Abstract:
A microfluidic system for performing fluid analysis is described having: (a) a submersible housing having a fluid analysis means and a power supply to provide power to said system; and (b) a substrate for receiving a fluid sample, having embedded therein a fluid sample inlet, a reagent inlet, a fluid sample outlet, and a mixing region in fluid communication with the fluid sample inlet, the reagent inlet, and the fluid sample outlet, and wherein the substrate includes a fluid drive means for moving the fluid sample through the substrate, and wherein the substrate interconnects with the housing. At least a portion of the fluid analysis means may be embedded in the substrate.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a chemical analysis system and, more particularly, to the use of self-supporting microfluidic systems for chemical analysis of water or mixtures of water and oil. 
     BACKGROUND 
     In oil well evaluation and aquifer management, quantitative analyses of formation fluid are typically performed in a laboratory environment, the samples having been collected remotely. Standard laboratory procedures are available for quantitative analyses by adding a reagent to chemically react with a specific target species in a sample to cause detectible changes in fluid property such as color, absorption spectra, turbidity, electrical conductivity, etc. See Vogel, A. I., “Text-Book of Quantitative Inorganic Analysis, 3rd Edition”, Chapter 10-12, John Wiley, 1961, incorporated by reference herein in its entirety. Such changes in fluid property may be caused, for example, by the formation of a product that absorbs light at a certain wavelength, or by the formation of an insoluble product that causes turbidity, or bubbles out as gas. For example, addition of pH sensitive dyes is used for colorimetric pH determination of water samples. A standard procedure for barium determination requires addition of sodium sulfate reagent to the fluid sample resulting in a sulfate precipitate that can be detected through turbidity measurements. Some of these standard laboratory procedures have been adapted for flow injection analysis (Ruzicka et al., Flow Injection Analysis, Chapters 1 and 2, John Wiley, 1981, incorporated by reference herein in its entirety). Flow injection analysis “is based on the injection of a liquid sample into a moving non-segmented continuous carrier stream of a suitable liquid” (see Ruzicka et al., Chapter 2, page 6). 
     Fluid samples collected downhole can undergo various reversible and irreversible phase transitions between the point of collection and the point of analysis as pressure and temperature conditions are hard to preserve. Concentrations of constituent species may change because of loss due to vaporization, precipitation etc., and hence the analysis as done in the laboratories may not be representative of true conditions. For example, water chemistry and pH are important for estimating scaling tendencies and corrosion; however, the pH can change substantially as the fluid flows to the surface. Likewise, scaling out of salts and loss of carbon dioxide and hydrogen sulfide can give misleading pH values when laboratory measurements are made on downhole-collected samples. Conventional methods and apparatuses require bulky components that are not efficiently miniaturized for downhole applications. 
     Further, fluid sample for water management requires very frequent (i.e. daily, twice daily, etc.) monitoring and measuring of fluid properties. These monitoring regimes include permanent subsurface systems that are designed solely to gather and store frequently acquired data over long periods of time. Accordingly, there is a need for a system that uses very low quantities of reagent, operates autonomously, and collects or neutralizes waste product. Traditional solutions include chemical sensors that tend to lose calibration over a relatively short period of time. 
     As will be described in more detail below, the present invention applies MEMS/MOEMS techniques to develop microfluidic devices overcoming the limitations of the prior art. Micro electromechanical systems (MEMS) are well known as microfluidic devices for chemical applications since the 1990&#39;s (see Manz et al., “Miniaturized Total Chemical and Analysis Systems: A Novel Concept for Chemical Sensing,”  Sensors and Actuators B , Vol. B1, pages 244-248 (1990), incorporated by reference herein in its entirety) and are typically fabricated from silicon, glass, quartz and poly(dimethylsiloxane) (PDMS) (see Verpoorte et al., “Microfluidics Meets MEMS”  Proceedings of the IEEE , Vol. 91, pages 930-953 (June 2003), incorporated by reference herein in its entirety). MEMS technology allows for miniaturized designs requiring smaller liquid volumes. In addition, MEMS devices are easy to mass produce having a very accurate reproducibility. MEMS also allows easy integration of different components, such as valves, mixers, channels, etc. Similarly, MEMS systems with optical devices are called MOEMS (micro optical electro mechanical systems, or Optical MEMS). MOEMS have also been used for chemical applications since the 1990&#39;s. Commercial (non-chemical) structures are used in the telecommunications field to make use of MEMS wave-guides to modify or route an optical signal. 
