Abstract:
A system for determining a parameter of interest of at least one particle in a sample of a fluid obtained from a formation, comprises a view cell containing at least a portion of the sample and at least one window for viewing the sample. A light source illuminates the sample. An imaging system captures at least one image of the illuminated sample. A program executing a set of instructions on a computer analyzes the at least one image and generates an output related to at least one parameter of interest of the at least one particle in said sample.

Description:
FIELD OF THE INVENTION 
   This invention generally relates to fluid sample analysis. More specifically this invention relates to a method and apparatus for determining characteristics of particles in a fluid sample. 
   BACKGROUND OF THE INVENTION 
   Problems encountered in crude oil production include the precipitation and/or agglomeration of particles or substances in solution and/or in suspension in the produced formation fluid. The term particles, is defined herein includes, but is not limited to solid particles, emulsion droplets, and gas bubbles. Asphaltenes are examples of solid particle components of crude oil that are often found in colloidal suspension in the formation fluid. If for any reason the colloidal suspension becomes unstable, such as with a drop in fluid pressure, the colloidal particles will precipitate, stick together and, especially in circumstances where the asphaltenes include resins, plug the well. Asphaltene precipitation during production causes severe problems. Plugging of tubing and surface facilities disrupts production and adds cost. Plugging of the formation itself is very difficult and expensive to reverse, especially for a deep water well. 
   Asphaltenes can precipitate from crude oils during production of the crude oil due to a drop in pressure. Crude oils which are somewhat compressible are particularly susceptible to this effect because the reduction in dielectric constant per unit volume which accompanies fluid expansion causes the asphaltene suspension to become unstable. The onset of asphaltene precipitation is difficult to predict, and when asphaltene plugging happens, it usually happens unexpectedly. Advance warning of asphaltene precipitation based on laboratory testing of formation fluid according to present techniques, while useful, is not optimally reliable. 
   Formation gas may be contained in solution in the produced formation fluid and may come out of solution as the fluid pressure is reduced during transit of the fluid out of the well. 
   Attempts have been made to determine the onset of the particle precipitation, particularly asphaltenes. U.S. Pat. No. 5,969,237 to Jones et al. describes a system for detecting scattered acoustic energy to determine particle size distribution of asphaltene particles. U.S. Pat. No. 6,087,662 to Wilt et al. uses mid-range infra red absorption spectroscopy to determine asphaltene concentrations in hydrocarbon feed. U.S. Pat. No. 5,420,040 to Anfindsen et al. provides a system to measure changes in the conductivity or capacitance of a petroleum fluid for determining asphaltene precipitation in the fluid. 
   All of the prior art systems infer particle precipitation and other related characteristics from related physical measurements. There is a demonstrated need for a system to view and analyze the particles to more definitively determine the characteristics of the particles. 
   SUMMARY OF THE INVENTION 
   The present invention is a method and system for determining characteristics of particles in a fluid sample. In one aspect of the present invention, a system for determining a parameter of interest of at least one particle in a sample of a fluid obtained from a formation, comprises a view cell containing at least a portion of the sample and at least one window for viewing the sample. A light source illuminates the sample. An imaging system captures at least one image of the illuminated sample. A program executing a set of instructions on a computer analyzes the at least one image and generates an output related to at least one parameter of interest of the at least one particle in said sample. 
   In another aspect, a method for determining at least one parameter of interest of at least one particle in a sample of a fluid obtained from a formation comprises;
     a. illuminating the sample with a light at a fluid pressure;   b. capturing an image of the sample;   c. changing the fluid pressure of the sample;   d. repeating steps a) and b) over a predetermined number of fluid pressure changes; and   e. analyzing the captured images to determine at least one parameter of interest of the at least one particle in said sample as a function of pressure.   

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features which are believed to be characteristic of the invention, both as to organization and methods of operation, together with the objects and advantages thereof, will be better understood from the following detailed description and the drawings wherein the invention is illustrated by way of example for the purpose of illustration and description only and are not intended as a definition of the limits of the invention, wherein: 
       FIG. 1A  shows a schematic diagram of a pressurized fluid imaging system according to one embodiment of the present invention; 
       FIG. 1B  illustrates detail A of  FIG. 1A ; and 
       FIG. 2  illustrates a flow chart of an analysis of a fluid sample according to one embodiment of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention is a method and system for determining characteristics, also called parameters of interest, of particles in a fluid sample. To the extent that the following description is specific to a particular embodiment or a particular use of the invention, this is intended to be illustrative and is not to be construed as limiting the scope of the invention. 
     FIG. 1A  shows a schematic diagram of a pressurized fluid imaging (PFI) system  1  according to one embodiment of the present invention. A sample of downhole formation fluid  24  is obtained and maintained at downhole pressure and temperature conditions. The fluid sample  24  is introduced into the sample side of buffer cells  20  and  21 . Buffer cells  20  and  21  have a piston  23  with a sliding seal (not shown) for isolating the sample fluid  24  from a pressurizing fluid  25 , commonly a mineral oil. The sample fluid sides of buffer cells  20  and  21  are hydraulically connected by fluid conduit  10  which may be a high pressure tubing. View cell  11  is disposed in conduit  10  such that sample fluid  24  passes through view cell  24  as sample fluid  24  is caused to flow between buffer cells  20  and  21  as described below. 
