examples of other interrogation methods include fluorescence measurement, electrochemical measurement, NMR or viscosity measurement, and/or other suitable measurement techniques. In some applications, the fluidic testing system  20  also may be configured to separate two immiscible fluids by using a membrane/filter or centrifugal separation. By way of further example, the separation system also may comprise a capillary array that is integrated into, for example, a microfluidic chip which may be located in a downhole environment. 
     In this example, the optical density (in the 300-700 nm range) measured in the reagent  36  after the slug flow mixing along channel  32  is illustrated graphically in  FIG. 7 . The optical density of the reagent  36  prior to slug flow mixing was considered as the baseline (zero H 2 S). The concentration of H 2 S in the gas phase  38 , after slug flow mixing along channel  32 , was measured to be zero. A corresponding spectral analysis of the liquid phase  36  collected from the slug flow after exiting channel  32  shows an increase in optical density which corresponds to an H 2 S concentration of 0.05 mM. Such results indicate a highly efficient component mass transfer of the H 2 S constituent between the gas phase  38  and the liquid phase  36  is a result of the mixing which occurred during the slug flow through channel  32 . 
     Similar to the microfluidic techniques described above, another embodiment involves a minifluidic approach which uses channel  32  in the form of a tube having a small inner diameter, e.g. less than 300 μm. The tube/channel  32  may be at least 10 cm long and can have a length up to at least 10 m. In some applications, the tube  32  may be formed as a stainless steel column that can be used in high pressure and high temperature environments. The interior of the tube  32  may be covered with a suitable coating  66  which makes the tube less sensitive to scavenging and/or corrosion. The coating  66  also may be selected to adjust tube properties, e.g. to make the tube more hydrophilic or hydrophobic. As with the previously described embodiments, the measurement system  88  can utilize a variety of interrogation methods including, for example, optical interrogation, fluorescence measurement, electrochemical measurement, NMR or viscosity measurement, an/or other suitable measurements. The fluidic testing system  20  again may be configured facilitate separation of two immiscible fluids using, for example, a membrane, a filter, and/or a gravitational separation. 
     The minifluidic set-up may be very similar to the microfluidic system illustrated in  FIG. 6 . In the minifluidic embodiment, the microfluidic device  72  is replaced by the tube/channel  32 . In a specific example, a maximum, inner cross-sectional dimension of the tube, e.g. inner diameter of the tube, is less than 500 μm and sometimes less than 300 μm and the tube has a length of about 2 m although other lengths may be used in other applications. The tube/channel should be configured so that laminar flow is created therein. By way of specific example, a back pressure regulator can be used to maintain the system at approximately 500 psi (or another suitable pressure) and the liquid phase  36  is collected after the back pressure regulator. In this example, the flow rate of the reagent/liquid phase  36  is on the order of 5 ml/minute and the flow rate of the gas is either a 2:1 or a 5:1 gas to reagent ratio for 50 ppm H 2 S gas. 
     Referring generally to  FIG. 8 , a graphical representation is provided of the absorption curves of the reacted reagent for the specific example. The ratio between the two absorbance values is illustrated as close to a ratio of 1:2.5. The measured optical intensities are about 15% lower than would otherwise be expected based on the sodium sulfide experiments being performed at room temperature. The lower intensity is accounted for by the difference in mixing methods. For example, bismuth sulfide particles formed by mixing via slug flow are smaller than those formed from the sodium sulfide analyses. As a result, there is a blue shift of the absorbance curve and thus a slightly lower intensity. 
     However, the results demonstrate the enhanced mass transfer between the gas phase  38  and the liquid phase  36  during slug flow along channel  32 . This enhanced mixing facilitates many hydrocarbon related sensing and analysis applications in which specific constituents are detected and evaluated based on the mass transfer of those constituents between two fluids. 
     Depending on the specifics of a given chemical constituent sensing and analysis application, the components of fluidic system  20  may be adjusted and/or changed. For example, various fluid injection systems and constituent measurement systems may be employed. Additionally, the configuration of the channel  32  may be selected according to the environment in which it is used and according to the parameters of a given application. The channel  32  may have a variety of cross-sectional shapes and sizes as well as a variety of lengths to accommodate testing for various constituents in many types of environments. In some applications, the channel  32  may be generally circular in cross-section while other applications may utilize cross-sectional configurations having multiple sides. For example, the channel may be defined by four sides arranged in a generally rectangular/trapezoidal pattern. The fluidic system  20  also may be constructed for testing and analyzing numerous types of fluids, e.g. hydrocarbon-based fluids, having a variety of chemical constituents which may be detected via the slug flow processes described above. 
     Although only a few example embodiments have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the example embodiments without materially departing from this disclosure. Accordingly, all such modifications are intended to be included within the scope of this disclosure as defined in the following claims. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures. It is the express intention of the applicant not to invoke 35 U.S.C. § 112, paragraph 6 for any limitations of any of the claims herein, except for those in which the claim expressly uses the words ‘means for’ together with an associated function.