Phase behavior analysis using a microfluidic platform

Methods and related systems are described for analyzing phase properties in a microfluidic device. A fluid is introduced under pressure into microchannel, and phase states of the fluid are optically detected at a number of locations along the microchannel. Gas and liquid phases of the fluid are distinguished based on a plurality of digital images of the fluid in the microchannel. Bi-level images can be generated based on the digital images, and the fraction of liquid or gas in the fluid can be estimated versus pressure based on the bi-level images. Properties such as bubble point values and/or a phase volume distribution ratio versus pressure for the fluid are can be estimated based on the detected phase states of the fluid.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation-in-part of International Patent Application No. PCT/IB09/50500, filed Feb. 7, 2009, which is incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This patent specification relates to an apparatus and method for measuring thermo-physical properties of a fluid. More particularly, the patent specification relates to an apparatus and method for analyzing phase behavior properties of a reservoir fluid flowing in a microfluidic device.

2. Description of Related Art

The measurement of reservoir fluid properties is a key step in the planning and development of a potential oilfield. It is often desirable to perform such measurements frequently on a producing well to provide an indication of the performance and behavior of the production process. Examples of such measurements are pressure, volume, and temperature measurements, often referred to as “PVT” measurements, which are instrumental in predicting complicated thermo-physical behavior of reservoir fluids. One important use of PVT measurements is the construction of an equation of state describing the state of oil in the reservoir fluid. Other properties of interest that may be determined using PVT measurements include fluid viscosity, density, chemical composition, gas-oil-ratio, and the like. Once a PVT analysis is complete, the equation of state and other parameters can be input into reservoir modeling software to predict the behavior of the oilfield formation.

Conventional PVT measurements are performed using a cylinder containing the reservoir fluid. A piston disposed in the cylinder maintains the desired pressure on the fluid, while the heights of the liquid and gaseous phases are measured using, for example, a cathetometer.

Despite wide application, conventional PVT measurements suffer from several significant limitations. Firstly, a conventional PVT analysis typically requires up to a few weeks to complete. Furthermore, a substantial volume of reservoir fluid, often as much as 4 liters, must be maintained at pressures up to about 1400 kilograms/square centimeter (20,000 pounds/square inch) from the wellsite to the testing laboratory. Shipping and handling such a large sample at these high pressures is costly and poses considerable safety issues.

While there are ways of characterizing properties of reservoir fluid known in the art, considerable shortcomings remain.

BRIEF SUMMARY OF THE INVENTION

According to embodiments, a system for analyzing phase properties in a microfluidic device is provided. The system includes a microchannel adapted to carry a fluid and having an entrance passageway and an exit passageway. A fluid introduction system in fluid communication with the entrance passageway, introduces the fluid under pressure via the entrance passageway. An optical sensing system is adapted and positioned to detect phase states of the fluid at a plurality of locations along the microchannel.

The optical sensing system preferably includes a processing system adapted and programmed to distinguish gas from liquid phases of fluid in the microchannel at a plurality of locations along the microchannel based on a plurality of digital images of the fluid in the microchannel. A plurality of bi-level images are preferably generated based on the digital images of the fluid in the microchannel, and values relating to the fraction of liquid or gas in the fluid is preferably estimated for a plurality of pressures based at least in part on the plurality of bi-level images.

Properties such as bubble point values and/or a phase volume distribution ratio versus pressure for the fluid are preferably estimated based at least in part on the detected phase states of the fluid.

Additionally, according to some embodiments a method for analyzing phase properties in a microfluidic device is provided. A microchannel adapted to carry a fluid is provided that has an entrance passageway and an exit passageway. Fluid is introduced under pressure into the microchannel via the entrance passageway, and phase states of the fluid are optically detected at a plurality of locations along the microchannel.

Further features and advantages of the invention will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

According to embodiments, techniques for measuring phase behavior of gas-liquid mixtures are provided. The techniques use a microfabricated chip made of a microchannel connected to thin silicon membranes that deform under the fluid pressure. The pressure inside the channel is measured using the membranes as further described in co-pending U.S. patent application Ser. No. 12/533,292, Patent Application Publication No. US 2010/0017135, entitled “PRESSURE MEASUREMENT OF A RESERVOIR FLUID IN A MICROFLUIDIC DEVICE,” filed on even date herewith, which is incorporated by reference herein. According to some embodiments, the liquid fraction along the channel is measured by capturing videos of the flow and processing them with a Matlab program. A phase behavior curve is obtained by plotting the liquid fraction against the pressure.

