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BACKGROUND OF THE INVENTION 
       [0001]    1. Field of Invention 
         [0002]    The present invention relates to a method and apparatus for separating solids from liquids in general and in particular to a method and apparatus for separating solids from liquids in an oil well drilling operation. 
         [0003]    2. Description of Related Art 
         [0004]    Wells for recovering oil, gas and the like are typically created by drilling into an underground source using a hollow drill string supported by a drilling rig. The drill string includes a drill bit at the lower end that is rotated into the ground to create a well bore. As the drill bit is rotated, drilling fluid is pumped down through the interior of the drill string to pass through the bit and return to the surface in the well bore external to the drill string. The drilling fluid acts to lubricate the drill bit and carries the loose solids or cuttings created by the drill bit to the surface. At the surface, the used drilling fluid is collected and recycled by removing some or all of the cuttings. The composition of the cuttings content in the drilling fluid can be varied depending on the state of the drilling process and the location of the drill bit below the surface. 
         [0005]    Presently, a mud storage tank to hold drilling fluid and a shale shaker to perform screening of larger cuttings tend to be standard equipment for a drilling rig. In normal well site operation, drilling fluid is circulated out of the borehole and passed over a shale shaker which is a screen to separate large solid particles from the drilling fluid. The shale shaker is generally positioned directly above the mud storage tank and the large particles are collected in a shale bin. The resulting collected large particles typically still have at least some drilling fluid on them after being deposited in the shale bin and are typically in the form of a slurry. A rotational particle separator such as for example a centrifuge or cyclonic separator is typically used to remove the smaller particles remaining in the drilling fluid in the storage tank. 
         [0006]    Drilling fluids are typically either water based or oil based. Regulations in many countries require that when an oil based drilling fluid is used, the cuttings in the shale bin be hauled away for disposal or blended with sawdust and canola for land spreading. This is because the remaining drilling fluid on the cuttings discussed above would contaminate any site at which the cuttings were disposed unless the drilling fluid was removed beforehand. Blending of the cuttings prior to land spreading increases the cost to dispose of cuttings as compared to the cost of disposal of dry cuttings alone. In addition, the resulting wet cuttings composition is greater in volume and weight than dry cuttings alone. This increased weight and volume of the wet cuttings further increases transportation and disposal costs. 
         [0007]    While the smaller particles in the drilling fluid are typically separated from the drilling fluid by a centrifuge, the larger particles removed by the shale shaker are not. The wet cuttings resulting from not centrifuging the larger particles results in the aforementioned problems with disposal of such wet cuttings. Previous attempts to pass all of the cuttings through a centrifuge have not been successful. 
         [0008]    Heretofore, it has been impractical to pass all of the solid materials removed by the shale shaker through a centrifuge to further remove any drilling fluid from the solid material. The larger particles removed by the shale shaker would constitute too dry of a composition to properly pass through a centrifuge or cyclonic separator without plugging the same. 
         [0009]    In addition, it has not been practical to pass all of the used drilling fluid through a centrifuge without first separating the drilling fluid from the larger particles with a shale shaker. Because of the relatively large volume of the mud storage tank, the velocity and agitation of the drilling fluid in this tank is relatively low. The lack of agitation of the mud storage tank allows the small and large particles to accumulate on the bottom of the tank. Because of the settling of the particles on the bottom of the tank, the larger particles would accumulate in the tank and thereby would not be removed quickly enough from the mud storage tank by the centrifuge. This allows for solids carry over between chambers of the mud storage tank and eventually allows for recirculation of the solids in the mud storage tank down the drill string which is undesirable. 
         [0010]    What is desirable is a solid separation system that produces a drier solid product that does not require blending prior to disposal. Specifically, a method and apparatus that enables all of the solid material to be passed through a centrifuge or cyclonic separator so as to produce a drier solid is desirable. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention provides a method and apparatus that enables all of the solid material in a drilling fluid to be passed through a centrifuge or cyclonic separator to remove all excess drilling fluid from the solid material. Specifically, the present invention permits the solid material removed from the drilling fluid by a shale shaker or screen to be passed through a centrifuge or cyclonic separator to further remove any drilling fluid from the solid material. 
         [0012]    According to a first embodiment of the present invention, there is provided an apparatus for separating solids from liquids in a drilling operation. In the drilling operation, used drilling fluid is screened into a substantially solid portion and a substantially liquid drilling fluid whereby the drilling fluid is stored in a storage tank. The apparatus comprises a recirculation tank having a second liquid for receiving the solid portion, and a first rotational particle separator for separating the solid portion from the second liquid. The apparatus further includes a pump for pumping the solid portion and the second liquid from the recirculation tank to the first rotational particle separator and a bin region to receive and retain the separated solids from the first rotational particle separator, wherein the separated second liquid is returned to the recirculation tank. 
