Abstract:
Liquids are removed from drill cuttings generated during drilling operations, recovered and recycled to the rig drilling fluid system, reducing the amount of waste generated and fluid lost. The process and apparatus to achieve this uses a fluid bed thermal desorption unit that vaporizes the liquids from the cuttings. A portion of this vapour is recycled to fluidize the bed, enhance heat transfer and maintain a low oxygen environment in the desorption chamber. The cuttings become the bed medium. The drilling fluid system is used to condense the vapours for re-use. The unit allows for varying rates of cuttings generation and varying composition of those cuttings.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a system for recovering drilling fluid from drill cuttings being produced by a drilling rig and recycling the recovered fluid to the rig drilling fluid storage and circulating system. More particularly the invention incorporates bed desorption, preferably fluid bed desorption, wherein forced gas convection heating is used to vaporize the drilling fluid. The vaporized fluid is then condensed and returned to the rig storage and circulating system. 
     BACKGROUND OF THE INVENTION 
     Drilling for oil and gas produces drill cuttings. As drilling progresses, the cuttings are brought to surface by the circulation of drilling fluid through the wellbore. The cuttings are usually separated from most of the drilling fluid using vibrating screens, referred to as shale shakers, and centrifuges. 
     The cuttings retain a significant volume of drilling fluid on them after separation. It is desirable to clean the cuttings by removing the fluid and to return the latter to the rig&#39;s drilling fluid storage and circulating system (herein referred to as the “rig fluid system”), for re-use. Otherwise the cuttings remain associated with drilling fluid and create a disposal problem involving environmental considerations. The drilling fluid often is oil based and may incorporate chemicals. 
     Current methods for disposing of cuttings contaminated with drilling fluid include: hauling the cuttings to a land fill and burying them; composting; bio-remediation; thermal desorption; and combustion. 
     Currently used thermal desorption processes for cleaning the drill cuttings are implemented after the rig has completed drilling. The processes commonly involve an eternally heated rotating drum for volatilizing the liquid. The off gases are burned rather than being recovered. 
     Another thermal method used involves fending the cuttings into kilns or fluidized bed reactors, wherein the fluid is directly burned off. Again, this is practised after drilling is finished. 
     These prior art thermal systems involve large scale, fixed capacity units that rely on constant rate, uniform composition feed. They are not adapted to clean cuttings while drilling is ongoing nor do they return separated drilling fluid to the drilling fluid system. Otherwise stated, they are not amenable to being integrated into the on-going drilling operation. 
     It is therefore the objective of the present invention to provide a thermal process for recovering drilling fluid from cuttings while drilling is on-going, so that the recovered fluid can be recycled to the rig drilling fluid system. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to method and apparatus for cleaning drill cuttings as they are produced by a rig involved in drilling a wellbore. Drilling fluid coating the cuttings is to be recovered and recycled to the rig&#39;s fluid system. 
     More particularly the coated drill cuttings are fed into a pressurized desorption chamber through which a stream of hot gas is pumped, to heat cuttings by convection and preferably fluidize them, thereby evaporating drilling fluid coating the cuttings. Optionally, indirect heat may be used to supplement the convective heat. Heating gas and drilling fluid vapours are removed from the chamber as an overhead stream. Part of the overhead stream is condensed to reclaim drilling fluid in liquid form. The reclaimed drilling fluid liquid is then recycled to the rig&#39;s fluid system. Cuttings are removed from the desorption chamber as an underflow, preferably al variable rates to control the solids volume in the chamber below a maximum allowable level. 
     A non-condensed portion of the overhead stream vapours is re-heated and recycled for use as the forced convection heating gas. The recycled gases, containing condensible vapours such as hydrocarbons and water, maintain a low oxygen atmosphere in the desorption chamber, which reduces the risk of explosion. 
     Preferably the temperature in the desorption chamber is maintained in the range of 400° F. to 600° F., more preferably 540° F. to 570° F. As a result the temperature in the chamber is sufficient for evaporation but below the temperature at which coking, cracking and oxidation reactions become a problem. 