According to embodiments, systems and methods for measuring pressure of a reservoir fluid in a microfluidic device are provided. For the purposes of this disclosure, the term “reservoir fluid” means a fluid stored in or transmitted from a subsurface body of permeable rock. Thus “reservoir fluid” may include, without limitation, hydrocarbon fluids, saline fluids such as saline water, as well as other formation water, and other fluids such as carbon dioxide in a supercritical phase. Moreover, for the purposes of this disclosure, the term “microfluidic” means having a fluid-carrying channel exhibiting a width within a range of a few to hundreds of micrometers, but exhibiting a length that is many times longer than the width of the channel. Similarly the term “microchannel” means a fluid-carrying channel exhibiting a width within a range of a few to hundreds of micrometers. Although many of the microchannels described herein are of rectangular cross section due to the practicalities of fabrication techniques, the cross section of a microchannel can be of any shape, including round, oval, ellipsoid, square, etc.

FIG. 1depicts a stylized, exploded, perspective view of a microfluidic device101for studying phase behavior, according to some embodiments of the invention. In the illustrated embodiment, microfluidic device101comprises a first substrate103defining a microchannel105, an entrance well107and an exit well109. Microchannel105extends between and is in fluid communication with entrance well107and exit well109. Microchannel105forms a serpentine pattern in first substrate103, thus allowing microchannel105to extend a significant length but occupy a relatively small area. According to one embodiment, microchannel105exhibits a length of one or more meters, a width of about 100 micrometers, and a depth of about 50 micrometers, although the present invention also contemplates other dimensions for microchannel105. Microfluidic device101further comprises a second substrate111having a lower surface113that is bonded to an upper surface115of first substrate103. When second substrate111is bonded to first substrate103, microchannel105is sealed except for an inlet117at entrance well107and an outlet119at exit well109. Second substrate111defines an entrance passageway121and an exit passageway123therethrough, which are in fluid communication with entrance well107and exit well109, respectively, of first substrate103. Also shown inFIG. 1are a number of cavities such as cavity150, each connected to the main microchannel105using a small side channel. As is explained in further detail below, each cavity such as cavity150is partially defined by a deformable membrane that allows for pressure measurement. According to preferred embodiments substrate103is fabricated with circular openings and the cavities are defined on the sides by the walls of the openings in substrate103, on the bottom with the deformable membrane, and on the top by the second substrate111.

InFIG. 1, first substrate103is preferably made of silicon and is approximately 500 micrometers thick, and second substrate111is made of glass, such as borosilicate glass, although the present invention contemplates other materials for first substrate103, as is discussed in greater detail herein. According some preferred embodiments substrate103is a conventional silicon on insulator (SOI) wafer. Exemplary borosilicate glasses are manufactured by Schott North America, Inc. of Elmsford, N.Y., USA, and by Corning Incorporated of Corning, N.Y., USA.

In operation, pressurized reservoir fluid is urged through entrance passageway121, entrance well107, and inlet117into microchannel105. The reservoir fluid exits microchannel105through outlet119, exit well109, and exit passageway123. Microchannel105provides substantial resistance to the flow of reservoir fluid therethrough because microchannel105is very small in cross-section in relation to the length of microchannel105. When fluid flow is established between inlet117and outlet119of microchannel105, the pressure of the reservoir fluid within microchannel105drops from an input pressure, e.g., reservoir pressure, at inlet117to an output pressure, e.g., atmospheric pressure, at outlet119. The flow rate is a function of the overall pressure drop between inlet117and outlet119, and viscosity. Fluid flow through microchannel105is laminar and, thus the pressure drop between inlet117and outlet119is linear when the reservoir fluid exhibits single-phase flow. For further details of microfluidic devices and method for measuring thermo-physical properties of reservoir fluid, see e.g. International Patent Application No. PCT/IB09/50500, filed Feb. 7, 2009, which is incorporated by reference herein. Once the flow is established, the membrane in each cavity, such as cavity150, deforms due to the fluid pressure and the deformation can be optically detected, as is described more fully in co-pending U.S. patent application Ser. No. 12/533,292, entitled “PRESSURE MEASUREMENT OF A RESERVOIR FLUID IN A MICROFLUIDIC DEVICE”, filed on even date herewith.

