Abstract:
A system and method of forming a mixture from two elements according to which the elements are introduced into a vessel and mixed in the vessel before being discharged from the vessel. The flow rate of one of the elements is controlled to maintain a constant level of the mixture in the vessel and the flow rate of the other element is controlled to maintain a predetermined ratio of the flow rate of the latter element and the discharge flow rate.

Description:
BACKGROUND  
         [0001]    In the drilling of oil and gas wells, a casing is usually placed in the well and cement, or some other similar material, is placed around the outside of the casing to protect the casing and prevent movement of formation fluids behind the casing. The cement is usually mixed in a mixer at the surface to form a slurry which is pumped down hole and around the outside of the casing. The mixing is typically done by mixing the cement ingredients, typically cement, 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 of primary importance because 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.  
           [0002]    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 below certain densities, the ratio of solids to water can change significantly with only minor changes in 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.  
           [0003]    Therefore, what is needed is a system and method for creating a relatively lightweight slurry that overcomes the above problems. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0004]    The drawing is a schematic diagram depicting the system of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0005]    Referring to the drawing, the reference numeral  10  refers to a mixing head which receives a quantity of water from an external source at a continuous volumetric rate Q 1 . The mixing head  10  communicates with a mixing vessel  12  for discharging the water into the mixing vessel  12 . A partition  14  is provided in the mixing vessel  12  to define a first vessel portion  12   a  which receives the water from the mixing head  10 , and a second vessel portion  12   b . The height of the partition  14  is such that the water flows, by gravity, from the first vessel portion  12   a  to the second vessel portion  12   b.    
         [0006]    A quantity of cement solids, also from an external source, is introduced into the mixing head  10  at a continuous volumetric rate Q 2 . The water and the cement solids mix in the first vessel portion  12   a  to form a mixture, also referred to herein as a slurry, which flows into the second vessel portion  12   b  and which is discharged from an outlet in the second vessel portion  12   b  at a continuous volumetric rate Q 3 .  
         [0007]    Three flow valves  16 ,  18 , and  20  operate in a conventional manner to control the water flow rate Q 1 , the cement solids flow rate Q 2 , and the slurry flow rate Q 3 , respectively, and thus control the ratio Q 1 /Q 3  so that it attains a predetermined value based on the flow rates Q 1 , Q 2 , and Q 3 . It is understood that actuators, or the like (not shown), may be associated with the flow valves  16 ,  18 , and  20  to control, in a conventional manner, the positions of the flow valves  16 ,  18 , and  20 , and therefore the flow rates Q 1 , Q 2 , and Q 3 .  
         [0008]    Two flow meters  22  and  24  are disposed upstream of the flow valves  16  and  20 , and measure the flow rates Q 1  and Q 3 , respectively. The flow meters  22  and  24  are conventional and could be in the form of turbine, magnetic, or coriolis meters. Although shown schematically for the convenience of presentation, it is understood that the flow valves  16 ,  18 , and  20  and the flow meters  22  and  24  are connected in flow lines, in the form of conduits, pipes, etc. through which the water, the cement solids, and the slurry flow.  
         [0009]    A measuring device  28  is provided in the second vessel portion  12   b  for measuring the slurry level. The measuring device  28  could be one of several conventional devices that are available for measuring liquid level including, but not limited to, radar, laser, ultrasonic, or float devices.  
         [0010]    The process is controlled through a control unit  30  that includes a microprocessor, or the like, and is electrically connected to the flow valves  16 ,  18 , and  20 , the flow meters  22  and  24 , and the measuring device  28 . Since the control unit  30  can be one of a number of conventional devices, it will not be described in great detail. The control unit  30  receives signals from the flow meters  22  and  24  and the measuring device  28 , processes the signals, and sends signals to the flow valves  16  and  20  to control same in a manner to be described. In this context, it is understood that a hydraulic control valve and an actuator can be associated with each flow valve  16 ,  18 , and  20  to operate same and, since these units are conventional, they are not shown and will not be described in detail.  
         [0011]    In operation, water is introduced at a flow rate Q 1  into the mixing head  10  while cement solids are introduced at a flow rate Q 2 . The water and the cement solids pass from the mixing head  10  into the first vessel portion  12   a  where they mix to form a slurry which flows, by gravity, into the second vessel portion  12   b  and discharges therefrom at a flow rate Q 3 . The flow meters  22  and  24  meter the flow rates Q 1  and Q 3 , respectively, and the measuring device  28  measures the slurry level in the second vessel portion  12   b . Signals from the flow meters  22  and  24  corresponding to the flow rates Q 1  and Q 3 , and signals from the measuring device  28  corresponding to the slurry level in the second vessel portion  12   b  are passed to, and processed in, the control unit  30 . The control unit  30  monitors the signals and sends corresponding signals to the flow valves  16 ,  18 , and  20  to control the flow through the flow valves  16 ,  18 , and  20 , and therefore the flow rates Q 1 , Q 2 , and Q 3 , accordingly.  
         [0012]    The introduction of the cement solids into the first vessel portion  12   a  at the flow rate Q 2  is controlled by the flow valve  18  to maintain a constant liquid level in the second vessel portion  12   b , and the Q 1 /Q 3  ratio is controlled by controlling the water flow rate Q 1  by the flow valve  16  and the slurry flow rate Q 3  by the flow valve  24 . At steady-state conditions, this will yield the correct proportion of water and cement slurry based on the equation Q 1 +Q 2 =Q 3 .  
         [0013]    In the event partial automatic control is desired, the flow rates Q 1  and Q 3  could be measured by the flow meters  22  and  24 , respectively, and the flow valves  16  and  20  controlled accordingly by the control unit  30  as described above, while the cement solids flow rate Q 2 , as well as the slurry level in the second vessel portion  12   b  could be controlled manually. Alternatively, slurry flow rate Q 3  could be controlled manually while flow rates Q 1  and Q 2  are controlled automatically by the control unit  30 . Other combinations of partial and manual control are possible.  
         [0014]    If it is desired to control the entire process manually, water flow rate Q 1  and slurry flow rate Q 3  would be measured and observed by an operator, preferably on a numeric display, along with the ratio Q 1 /Q 3 . The operator would set the flow rates to maintain the proper ratio and mixing rate and would also observe the slurry level in the second vessel portion  12   b  and add the cement solids by manually adjusting the flow valve  18  in order to keep the level constant.  
         [0015]    It is understood that variations may be made in the foregoing without departing from the scope of the invention. For example, the elements forming the slurry can be varied within the scope of the invention and do not have to include cement. Also, the elements may be such that the slurry density becomes insensitive to changes in the solids-to-water ratio (Q 2 /Q 1 ), a situation that will occur when the specific gravity (or density) of the slurry and the specific gravity (or density) of the one or more of the elements forming the slurry become sufficiently close in value. Besides lightweight slurries described above, this would also include high-density cement slurries such as those above 20 pounds per gallon.  
         [0016]    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.