     For example, U.S. Pat. No. 5,116,759 to Klainer et al. (incorporated by reference herein in its entirety) discloses a laboratory-based system utilizing a MEMS device. In particular, the MEMS device is a cell that receives the sample for analysis. All associated analytical devices, including optical interrogation, power supply, reagent sources, and processing means, are typical laboratory-sized devices not suitable for remote interrogation. 
     Accordingly, it is one object of the present invention to provide a novel system to autonomously perform remote chemical analysis. 
     It is another object of the present invention to provide a microsystems that will regulate the amounts of sample and reagent to be consumed during each measurement, allowing the use of a reagent reservoir in the downhole instrument and the storage of waste within the instrument. 
     It is yet another object of the present invention to provide a microsystem having a total flow rate in the order of microliters per minute, enabling the measurement of pH and use with other reagents for determining the concentration of species like nitrate, heavy metals, scaling ions and hydrocarbons. 
     It is yet a further object of the present invention to provide an autonomous system having low power consumption, minimum consumables, neutralized waste material and data logging for in-situ measurements of fluid parameters on a multi-year permanent basis. 
     SUMMARY OF THE INVENTION 
     In a first embodiment of the present invention, a microfluidic system for performing fluid analysis is disclosed having: (a) a submersible housing having a fluid analysis means and a power supply to provide power to the system; and (b) a substrate for receiving a fluid sample, having embedded therein a fluid sample inlet, a reagent inlet, a fluid sample outlet, and a mixing region in fluid communication with the fluid sample inlet, the reagent inlet, and the fluid sample outlet, and wherein the substrate includes a fluid drive means for moving the fluid sample through the substrate, and wherein the substrate interconnects with the housing. At least a portion of the fluid analysis means may be embedded in the substrate. 
     Fluid is moved through the system using a fluid drive means which may be passive or active. A passive fluid drive system includes a system wherein the fluid is driven due to the differential in pressure between the sampling environment and the internal pressure of the microfluidic device. Active fluid drive systems may include a pump in the housing or embedded in the substrate. Preferably, the pump is a piezo-electric pump embedded in the substrate; most preferably, it is pressure-balanced. At least one reagent reservoir may be connected to the reagent inlet to provide reagents to perform the fluid analysis. It is noted that the substrate may include more than one reagent inlet, wherein each additional inlet has at least one reagent reservoir. Preferably, the reagent reservoirs are collapsible bags, and, most preferably, they are threaded bags. 
     To fully pressure-balance the system and ensure efficient fluid handling, the fluid sample inlet and fluid sample outlet may be in fluid communication with the fluid to be sampled. In addition, a separator system may be positioned at the fluid sample outlet to remove particulate from the fluid prior to analysis. The separator system may be embedded in the substrate and may include activated charcoal, an ion exchange membrane, or other means commonly used in the field. 
     The system may further comprise a control means to control fluid analysis means to assist in the remote operation of the system. Likewise, data processing means may be used to receive, store, and/or process data from the fluid analysis means. The control means may include data transmission means to transmit data received from the fluid analysis means. 
     A second embodiment is a method of performing fluid analysis comprising: (a) remotely deploying a microfluidic system in or proximate to the fluid to be sampled (also referred to as a sampling environment), wherein the microfluidic system is comprised of a submersible housing having a fluid analysis means and a power supply to provide power to the system; and a substrate for receiving a fluid sample, having embedded therein a fluid sample inlet, a reagent inlet, a fluid sample outlet, and a mixing region in fluid communication with the fluid sample inlet, the reagent inlet, and the fluid sample outlet, and wherein the substrate includes a fluid drive means for moving the fluid sample through the substrate, and wherein the substrate interconnects with the housing; (b) receiving a fluid sample into the fluid sample inlet; (c) mixing the fluid sample with reagent from the reagent inlet in the mixing region; and (d) analyzing the fluid sample using the fluid analysis means. The fluid sample may then be stored in the housing for later disposal or discharged back, into the sampling environment. 
     The device of the present invention may be manufactured by (a) providing two or more substrates; (b) forming fluid mixing channels and fluid analysis channels within at least one of the substrates; (c) forming an inlet and an outlet within at least one of the substrates; (d) embedding a piezoelectric pump within at least one of the substrates; and (e) bonding the substrates to one another. It is preferred that the optical fibers and electrical wires required for the operation of the pump and the fluid analysis region be embedded within at least one of the substrates. 
     The overall system has limited dimensions (such as in diameter and length) and is completely self supporting, enabling remote analysis or monitoring such as in standpipes, aquifers, groundwater, hazardous sites, chemical plants and boreholes. The device is submersible and autonomous. Because the device remains robust over an extended period of time it may be permanently (or semi-permanently) installed in remote locations for extended monitoring. 
