WO2013075252A1

WO2013075252A1 - Solids control system and method

Info

Publication number: WO2013075252A1
Application number: PCT/CA2012/050851
Authority: WO
Inventors: James Ingalls; John MARTY
Original assignee: Strad Energy Services Ltd.
Priority date: 2011-11-23
Filing date: 2012-11-23
Publication date: 2013-05-30

Abstract

A solids control system separates solids from used drilling mud. The system is a closed loop system arranged on a single skid. First and second separators are located at opposing ends of the skid. Depending upon the nature of the solids content, the used drilling mud is directed either to the first and second separator is series, where the drilling fluid is weighted, or to at least the second separator if the drilling fluid is light. The at least second separator could be both the first and second separators in parallel. Dewatering agents can be added to the used drilling mud before one or both of the separators. The elevation of the separators can be raised and lowered over the collection vessels so as to accommodate various rig equipment.

Description

SOLIDS CONTROL SYSTEM AND METHOD

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefits under 35 U.S.C 1 19(e) of US Provisional Application Ser. No. 61/563,473, filed November 23, 201 1 , which are incorporated fully herein by reference.

FIELD

The embodiment herein relate generally to systems and methods for removing solids from drilling fluid having variable solids content, the system having a minimum footprint and producing component for drilling fluid reuse.

BACKGROUND

Drilling fluids are used in hydrocarbon well drilling, as well known in the prior art, to collect solid cuttings and debris. The solids and debris must be removed if the fluids are to be re-used. Drilling fluid, also called "mud," is typically either water-based or oil-based. "Oil" includes, but is not limited to, diesel, crude oil, mineral oil and synthetic oil. Typically a mud is mixed with various additives and is pumped downhole through a hollow drill string, into a well being drilled, and exits through the drill bit. The mud picks up drilled solids including cuttings (rock), other solids, and various contaminants, such as, but not limited to, crude oil, water influx, and salt from the well and carries them upwardly away from the bit, up an annulus between the well walls and the drill string, and out of the well in a space. At surface, the contaminated or used solids-laden mud is typically discharged over a shale shaker which has a series of screens that catch and remove solids from the mud as the mud passes through them. If drilled solids are not removed from the used mud before return to the well, recirculation of the used drilled solids can create weight, viscosity, and gel problems in the mud, as well as increasing wear on mud pumps and other mechanical equipment used for drilling.

Various systems and technologies are known for separating solids from recovered drilling fluid before re-use of the drilling fluids. Typically the separation process is a two-staged operation.

One can refer to Canadian Patent Application No. 2,626,814 to Wick for a description of a two-staged operation in which the used drilling fluid from the rig is first passed through a shale shaker for separating the larger particulates from the drilling fluid. The remaining drilling fluid is then passed through at least one centrifuge for separating solids, the fluid being returned to a mud tank for reuse. Depending on the nature of the recovered drilling fluid, additional drilling fluid can be recovered using as second centrifuge. The first centrifuge separates solids material from the drilling fluid, the drilling fluid being returned to the mud tank. Further, shaker cuttings and centrifuged solids can be separated from additional drilling mud in a second centrifuge. In Wick's methodology, a first centrifuge is used to process the fluid output from the shale shaker and return fluid to the mud tank while the second centrifuge reprocesses shaker solids output and first centrifuge solids output for directing an additional fluid stream to the mud tank. The system employs first and second centrifuges regardless of the condition of the drilling fluid from the rig. Wick's system is designed for a general form of used drilling fluid and not readily able to manage drilling fluids of variable solids content including those having weighted solids.

There is a need for an automated system which can treat a wide variety of drilling fluids depending on the nature of the solids content therein.

SUMMARY OF THE INVENTION

Embodiments described herein relate to a solids control system comprising two separators, such as centrifuges, mounted on a platform of a single stand or skid. The stand is incorporated into a single unit for treatment of drilling fluids. The single stand has a small footprint, eliminates hauling a variety of conventional treatment equipment onto the well site depending on which drilling fluid is use, and reduces the overall equipment inventory required in a fleet. As a result, the system can be operated as a closed loop zero discharge system. The system is configurable depending upon the nature of the used drilling fluids. Some used drilling fluids contain heavier solids such as barite and others have lighter solids content. Barite and additives required in the drilling process can be recovered for reuse.

