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You are an expert at summarizing long articles. Proceed to summarize the following text: 
BACKGROUND OF INVENTION  
       [0001]     In the process of rotary drilling a well, drilling fluid, or mud, is circulated down the rotating drill pipe, through the bit, and up the annular space between the pipe and the formation or steel casing, to the surface. The drilling fluid performs different functions such as removal of cuttings from the bottom of the hole to the surface, to suspend cuttings and weighting material when the circulation is interrupted, control subsurface pressure, isolate the fluids from the formation by providing sufficient hydrostatic pressure to prevent the ingress of formation fluids into the wellbore, cool and lubricate the drill string and bit, maximize penetration rate, etc.  
         [0002]     The required functions can be achieved by a wide range of fluids composed of various combinations of solids, liquids and gases and classified according to the constitution of the continuous phase mainly in two groupings: aqueous drilling fluids, and oil-based drilling fluids. In drilling water-sensitive zones such as reactive shales, production formations, or where bottom hole temperature conditions are severe or where corrosion is a major problem, oil-based drilling fluids are preferred.  
         [0003]     Oil-based drilling fluids typically contain oil-soluble surfactants that facilitate the incorporation of water-wet clay or non-clay formation minerals, and hence enable such minerals to be transported to surface equipment for removal from circulation before the fluid returns to the drill pipe and the drill bit. The largest formation particles are rock cuttings, the size typically larger than 0.1 to 0.2 mm, removed by shale-shaker screens at the surface. Smaller particles, typically larger than about 5 μm, will pass through the screens, and must be removed by centrifuge or other means.  
         [0004]     Oil-based drilling fluids have been used for many years, and their application is expected to increase, partly owing to their several advantages over water based drilling fluids, but also owing to their ability to be re-used and recycled, so minimizing their loss and their environmental impact.  
         [0005]     As mentioned above, during drilling, formation particles become incorporated into the drilling fluid. Unless these are removed, they eventually alter the fluid&#39;s properties, particularly the Theological parameters, out of the acceptable range. However, formation particles that are less than about 5 to 7 μm in size are more difficult to remove than larger particles. These low gravity solids can build up in a mud system, causing inefficient drilling problems such as drill pipe sticking, increased pipe torque, and other high viscosity issues.  
         [0006]     While low gravity solids may be removed from drilling fluids using mechanical means such as a centrifuge, it has been found that longer run-times are required to remove the colloidal particles, if the low gravity solids can be removed at all. Thus, there is a need for an apparatus that can be used with traditional solids separation equipment to reduce the run-time required to remove low gravity solids. Further, it would be an improvement in the art to have an apparatus that can be utilized both on active drilling projects to facilitate solids control equipment efficiency as well as by mud plants in reclaiming and/or reconditioning mud returned from field operations.  
       SUMMARY  
       [0007]     In one aspect, the claimed subject matter is generally directed to an apparatus for preparing an oil-based drilling fluid for recovery. The apparatus includes a first static mixer in which the oil-based drilling fluid and a surfactant are mixed. In a second static mixer a flocculant and a base fluid may be mixed. The flocculant mixture is added to the drilling fluid mixture and further mixing occurs through a series of additional mixers. Upon exiting the final mixer, the drilling fluid mixture is prepared to have solids separated therefrom so that the oil-based drilling fluid may be further processed for recovery. A centrifuge may be used to separate solids from the remaining effluent.  
         [0008]     In another illustrated aspect, the claimed subject matter is directed to an apparatus for reclaiming oil-based drilling fluid and recovering valuable weighting agent. The apparatus includes an additional centrifuge to remove the weighting agent prior to the injection of polymer to the oil-based drilling fluid.  
         [0009]     In another illustrated aspect, a method for preparing an oil-based drilling fluid for recovery is claimed. The method includes demulsifying the drilling fluid with a surfactant and preparing a flocculant mixture. The flocculant mixture is then mixed with the drilling fluid mixture. The next step includes separating solids from the drilling fluid mixture and collecting them. Effluent from the separating solids step may be collected for further processing.  
         [0010]     Other aspects and advantages of the claimed subject matter will be apparent from the following description and the appended claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a schematic of an apparatus for preparing an oil-based drilling fluid for recovery.  
         [0012]      FIG. 2  is a schematic of an alternative embodiment of an apparatus for preparing an oil-based drilling fluid for recovery.  
         [0013]      FIG. 3  is a layout of the apparatus mounted on a skid.  