     The instrument is particularly useful, for example, in oilfield applications for the detection of scale forming ions and dissolved gases and in water applications for the detection of hazardous chemicals. Chemical measurements of interest in the water business include, but is not limited to, pH and toxic chemicals, such as nitrate, arsenic and other heavy metals, benzene and other organic compounds. Chemical measurements of interest in the oilfield include, but is not limited to, the determination of pH, the detection of H 2 S and CO 2 , as well as scale forming ions such as Ca, Ba, Sr, Mg, and SO 4 . 
     Further features and applications of the present invention will become more readily apparent from the figures and detailed description that follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of the microfluidic system of the present invention. 
         FIGS. 2(   a ), ( b ) and ( c ) are schematic diagrams of the substrate of the microfluidic system of the present invention. 
         FIG. 3  is a schematic showing a detail of a reagent reservoir having a spiral channel. 
         FIG. 4  is a schematic diagram showing one method of manufacturing the present invention. 
         FIG. 5  is a schematic diagram of one application of the present invention, useful in the oilfield and water management areas. 
         FIG. 6  is a schematic diagram showing various suitable telemetry methods. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a schematic of the autonomous microfluidic system  10  of the present invention having a microfluidic substrate  200  in communication with a housing  100 . Preferably, the substrate  200  is hermetically sealed to the housing  100  such that the sample inlet  205  extends outside of the housing  100  and the electrical connections  120  are within the housing  100 . The housing  100  further includes a power supply  105  and control electronics  110  in electrical connection with the substrate  200 . It is noted that while reservoir  210  is shown in this figure outside the housing  100  and the waste collector  225  is shown inside the housing  100 , the location of these components relative to the housing will depend on the desired configuration of the system. Alternatively, the waste fluid may be discharged via outlet  235 . Accordingly, the configuration of  FIG. 1  is intended to be illustrative and non-limiting. Most preferably, the housing  100  is bonded  115  directly to the substrate  200  avoiding electrical feedthroughs. 
       FIGS. 2(   a )-( c ) are detailed schematics showing non-limiting embodiments of the substrate  200 . More particularly,  FIG. 2(   a ) depicts the substrate  200  having fluid channels (dashed lines), optical fibers (dotted lines), and electrical wires (grey lines) embedded therein. Fluids enter the system via sample inlet  205  and mixes with reagent stored in the reagent reservoir  210  in mixing region  215 . To minimize particulate in the system, a filter (not shown) may be placed over, attached to, or embedded in, the inlet. The fluid in the system is subject to a driving force, which may be passive or active. As shown in  FIG. 1(   a ), the fluid may be moved through the system using a pump  220  (such as an ultrasonic pump or a piezo-electric pump) operated by control electronics  110  and a power source  105 . Preferably, the pump is a piezo-electric pump that is pressure-balanced, such as by applying a water impervious, electrically isolating gel on the surface of the piezo. The system may be designed such that the pump pulls or pushes the fluid through the system, or designed such that the pump pulls a portion of the fluid and pushes another portion of the fluid. The arrows are intended to show the direction of fluid flow. Alternatively, fluid may be moved through the substrate using a passive fluid drive means wherein the differential in pressure between the sampling environment and the pressure within the tool housing is used to move the fluid through the system (such as by lowering the pressure within the submersible housing relative to the sampling environment). 
     The sample may be stored in a collector  225  for later use or disposal, or discharged back into the borehole via outlet  235 . The sample may be ‘cleaned’ (i.e., reagents or precipitates removed to an acceptable level) prior to discharge using a separator means  230 , having, for example, activated charcoal or an ion membrane. The separator means  230  may be embedded on the substrate or may be positioned to the outside of the outlet such that the sample passes through the separator means prior to discharge. 
     The reagent reservoir  210  preferably has a pressure-balanced contact with the environment to ensure that the reagent is subject to the same pressure as the sample. This pressure-balanced contact might be, for example, a flexible impermeable foil or a mechanical pressure adapter. The pressure equilibrium prevents back flow through the microfluidic device and reduces the pressure difference to be overcome by the pump. The reagent in the reservoir can be, for example, a pH-sensitive color indicator or other reagents or catalysts applicable to the chemical analyses desired. The reagent reservoir  210  is connected to the fluid handling system, such as through a permanently open connection or a controlled connection such as with a valve. It is noted that the overall system is inherently pressure-balanced as the inlet and the outlet are exposed to the sampling environment. 