     Broadly stated, in one aspect the invention is concerned with a method for cleaning drill cuttings being produced by a drilling rig having a drilling fluid storage and circulating system, the cuttings being coated with drilling fluid, comprising: feeding the cuttings into a pressurized desorption chamber having an overhead vapor outlet and an underflow cuttings outlet; pumping hot pressurized healing gas through the chamber to contact and directly heat cuttings, by convection, so that drilling fluid on the cuttings is vaporized; discharging a gaseous mixture of heating gas and drilling fluid vapours through the overhead outlet; condensing part of the gaseous mixture to recover drilling fluid vapours in liquid form, thereby leaving part of the gaseous mixture as non-condensed gas containing condensible vapours; recycling the recovered liquid to the rig drilling fluid storage and circulating system; heating non-condensed gas and recycling it to the desorption chamber as heating gas; and removing cleaned cuttings from the desorption chamber through the underflow cuttings outlet. 
     In another aspect, the invention is concerned with a circuit for cleaning drill cuttings produced by a drilling rig having a drilling fluid storage and circulating system, the cuttings being coated with drilling fluid, comprising: a vessel forming a pressure desorption chamber having an overhead vapour outlet and underflow cuttings outlet; means for feeding drill cuttings into the chamber as they are produced by the rig; means for pumping hot pressurized heating gas through the chamber to heat cuttings by convection so that drilling fluid on the cuttings is vaporized to produce a mixture of heating gas and drilling fluid vapour, means for condensing part of the mixture leaving the chamber through the overhead vapour outlet, to produce liquid drilling fluid, thereby leaving part of the gaseous mixture as non-condensed gas containing condensible vapours, means for recycling liquid drilling fluid to the drilling fluid storage and circulating system; means for heating and recycling non-condensed gas to the chamber as heating gas; and means for removing cleaned cuttings from the desorption chamber. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic showing a system or circuit for cleaning drill cuttings, tied in with the shale shaker centrifuge and drilling fluid system of a rig drilling a wellbore; and 
     FIG. 2 is a process flow diagram showing the circuit of FIG.  1 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The circuit  1  comprises a closed pressure vessel  2  forming an internal desorption chamber  3 . The chamber  3  has an overhead vapour outlet  4  and underflow cuttings outlet  5 . The bottom  19  of the vessel  2  is conical with the outlet  5  at its apex. 
     A drill cuttings recovery means  6 , such as a shale shaker  7  and centrifuge  8 , is connected with and forms part of the drilling fluid system  9  of a rig  10 . The means  6  separates drill cuttings  11  from drilling fluid  12  being circulated out of the wellbore  13 . The cuttings  11  are collected in a buffer storage hopper  14 . 
     The hopper  14  is connected by a bottom discharge line  15  and variable rate valve  16  with a screw conveyor  17 . The conveyor  17  is connected through an inlet  18  with the desorption chamber  3 . 
     From the foregoing, it will be understood that drill cuttings  11 , coated with drilling fluid and being produced by the rig  10  in the course of drilling the wellbore  13 , are collected in the hopper  14  and are fed by the screw conveyor  17  into the chamber  3 . The rig will produce the cuttings at variable rates. The speed of the screw conveyor  17  can be varied to cope with variations in the production rate of the cuttings. 
     The cuttings  11  typically have a fluid content of around 20%, but this can vary widely, for example between 6 and 45%. 
     The cuttings  11  form the bed  20  in the chamber  3 . Here they are heated by convection, using recycled hot vapours  21  pumped by a fan  22  into the bed  20  through a manifold  23 , line  24 , and pre-heater  25 . The hot vapours  21  are typically at 500° F. In the embodiment shown, additional heat is supplied by a hot oil coil  26  receiving hot oil from an oil beating unit  27  through line  28 . The hot oil is heated to between 450° F.-650° F., depending on the through put of cuttings. The temperature within the chamber  3  is maintained within the range 400° F. to 600° F., preferably at 540° F. to 570° F. Preferably the recycled hot vapours are supplied at a rate sufficient to maintain partial or complete fluidization. However it has been found that low rates can satisfactorily be used with virtually no fluidization. 
     The resulting heat input vaporizes the drilling fluid liquid attached to the cuttings  11 , separating the liquid from the cuttings. 
     A mixture of heating gas and drilling fluid vapours leaves the chamber  3  through vapour outlet  4  and line  30 . The gas mixture is preferably processed in a cyclone  31  to remove fine particles. 