     
    
     DETAILED DESCRIPTION  
       [0014]     The claimed subject matter relates to an apparatus and method for preparing an oil-based drilling fluid for recovery. The oil-based drilling fluid includes oil, water, and solids in relative proportions consistent with used drilling fluid that has been subjected to preliminary processes to remove large solids from the fluid. The solids remaining in the drilling fluid typically include a percentage of high gravity solids and a percentage of low gravity solids. High gravity solids are those solids that are dense, as in barite or hematite, while low gravity solids are those solids that have a lower density than barite. The oil and water in the used drilling fluid are present in proportionate amounts, the relationship between them often being expressed as an oil-to-water ratio.  
         [0015]     In a first embodiment, shown in  FIG. 1 , the apparatus  10  includes a plurality of mixers  12 - 20  that may be mounted to a common skid  22 . Oil-based drilling fluid  24  is pumped from a mud plant  26  to the first mixer  12 . A pump  28  may be used to introduce the drilling fluid  24  to the first mixer  12  with a predetermined pressure and flow rate. A surfactant  32  is pumped into the first mixer  12  from a surfactant tank  34 . The surfactant  32  may be diluted with water  30  from water tank  38  prior to its introduction to the first mixer  12 . A dose pump  36  may be used to introduce the surfactant  32  to the mixer with a predetermined pressure and flow rate. The surfactant  32  acts on the mud solids, improving their hydrophilicity so that the polymer, which is very hydrophilic and added downstream, can flocculate the solids.  
         [0016]     The first mixer  12  preferably is a static shear mixer including an insert (not shown) that provides shear to the fluid passing through the first mixer  12  sufficient to mix the surfactant  32  and the drilling fluid  24 . The surfactant  32  and the drilling fluid  24  are introduced to the first mixer  12  upstream from the insert and exit the mixer  12  as a surfactant treated mud  40 .  
         [0017]     A flocculant polymer  42  is stored in a flocculant storage tank  44  and ma) be mixed with a base fluid  46 , when necessary, to form a flocculant mixture  48 . The dilution of the flocculant polymer  42  with the base fluid  46  can improve the dispersal of the polymeric droplets into the mud. The decision to do this or not is based on the type of dosing equipment, the viscosities of the mud  24  and the flocculant polymer  42 , and the strength to the mixing employed.  
         [0018]     Dosing pumps  50 ,  52  may be used to introduce the flocculant  42  and the base fluid  46 , respectively, to the second mixer  14  in predetermined relative quantities. The second mixer  14  preferably is a static shear mixer including an elongated insert to enhance the dispersion of flocculant  42  within the base fluid  46  and to provide turbulence to the flow. The turbulence created by the insert causes the flocculant  42  and the base fluid  48  to form the flocculant mixture  48 .  
         [0019]     The flocculant mixture  48  is mixed with the surfactant treated mud  40  in a third mixer  16 . Like the first mixer  12 , the third mixer  16  preferably is a static mixer including an insert to provide shear to the passing fluids sufficient to mix the fluids together. The addition of flocculant  48  to the surfactant treated mud  40  causes solid material in the surfactant treated mud  40  to coagulate around the flocs. Creating larger solid masses aids in their later removal from the drilling fluid.  
         [0020]     The treated mud  54  is mixed further in additional downstream mixers  18 ,  20 . Preferably, a fourth mixer  18  is a dynamic mixer. In the dynamic mixer  18 , the treated mud  54  is subjected to agitation providing additional shearing to facilitate the coagulation of solids and floc. Additional mixers  20 ,  21  may be included. The additional mixers  20 ,  21  preferably are in-line mixers, providing additional mixing by subjecting the drilling fluid and polymer mixture  54  to shear as in the second mixer  14  discussed earlier. By including a plurality of mixers downstream from the injection of flocculant polymer  48 , the exposure of solids to the flocculant is enhanced prior to directing the treated mud  54  to a separation process.  
         [0021]     Upon exiting the final mixer  21 , the treated mud  54  is a prepared mud mixture  56  ready for further processing to remove the solids from the fluid. The prepared mud mixture  56  may be directed to equipment outside of the skid  22  for additional processing. Such equipment may include a centrifuge  58  to which the prepared mud mixture  56  is directed. The centrifuge  58  includes a bowl that is rotated at a speed sufficient to separate the solids  60  in the prepared mud mixture  56  from the fluid, or effluent  62 . As the solids  60  are discharged from the centrifuge  58 , they may be collected in a cuttings box  64 . Effluent  62  may be released to a fluid storage area  66 , or directed to additional equipment (not shown) for further processing.  
