Abstract:
A transportable drilling fluid cleaning system for removing solids from drilling fluid at a drill site comprises a platform for transporting the system. A bin region on the platform retains solids from the drilling fluid. A settling tank on the platform separates the drilling fluid into an upper fluid fraction having a reduced concentration of solids and a lower solids fraction having a higher concentration of solids as the drilling fluid flows from an inlet chamber for receiving drilling fluid to at least one other chamber. A stand on the platform supports at least one centrifuge for separating the solids from the drilling fluid, the stand being movable between stored and operating positions. The system provides a self-contained unit that is easily transportable on a flat bed truck to provide all the ancillary equipment necessary for solids control at the drill site.

Description:
This application claims priority to Canadian Patent Application No. 2,414,321, entitled “Shale Bin/Settling Tank/Centrifuge Combination Skid,” filed on Dec. 13, 2002, which is hereby incorporated by reference. 
   FIELD OF THE INVENTION 
   This invention relates to apparatus for separating solids from fluids. In particular, the apparatus is useful for removing solids from drilling fluids used in well drilling operations. 
   BACKGROUND OF THE INVENTION 
   Wells for recovering oil, gas and the like are typically created by drilling into an underground source using a hollow drill string supported in 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 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 solids. The viscosity or solids content of the drilling fluid can be varied depending on the stage of the drilling process and the location of the drill bit below the surface. 
   Equipment and methods for handling the drilling fluid to remove solids in order to recycle the fluid are well known. Settling tanks, shale shakers, flocculating tanks and centrifuge or cyclone separators can be interconnected to handle the drilling fluids from a drill site and separate the used drilling fluid and undesirable solids for subsequent recycling of the drilling fluid. Canadian Patent No. 2,219,053 issued May 25, 1999 to Grand Tank (International) Inc. discloses an arrangement of settling tanks and centrifuges to achieve this result. 
   Presently, a mud storage tank to hold drilling fluid and a shale shaker to perform screening of larger solids tend to be standard equipment for a drilling rig. In normal well site operation, drilling fluid is circulated out of the a 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. The rest of the equipment for solids handling including a settling tank, a shale bin for collecting solids for disposal, centrifuges and a flocculent tank tends to be available as individual pieces of equipment that must be delivered to the well site in separate loads. It is important to choose compatible equipment that is interconnectable and that is properly sized to be of appropriate capacity to work with other selected equipment. Once delivered to the site, the various pieces of equipment must be assembled together. Generally, a picker truck is needed to lift the centrifuge equipment onto a conventional raised centrifuge stand. It requires superior organization and scheduling skills to ensure the components of a solids handling system are delivered in a timely manner to the work site and assembled into an efficient and reliable solids handling system. 
   SUMMARY OF THE INVENTION 
   There is a need for a solids handling system for drilling fluid at a well site that avoids the logistical and assembly problems of the prior art. 
   The present invention provides a self-contained apparatus for cleaning drilling fluids that is mountable on a platform for convenient transport by a conventional truck trailer between well sites as a single load. The components of the apparatus are sized and positioned to operate together in an efficient manner under various working configurations in which some or all of the components are used depending on the stage of the well being drilled. 
   Accordingly, the present invention provides a transportable drilling fluid cleaning system for removing solids from drilling fluid at a drill site comprising: 
   a platform for transporting the cleaning system to a drill site; 
   a bin region on the platform to retain solids from the drilling fluid; 
   a settling tank on the platform having an inlet chamber to receive drilling fluid and at least one other chamber, the settling tank acting to separate the drilling fluids into an upper fluid fraction having a reduced concentration of solids and a lower solids fraction having a higher concentration of solids as the drilling fluid flows from the inlet chamber to at least one other chamber; and 
   a stand on the platform to support at least one centrifuge for separating the solids from the drilling fluid, the stand being movable between a stored position during transport of the platform and an operating position. 
   The platform is preferably in the form of a skid sized to be readily transportable on a conventional truck semi-trailer. The maximum dimensions for a loaded trailer in the province of Alberta, Canada to permit navigation on roads and highways are a length of about 63 feet, a width of about 12.5 feet and a height of about 17.5 feet. The components of the present invention are sized to take maximum advantage of these dimensions. 
