Abstract:
A method of forming a mixture of two or more elements for discharge from a vessel. The change in the volume of the mixture in the vessel, as well as the flow of at least one of the elements and the mixture are measured so that the flow of an unmeasured element into the vessel can be calculated.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This application is a continuation-in-part application of co-pending application Ser. No. 10/085,443 filed Feb. 28, 2002. 
     
    
     BACKGROUND  
       [0002]     In the drilling of oil and gas wells, a casing is usually placed in the well and cement, or other similar material, is mixed with a liquid, such as water, at the surface to form a slurry which is pumped down hole and around the outside of the casing to protect the casing and prevent movement of formation fluids behind the casing. The mixing is typically done by mixing the cement ingredients, typically cement, with water, chemicals, and other solids, until the proper slurry density is obtained, and then continuing to mix as much material as needed at that density while pumping the slurry down hole in a continuous process. Density is important since the resulting hydrostatic pressure of the slurry must be high enough to keep pressurized formation fluids in place but not so high as to fracture a weak formation.  
         [0003]     Some wells require lightweight slurries that will not create enough hydrostatic pressure to fracture a weak formation. One way of creating light-weight slurries is to use low specific gravity solids in the blend. The problem with such slurries is that the density of the solids can be close to, or the same as, the density of the slurry. When this happens, the ratio of solids to liquid can change significantly with little or no change in slurry density. Changes in solids-to-water ratio can affect slurry viscosity, compressive strength, and other properties. In these situations, density-based control systems do not work well.  
         [0004]     As a result of the above it is important to be able to measure the flow rates of the liquid, the solids, and the slurry so that the density of the slurry can be determined and controlled. However, the flow rate of the solids can not be measured directly.  
         [0005]     Therefore, what is needed is a system and method for creating a slurry of the above type that overcomes the above problems. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0006]     The drawing is a schematic diagram depicting a system according to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0007]     Referring to the drawing, the reference numeral  10  refers to a mixing head which receives a quantity of liquid, such as water, from a flow line  12  at a continuous volumetric flow rate Q 1 . The mixing head  10  communicates with a vessel  14  that includes a partition  14   a  that divides the vessel into a first portion  14   b  which receives the liquid from the head  10 , and a second portion  14   c . The height of the partition  14   a  is such that the liquid flows, by gravity, from the first vessel portion  14   b  to the second vessel portion  14   c.    
         [0008]     A quantity of solids, such as cement and possibly other chemicals, is passed from an external source, via a flow line  16 , into the mixing head  10  at a continuous volumetric flow rate Q 2 . The liquid and the solids flow from the head  10  to the vessel portion  14   b  and mix to form a slurry that flows into the vessel portion  14   c  before discharging from an outlet in the vessel portion  14   b  through a flow line  18  at a continuous volumetric flow rate Q 3 .  
         [0009]     Three flow valves  20   a ,  20   b , and  20   c  are mounted in the flow lines  12 ,  16 , and  18 , respectively, and operate in a conventional manner to control the liquid flow rate Q 1 , the solids flow rate Q 2 , and the slurry flow rate Q 3 , respectively, in a manner to be described. It is understood that actuators, or the like (not shown), may be associated with the valves  20   a ,  20   b , and  20   c  to control, in a conventional manner, the positions of the valves, and therefore the rates Q 1 , Q 2 , and Q 3 .  
         [0010]     Two flow meters  22   a  and  22   b  are disposed in the flow lines  12  and  18 , respectively, upstream of the valves  20   a  and  20   c , respectively, and measure the flow rates Q 1  and Q 3 , respectively. The meters  22   a  and  22   b  are conventional and could be in the form of turbine, magnetic, or Coriolis meters.  
         [0011]     A measuring device  24  is provided in the vessel portion  14   c  for measuring the level of the slurry in the vessel portion. The device  24  can be one of several conventional devices that are available for measuring liquid level including, but not limited to, radar, laser, or ultrasonic devices.  
         [0012]     The volume of slurry in the vessel portion  14   c  is determined by monitoring the level of the slurry in the vessel portion and calculating the volume of slurry in the vessel portion utilizing the measured value and the vessel dimensions, or geometry, in a conventional manner. The slurry level in the vessel portion  14   c  is monitored continuously so that any changes in the slurry volume with respect to time can be determined.  
         [0013]     An electronic control unit  30  is provided that includes a microprocessor, or the like, and is electrically connected to the valves  20   a ,  20   b , and  20   c , the meters  22   a  and  22   b , and the measuring device  24 . Since the control unit  30  can be one of a number of conventional devices, it will not be described in great detail and its operation will be described below.  
         [0014]     In operation, liquid is introduced at a rate Q 1  into the head  10  while solids are introduced at a rate Q 2 . The liquid and the solids mix in the head  10  to form a slurry that flows into the vessel portion  14   b , and then, by gravity, into the vessel portion  14   c  before discharging from the latter vessel portion at a rate Q 3 . The meters  22   a  and  22   b  meter the flow rates Q 1  and Q 3 , respectively, while the measuring device  24  measures the slurry level in the vessel portion  14   c . Electrical signals from the meters  22   a  and  22   b , corresponding to the flow rates Q 1  and Q 3 , and signals from the measuring device  24 , corresponding to the slurry level in the vessel portion  14   c , are passed to, and processed in, the control unit  30 .  
         [0015]     The control unit  30  calculates the change in the volume of the slurry in the vessel portion  14   c , and sends corresponding signals to the valves  20   a ,  20   b , and  20   c  to control the flow through the valves, and therefore the rates Q 1 , Q 2 , and Q 3 , accordingly.  
         [0016]     Although the flow rate at which the solids are being added to the vessel  14  cannot be measured directly, the flow rate can be determined by performing a volume balance on the vessel  14 . The volume balance involves the following equation: 
 