The system cleans used drilling mud, responsive to the nature of the solids in the drilling fluid. The system is preconfigured to handle the flow of fluids with no on-site set up, piping or re-configuration. The system is a closed loop system for removing unwanted drilled solids while minimizing dilution rates of the fluid. The system can also provide cuttings and solids management in two alternate cleaning applications. Further, a cuttings bin or "roll-off" box can be removably installed in the center of the system for cuttings containment or alternatively a low wall can be installed on the system for on-board containment.

Generally a solids control system comprises separators, such as centrifuges, mounted on a single stand or skid having a first end and an opposing second end. A first and second separator is located at the skid's first and second ends respectively. Redirection devices, such as a two-way valves, direct used drilling mud for treatment in either a series separation of a first separator then a second separator, a parallel single stage separation using both separators, or a single separator, single stage separation, the selection of which being based upon solids content. Containers are provided on or incorporated into the skid for collecting discharged components of the separated used drilling mud. A controller receives a signal representative of the solids content of the used drilling mud and transmits a signal to control the redirection device or devices, for example to select either both separators in series, the separators in parallel, or simply the second separator. When the solids content is at or above a threshold, such as those containing barite, the controller directs the heavy used drilling mud to the first separator. A first heavy solids stream is directed to a first weighted solids container, and a first light solids stream is directed to the second separator. When the solids content is below the threshold, having lighter solids, the controller directs the used drilling mud to the second separator, bypassing the first separator, or to both separators in parallel aiding in throughput. In both instances, the second separator outputs a second light solids stream to a second clean fluid container and a second heavy solids stream to a third cuttings container.

The system may be controlled by a single operator and may be thought of as a black having one or two input streams, a used drilling mud and an optional second input stream for a dewatering agent, and two solids streams, a weighted solids slurry, and clean drilling fluids.

In one broad aspect a system is provided for treating used drilling fluids. The system comprises a skid having a first separator located for producing a first heavy solids stream and a first light solids stream. A second separator for producing a second heavy solids stream and a second light solids stream. A first redirection device is provided for receiving used drilling mud and directing the used drilling mud at least to the first separator, or to the second separator or to both the first and second separators. A first collection box receives the first heavy solids stream. A second collection box, and a third collection box are provided wherein, the second heavy solids stream is directed to the third collection box and the second light solids stream is directed to the second collection box. When the solids content is at or above a first threshold value the first redirection device directs the used drilling mud to the first separator, the first heavy solids stream being directed to the first collection box, the first light solids stream being directed to the second separator, and when the solids content is below the first threshold, the first redirection device directs the used drilling mud to at least the second separator. In another broad aspect a method is provided for treating used drilling fluids. The method comprises determining a first threshold value for solids content of a used drilling fluid. If the solids content is at or above a first threshold value the used drilling fluid is directed to a first separator for separating the used drilling fluid for outputting a first heavy solids stream to a first collection box and a first light solids stream to a second separator. The second separator separates the first light solids stream and outputs a second light solids stream to a second collection box and a second heavy solids stream to a third collection box. If the solids content is below the first threshold value the used drilling mud is directed to at least the second separator for outputting the second heavy solids stream to the third collection box, and the second light solids stream to the second collection box.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a perspective view of a single stand for drilled solids control according to one embodiment;

Figure 2A is a side view of the system of Fig. 1 ;

Figure 2B is an end view of the system of Fig. 1 ;

Figure 3 is a fanciful perspective view of a single stand embodying several optional operating arrangements of the various components of the system of Fig. 1 ;

Figure 4A is a schematic illustrating flow paths of a used drilling fluid through the stand of Fig. 3, a first two-stage series arrangement for weighted solids and a single stage arrangement for lighter solids;

Figure 4B is a schematic illustrating flow paths of a used drilling fluid through the stand of Fig. 3, a parallel separator, single stage arrangement for lighter solids;

Figure 5A is a logic flow diagram illustrating the optional sequence of flow of a heavy solids or lighter solids drilling fluid through the system of Fig. 3;

Figure 5B is a logic flow diagram illustrating the flow of a heavy lighter solids, used drilling fluid through a parallel arrangement of separators;