   In a further aspect, the present invention provides apparatus for removing solids from a fluid/solid mixture comprising, in combination: 
   a platform; 
   a region defined on the platform to retain solids; 
   a settling tank on the platform having at least an inlet chamber for receiving the mixture and an outlet chamber, the settling tank acting to separate the mixture into an upper fluid fraction and a lower solids fraction as the mixture flows from the inlet chamber to the outlet chamber; and 
   at least one centrifuge on the platform in fluid communication with the settling tank and movable between a stored position and an operating position. 
   In a preferred arrangement, a flocculent source is also provided on the platform for adding a flocculating agent to the drilling fluid to promote removal of solids from the drilling fluid. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Aspects of the present invention are illustrated, merely by way of example, in the accompanying drawings in which: 
       FIG. 1  is a schematic view of a drilling system incorporating the transportable drilling fluid cleaning system of the present invention; 
       FIG. 2  is a perspective view of a preferred embodiment of the transportable drilling fluid cleaning system; 
       FIG. 3  is a top plan view of the cleaning system of  FIG. 2 ; 
       FIG. 4  is a longitudinal section view taken along line  4 — 4  of  FIG. 3 ; 
       FIGS. 5   a - 5   g  are section views taken along the indicated section lines of  FIG. 3  showing various details of the cleaning system; and 
       FIGS. 6   a - 8   c  are detail views of various elevatable platforms for supporting the centrifuge according to the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIG. 1 , there is shown a schematic view of a well drilling system  2  which includes a drill rig  4  and a transportable drilling fluid cleaning system  6  according to the present invention. Drill rig  4  includes a drill derrick  8  supporting a drill string  10  which is rotated to drill a well bore into the ground. A tank  12  associated with drill rig  4  stores drilling fluid  14 . The viscosity of drilling fluid  14  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 undesired solids from the drilling mud and to recycle the drilling fluid to tank  12  at a desired viscosity. 
   Drilling fluid  14  is pumped in a conventional manner through conduit  16  into drill string  10 . Fluid  14  flows downwardly through the drill string to exit from the lower end of the string at bit  11 . Drilling fluid  14  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 fluid/solids mixture passes through conduit  18  to be delivered to shale shaker  20 . Shale shaker  20  is a vibrating screen positioned above tank  12  that removes solids from the drilling fluid and delivers cleaned drilling fluid back to tank  12 . Depending on the stage of drilling and the volume of drilling fluid being used, shaker  20  may be able to remove solids to an extent such that the drilling fluid simply drains through the shaker directly into tank  12  for re-use. In other cases, some or all of the drilling fluid that passes through shaker  20  is diverted via conduit  18 a for delivery to the drilling fluid cleaning apparatus  6  of the present invention. Apparatus  6  removes solids from the drilling fluid  14  by settling, centrifuging or a combination of methods to deliver cleaned drilling fluid back to tank  12  via conduit  22 . 
     FIG. 2  shows a perspective view of a preferred embodiment of the transportable drilling fluid cleaning apparatus  6  of the present invention. The system includes a settling tank  30 , a bin  32  to retain solids extracted from the drilling fluid, at least one centrifuge  34 , and, optionally, a flocculent source  36 . The above components are arranged on a platform, preferably in the form of a skid  38 , to permit loading of the system onto a trailer towable by a truck for transport of all the components as a unit in a single trip by road between drilling sites. In the province of Alberta, Canada, where the inventors are based, skid  38  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 or follow the truck is not required. Skids of different sizes may be permitted in other jurisdictions. 
     FIG. 3  is a plan view of apparatus  6  with the centrifuges removed showing details primarily of settling tank  30 . Settling tank  30  occupies the largest area on skid  38  extending over most of the length of the skid from bin  32  to an enclosure  40  housing at least one pump that acts to move drilling fluid from the settling tanks to the centrifuges of the apparatus. Settling tank  30  is divided into an inlet chamber  30   a  and at least one other chamber by baffle walls  42  that extend transversely between tank side walls  45  at intervals between end walls  46  and  47 . In the illustrated embodiment, settling tank  30  is formed with three chambers: an inlet chamber  30   a , an intermediate chamber  30   b , and an end chamber  30   c . Depending on the size of skid  38  and the solids content of the drilling fluid, additional or fewer chambers can be formed as will be apparent to a person skilled in the art. The chambers of the settling tank can be formed to be substantially equal in volume. If chambers of differing size are used, inlet chamber  30   a  may be the largest chamber as it experiences the largest solids load. By way of example, it is possible to construct a settling tank having a volume ranging between about 1050 to about 2100 cubic feet on a skid that is dimensioned to be transportable on roads by truck trailer. Such a tank can be divided into three equal chambers of about 350 to 700 cubic feet. The overall dimensions of such a settling tank are 40 feet long and 11.5 feet wide with the height being varied between about 2.5 feet and about 5 feet to accommodate the range of tank volumes. 