   The pressurizing fluid sides of buffer cells  20  and  21  are hydraulically connected to hydraulic pumps  5  and  6  respectively by conduits  50  and  51 . Precision hydraulic pumps  5  and  6  are precision pumps having an internal stepper motor driven piston (not shown). Pumps  5  and  6  are controlled by controller  9 . Such a pump and a controller are commercially available, for example from Quizix, Inc. of North Highlands, Calif. In one mode, one pump extends at a first predetermined rate while the other pump retracts at a second predetermined rate, thereby causing sample fluid  24  to flow between buffer cells  20  and  21 . The pumps  5  and  6  may be controlled by controller  9  to cause the sample fluid  24  to flow back and forth between buffer cells  20  and  21 . In operation, when the first predetermined rate is equal to the second predetermined rate, the system pressure remains substantially constant. In another mode, the first predetermined rate is less than the second predetermined rate, or vice versa, causing the system pressure to be controllably reduced or increased. Alternatively, the system pressure may be detected using pressure sensor  3 . Controller  9  may be used to adjust the first and second predetermined rates to maintain the system pressure at a predetermined value. The predetermined pressure may also vary with time, with the processor adjusting the first and second rates according to programmed instructions in controller  9 . When the fluid from one buffer cell is substantially all transferred to the other buffer cell, the pumps may be reversed, allowing substantially continuous flow through the view cell  11 . 
   As shown in  FIG. 1A , buffer cells  20  and  21  are positioned within thermal chamber  12  that is maintained at substantially downhole temperature using temperature sensor  4  and commercially available heaters. Alternatively, the temperature of thermal chamber  12  may be controlled such that the chamber temperature and pressure profiles are coordinated to simulate the profiles of a fluid as it is being pumped from a well. 
   The operation of pumps  5  and  6  causes sample fluid  24  to pass through view cell  11  that is shown in more detail in  FIG. 1B . As shown in  FIG. 1B , view cell  11  has ports  41  and  43 , in housing  40 , connected to a chamber  46  having windows  42  positioned on either side of chamber  46  to allow visual examination of sample fluid  24  as it traverses, in either direction, chamber  46 . Windows  42  are designed to operate with sample fluid  24  pressures of 20,000 psi. The size of chamber  46  may be adjustable to maintain a predetermined light transmission through the sample fluid  24  as the properties of the fluid samples  24  change. Such a view cell is commercially available from Temco, Inc. of Tulsa, Okla. Sample fluid  24  is illuminated in chamber  46  by light  45  from light source  13  (see  FIG. 1A ) transmitted to visual cell  11  along light pipe  14 . Light source  13  provides light from a halogen source. Alternatively, a xenon source may be used. Both types of sources are commercially available and will not be described further. 
   Light  45  passes through sample fluid  24  in chamber  46  and through window  47  in a wall of thermal chamber  12 . Light  45  passes through an optical magnifier  33  and is captured by imaging detector  32 . Optical magnifier  33  is a microscope that is controllable by program instructions stored in memory in processor  30 . In one embodiment, optical magnifier  33  is a stereo-microscope. In one embodiment, imaging detector  32  is a digital still camera connected by cable  31  to processor  30 . Processor  30  may be a personal computer of a type known in the art having a processing unit, memory, internal magnetic and/or optical storage devices, and interface circuitry to communicate with digital camera  31  and optical magnifier  33 . Digital camera  31  takes images according to programmed instructions controlled by processor  30 . The images may be taken at fixed time intervals at rates greater than one image per second and as fast as about 1.4 images per second. Alternatively, the images may be captured by a video camera at suitable speeds for continuous playback. The images may be correlated with readings from pressure sensor  3  and/or temperature sensor  4 . The images may be stored in at least one of internal memory, internal storage media, and external storage media. 
   Images from digital camera  31  may be visually analyzed by programmed instructions stored in processor  30  to determine various characteristics of particles present in sample fluid  24 . Such characteristics include, but are not limited to, (i) particle size, (ii) particle shape, (iii) particle size distribution, and (iv) number of particles. The analysis may be accomplished by a commercially available software product such as the Image-Pro Plus brand of analysis software by Media Cybernetics, Inc. of Silver Spring, Md. The output may be in visual, tabular, and/or graphical form. The output may be correlated to the sample fluid pressure for providing, for example, an estimate of the pressure at which asphaltenes begin to substantially precipitate. The system as described above may be adapted, using techniques known in the art, for use in a laboratory and/or a field environment. 
     FIG. 2  is a flow chart of the PFI operation according to one embodiment of the invention. At step  105  a downhole fluid sample is obtained. The fluid sample is commonly kept at downhole temperature and pressure conditions during transport to the PFI. At step  115 , the downhole fluid sample is caused to flow through the view cell. At least one image of the downhole fluid sample is acquired at step  120 . The image is analyzed at step  125  to determine characteristics of the particles. As described previously, these characteristics include, but are not limited to, (i) particle size, (ii) particle shape, (iii) particle size distribution, and (iv) number of particles. The pressure of the fluid sample is reduced a predetermined amount in step  130 . At each pressure change, steps  115  through  125  are repeated. The characteristics of the particles are output as a function of fluid sample pressure in step  135 . The output may be in visual, tabular, and/or graphical form. As described previously, the downhole fluid sample may be analyzed in the laboratory or at a field location, using the system of the present invention. Alternatively, all of the images may be taken, stored, and analyzed at a later time. 
   The foregoing description is directed to particular embodiments of the present invention for the purpose of illustration and explanation. It will be apparent, however, to one skilled in the art that many modifications and changes to the embodiments set forth above are possible without departing from the scope and the spirit of the invention. It is intended that the following claims be interpreted to embrace all such modifications and changes.