         [0013]    The rotational particle separator may comprise a centrifuge. The rotational particle separator may comprise a cyclonic separator. The apparatus may be further mounted on a field transportable skid. The skid may include a raised platform for mounting the rotational particle separator. The apparatus may further comprise a second rotational particle separator for removing particles from the drilling fluid supplied from the storage tank wherein the particles are deposited in the bin region wherein the drilling fluid is returned to the storage tank. 
         [0014]    The recirculation tank may include an angled bottom having a high end and a low end relative to each other. The pump may draw the solid portion and the second liquid proximate to the low end. The recirculation tank may further include a closable top. The second liquid may comprise drilling fluid. The apparatus may further include at least one tank for supplying a particle separation assisting agent to the input of the second rotational particle separator when said drilling fluid is a water based drilling fluid. 
         [0015]    According to a further embodiment of the present invention, there is provided a method for separating solids from liquids in a drilling operation. In the drilling operation, used drilling fluid is screened into a substantially solid portion and a substantially liquid drilling fluid, wherein the drilling fluid is stored in a storage tank. The method comprises depositing the solid portion in a recirculation tank having a second liquid and drawing off the solid portion and the second liquid from the recirculation tank for delivery to a first rotational particle separator. The method further comprises separating the solid portion from the second liquid in a first rotational particle separator and collecting the separated solids in a bin region and deposing the second liquid back in the recirculation tank. The method may further include removing solid particles from a portion of the screened drilling fluid in a second rotational particle separator and returning the liquid portion to a collection tank, wherein the solids are deposited in a bin region. 
         [0016]    Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    In drawings which illustrate embodiments of the invention, 
           [0018]      FIG. 1  is a schematic view of a drilling system incorporating the drilling fluid cleaning system of the present invention. 
           [0019]      FIG. 2  is a perspective view of a preferred embodiment of the drilling fluid cleaning system according to the present invention. 
           [0020]      FIG. 3  is a top plan view of the drilling fluid cleaning system of  FIG. 2 . 
           [0021]      FIG. 4  is cross sectional view of the recirculation tank of  FIG. 2  taken along the line  4 - 4  of  FIG. 3 . 
           [0022]      FIG. 5  is cross sectional view of the shale bin and two polymer tanks of  FIG. 2  as taken along the line  5 - 5  of  FIG. 3 . 
           [0023]      FIG. 6  is cross sectional view of the shale bin of  FIG. 2  as taken along the line  6 - 6  of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0024]    Referring to  FIG. 1 , there is shown a schematic view of a well drilling system  10 , which includes a drill rig  12 , and a transportable drilling fluid cleaning system  40 . The drilling rig includes a drill derrick  14  supporting a drill string  16 , which is rotated to drill a well bore into the ground. A mud storage tank  20  is associated with the drilling rig and stores drilling fluid  18 . The composition of the drilling fluid  18  can be adjusted depending on the stage to which the well bore is drilled. The apparatus of the present invention can be used to remove undesirable solids from the drilling mud and to recycle the drilling fluid to the tank  20  at a desired viscosity. 
         [0025]    The drilling fluid is pumped in a conventional manner from the tank  20  through a conduit  22  into drill string  16 . Drilling fluid  18  flows downwardly through the drill string to exit from the lower end of the string at bit  17 . The drilling fluid  18  acts to lubricate the drill bit and collect cuttings created by the drilling action of the bit. The drilling fluid with additional solids flows upwardly in the well bore externally to the drill string to be collected near the surface. The collected drilling fluids/solids mixture passes through conduit  24  to be delivered to a shale shaker  26 . Shale shaker is a vibrating screen positioned above the tank  20  that removes larger solids from the drilling fluid and delivers cleaned drilling fluid back to the tank  20 . The larger solids are delivered to the apparatus for further removal of drilling fluid for collection. Drilling fluid from the tank  20  may also be pumped by conventional means to the cleaning system  40  for further removal of smaller particles via conduit  28  wherein cleaned drilling fluid is returned to the tank  20  via conduit  30 . 
         [0026]      FIG. 2  shows a side elevation view of a preferred embodiment of the drilling fluid cleaning system  40 . The system includes a recirculation tank  50 , a shale bin  70 , a first centrifuge  92  and a pump house  100 . The system may also include a second centrifuge  94  and a plurality of strip tanks and polymer tanks  102  and  104  respectively. The above components may be arranged on a platform, preferably in the form of a skid  42 , to permit loading of the system onto a trailer towable by a truck for transport of all of the components as a unit in a single trip by roads between drilling sites. In the province of Alberta, Canada, where the inventors are based, skid  42  can be dimensioned to a maximum size of 63 feet long, 12.5 feet wide and 17.5 feet high in order to be transportable on roads and highways as a non-divisible load. A skid of this size requires a special permit for oil field hauling, however, a pilot vehicle to lead the truck is not required. Skids of different sizes may be permitted in other jurisdictions. 