   Inlet chamber  30   a  receives drilling fluid via conduit  18   a  as best shown in FIG.  1 . Conduit  18   a  can be a large diameter hose from mud storage tank  12  through which drilling fluid  14  is pumped. As drilling fluid flows from inlet chamber  30   a  through the various additional chambers, settling tank  30  acting to separate the drilling fluid into an upper fluid fraction having a reduced concentration of solids and a lower solids fraction having a higher concentration of solids. Each baffle  42  is formed with at least one passage  49  formed thereon adapted to deliver drilling fluid that overflows into the passage from a first upstream chamber to adjacent a floor of a second downstream chamber. For example, as best shown in  FIG. 4 , which is a longitudinal section view taken along line  4 — 4  of  FIG. 3 , drilling fluid which fills chamber  30   a  to level  50  overflows into passage  49  adjacent baffle  42  and flows in the direction indicated by arrow  52  to emerge at the bottom of chamber  30   b . Baffles  42  and passages  49  serve to slow the flow of drilling fluid through the chambers of settling tank  30  to allow solids in the drilling fluid time to settle due to gravity and concentrate toward the bottom of each chamber. Therefore, the drilling fluid at the top of each chamber tends to have a lower solids concentration, and it is this fluid that overflows to the next chamber with the result that the drilling fluid has less solids as it travels to downstream chambers. Inlet chamber  30   a  tends to have the greatest solids load while subsequent chambers see increasingly smaller loads. 
   As best shown in  FIGS. 3 and 4 , it is preferable that each baffle  42  includes two spaced, vertically aligned passages  49  although other arrangements are possible depending on the volume of fluid to be treated and the desired residence time in each chamber. Preferably, each passage  49  includes a cover  49   a  which is pivotable to seal or open the passage to control flow therethrough. Passages  49  are also shown in section view  5   a  taken along line  5   a — 5   a  of FIG.  3 . 
   Referring to  FIG. 3 , each chamber of the settling tank includes a collection area or sump  65  in the floor of the chamber in which the lower solids fraction of the drilling fluid tends to concentrate. As best shown in  FIGS. 5   a - 5   e , the lower portion  45   a  of tank side walls  45  are preferably angled inwardly to direct settling solids to sump  65 . Sump  65  provides a convenient location into which various conduits extend for collection of drilling fluid for further processing. 
   Adjacent each sump  65 , there is an inlet  66  to a collection conduit  68  that preferably runs along a side wall  45  of the settling tank.  FIG. 5   b , which is a section taken along line  5   b — 5   b  of  FIG. 3 , shows in detail inlet  66  extending from sump  65  to collection conduit  68 . Each inlet  66  includes a manually operable valve  67  to open or close the inlet. Valve  67  can be operated from above the settling tank by control  67   a  extending upwardly from the valve.  FIG. 3  shows collection conduit  68  extending through baffles  42  to communicate sump  65  of each chamber with at least one pump located in pump enclosure  40  at the end of the skid. In the illustrated embodiment, enclosure  40  houses first and second pumps  69 ,  70 , respectively, which act to collect the lower solids fraction of the drilling fluids from the chambers and deliver the fluid to the centrifuges for further solids separation. Drilling fluid is pumped to the centrifuges via lines  72 , 73  which extend generally along the longitudinal axis of skid  38  above settling tank  30  back to the platform that supports the centrifuges over inlet chamber  30   a . Depending on the volume of drilling fluid to be handled only a single pump and/or centrifuge may be necessary. 