     The system may be designed to control the flow rate, sample volumes, and mixing ratios by adjusting the fluid resistance of the system. Because the total flow rate is dependent on the fluid resistance of the complete circuitry, dimensional variation (shape and geometry of the channels, for example) in the system will influence the total fluid resistance and thus the flow rate. To ensure that adequate mixing of the sample with the reagent over a relatively short channel length, various mixing and channel geometries may be used. One useful geometry is the herringbone geometry as described by Strook et al. in “Chaotic Mixer for Microchannels”,  Science , Vol. 295, pages 647-651 (2002) (incorporated by reference herein in its entirety). 
     While only one reagent and mixing region are shown in  FIG. 2(   a ), the fluid circuitry may be adapted to generate certain reaction time before interrogation. Accordingly, the fluid circuitry may contain multiple reagent reservoirs, fluid resistors and mixers to control fluid flow and mixing or to create subsequent reactions (such as multistage reactions with variable reaction times). 
       FIGS. 2(   b ) and  2 ( c ) show alternate embodiments of the present invention.  FIG. 2(   b ) shows the microfluidic device of  FIG. 2(   a ) with a fluid analyzing means  245  inside housing  100  (such as part of the analysis module of  FIG. 5) . Again, more than one reagent reservoir may be used (i.e., positioned in parallel or series) to allow more than one analyses to be performed using a single microfluidic system. Further, the reagents may be stored in a collapsible bag, or a threaded bag as shown in  FIG. 3 , to minimize backflow through the substrate. While this embodiment shows the fluid analysis means  245  in the housing  100  and connected to the substrate  200 , the fluid analysis means  245  may be embedded directly into the substrate  200  (see, for example,  FIG. 2(   c )). 
     In  FIG. 2(   c ) the fluid analyzing means  245  is an optical interrogation zone  245   a  having a light source  245   b  and a detector  245   c . The light source  245   b  and detector  245   c  may be either embedded in the substrate or connected via optical fibers (as shown). The light source  245   b  transmits lights through the optical interrogation zone  245   a  to the detector  245   c . The light source  245   b , may be any incandescent lamp, LED, laser, etc. suitable for the analysis to be performed. Likewise, the detector  245   c  measures the transmitted light at a defined wavelength depending on the analysis performed and the source  245   b  used. For example, the detector  245   c  can be a spectrum analyzer or a combination of appropriate filters and photodiodes. Light source  245   b  and detector  245   c  are controlled by electronics  110 , which may include a microprocessor to process the data and store the measurement values. It is noted that if cyclic olefin copolymer (COC) or any optically clear material is used as the substrate, then no separate optical windows are needed; COC may be used as the optical window. 
     As mentioned above, the reagent reservoir  210  should be pressure balanced with the sampling environment.  FIG. 3  is a schematic of a most preferred embodiment of the reagent reservoir  210 , hereinafter referred to as a threaded reagent reservoir. This embodiment includes a spiral channel  250  having an opening at the top at  255  such that the channel is pressure balanced relative to the sampling environment. A channel  260  extends through the threaded portion to allow the reagent reservoir to be filled and capped  265 . Reagent passes from the reservoir into the channels of the substrate via outlet  270 . 
     Alternatively, the fluid analyzing system may be designed to perform resistivity tests, determine the presence of specific precipitate (such as metal or salt precipitates) or perform other chemical analyses. 
     It is noted that fluid analyses may take place at more than one interrogation zone (not shown), placed in parallel or in series. As described above, multiple reagents may be used to allow for multiple analyses. 
     One particularly useful downhole fluid analysis is pH indication. The present invention was tested wherein the interrogation zone was a colorimetric (i.e. optical) pH indicator. The results of this test are provided in Table 1, wherein a sample with a known pH was measured using the present invention and compared to measurements taken with standard laboratory equipment (in this case a Spectroquant® Vega 400 photometer): 
                             TABLE 1               Certified   Measurement using   Measurement using the       Buffer pH   Vega 400   present invention                   4.00   3.98   3.97       5.00   4.90   5.01       6.00   not taken   5.98       6.86   6.78   6.84       7.00   6.90   6.97       7.70   7.63   7.67       8.00   7.99   7.97                    
As can be seen by the data of Table 1, the system of the present invention can take measurements that are comparable to standard laboratory measurements.
 
     One skilled in the art would recognize that the presence of bubbles in the fluid sample may interfere with optical measurements and capillary pressure. Accordingly, a bubble trap  240  may be positioned between the mixing region  215  and the optical interrogation zone  245   a . The entire system is preferably manufactured using MEMS/MOEMS techniques such that all or nearly all connections are eliminated. Accordingly, most bubble sources are naturally eliminated in the design. However, the bubble trap  240  may be used to remove any remaining bubbles and ensure the integrity of the optical measurements. 