FIG. 2provides a stylized, schematic representation of the reaction of reservoir fluid201as the reservoir fluid flows through microchannel105in a direction generally corresponding to arrow202, according to some embodiments. When the reservoir fluid enters inlet117of microchannel105, the reservoir fluid is at a pressure above the “bubble point pressure” of the reservoir fluid. The bubble point pressure of a fluid is the pressure at or below which the fluid begins to boil, i.e., bubble, at a given temperature. When the reservoir fluid exits outlet119of microchannel105, the reservoir fluid is at a pressure below the bubble point pressure of the reservoir fluid. Thus, a “first” bubble203forms in the reservoir fluid at some location, e.g., at205inFIG. 2, within microchannel105where the reservoir fluid is at the bubble point pressure. Downstream of location205, multi-phase flow, e.g., gas and liquid flow, of reservoir fluid201occurs in microchannel105. Previously-formed bubbles, e.g. bubbles207,209,211,213,215, and the like, grow in size as reservoir fluid201flows within microchannel105beyond the location corresponding to the formation of the first bubble due to decreased pressure in this portion of microchannel105and more evaporation of the lighter components of reservoir fluid201. The bubbles are separated by slugs of liquid, such as slugs217,219,221,223,225, and the like. Expansion of the bubbles, such as bubbles207,209,211,213,215, results in an increased flow velocity of the bubbles and liquid slugs, such as slugs217,219,221,223,225, within microchannel105. The mass flow rate of reservoir fluid201is substantially constant along microchannel105; however, the volume flow rate of reservoir fluid201increases as reservoir fluid flows along microchannel105. The reservoir fluid also enters cavity150through small channel152. According to some embodiments the width of small side channel152is approximately 50 micrometers, or about half of the width of microchannel105, and is about 50 micrometers deep.

Thermo-physical properties of the reservoir fluid, such as reservoir fluid201ofFIG. 2, for example gas-oil-ratio, phase envelope, and equation of state, can be determined by measuring the size and concentration of bubbles within microchannel105. Referring now toFIG. 3, the flow of the reservoir fluid through microchannel105is depicted in three regimes. A first bubble, such as first bubble203ofFIG. 2, is formed at301along microchannel105. From inlet117of microchannel105to location301of the first bubble, indicated inFIG. 3as a first region303, the pressure of the reservoir fluid is above the bubble point. No bubbles are observed within first region303. In first region303, the flow of the reservoir fluid is laminar due to a low Reynolds number and the pressure drops linearly therein. Once bubbles are formed, the bubbles move along within microchannel105toward outlet119and the volumes of the bubbles increases. In a second region305, the void fraction, i.e., the volume of gas to total volume, of the reservoir fluid is less than one. In a third region307, the flow of the reservoir fluid is dominated by high-speed gas flow. The gas bubbles are separated by small droplets of liquid, such as water. The pressure of the reservoir fluid within third region307decreases rapidly. Gas bubbles flow within second region305at a slower rate than in third region307, where they are often nearly impossible to follow with the naked eye.

Once a stabilized flow of reservoir fluid is established in microchannel105, a camera401is used to capture snapshots of the flow, as shown inFIG. 4. Note that the flow of reservoir fluid into inlet117(shown inFIGS. 1 and 3) is represented by an arrow403and that the flow of reservoir fluid from outlet119(shown inFIGS. 1 and 3) is represented by an arrow405. In one embodiment, camera401is a charge-coupled device (CCD) type camera. The images produced by camera401are processed using image analysis software, such as ImageJ 1.38x, available from the United States National Institutes of Health, of Bethesda, Md., USA, and ProAnalyst, available from Xcitex, Inc. of Cambridge, Mass., USA, to measure the size and concentration of the bubbles in the reservoir fluid disposed in microchannel105. Using this technique, many thermo-physical properties of the reservoir fluid, such as gas-oil-ratio, phase envelope, and equation of state, can be determined.