     A portion of the gas mixture, containing condensible vapours such as water and hydrocarbons, is preferably drawn off for recycle to chamber  3 . It is passed through line  24  and heated by heat exchange with the hot oil in line  28 . The flow rate of the recycled vapour is preferably kept constant to maintain the chamber temperature and fluidizing characteristics constant, regardless of cuttings flow rates. For example, if no cuttings are being fed into the chamber  3 , or if the cuttings are deficient in liquids, the same gas will recycle until there are sufficient vapours to require flow to the condensing step. This keeps a constant recycling flow during variable operating conditions. 
     The remaining vapours are then condensed, preferably using the drilling fluid system  9 . The vapour flow is typically less than 0.1% of the fluid flow in the system  9 . Condensation is carried out by circulating a fluid flow from the fluid system  9  through heat exchanger  32  which is connected with gas mixture lien  30 . The drilling fluid us for cooling and the condensed vapours are returned to the drilling fluid system  9  through lines  50 ,  51  respectively. 
     The cuttings  11 , after vaporization of liquids, are expelled from the bottom of the chamber  3  through line  33  and valve  34 . The latter may be a slide or rotary valve. 
     The rate of expulsion is adjusted to match the rate of introduction of cuttings. This provides a means of varying the through put rate depending on the cuttings generation rate. The exiting cuttings are partially cooled by ambient air pumped by pump  36  through line  36  and heat exchange tubes  37 . The tubes  37  isolate the air flow from the interior of the chamber  3 . The cooling air exits the vessel  2  to atmosphere or may be recovered for its heat value for use in heating the cuttings as they enter chamber  3 . 
     Due to the nature of the drilling operation, the adjustments will be to lower the flow from maximum throughput rate. As drilling progresses to greater depths, the hole size is normally reduced at intervals. This reduces the rate at which cuttings are generated. Exceeding the maximum rate will diminish the effectiveness of the fluid removal. The buffer hopper  14  handles surges above the maximum rate, and is to be sized based on expected maximum cuttings generation rates versus system processing capacity. The recycle gas is at a constant rate, with the hot oil temperature control used to prevent overheating. 
     The fines are expelled from the bottom of the cyclone  31  via valve  40  and line  41 . The valve  40  is similar to that at the bottom of the fluid bed  20 . All solids are conveyed to a storage pile if on land or overboard if off-shore. It may be necessary to reduce the temperature of the solids. This can be achieved by using methods such as a water quench in the output conveyor. This is external to the process of removing the liquids. 
     The preferred form of the apparatus and corresponding process provide an integrated system for removing and recovering drilling fluid liquids from drill cuttings. The characteristics of the fluid bed system include the use of the drill cuttings as the bed material, hot oil for heating the bed and recycled vapour and/or oil heater exhaust gas for hearing and fluidizing the bed, with a constant recycle rate used to maintain bed characteristics regardless of feed drill cuttings flow and content. Further characteristics of the preferred process are the use of cyclones when necessary for particulate removal instead of more complicated fines removal methods, use of the drilling fluid system for cooling rather than more expensive, complicated alternatives, and the use of variable rate valves to adjust the material flow to the rate generated by the drilling operation. The invention enables in-line recovery of drilling fluids and minimizes the amount of waste generated in the form of contaminated drill cuttings and solids. 
     The invention can be sized for any throughput requirement through scaling of the components. The system can also be adapted for use on both land and off-shore with minor changes. In both cases the basic components of both the apparatus and process are retained. 
     An example of the embodiment described follows. This is a land based application requiring 2 metric tonnes per hour through put cepacity. 
     The process and equipment parameters selected for this example are detailed below. For this application a residence time of 1 hour was selected as the basis for the maximum sustained material input of 4410 lb/hr. The system can be turned down from this capacity to suit the cuttings generation rate. A fluid content of 15% by weight was used in this example. t, 110   
     The cyclone design specifications are shown below. These rely on standard methods based on expected material composition and proportion in the flow. 
     