   Collection conduit  68  is preferably formed from two pipelines: a first pipeline  68   a  communicates inlet chamber  30   a  to first pump  69  and a second pipeline  68   b  communicates the remaining chambers  30   b  and  30   c  with second pump  70 . As best shown in the section views of  FIGS. 5   a  to  5   e , pipelines  68   a  and  68   b  are preferably stacked vertically as they extend along the settling tank to pumps  69 ,  70 . Pipeline  68   a  and pump  69  service only inlet chamber  30   a  as this chamber experiences the greatest solids loads and will tend to require a dedicated centrifuge at the end of line  73 . In contrast, intermediate chamber  30   b  and end chamber  30   c , downstream from inlet chamber  30   a , are subjected to progressively lesser solids loads since they are processing drilling fluid that has already had solids removed. Therefore, these subsequent chambers are adequately serviced by second pipeline  68   b  and pump  70  which feed the combined flows from chambers  30   b  and  30   c  to a second centrifuge via line  72 . 
   Referring to FIG.  3  and  FIG. 5   b , depending on the solids load of the drilling fluid, some or all of the chambers of settling tank  30  may not be required. To accommodate this situation, at least one chamber of the settling tank preferably includes a bypass conduit  80  having an inlet  82  external to the tank that communicates with collection conduit  68   a  or  68   b . Drilling fluid  14  from mud tank  12  would be pumped via a hose directly into a bypass conduit  80  to bypass the chamber and feed directly into the conduit  68  for delivering to the centrifuges. In the illustrated embodiment, inlet chamber  30   a  and intermediate chamber  30   b  are provided with bypass conduits  80 . 
   Referring to FIG.  3  and  FIG. 5   c , it is preferable that each chamber  30   a ,  30   b  and  30   c  of the settling tank include a drainage conduit  60  to permit drainage of drilling fluid from the chamber.  FIG. 5   c  which is a section view taken along line  5   c — 5   c  of  FIG. 3  shows a typical drainage conduit  60  which extends from the floor of the chamber adjacent sump  65  to the top edge  62  of side wall  45  of the settling tank. Each drain conduit  60  includes a terminal coupling  63  with a manually operable valve by which the conduit can be connected to a hose for withdrawing drilling fluid from the chamber. Drain conduits  60  are typically used to drain the chambers of the settling tank prior to moving the system of the present invention to a new drilling site. 
   Drilling fluid that is cleaned within settling tank  30  needs to be pumped back to mud tank  12  via conduit  22  ( FIG. 1 ) to complete one possible path of the solids removal cycle of the present invention. This pumping action is performed by at least one pump in the settling chamber. Preferably, the pump comprises a floating pump  85  situated in end chamber  30   c  as best shown in  FIG. 5   d  which is a cross-section taken along line  5   d — 5   d  in FIG.  3 .  FIG. 5   d  also shows a pivoting boom structure  88  by which floating pump  85  can be lifted into and out of end chamber  30   c.    
   Depending on the volume of drilling fluid that requires recycling generated at a well site, it may be necessary to use more than one system according to the present invention. It is possible to connect together multiple cleaning systems according to the present invention in modular fashion to increase the drilling fluid handling capacity. At least one chamber of the settling tank other than inlet chamber  30   a  includes an outlet conduit  90  to permit connection in series to an additional drilling fluid cleaning system. Referring to FIG.  3  and  FIG. 5   e , which is an end section taken along line  5   e — 5   e  of  FIG. 3 , outlet conduit  90  preferably extends from end chamber  30   c  and is housed within pump enclosure  40 . Outlet conduit  90  includes a valve assembly  91  to control flow of fluid through an attached hose which feeds directly into the inlet chamber of an additional adjacent drilling fluid cleaning system according to the present invention. 
   Drilling fluid pumped by pumps  69  and  70  is directed to centrifuges for further solids separation. Referring to  FIG. 2 , the illustrated embodiment employs two centrifuges  34   a  and  34   b  which are conventional units. As previously mentioned, depending on the volume of drilling fluids to be processed and the solids content of the drilling fluid, only a single centrifuge may be necessary. Centrifuges  34   a  and  34   b  are supported on a stand  120  that is movable between a stored position during transport of the cleaning system and an operating position when the system is working at a well site. Centrifuges  34   a  and  34   b  are sized to match the capacity of the pump delivering drilling fluid via conduits  72  and  73 . For example, if pump  69 , which pumps drilling fluid from inlet chamber  30   a  is rated for 1000 litres/minute, centrifuge  34   a  to which the fluid is delivered should be able to handle this volume. Pump  70  delivers drilling fluid from the other chambers  30   b  and  30   c  to centrifuge  34   b . The cleaned drilling fluid from the centrifuges is preferably fed back into mud storage tank  12  via line  114  as best shown in FIG.  1 . Alternatively, the cleaned fluid from the centrifuges can be directed back into inlet chamber  30   a  to reduce the solids loading on the inlet chamber. 