Figure 6 is a logic flow diagram illustrating the used drilling fluid through the system of Fig. 5A wherein a dewatering agent is added to the used drilling mud inlet; Figure 7A is a logic flow diagram illustrating the used drilling fluid through the system of Fig. 5A wherein a dewatering agent is added to each the separators;

Figure 7B is a logic flow diagram illustrating the used drilling fluid through the system of Fig. 5B wherein a dewatering agent is added to each the separators; and

Figure 8 is a logic flow diagram illustrating the treatment of used drilling fluid wherein a dewatering agent is added to the used drilling fluid in a drilling mud holding tank, prior to the separators.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of a solids control system comprise at least first and second separators, such as centrifuges, mounted on a single stand or skid. Incoming used drilling mud from a drilling rig is redirected, such through a two- way valve, either to the first or the second separator of both separators in parallel depending upon the nature of the solids of the used drilling mud. Clean fluids from the system can be recycled for reuse by the rig.

With reference to Figs. 1 and 2, one embodiment of a closed solids control system 10, for separation of solids from used drilling mud, comprises a housing 12 supported on a generally rectangular transportable skid 14. The skid 14 has a first end and an opposing second end. The housing 12 comprises a first enclosure 16 located at the first end of the skid 14, and a second enclosure 18 located at the second end of the skid 14. The first and second enclosures 16,18 are spaced apart for forming a third enclosure 32 in a gap 20 formed therebetween. A structure or platform 22 is provided above the housing 12 for supporting a first separator 28 and a second separator 30. The platform 22 can bridge the gap 20 between the two enclosures 16, 18. The platform 22 may be equipped with hand rails 24 for safety worker safety.

The platform 22 supports the first separator 28 and the second separator 30 above the first, second and third enclosures. The first separator 28 and the second separator 30 may be centrifuges such as decanter centrifuges.

When employed for treating used drilling mud having weighted solids, such as those including barite, the first separator 28 produces a first heavy solids stream H 1 for discharge to the first enclosure 16 and a first light solids stream L1 for discharge to the second separator 30. The second separator 30 produces a second heavy solids stream H2 for discharge to the third enclosure 32 and a second light solids stream L2 for discharge to the second enclosure 18.

On operations for cleaning used drilling fluids having a high or heavy solids content, the system is operated in series, the first separator 28 being employed for separating the first heavy solids stream H 1 , being a weighted solid slurry, and discharging same to a first collection box in the first enclosure 16, such as to a weighed solids hopper 34 where the first heavy solids stream H 1 contains barite, the solids can be recovered in the hopper 34 for reuse. The hopper 34 might be housed within the first enclosure 16 or be integrated with or formed as part of the first enclosure 16. The first separator 28 also discharges the first light solids stream L1 to the second separator 30. The second light solids stream L2 discharges to the second enclosure 18, or a second collection box such as a clean fluids tank 36. The clean fluids tank 36 might be housed within the second enclosure 18, or be integrated therewith. The second separator 30 is employed for separating its input stream into the second heavy solids stream H2, being cuttings, and discharging same to the third enclosure, or a third collection box such as a cuttings bin 32. The cuttings bin 32 can also be integrated with the housing 12 or skid 14, or may be a removable bin as shown.

The cuttings bin 32 is located on the skid 14 within the gap 20 provided between the separated enclosures 16, 18. The cuttings bin 32 may be integrated with the skid or removable for disposing of the contents therein.

The weighted solids hopper and clean fluid holding tanks 34,36 typically can have a holding capacity of approximately twenty barrels, for containing the first heavy solids stream H 1 discharged from the first centrifuge 28 and the second light solids stream L2 discharged from the second centrifuge 30 respectively.

The first separator 28 can be a centrifuge. More particularly, the centrifuge can be a decanter centrifuge for producing the first heavy stream H 1 or weighted solids slurry 38, and the first light stream L1 or light solids slurry 40. The first decanter centrifuge may be a big bowl centrifuge such as a STRAD Lynx 40 decanter centrifuge developed for oilfield applications by Alfa-Laval having offices at Toronto, Canada. The centrifuge having a variable frequency drive (VFD) with a variable frequency main drive, back drive and feed pump. The second separator 30 can also be a decanter centrifuge. The second decanter centrifuge may be a high speed centrifuge for producing the second heavy solids stream H2 or drill cuttings 42, and a second light solids stream L2, or clean drilling fluid 44. The second centrifuge may be available from Kemtron model KT-1448 HD centrifuge available from Kemtron Technologies Inc., headquartered at Stafford, Texas, USA. The Kemtron centrifuge is suitable for high speed applications and low gravity solids removal.