FIGS. 5 and 6depict a microfluidic device501, according to some embodiments. As in microfluidic device101ofFIG. 1, microfluidic device501comprises a first substrate503defining a microchannel505, an entrance well507, and an exit well509. Microchannel505extends between and is in fluid communication with entrance well507and exit well509. In the illustrated embodiment, first substrate503is made from silicon; however, first substrate503may be made from glass. Microchannel505, entrance well507, and exit well509are, in one embodiment, first patterned onto first substrate503using a photolithography technique and then etched into first substrate503using a deep reactive ion etching technique. As in the first embodiment shown inFIG. 1, in a preferred embodiment, microchannel505exhibits a length of one or more meters, a width of about 100 micrometers, and a depth of about 50 micrometers, although the present invention also contemplates other dimensions for microchannel505. A number small side channels, such as side channels552and556lead from the main microchannel505to circular cavities such as cavities550and554. Also shown in a side channel560that leads to cavity558. According to some embodiments, twelve cavities are spaced out along the length of microchannel505and each of the cavities are about 2 mm in diameter, although the present invention also contemplates other numbers of cavities and diameters for each cavity. Each cavity is partially defined by a flexible membrane on the under side of the device501. The membranes deform under the local static pressure. The deformation is measured using a Confocal PolyChromatic Sensor (CCS), and after calibration, gives the pressure value inside the channel.

Microfluidic device501further comprises a second substrate511defining an entrance passageway513and an exit passageway515in fluid communication with entrance well507and exit well509. Second substrate511is made from glass, as discussed herein concerning second substrate111(shown inFIG. 1). By making the front of the device501transparent, observation of the flow and video capturing of the flow inside the microchannel505is provided. In one embodiment, entrance passageway513and exit passageway515are generated in second substrate511using a water jet or abrasive water jet technique. First substrate503and second substrate511are preferably fused using an anodic bonding method after careful cleaning of the bonding surfaces of substrates503and511.

The present invention contemplates microfluidic device501having any suitable size and/or shape needed for a particular implementation. In one embodiment, microfluidic device501exhibits an overall length A of about 80 millimeters and an overall width B of about 15 millimeters. In such an embodiment, passageways513and515are spaced apart a distance C of about 72 millimeters, cavities558and550are spaced apart a distance D of about 3 millimeters, and cavities along the serpentine section of microchannel505, such as cavities550and554are spaced apart by a distance E of about 5 millimeters. It should be noted that microfluidic device101may also exhibit dimensions corresponding to microfluidic device501. However, the scope of the present invention is not so limited.

Referring toFIG. 7, one or more portions of microchannel505include zones of reduced cross-sectional area to induce the formation of bubble nuclei in the reservoir fluid. For example, as shown inFIGS. 7 and 8, a micro-venturi701is incorporated into an inlet of microchannel505. Micro-venturi701includes a nozzle opening801having a width W1, which is smaller than a width W2of microchannel505. The contraction provided by micro-venturi701causes a substantial pressure drop in the reservoir fluid at nozzle opening801along with an increased velocity of reservoir fluid flow. The combined effect of the pressure drop and the increased velocity induces formation of bubble nuclei in the reservoir fluid. Preferably, microchannel505further includes one or more additional constrictions703, as shown inFIGS. 7 and 9. Constrictions703exhibit widths W3, which are smaller than a width W4of microchannel505. Preferably, width W1of nozzle opening801and widths W3of constrictions703are about 20 micrometers, whereas the preferred width W2and W4of microchannel505is 100 micrometers. These restrictions increase the velocity of the reservoir fluid by up to about 500 percent.

FIG. 10is a stylized, schematic representation phase behavior analysis system, according to some embodiments. A high capacity syringe1054pump electronically controlled by computer system1030and pushes a testing fluid stored under pressure in sample bottle1052. The fluid is flowed from sample bottle1052, through valve1050and into the serpentine channel of microfluidic device501. A constant input pressure is maintained, and measured with a pressure gauge1056. A strong light1062illuminates the transparent face511of the microfluidic device501and a camera1060captures videos of the flow inside the microchannel. When gas bubbles and liquid slugs are present in the same time in the channel there is a strong difference in brightness between these two phases. The images captured by the camera1060provide then the distribution of slugs and bubbles along the flow. The optical sensor1010is mounted on a high-precision stage1014. The optical sensor1010moves along the back face of the microfluidic device501and measures the deformation of the membrane for each cavity on device501. A spectrometer1020receives signals from the optical sensor1010via optical fiber link1012. The results of the spectrometer are fed to the computer system1030, thus giving a record of the pressure inside the channel at the locations of the cavities on device501. Computer system1030includes a one or more processors, a storage system1032(which includes one or more removable storage devices that accept computer readable media), display1036, and one or more human input devices1034, such as a keyboard and/or a mouse. Computer system1030also includes a data acquisition system for collecting data from the spectrometer1020.