   Stand  120  that supports centrifuges  34   a  and  34   b  includes an elevatable surface  122  mounted to an elevating system for the surface between the operating position in which the surface is raised above the settling tank and the stored position in which the surface is lowered adjacent the settling tank to reduce the overall height of the unit so that the apparatus is sufficiently compact for transportation by semi-trailer on roads or highways. The elevating system for surface  122  can be constructed according to various different arrangements. Some specific preferred arrangements are described below, however, the system of the present invention is not limited to these specific arrangements. 
     FIGS. 6   a  and  6   b  show the operation of a centrifuge stand  120  having an elevating system that relies on a plurality of telescoping legs  124 . Four legs  124  extend between the side walls  45  of settling tank  30  and the corners of elevatable surface  122 . Each leg includes an internal hydraulic cylinder  126  that is extendable to move an upper leg portion  128  to which surface  122  is mounted upwardly with respect to a lower fixed leg portion  130  as shown in  FIG. 6   a . When cylinder  126  is collapsed, surface  122  is moved downwardly to the stored position as shown in  FIG. 6   b.    
     FIGS. 7   a  to  7   c  illustrate an alternative arrangement in which the elevating system comprises a plurality of pivotable legs  132  pivotable by an actuating system between a generally horizontal alignment corresponding to surface  122  being in the lowered, stored position ( FIG. 7   c ) and a generally vertical alignment corresponding to the surface being in the raised operating position ( FIG. 7   a ).  FIG. 7   b  shows an intermediate position to clearly show the operation of the elevating system. Preferably, surface  122  supporting the centrifuges is supported by four pivotable legs  132  at the corners of the surface. Each pivotable leg  132  is pivotally connected to the settling tank at a lower end  134  and pivotally connected to the surface at an upper end  136 . Pivoting of the legs is controlled by an actuating system comprises at least one cable  138  of variable length. A first end  140  of cable  138  is attached to at least one of the legs. Preferably, the cable is attached to two of the legs supporting surface  122 . Cable  140  is extendable to allow legs  132  to pivot to their generally horizontal alignment ( FIG. 7   c ) and retractable to pull the legs to their generally vertical alignment ( FIG. 7   a ). 
   Cable  140  is attached to a piston arm of a cylinder  142  at a second end  144  of the cable, and the cable extends and retracts by virtue of extending and retracting of the piston arm. Preferably, cylinder  142  is mounted externally to the end wall  46  of settling tank  30  in a horizontal orientation. Movement of cable  140  is guided by appropriately placed sheaves along the cable path. 
     FIGS. 8   a  to  8   c  illustrate a still further elevating system that is similar to the elevating system of  FIGS. 7   a  to  7   c  in that the arrangement relies on pivotable legs  132  that are pivotally connected to the settling tank at a lower end  134  and pivotally connected to surface  122  at an upper end  136 . The cable actuating system is replaced by at least one telescoping cylinder  150  pivotally connected at each end of the cylinder to surface  122  and at least one of the pivotable legs  132 . The telescoping cylinder is extendable and retractable to control the angle between surface  122  and the at least one pivotable leg in order to raise ( FIG. 8   a ) and lower the surface ( FIG. 8   c ). Preferably, there are at least two telescoping cylinders  150  associated with two pivotable legs. 
   As shown in  FIG. 8   a , it is preferably that an access ladder  154  and walkway  156  are mounted to at least one of the pivotable legs  132  to permit access to the centrifuge stand when it is in the raised, operating position. Ladder  154  and walkway  156  are mounted to legs  132  such that the ladder and walkway are positioned within the settling tank when the legs are pivoted to the generally horizontal alignment shown in  FIG. 8   c.    