         [0022]     As previously stated, the equipment required to process the drilling fluid  24  prior to its being directed to the centrifuge  58  may be housed on a skid  22 . To consolidate the equipment onto a single skid  22 , attention must be given to the layout of the equipment. In a preferred embodiment, shown in  FIG. 3 , water and base oil tanks  38 ,  47  are positioned directly above the surfactant and polymer tanks  34 ,  44 . The water and base totes  38 ,  47  may be placed on rails so that they are movable to an outward position, away from the polymer and surfactant tanks  34 ,  44  for refilling.  
         [0023]     Dosing pumps  36 ,  39 ,  50 ,  52  may be positioned on the skid  22  such that the polymer and base oil pumps are directly beside their respective tanks with one pump placed atop another to conserve space. Likewise, the surfactant and water pumps may be stacked to conserve space.  
         [0024]     The flocculant polymer  42  or flocculant mixture  48  added to the drilling fluid enhances removal of the solids  60  by the centrifuge  58  by forming larger solid particles. The polymer droplets have to be well dispersed into the mud to be flocculated, without dissolving the polymer. The droplets remain intact and adhere the solids in the mud together, thus greatly improving the solid-liquid separation efficiency upon centrifugation. In order to derive the most benefit from the polymeric droplets as a flocculant, it is necessary that they be well mixed into the mud, and at an efficacious dose. The amount of flocculant polymer  48  added to the surfactant treated mud  40  should be that sufficient to leave the polymeric droplets homogeneously dispersed throughout the mud  24  to be flocculated.  
         [0025]     A second embodiment of the apparatus  10 ′ is shown in  FIG. 2 . In this embodiment, the drilling fluid  24  is pumped from the mud plant  26  into a first centrifuge  70 . The first centrifuge  70  is optimized to recover the weighting agent  72 , such as barite, from the drilling fluid  24 . The weighting agent  72  is discharged from the first centrifuge  70  to a cuttings box  74  or a storage tank  66 ′ to be reintroduced to the recovered drilling fluid  62 ′ discharged from the apparatus  10 ′. Effluent  76  from the first centrifuge  70  is pumped into the first mixer  12 . As previously described, surfactant  32  is injected into the first mixer  12  and the effluent  76  and surfactant  32  are subjected to static shear sufficient to distribute the surfactant through the drilling fluid to form a surfactant treated effluent  40 ′.  
         [0026]     A polymer mixture  48  is made by mixing a flocculant  42  and a base fluid  46  in a mixer  14 , if a base fluid is needed. The polymer mixture  48  is directed to mixer  16  where it is mixed with the surfactant treated effluent  40 ′, as previously described. If a base fluid is not needed, flocculant  42  may be directed to the mixer  16 , in which it is mixed directly with the surfactant treated effluent  40 ′ to form a treated mud  54 ′.  
         [0027]     The treated mud  54 ′ from the mixer  16  is directed through a series of additional mixers  18 ,  20 ,  21  to ensure there is sufficient mixing to prepare the treated mud  54 ′ for separation and further processing. As previously described, a dynamic mixer  18  and one or more inline mixers  20 ,  21  are preferred to ensure sufficient mixing of the flocculant  42  within the surfactant treated effluent  40 ′.  
         [0028]     A centrifuge  58  may be used to separate solids  60 ′ and effluent  62 ′. The recovered weighting agent  72  from the first centrifuge  70  may be added to the effluent  62 ′ as needed to reproduce drilling fluid to be used in drilling operations.  
         [0029]     While the claimed subject matter has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the claimed subject matter as disclosed herein. Accordingly, the scope of the claimed subject matter should be limited only by the attached claims.

Summary:
Oil-based drilling fluid is prepared for further processing to recover the drilling fluid by pumping the drilling fluid through a flow meter. Surfactant may be added to the drilling fluid by using a dose pump and a flow meter. The drilling fluid and surfactant are then blended by passing them through a static mixer. A flocculating polymer is transferred via dose-pumps to another static mixer where it is blended with the surfactant and drilling fluid mixture. To ensure adequate mixing and reaction, additional mixers are included through which the mixture passes. A centrifuge is used to separate solid particles from the fluid.