Having reference to Figs. 2A, 2B, lift structures 26 connect and support between the platform 22 and the skid 14 and housing 12 and are capable of raising and lowering the platform 22. The lift structures 26 may be actuated by hydraulics, rack and pinion or other means known in the art. The lift structures 26 can vary the height of the platform 22 and separators 28,30 above the skid 14 for accommodating various heights of surrounding rig equipment.

Returning to Fig. 1 , the system 10 may further comprise one or more telescopic lights 46 strategically located on the platform 22 for providing light during night time operations. An on-board power generator that can be housed in the housing 12 such as in the enclosure 16, (not shown) may be provided, which meets most noise suppression requirements. The power generator may be a diesel powered 320 KW generator which functions independently of the drilling rig. On board, intrinsically safe heaters (not shown) can also be provided for preventing equipment from freezing in cold climates and arctic conditions. With reference to Figs. 3, 4A,4B and 5A,5B, various optional embodiments of the flow paths of the used drilling mud through the system 10 are demonstrated. The system 10 comprises an inlet 50 for accepting used drilling mud from a drilling rig (not shown). The inlet 50 has a first redirection device R1 , such as a two-way valve, comprising outlet ports for directing the flow of fluid to the first separator 28, to the second separator 30 or to both separators 28,30. A first solids content threshold value "X" is pre-determined prior to or determined during fluid separation operations. Drilling operations may predetermine the solids content and the redirection device R1 can be set or controlled manually. Alternatively, a control system or controller 54 can be provided for actuating the first redirection device R1 . The controller 54 receives a signal, such as a specific gravity sensor, the signal being representative of the nature of the solids content of the used drilling mud and provides a solids signal to the first redirection device R1 . The solids signal provided by the controller 54 is dependent upon a comparison of the received signal and the first solids content threshold. If the used drilling fluid contains heavy solids, such as barite or a high concentration of solids, the controller 54 provides a signal to direct the used drilling fluids to the first separator 28, an initial flow path being represented by a dotted line in Figs. 3 and 4A. The first and second separators 28,30 are configured to operate in series for removing heavy solids from the used drilling mud. In this series, complete solids removal mode, typical combined throughput of the separators 28,30 is rated at about 75 gpm. When the system 10 is operating in series the solids are removed in two stages. For centrifuge type separators, the speed of the first and second centrifuges may be configured to achieve a desired performance. The first separator 28 accepts the used drilling mud from the inlet 50 and discharges the weighted solids slurry 38 and the light solids slurry 40. The weighted solids slurry 38 is deposited into the weighted solids hopper 34 located in the first enclosure 16 of the skid's 14 first end for treatment and reuse during drilling operations. These weighted solids may contain barite and additives introduced to the drilling mud prior to drilling or added to the weighted solids hopper during separation. The light solids slurry 40 is discharged to the second separator 30. The second separator 30 accepts the light solids slurry 40 from the discharge of the first separator 28 and discharges light solids 42 such as drill cuttings, and clean drilling fluid 44. The light solids 42 are deposited in the cuttings bin 32. The clean drilling fluid 44 is discharged to the clean fluid holding tank 36 for temporary storage. For reuse, the clean drilling fluid 44 may be mixed with the weighted solids slurry 38 in, or from, the weighted solids hopper 34. The mixture of the treated weighted solids slurry 30 and the clean drilling fluid 44 can be transferred to a rig tank 58 or the clean drilling fluid 44 may be transferred to a separate storage 60 for reuse, treatment, or disposal.