       
         
               
             
               
               
               
               
             
               
               
               
             
               
               
               
               
             
           
               
                   
               
               
                 Cyclone Fractional Efficiency (Koch &amp; Licht Eqns) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Input Data 
                   
                   
               
               
                   
                 Body Diameter (Dc) 
                 2 
                 ft 
               
               
                   
                 Inlet Height (a) 
                 1 
                 ft 
               
               
                   
                 Inlet Width (b) 
                 0.4 
                 ft 
               
               
                   
                 Outlet Width (S) 
                 1.3 
                 ft 
               
               
                   
                 Outlet Diameter (De) 
                 1 
                 ft 
               
               
                   
                 Cylinder Height (h) 
                 3 
                 ft 
               
               
                   
                 Overall Height (H) 
                 6 
                 ft 
               
               
                   
                 Dust Outlet Diameter (B) 
                 1 
                 ft 
               
               
                   
                 Gas Pressure 
                 3 
                 in wc 
               
               
                   
                 Dust Abs Density (Rho) 
                 100 
                 lb/cu ft 
               
               
                   
                 Air Temp 
                 500° 
                 F. 
               
               
                   
                 Gas Flowrate 
                 3859.0847 
                 lb/hr 
               
               
                   
                 Grain Loading 
                 208.50 
                 gr/ACF 
               
               
                   
                 Ash Rate 
                 2760.3 
                 lb/hr 
               
             
          
           
               
                   
                 Intermediate Results 
                   
               
               
                   
                 Natural Length: 
                 4.954619 
               
               
                   
                 Cyclone Volume: 
                 7.147123 
               
               
                   
                 Config Factor: 
                 545.8517 
               
               
                   
                 Velocity Heads: 
                 6.4 
               
               
                   
                 Air Flow (ACFS): 
                 25.7425 
               
               
                   
                 Air Density 
                 0.041642 
               
               
                   
                 Air Viscosity: 
                 1.9E-05 
               
               
                   
                 Vortex Exponent: 
                 0.550829 
               
               
                   
                 Saltation Velocity (ft/s): 
                 70.30178 
               
               
                   
                 Pressure Drop (in wc): 
                 3.311417 
               
               
                   
                 Internal Plumbing 
               
               
                   
                 d: 
                 0.91506 
               
               
                   
                 Vol (natural) 
                 7.130605 
               
               
                   
                 Vol (below exit): 
                 7.147123 
               
               
                   
                 Vol (annular): 
                 1.884956 
               
               
                   
                 K (c): 
                 0.682315 
               
               
                   
                 K (a): 
                 0.5 
               
               
                   
                 K (b): 
                 0.2 
               
               
                   
                 Inlet Velocity (ft/s): 
                 64.35626 
               
               
                   
                 w (ft/s): 
                 3.593214 
               
             
          
           
               
                   
                 General Dimensions 
                   
                   
               
               
                   
                 Top Surface 
                 2.4 
                 sq ft 
               
               
                   
                 Cylinder Surface 
                 18.8 
                 sq ft 
               
               
                   
                 Cone Surface 
                 22.7 
                 sq ft 
               
               
                   
                 Total 
                 73.9 
                 sq ft 
               
               
                   
                 Outlet shroud Surface 
                 4.1 
                 sq ft 
               
               
                   
                   
               
             
          
         
       
     
     The equipment in the designed process configuration is to be mounted on a trailer to be transported to site by tractor unit. The major components of the processing unit are shown. 
     When moved to location the surge hopper is set up with the outlet flowing into the desorption vessel. A conveyor of either belt or screw type transports the shale shaker overflow and centrifuge underflow to the unit. The lengths of these are dependent on the site layout. These are to be sized to suit the specific need. Similar conveyors are attached to the outlets to move the cuttings into a pile. If conditions require, a water quench or other cooling method may be utilized to cool the cuttings. 
     A fuel source supplies diesel fuel for the heater unit, and power is supplied from either the rig generation system or by a stand alone generator. 
     The best mode of the invention has been explained in detail. However it is to be understood that the invention is not limited in its application to the details to the construction and the arrangement of components set forth. The invention is capable of other embodiments and of being praised in various ways. The scope of the invention is defined in the claims now following. 
     The entire disclosure of all applications, patents and publications, cited above, and of corresponding application Canadian application No. 2,262,192, filed Feb. 17, 1999, is hereby incorporated by reference. 
     From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.