     The microfluidic device described herein is preferably designed and manufactured so that all channels, tubes and fibers are embedded in a single substrate, such as that possible using MEMS/MOEMS techniques. Suitable substrates include (but are not limited to) silicon, quartz, and plastic. For downhole applications, including oilfield and water management applications, the substrate may be constructed of plastic using micro-molding techniques wherein a mold is made by machining a piece of metal. The plastic is then formed using the mold and appropriately cured, if needed. As shown in  FIG. 4 , to close the channel  250  in substrate  200   a , a second substrate  200   b  may be attached to  200   a  where a surface-to-surface bond is applied such that the channels  250  are preserved. Adhesive, such as UV curable adhesive, may be used. If UV curable adhesive is used, a mask may be used to selectively cure the glue in areas of interest. The mask allows preferential transmission of UV light such that the glue does not cure in the area of the channels, but cures where desired. In addition, laser welds may be used. Preferably, substrate is formed of plastic and chemical bonds are used which minimizes dimensional variations due to the layer of glue and complexity of laser welding. 
     It is noted that while only two substrate segments are shown in  FIG. 4 , additional substrate segments may be used to form the microfluidic device of the present invention. 
     Depending on the analysis to be performed, it may be preferable to achieve highly polished channel surfaces. For example, if the microfluidic device is to be used for optical interrogation, channel surfaces within the optical interrogation zone may require optical grade polishing to nano-meter scale. For plastic molding, this can be achieved by making the corresponding surface of the mold to be of optical quality polish. 
     All tubes and fibers should preferably extend from the substrate at a common end such that they may be isolated in a common waterproof housing. This configuration also allows the device to be easily adapted for fitting in various sampling tools, such as those typically used to monitor aquifers and groundwater as well as those used in the oilfield. 
     The present invention may be implemented in a laboratory or in various downhole fluid analysis tools. For example, the apparatus described in commonly owned co-pending U.S. patent application Ser. No. 10/667,639 filed Sep. 22, 2003, entitled “Determining Fluid Chemistry of Formation Fluid by Downhole Reagent Injection Spectral Analysis” (incorporated by reference herein in its entirety) is a preferred implementation of the present reagent mixture. 
     One non-limiting embodiment of the present invention, as shown in  FIG. 5 , is a wireline formation tester  310 , including fluids analyzer  320 . The formation tester is shown downhole within fluid-filled borehole  305  in formation  300  suspended by logging cable  315 . Logging cable  315  also couples the formation tester to surface system. The housing in this example is the formation tester  310  having a fluids analyzer module  320  with the substrate  200 . As shown in this figure, the substrate  200  is affixed to the formation tester  310  in the area of the fluids analyzer module  320  such that the electrical connections  120  are isolated within the tool and the inlet of the microfluidic device (not shown) extends into a fluid flow line  325 . The power supply and control electronics (not shown) are within the formation tester  310 . This configuration eliminates the need to separate pumps, probes and reagent containers. 
     It is noted that  FIG. 5  is intended to depict a non-limiting embodiment useful for deploying the present invention in the oilfield. Other suitable elements may be included as dependent upon the specific application. For example, other configurations may be used to extract fluids such as in water or waste water management. The substrate may be affixed to tools usually deployed in groundwater monitoring wells such as the Diver® by Van Essen Instruments, chemical processes plants, or producing wells. Likewise, the device may be permanently or semi-permanently installed in these environments. 
     It is envisioned that the microfluidic device can be used to perform fluid analysis on any fluid sample obtained remotely where space and sample volume is of concern. For example, the device may be used in processing plants, for space applications or in a downhole oilfield or water management applications. In addition, the microfluidic system of the present invention is robust for long term, semi-permanent and permanent applications (on the order of days, months, and years). Accordingly, as shown in  FIG. 6 , the microfluidic device  100  may communicate with remote equipment via one of the many telemetry schemes known in the art, such as over electronic conductors, optical fibers or other suitable medium to a computer or other remote processing/data storage means  110 ; it may store the data retrieved from the sensors in the incorporated memory (not shown) to be later retrieved; or it may be transmitted wirelessly  415 ; or it may be downloaded to a local or remote computer  410 . 
     While the invention has been described herein with reference to certain examples and embodiments, it will be evident that various modifications and changes may be made to the embodiments described above without departing from the scope and spirit of the invention as set forth in the claims.