 Q   1 + Q   2 = Q   3 + dV/dT  
 
 Where: 
        Q 1 =flow rate of the liquid into the mixing head  10  (in terms of volume per unit time, e.g. gallons per minute)     Q 2 =flow rate of the solids into the mixing head  10  (in terms of volume per unit time, e.g. gallons per minute)     Q 3 =flow rate of the slurry discharged from the vessel portion  14   c  (in terms of volume per unit time, e.g. gallons per minute)     V=volume of slurry in the vessel  14  (in terms of gallons)     T=time     dV/dT=change in volume of the slurry in the vessel  14  with respect to time (in terms of volume per unit time, e.g. gallons per minute). 
 
 Thus: 
 
 Q   2 = Q   3 − Q   1 + dV/dT  
       
 
         [0023]     As a result, continuous measurement of dV/dT enables the flow rate Q 2  of the solids into the head  10 , and therefore into the vessel  14 , to be determined on a continuous basis, allowing the operator or the control unit  30  to adjust and maintain the solids flow rate Q 2  at a desired value.  
         [0024]     If it is desired for the solids flow rate Q 2  to be proportional to either the liquid flow rate Q 1  or the slurry discharge flow rate Q 3 , then the solids flow rate Q 2  could be maintained as a percentage of either of the liquid flow rate Q 1  or the slurry flow rate Q 3 . Alternatively, the solids flow rate Q 2  could be maintained at a desired value independent of the liquid flow rate Q 1  or the slurry discharge flow rate Q 3 , or the system could be used as a solids flow meter to simply measure the solids flow rate without any attempt to control the rate to a given value.  
         [0025]     It is also possible (but not necessary) to control the ratio of the liquid flow rate Q 1  to the slurry discharge flow rate Q 3  simultaneously with the solids flow rate Q 2 . For example, if a solids slurry is being mixed where the desired slurry was X% liquid and Y% solids, the liquid flow rate Q 1  and the solids flow rate Q 2  could be maintained at the rates: 
 
 Q   1 =( X /100)× Q   3  
 
 Q   2 =( Y /100)× Q   3  
 
         [0026]     If it were desirable to maintain the solids flow rate, Q 2 , as a percentage, Z%, of the liquid flow rate, Q 1 , then the solids flow rate could be maintained at the rate calculated by: 
 
 Q   2 =( Z /100)× Q   1  
 
 In this case, the relationship of Q 1  to Q 3  would not need to be maintained at a specified ratio. 
 
         [0027]     Other combinations of inflow and outflow proportions could be controlled.  
         [0028]     Thus, according to the above, it is not necessary to maintain a certain ratio between Q 1  and Q 3  (although it can be done), and the solids can be added at a rate that is independent of one or both of the other rates, Q 1  and Q 3 . Also, the solids flow rate Q 2  can be determined and controlled during non-steady state conditions, i.e. when the level of the vessel portion  14   c  (and therefore the vessel volume) is fluctuating. Further, manual control can be utilized if the automatic control of one or more of the flow rates Q 1 , Q 2 , and Q 3  cease to function.  
         [0029]     In the event partial automatic control is desired, the flow rates Q 1  and Q 3  could be measured by the meters  22   a  and  22   b , respectively, and the valves  20   a  and  20   c  controlled accordingly by the control device  30  as described above, while the solids rate, Q 2 , could be controlled manually. Alternatively, Q 3  could be controlled manually while Q 1  and Q 2  are controlled automatically by the control device  30 . Other combinations of partial and manual control are possible.  
         [0030]     If it is desired to control the entire process manually, Q 1 , Q 2 , and Q 3  would be observed by an operator, preferably on a numeric display, and the operator would set the rates to maintain the proper ratios and mixing rate.  
         [0031]     It is understood that variations may be made in the foregoing without departing from the scope of the invention. For example, the number and the type of elements forming the slurry can be varied within the scope of the invention and do not have to include solids.  
         [0032]     Although only one exemplary embodiment of this invention has been described in detail above, those skilled in the art will readily appreciate that many other modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the following claims.