         [0027]    The centrifuges  92  and  94  may be located on a secondary platform  96  above the shale bin and recirculation tank. The secondary platform  96  may also be moveable between a stored position during transportation and an operating position. The secondary be located above the shale bin  70  and recirculation tank  50  by means of telescoping arms  98  wherein the telescoping arms facilitate movement between the stored and operating positions. It will be appreciated that other means of moving the secondary platform between the stored and operating positions, such as, for example, by means of supporting the secondary platform on rotatable arms, will also be acceptable. 
         [0028]      FIG. 3  shows a plan view of the fluid cleaning system  40  with the centrifuges removed showing details primarily of recirculation tank  50 , shale bin  70  and strip tanks and polymer tanks  102  and  104 , respectively. The recirculation tank and shale bin are located at a first end  44  of the skid  42 . The pump house  100  is located at a second end  46  of the skid  42  and includes various pumps and ancillary equipment for use in the fluid cleaning operation including pump  48 . The strip tanks and polymer tanks  102  and  104 , respectively, are located intermediate the pump house  100  and the shale bin  70  and recirculation tank  50 . 
         [0029]    In the preferred embodiment shown in  FIG. 3 , the recirculation tank  50  is located longitudinally along the skid  42  adjacent to the first end  44  of the skid. The shale bin  70  is located adjacent to the recirculation tank  50  and is substantially coterminous along the longitudinal length of the skid  42 . The recirculation tank  50  comprises a substantially rectangular container region defined by first and second longitudinal walls  52  and  54  respectively, first and second end walls  56  and  58 , respectively, and a bottom  60 . 
         [0030]    Turning now to  FIG. 4 , a cross sectional view of the recirculation tank is shown along the line  4 - 4  from  FIG. 3 . As shown in  FIG. 4 , the recirculation tank includes an opening  62  which may be connected to a conduit  47  as shown in  FIG. 5 . The conduit connected to the opening  62  may be further connected to a pump  48  of conventional means which serves to supply a flow of the solid particles and drilling fluid in the recirculation tank to the first centrifuge  92 . In a preferred embodiment as shown in  FIG. 4 , the bottom  60  may be angled so as to direct any particles settling on the bottom to the opening  62  for pickup and processing by the centrifuges. The recirculation tank  50  may also include a cover  64  that is positionable over the recirculation tank as shown in  FIG. 6 . Cover  64  may be connected to the recirculation tank by a hinge or other suitable pivot  66  so that the cover may be opened to permit depositing of solid material in the recirculation bin or closed so as to aid in depositing of solid material directly into the shale bin  70 . In a preferred emobidment as shown in  FIG. 6 , the cover  64  opens inwardly into the recirculation tank  50 . The cover  64  may be retained in a closed position over the recirculation tank  50  by any suitable means, such as, for example, by a chain (not shown) suspended from the secondary platform  96  connected to the free end  67  of the cover  64  at an appropriate position to substantially cover the recirculation tank. It will be appreciated by those of skill in the art that other methods of retaining the cover in a closed position may also be applied to the present apparatus. 
         [0031]    The recirculation tank  50  is sized such that the addition of solid particles and cleaned drilling fluid at the top and the removal of the same from the bottom produces a sufficient agitation as to prevent the settling of a large quantity of particles before being drawn into the opening  62 . In practice, the applicant has found that a distance of approximately about  18  inches or less between the first and second longitudinal walls  52  and  54 , respectively, is sufficient to prevent excessive settling of any solid particles in the recirculation tank with a distance of  12  inches being preferred. 
         [0032]    Referring back to  FIG. 1 , drilling fluid pumped out of the recirculation tank  50  is directed to the first centrifuge  92  for further solids separation. The first centrifuge separates the solids from the drilling fluid/solid particles mixture and deposits the solid particles in the shale bin  70 . The cleaned drilling fluid is then returned to the recirculation tank  50 . Furthermore, in a preferred embodiment, the system also includes a second centrifuge for separating the solids from the drilling fluid in the mud storage tank  20  of the drilling rig. The solid/drilling fluid in the mud storage tank  20  is pumped by a conventional pump to the centrifuge. The centrifuge separates the solids from the liquids and deposits the solids in the shale bin  70 . The cleaned drilling fluid is then returned to the mud storage tank  20 . 
         [0033]    Solids removed from the drilling fluid by centrifuges  92  and  94  as well as solids not requiring centrifuging are preferably stored in a shale bin  70  adjacent to the first end  44  of the skid  42 . Bin  70  is defined by four walls and floor  80  at a region of the skid adjacent to the recirculation tank  50 . The first and second walls  72  and  74  respectively of the bin  70  are transverse to the longitudinal length of the skid  42  while the third and fourth walls  76  and  78  are substantially parallel the longitudinal length of the skid. Preferably, as shown in  FIGS. 5 and 6 , the bottom of each of the walls of the bin  70  may be offset towards the center of the bin so as to angled the wall. Alternatively, the walls of the bin  70  may be substantially vertical or the fourth wall  78  may alternatively include a hinged bottom to facilitate access for removing solid particles from the bin  70 . 