A quality check can be performed on the clean drilling fluid 44. The quality check comprises a recycling system 70 having an outlet 72 at the holding tank 36 for recycling to an inlet 74 to the second separator 30. The outlet 72 of the holding tank 36 has a recycle device 76, such as a two-way valve, for directing the flow of clean drilling fluid 44 either for reuse or recycling to further solids removal. A second solids content threshold value Ύ" is determined prior to or during fluid separation operations. A second control system or controller 78 is provided for actuating the recycle device 76. The second controller 78 receives a signal, representative of the solids content of the clean drilling fluid 44 and provides an output signal to the recycle device 76. The second output signal provided by the second controller is dependent upon a comparison of the received signal and the second solids content threshold Ύ" (see Fig. 5A). If the clean drilling fluid 44 contains solids content in greater than the second threshold Ύ", the controller provides a signal to the recycle device 76 for directing the clean drilling fluid 44 to the second separator inlet 74. If the clean drilling fluid 44 contains solids content in less than the second threshold Ύ", the second controller 78 provides a signal to device 76 for directing the clean drilling fluid 44 to the weighted solids hopper 34 to be treated and transferred to the rig tank 58, or transferred to the separate storage location 60 for further disposal.

With reference to Figs. 3, 4A, 4B, when used drilling mud input into the system 10 contains substantially only lighter solids or comprises a solids content less below the first threshold "X", the controller configures the first redirection device R1 to direct the used drilling fluid to the second separator 30 or both separators in parallel. A second redirection device R2 redirects the first light solids stream from the second separator 30 to the clean fluid holding tank 36. A third redirection device R3, redirects the first heavy solids stream from the weighted solids hopper 34 to the cuttings bin 32. For example, the first heavy solids stream H 1 from the first separator 28 may be directed away from the weighted solids hopper to the cuttings bin by a diverter box.

First, as shown in Figs. 3 and 4A, the first redirection device causes the separator 28 to be bypassed and the used drilling mud is transferred directly and exclusively to the second separator 30. The used drilling mud follows the path indicated by the dashed line 80 in Figs. 3 and 4A. In the bypass mode of operation, the first centrifuge 28 is inactive. In the parallel mode of operation, the first separator acts as an additional second separator.

The second separator 30 accepts the used drilling mud and discharges the light solids such as drill cuttings 42, and clean drilling fluid 44. The drill cuttings 42 are deposited in the cuttings bin 32. The clean drilling fluid 44 is discharged to the holding tank 36 for temporary storage. As shown in Fig. 4B, in parallel operations, the first separator 28 similarly discharges light solids such as drill cuttings 42 for deposit to the cuttings bin 32 and the clean drilling fluid 44 to the holding tank 36.

The clean drilling fluid 44 may be reintroduced to the rig tank 58, or may be transferred to a separate storage 60.

The solids control system 10 may be configured either manually, using a supervisory control and data acquisition (SCADA) control system, or may be automated with the use of sensors and an automated valve system (SCADA under control of sensors in the fluid stream). The solids control system 10 may be operated either via local panel located on the system or remotely over a communications link (satellite, landline, cellular or proprietary network). The first controller 54 receives the threshold information from the system operator for which the solids content of the used drilling mud will be compared. During the solid separation operation and input signal is received either manually or from a solids content sensor. The first controller 54 compares the input data concerning the solids content of the used drilling mud to the threshold value. The first controller 54 responds by sending a signal to the first redirection device R1 at the inlet 50. The first redirection device R1 either directs the used drilling mud to the first separator 28 to operate in series with the second separator 30, to bypass the first separator 28 and direct the used drilling mud to the second separator 30, or direct the used drilling mud to both the first and second separators 28,30. As shown in Fig. 4B, the light and heavy solids streams L1 ,H 1 of the first separator 28 are similarly fit with the second and third redirection device R2,R3 for diverting the streams L1 ,H 1 to the clean fluid holding tank 36 and cutting bin 32 respectively.

In operation, as set forth in Fig. 5A, the system 10 receives used drilling mud at Block 500. At Block 501 , the nature of the used drilling fluid is determined and if above a threshold X, the flow is directed to the first separator at Block 502, first heavy solids flowing for collection at Block 503. Alternatively, instead, if at Block 501 , the nature of the used drilling fluid is determined and the solids content is at or below threshold X, the flow is directed to the second separator at Block 504, second heavy solids flowing for collection at Block 505 and clean fluid flowing for collection at Block 506. The quality of the clean fluids can be monitored at Block 507, and if suitably light can be directed a rig tank Block 508 for storage, or for reuse and recombination with weighted solids at Block 503, else if too heavy below threshold Y, the clean fluids are recycled back to the second separator at Block 504.