The videos from camera1060are stored on computer system1030using a video acquisition program, such as is available from EPIX, Inc. of USA. According to some embodiments, a video of the full image of the microchannel is made of approximately 300 frames. According to some embodiments, the controller of pump1054, the pressure gauge1056, the stage1014and the optical sensor1012are all in communication with a control application on computer system1030such as the LabVIEW program from National Instruments Corporation, which controls all the devices and records the measurements.

FIG. 11shows an example of a frame of captured video from a fluid flowing through a microfluidic device, according to some embodiments. A measurement is made up of one or more videos of the flow plus the measured pressure values at the different cavities of the microfluidic device using the optical sensor. Frame1102is a frame from a captured video of the flow, while frame1104is an image resulting from its transformation into a binary, or black and white image. As used herein the terms “binary image” or “bi-level image” means a digital image that has only two possible values for each pixel. At the first segments (near the left side of the frames), just after the input, the pressure is still high and not much gas has gotten out of the liquid. However, further downstream (to the right side of the frames), as the pressure decreases, more and more gas gets out of the liquid.

An image processing routine running on computer system1030, for example, programmed under Matlab, transforms the original grayscale images such as1102into binary images such as1104. The process involves the sensible choice of some image processing parameters. The binary image itself is then analyzed by a computation routine, for example also programmed under Matlab. The output of the computation is the liquid fraction in each of the segments composing the microchannel. This liquid fraction is then averaged on all the frames of the captured video, thus giving a more precise measurement and a value of the standard deviation. This process thus provides the evolution of the liquid fraction along the channel.

FIG. 12Ais a plot showing pressure drop in a microchannel versus channel length for C1and C10, according to some embodiments.FIG. 12Bis a plot showing phase volume distribution versus pressure for a mixture of C1and C10, according to some embodiments.FIGS. 12A and 12Bdepict results of the measurements conducted on a live fluid in the microfluidic device shown inFIG. 5and the setup shown inFIG. 10. The fluid is a mixture of methane and decane saturated at 500 psig. The pressure measurements of curve1210show a linear pressure drop inside the device. Combining the pressure measurements with phase volume distribution inside the channel provides the phase volume distribution of the fluid at different pressures as shown inFIG. 12B. InFIG. 12B, the round circles, such as point1212, depict the measurements using the microfluidic device in the setup shown inFIG. 10, while the solid squares, such as point1214, show the measurements conducted by a conventional PVT apparatus.

FIG. 13Ais a plot showing pressure drop in a microchannel versus channel length for a mixture of a multicomponent gas and C10, according to some embodiments.FIG. 13Bis a plot showing phase volume distribution versus pressure for a multicomponent gas and C10, according to some embodiments. InFIGS. 13A and 13B, shows the results of measurements on a multicomponent gas is recombined with decane at 600 psig. InFIG. 13A, the pressure measurements of curve1310show a linear pressure drop inside the device. InFIG. 13B, the round circles, such as point1312, depict the measurements using the microfluidic device in the setup shown inFIG. 10, while the solid squares, such as point1314, show the measurements conducted by a conventional PVT apparatus.

FIG. 14Ais a plot showing pressure drop in a microchannel versus channel length for a mixture of a light oil and C1, according to some embodiments.FIG. 14Bis a plot showing phase volume distribution versus pressure for a light oil and C1, according to some embodiments.FIGS. 14A and 14Bshows the results of measurements on a light oil recombined with methane at 500 psig saturation pressure. InFIG. 14A, the pressure measurements of curve1410show a linear pressure drop inside the device. InFIG. 14B, the round circles, such as point1412, depict the measurements using the microfluidic device in the setup shown inFIG. 10, while the solid squares, such as point1414, show the measurements conducted by a conventional PVT apparatus. As can be seen fromFIGS. 12B,13B and14B, there is good agreement between measurements with the microfluidic device and conventional PVT.

FIG. 15shows an example of a line-scan method for measuring the liquid fraction in a microfluidic device, according to some other embodiments. The camera, such as camera1060ofFIG. 10, can be set up to capture only a selected line in the image of the channel. In a way, the camera is working in a similar fashion as a barcode reader. Each frame, highlighted in with the dashed rectangle, is essentially a line that regroups the phase states at the same position in all the segments of the serpentine microchannel. For a given segment at the framed position is essentially a point, and the phase state can be either liquid, in which case the point corresponding to the segment in the line is bright (and is assigned a value of 1), or gas in which case the same point is dark (and is assigned a value of 0). A simplified example of the assigned values resulting from a single frame is shown as the binary string1510.