         [0034]    In certain circumstances during drilling, for example when the drilling fluid is being changed from a water based drilling fluid to an oil based drilling fluid, it may be desirable to use a flocculating agent to promote the removal of solids from the drilling fluid. To address this need, the system of the present invention may include a flocculent source for adding a flocculating agent to the drilling fluid. Preferably, the flocculent source comprises at least one compartment for holding and mixing a flocculating agent and a delivery system to deliver flocculating agent to the centrifuges. Preferably, flocculating agent is added to the drilling fluid at the inlets of pumps supplying the centrifuges so the agent is mixed with the drilling fluid prior to centrifuging. Flocculating agents are conventional and may include a calcium water solution or a polymer based flocculating agent. According to a preferred embodiment, the fluid cleaning system  40  of the present invention includes two strip tanks  102  for holding a calcium water solution and two polymer tanks  104  for holding a polymer flocculating agent. 
       Operation 
       [0035]    In use, the drilling fluid cleaning system  40  of the present invention is operated according to different schemes depending on the drilling stage. 
         [0036]    During drilling of the “surface hole” (the first portion of the borehole), water based drilling fluid is commonly used to protect groundwater aquifers. During drilling of the surface hole, larger particles may be deposited directly into the shale bin  70  and the cover  64  of the recirculation tank positioned over the recirculation tank. The large particles may thereby slide over the cover  64  and into the shale bin  70 . The second centrifuge  94  may also be used to remove smaller particles from the drilling fluid  18  from the mud storage tank  20  whereby the particles are deposited in the shale bin  70  and the cleaned drilling fluid returned to the mud storage tank  20 . 
         [0037]    During changeover of the drilling fluid from a water based drilling fluid to an oil based drilling fluid, the recirculation tank  50  may remain covered by cover  64 . The drilling fluid  18  in the mud storage tank  20  may be pumped into the second centrifuge  94  to remove any particles in the drilling fluid. In addition, calcium water from the strip tanks  102  or a polymer flocculating agent from the polymer tanks  104  may be added to the inlet of the centrifuge  94  along with the drilling fluid to enhance the separation of the solid particles from the drilling fluid. Thereafter the solid particles may be deposited in the shale bin  70  while the drilling fluid is returned to one or more of the strip tank  102 , polymer tank  104  or mud storage tank  20 . When the water based drilling fluid is sufficiently cleaned of particles it may be disposed of in a sump on or off site. 
         [0038]    During drilling with oil based drilling fluid, the cover  64  to the recirculation tank  50  may be positioned off of the recirculation tank and the larger particles from the shale shaker  26  may be deposited in the recirculation tank. As previously indicated, the recirculation tank  50  contains a second fluid, which may be a drilling fluid similar to the drilling fluid as is used to drill the well. The drilling fluid and solid particles are drawn out of the recirculation tank  50  at opening  62  and passed through the first centrifuge  92 . The centrifuge removes the solids and deposit them into shale bin  70  and returns the drilling fluid to the recirculation tank  50 . The second centrifuge  94  draws the drilling fluid  18  from the mud storage tank  20 , removes the solids from the drilling fluid and returns the drilling fluid to the tank  20 . The solids are thereafter deposited in the shale bin  70 . The solids in the shale bin may thereafter be removed for land spreading or disposal by other means. 
         [0039]    It will be appreciated that as the foregoing equipment is located on a field transportable skid, transportation to and from a drilling site is greatly simplified as compared to the transportation of various equipment separate from each other. In addition, the close proximity of all of the equipment set out above will greatly facilitate the switch over between different types of drilling fluid as well as switching over from removing solids from the drilling fluid to removing the water based drilling fluid from the drilling system. 
         [0040]    While specific embodiments of the invention have been described and illustrated, such embodiments should be considered illustrative of the invention only and not as limiting the invention as construed in accordance with the accompanying claims.

Summary:
In a drilling operation, where used drilling fluid is screened into a substantially solid portion and a substantially liquid drilling fluid and the drilling fluid is thereafter stored in a storage tank. A method and apparatus for of separating solids from liquids. The apparatus comprises a recirculation tank having a second liquid for receiving the solid portion, and a first rotational particle separator for separating the solid portion from the second liquid. The apparatus further includes a pump for pumping the solid portion and the second liquid from the recirculation tank to the first rotational particle separator and a bin region to receive and retain the separated solids from the first rotational particle separator, wherein the separated second liquid is returned to the recirculation tank.