As shown in Fig. 5B, where the solids content of the used drilling mud is determined to be below threshold X, the flow is directed to both the first separator at Block 510, and the second separator at Block 512 operating in parallel. Heavy solids, from each separator, flow for collection in the cuttings bin at Block 514 and clean fluid flows into the holding tank 36 for collection at Block 516. If the clean fluid is suitably light, it can be directed for storage or immediate reuse in the drilling process at Block 520, else if too heavy, being below threshold Y at Block 518, the clean fluids are recycled back to the first separator at Block 510 and second separator at Block 512.

With reference to Figs. 6 to 8, the system 10 may also remove ultra fine solids by means of chemically enhanced dewatering techniques to provide visually clear water discharge. In operation, as set forth in Fig. 6, the system 10 receives used drilling mud at Block 600. A dewatering process may be employed by treating the used drilling mud entering the system with a coagulant or polymer at Block 602. The separators may operate in any configuration as described in the alternative embodiments such as in Figs. 5A & 5B.

In an alternative embodiment, and as set forth in Fig. 7A, the system 10 receives used drilling mud at Block 700. At Block 701 , the nature of the used drilling fluid is determined and if above threshold X, the flow is directed to the first separator at Block 704. A dewatering process may be employed by treating the used drilling mud with a coagulant or polymer at block 702. The dewatering agent may be directed to one or both separators at block 704 and block 706 via a pump and valve system. The valve operation may be manual, under control of servo motor system or under control of a computer system. A first heavy solids flow is collected in the hopper at Block 710. The coagulated ultra fine solids flow with the first light solids stream to the second separator 30. The coagulated ultra fine solids are collected with the second heavy solids in the cuttings bin 32 for collection at Block 708. Alternatively, instead, if at Block 701 , the nature of the used drilling fluid is determined and the solids content is at or below threshold X, the flow is directed to the second separator at Block 706, a second heavy solids, including the coagulated ultra fine solids, are collected in the cuttings bin 32 for collection at Block 708 and clean fluid flowing to the holding tank 36 for collection at Block 712. The quality of the clean fluids can be monitored at Block 714, and if suitably light can be directed to a rig tank Block 716 for storage, or for reuse and recombination with weighted solids at Block 710, else if greater than threshold Y, the clean fluids are recycled back to the second separator at Block 706.

In an alternative embodiment, and as set forth in Fig. 7B, the system 10 receives used drilling mud at Block 750. At Block 752, the nature of the used drilling fluid is determined where the used drilling mud is determined to be below threshold X, the flow is directed to both the first separator at Block 754, and the second separator at Block 756 operating in parallel. A dewatering process may be employed by treating the used drilling mud with a coagulant or polymer at block 758. The dewatering agent may be directed to the separators at block 754 and block 756 via a pump and valve system. The valve operation may be manual, under control of servo motor system or under control of a computer system. Heavy solids, including the coagulated ultra fine solids, from each separator flow to the cuttings bin 32 for collection at Block 760. Clean fluid flows to the holding tank 36 for collection at Block 762. If the clean fluid is suitably light, it can be directed for storage for later reuse in the drilling process at Block 764, else if greater than threshold Y at Block 766, the clean fluids are recycled back to the first separator at Block 754 and second separator at Block 756.

In an alternative embodiment, and as set forth in Fig. 8, a dewatering process may be employed by treating used drilling mud with a coagulant or polymer at block 800, at a used drilling mud holding tank at block 802. The polymer may be pumped into the used drilling mud holding tank via a pump and valve system from a storage tank. The system 10 receives the treated used drilling mud at Block 804. The separators may operate in any configuration as described in the alternative embodiments such as in Figs. 5A & 5B.

Claims

THE EMBODIMENTS OF THE INVENTION FOR WHICH AN EXCLUSIVE PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1 . A system for treating used drilling fluids comprising:

a skid having a first separator located for producing a first heavy solids stream and a first light solids stream and a second separator for producing a second heavy solids stream and a second light solids stream;

a first redirection device for receiving used drilling mud and directing the used drilling mud at least to the first separator, or to the second separator or to both the first and second separators;

a first collection box for receiving the first heavy solids stream; a second collection box, and a third collection box wherein, the second heavy solids stream is directed to the third collection box and the second light solids stream is directed to the second collection box; and

when the solids content is at or above a first threshold value the first redirection device directs the used drilling mud to the first separator, the first heavy solids stream being directed to the first collection box, the first light solids stream being directed to the second separator, and

when the solids content is below the first threshold, the first redirection device directs the used drilling mud to at least the second separator.