Each measured line is at first a grayscale image then undergoes the same image processing as described above with respect toFIG. 11. A similar computation gives then the phase state (0 or 1) at the line position for each segment in the processed frame. Finally, this binary value is averaged on all the video frames to have the liquid fraction along the channel. This line-scan technique allows the capturing of approximately 20,000 frames, improving thus the averaging on the video frames and reducing the error. According to an alternative embodiment, an array of optical fibers connected an array of photo diodes is used instead of a conventional camera. Each optical fiber in the array is directed to a single vertical segment of the serpentine microchannel505.

FIG. 16shows an example of a matrix of phase states, according to some embodiments. The frames of the line scan video as described with respect toFIG. 15, after being converted to a binary image, can be put in a vertical sequence so as to form a matrix1610. The obtained matrix1610displays the phase state in all the segments at all the instants of the video. The Y-axis is time and moves forward downward—the frame period separates two lines. The X-axis is segment number as it comes in the full image. The microchannel input is on the left, and the output is on the right. This representation constitutes a type of “fingerprint” that is specific to the flow in the channel and gives valuable information on it, as the frequency that can be observed in the matrix.

FIGS. 17A and 17Bare plots showing the results of the line scan videos, according to some embodiments. The line scan technique gives liquid fraction measurements very close to those obtained with the full image video. Here again, the liquid fraction is plotted against the pressure profile in the channel and the obtained curve matches the conventional measurements one more time. InFIG. 17A, the result of line scan measurements on a methan-decane mixture saturated at 500 psig are shown in the solid squares, such as point1710, and the conventionally measured data is shown in the open triangles, such as point1712. InFIG. 17Bthe result of line scan measurements on a multicomponent gas saturated at 600 psig with decane are shown in the open circles, such as point1720, and the conventionally measured data is shown in the solid squares such as point1722.

FIGS. 18A and 18Bshow a microchannel according to an alternative embodiment. Although much of the discussion herein refers to microchannels as being fabricated from a conventional silicon etching process, other types of microchannels can be used with the microfluidic devices and related techniques described herein. For example, the microchannel1805is made from a glass tube formed in a serpentine shape.FIG. 18Bshows a cross section of the glass tube microchannel, which is round. Furthermore, layout patterns of the microchannel other than serpentine can be used the microfluidic devices.FIG. 19shows an example of a spiral microchannel layout pattern, according to an alternative embodiment. Microchannel1905can be fabricated with conventional silicon processing or can be made using other techniques, for example it could be a glass tube as shown inFIGS. 18A and 18B.

Although many embodiments have been described herein with respect to analysis of reservoir fluids, the present invention is also applicable to the analysis of many other types of fluids. According to some embodiments analysis of one or more types of biomedical fluids is provided including but not limited to bodily fluids such as blood, urine, serum, mucus, and saliva. According to other embodiments analysis of one or more fluids is provided in relation to environmental monitoring, including by not limited to water purification, water quality, and waste water processing, and potable water and/or sea water processing and/or analysis. According to yet other embodiments, analysis of other fluid chemical compositions is provided.

Whereas many alterations and modifications of the present invention will no doubt become apparent to a person of ordinary skill in the art after having read the foregoing description, it is to be understood that the particular embodiments shown and described by way of illustration are in no way intended to be considered limiting. Further, the invention has been described with reference to particular preferred embodiments, but variations within the spirit and scope of the invention will occur to those skilled in the art. It is noted that the foregoing examples have been provided merely for the purpose of explanation and are in no way to be construed as limiting of the present invention. While the present invention has been described with reference to exemplary embodiments, it is understood that the words, which have been used herein, are words of description and illustration, rather than words of limitation. Changes may be made, within the purview of the appended claims, as presently stated and as amended, without departing from the scope and spirit of the present invention in its aspects. Although the present invention has been described herein with reference to particular means, materials and embodiments, the present invention is not intended to be limited to the particulars disclosed herein; rather, the present invention extends to all functionally equivalent structures, methods and uses, such as are within the scope of the appended claims.