   In certain circumstances during drilling, 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  36  for holding and mixing a flocculating agent and a delivery system to deliver flocculating agent to the centrifuges and/or the settling tank. Preferably, flocculating agent is added to the drilling fluid at the inlets of pumps  69  and  70  so the agent is mixed with the drilling fluid prior to centrifuging. Alternatively, flocculating agent can be added to one or more chambers of the settling tank. 
   As best shown in  FIGS. 2 and 3 , the flocculent source preferably comprises two compartments  36   a ,  36   b  for holding and mixing flocculating agent. In the illustrated example, compartments  36   a ,  36   b  both deliver flocculating agent to the inlet of pumps  69 ,  70 . Alternatively, first compartment  36   a  may deliver flocculating agent to the inlets of pumps  69  and  70  while second compartment  36   b  may deliver flocculating agent to the inlet chamber of the settling tank. Compartments  36   a , 36   b  are preferably positioned above the settling tank over end chamber  30   c.    
   Solids removed from the drilling fluid by centrifuges  34   a  and  34   b  and solids from shale shaker  20  are preferably stored in a bin  32  adjacent the same end of skid  38  as the centrifuges. Collection pipes  68   a  and  68   b  within the settling tanks tend to collect all the settled solids from chambers  30   a ,  30   b  and  30   c  for delivery to pumps  69 ,  70  and the centrifuges. Bin  32  is defined by four walls and a floor at a region of the skid adjacent inlet chamber  30   a  and centrifuges  34   a , 34   b .  FIG. 5   f  is an end view of skid  38  taken along line  5   f — 5   f  of  FIG. 3  showing an end wall  100  of bin  32 . The opposite end wall of the bin is wall  46  shared with inlet chamber  30   a . Side wall  102  of bin  32  is fixed while the opposite side wall  104  is pivotally mounted to move between a first raised position to retain solids dumped into the bin ( FIG. 5   f ) and a second, lowered position to define a ramp for access to the bin for periodic removal of solids ( FIG. 5   g ). When wall  104  is in its lowered position as a ramp, a front end loader or other suitable equipment can be driven up the ramp to permit efficient access to the bin interior. Preferably, wall  104  is formed with transverse ridges  105  that act as traction bars for vehicles entering the bin when in the lowered position. Preferably, the floor  106  of bin  32  is sloped away from pivotally mounted wall  104  to ensure that solids tend to collect away from the pivoting connection of wall  104 . 
   In use, the drilling fluid cleaning system of the present invention is operated according to different schemes depending on the drilling stage. 
   During drilling of the “surface hole” (the first portion of the borehole), relatively high viscosity drilling fluid is used to protect groundwater aquifers. During drilling of the surface hole, only the shale bin of the system is typically used. This involves sending solids retained on the shale shaker to the shale bin as indicated by arrow  110  on FIG.  1 . 
   During drilling of the “floc-water section” (the middle portion of the borehole), relatively low viscosity drilling fluid is used in order to maximize the rate of penetration. During drilling of the floc-water section, all of the components of the system are typically used. This involves sending solids from the shale shaker to the shale bin  32 , and also circulating drilling fluid from mud storage tank  12  to settling tank  30 , and centrifuges  34 , and then back to the mud storage tank. Solids from the centrifuges  34  are also sent to the shale bin as indicated by arrow  112  in FIG.  1 . 
   During drilling of the “mud-main hole” (the bottom portion of the borehole), the producing formation is penetrated and the viscosity and weight of the drilling fluid must be carefully monitored. During the drilling of the main hole, only the centrifuges  34  and the shale bin  32  of the system are typically used. This involves sending drilling fluid from mud storage tank  12  directly to the centrifuges  34  for treatment using bypass conduits  80 , and then returning the treated drilling fluid to the mud storage tank as shown by arrow  114  in FIG.  1 . Solids collected on the shale shaker and solids from the centrifuges are sent to shale bin  32 . 
   While the apparatus of the present invention has been described in an environment in a well drilling environment, it is contemplated that the apparatus can be used in any situation where there is a requirement for high volume removal of solids from a fluid/solid mixture in order to recycle the fluid portion. 
   Although the present invention has been described in some detail by way of example for purposes of clarity and understanding, it will be apparent that certain changes and modifications may be practised within the scope of the appended claims.