2. The system of claim 1 wherein when the solids content is below the first threshold, the first redirection device directs the used drilling mud only to the second separator.

3. The system of claim 1 further comprising a second redirection device for directing the first heavy solids stream to the first collection box or to the third collection box,

a third redirection device for directing the first light solids stream to the second separator or to the second collection box, and

wherein when the solids content is below the first threshold, the first redirection device directs the used drilling mud to both of the first and second separators,

the second redirection device directs the first heavy solids stream to the first collection box; and

the third redirection device directs the first light solids stream to the third collection box.

4. The system of claim 1 , 2 or 3 further wherein: the skid has a first end and an opposing second end; the first separator is located at the skid's first end above the first collector box;

the second separator is located at the skid's second end above the second collector box; and

the third collector box is located between the first and second collection boxes.

5. The system of any one of claims 1 to 4 wherein the third collector box is removable from the skid.

6. The system of any one of claims 1 to 5 further comprising a recycle from the second collector box to the second separator.

7. The system of any one of claims 1 to 6 wherein when the solids content is at or above a first threshold value, the first heavy solids stream from the first collection box is recycled to a rig tank.

8. The system of any one of claims 1 to 6 wherein when the solids content is at or above a first threshold value, the second light solids stream from the second collection box is mixed with the first heavy solids stream from the first collection box for recycling to a rig tank.

9. The system of any one of claims 1 to 8 wherein the first and second separators are decanter centrifuges.

10. The system of any one of claims 1 to 9 further comprising: structure between the skid and the separators; and

wherein the structure is capable of varying the height of the separators above the skid for accommodating various heights of surrounding rig equipment.

1 1. The system of any one of claims 1 to 10 further comprising a first sensor for producing signals representative of the solids content of the used drilling fluid for controlling at least the first redirection device.

12. The system of claim 7 further comprising a second sensor for producing the signals representative of the quality of clean fluid in the second collection box for controlling the recycle to the second separator.

13. A method for treating used drilling fluids, the method comprising:

determining a first threshold value for solids content of a used drilling fluid, and

if the solids content is at or above a first threshold value directing the used drilling fluid to a first separator and separating the used drilling fluid for outputting a first heavy solids stream to a first collection box and a first light solids stream to a second separator,

separating the first light solids stream at the second separator for outputting a second light solids stream to a second collection box and a second heavy solids stream to a third collection box, and

if the solids content is below the first threshold value

directing the used drilling mud to at least the second separator for outputting the second heavy solids stream to the third collection box, and directing the second light solids stream to the second collection box.

14. The method of claim 13 when if the solids content is at or above the first threshold value, then further comprising:

mixing clean fluid from the second collection box with the first heavy solids stream from the first collection box for reuse as drilling mud.

15. The method of claim 13 or 14 wherein if the solids content is below the first threshold value,

directing the used drilling mud to both of the first and second separators for outputting both of the first and second heavy solids streams to the third collection box, and directing both of the first and second light solids streams to the second collection box.

16. The method of claim 13, 14, or 15, wherein separating further comprises centrifuging.

17. The method of claim 15 further comprising:

directing clean fluid from the second collection box for reuse as drilling mud.

18. The method of any one of claims 13 to 17 further comprising determining the solids content of clean fluid in the second collection box, and if the solids content is above a second threshold, recycling clean fluid from the second collection box to the second separator.

19. The method of any one of claims 13 to 18 wherein the clean fluid from the second collection box is directed to a separate storage tank.

20. The method of any one of claims 13 to 18 further comprising adding a dewatering agent to the used drilling mud for coagulating ultra fine solids contained therein.

21 . The method of claim 20 wherein adding a dewatering agent to the used drilling mud further comprises adding the dewatering agent to the first separator, the second separator or both.

22. The method of claim 20 or 21 further comprises separating the coagulated ultra fine solids from the used drilling mud in the first and second separator.