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BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates generally to fluid clarification systems and, more particularly, to a system and method of treating drilling mud, while retaining certain desirable solids in the fluid so that the fluid can be subsequently used. Further, the present invention relates to a fluid clarification system including a subsystem for dynamically measuring mass flow rate. The present invention further provides a purification step, preferably using a vertical centrifuge, that removes fine suspended solids which have in the past been returned to the drilling mud system. 
     (2) Description of Related Art 
     The present invention provides a drilling mud treatment system used with a drilling rig. When an oil well is drilled, it is necessary to drill the well with drilling fluid, commonly referred to in the art as drilling mud. The drilling mud is provided to lubricate and cool the drill bit and to carry away cuttings as the mud flows upwardly in the annular flow space around the drill string. The drilling mud is pumped down the drill string to pick up the cuttings and other debris. Commonly, the drilling mud is either water or an oil-based carrier. 
     When drilling into a high pressure formation or at great depths, safety is enhanced by incorporating a weight component, such as barium sulfate, barite, or hematite, for example, to the drilling mud to increase the weight of the drilling mud. The additives are expensive and various systems have been proposed for the recovery and recycling of drilling mud additives. Also, when drilling mud circulates through the well it picks up particles or cuttings of the earth formations cut by the drill bit. Various systems have therefore been proposed to remove the cuttings from the drilling mud so that the drilling mud can be recycled for further use in drilling operations. 
     It is relatively easy to clean the drilling mud if the cuttings are primarily heavy rock. Also, large particle cuttings are easily removed from the mud by passing the drilling mud through a set of screens and other components, such as including shale shakers, desanders, degassers, and other cleaning devices. As used herein, such early-stage cuttings separators are referred to as coarse cuttings separators. Centrifuge systems are often used to further treat drilling mud by removing the finer cuttings. Unfortunately, very fine low density solids, which are not as easily removed from the drilling mud, have simply been accepted in the past and the drilling mud has been routinely returned to the mud system with such very fine solids entrained in the mud. This practice is particularly deleterious to the mud system because the very fine solids have an adverse impact on the viscosity of the drilling mud. Thus, there remains a need for further treatment of drilling mud to remove these very fine suspended low gravity solids, while returning drilling mud additives to the drilling mud system. 
     There is a direct economic benefit in removing as much of the undesirable solids from the drilling mud while retaining the additives in the mud. The natural inclination of operators of drilling mud treatment systems in the field is to maximize the flow rate of drilling mud through the system. However, running the system at maximum flow rate does not necessarily remove the greatest amount of the cuttings. So, there remains a need for a system with installed controls to operate the system for the maximum efficiency in the removal of the cuttings from the drilling mud. Further, there remains a need for a system which demonstrates the cost savings to the operator if the system is operated at such a maximum efficiency operating point. Such a system should provide a dynamic measurement of mass flow throughout the system in order for operators to determine the most efficient flow through the system. 
     In our co-pending U.S. patent application Ser. No. 09/579,702, filed May 26, 2000, incorporated herein by reference, we described a batch system for measuring mass flow through the system. That batch system was based on the realization that measuring the rate of change of volume in a measurement tank, and the concomitant change in the weight at two measured volumes of drilling mud, provided a direct measurement of mass flow rate in the system. Measurement of mass flow at two points in the treatment system provided a technique for measuring the efficiency of the system in removing undesirable solids from the drilling mud. The present invention improves on that technique by providing in-line measurement surge tanks in the treatment system to dynamically measure mass flow rate at selected points in the system. The present invention eliminates the need for batch measurement of mass flow rate by sampling outside the treatment system. 
     The present invention is further directed to another long felt need in the drilling art. It is known that mud systems are not completely effective in cleaning all the drill cuttings from down hole. Consequently, cuttings tend to build up down hole over time, and periodically operators typically stop the drilling operation and increase mud flow rate, sometimes as much as double the usual flow rate, to clean out the accumulated cuttings. This is known in the art as “sweeping” the well. With current mud systems, however, there is no way to tell how long to “sweep” the well, since there is currently no effective way to determine total solids removed by current mud purification systems. Consequently, operators tend to either under sweep a well, and thereby do an inadequate job of removing accumulated cuttings, or they tend to over sweep a well, losing valuable drilling time at substantial expense. The present invention addresses this need in the art. 
     SUMMARY OF THE INVENTION 
     The present invention addresses these and other needs in the art by providing an additional stage in the treatment system, in addition to that shown and described on our application Ser. No. 09/579,702, for maximum efficiency in removing these undesirable very fine, low gravity solid components. The system comprises a primary decanter centrifuge adapted for the removal of high density solids, the type commonly added to drilling mud as weight components. The liquids discharge of the primary decanter centrifuge is fed to the inlet of a secondary decanter centrifuge, which is adapted to remove low gravity cuttings from the drilling mud. The solids discharge of the primary decanter centrifuge is recirculated back to the mud system for reuse. The liquids discharge of the secondary decanter centrifuge is preferably directed back to the system for reuse, although a portion of the liquids discharge from the secondary decanter centrifuge may be directed to the influent of a cuttings dryer, as shown and described in our U.S. patent application Ser. No. 09/620,844, filed Jul. 21, 2000, now U.S. Pat. No. 6,432,299, and incorporated herein by reference. The cuttings dryer is available from Hutchison-Hayes International under the trademark DUSTER™. The cuttings dryer further treats the drilling mud, reducing the drilling fluids associated with the solids to a point where the solids can be safely discharged within government regulations for discharge of oil-based drilling mud offshore. 
     The liquids discharge of the cuttings dryer is directed to the inlet of a dryer recovery decanter centrifuge for further treatment. The liquids discharge of the dryer recovery decanter centrifuge may preferably be directed to a de-sludging high speed vertical disc centrifuge, available from Hutchison-Hayes as model number SEA-1200. The vertical disc centrifuge removes the very fine low gravity solids which can adversely effect the viscosity of the drilling mud if recycled to the drilling mud system. The liquids from the vertical disc centrifuge are returned to the drilling mud system. 
     The present invention provides the additional feature of a plurality of mass balance units in the drilling mud flow path at selected points in the system. The mass balance units provide a direct measurement of the solids being removed by the various centrifuges and the cuttings dryer in the system, so that the system controls maintain the operating points for the system for the maximum efficiency in the removal of undesirable cuttings from the drilling mud. 
     These and other features and advantages of the present invention will be apparent to those skilled in the art from a review of the following detailed description along with the accompany drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
     FIG. 1 is an overall schematic diagram of the drilling mud treatment system of this invention, including a plurality of in-line mass balance units. 
     FIG. 2 is a schematic diagram of the system including an additional stage vertical centrifuge. 
     FIG. 3 a  is a front elevation view of a set of in-line mass flow detectors in accordance with this invention. 
     FIG. 3 b  is a side elevation view, and 
     FIG. 3 c  is a top view of the detectors. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIG. 1 depicts a mud clarification or processing system  10  of the present invention. The system is temporarily assembled adjacent to a drilling rig (not shown) and typically includes a set of mud pits which receive the used mud from the well borehole. The mud delivered to the mud pits is transferred to a shale shaker. The supply line from the shale shaker is shown schematically in FIG. 1 with the reference number  12 . The shale shaker picks up large particles which are collected on a screen in the shale shaker for removal from the mud. From the shale shaker, a mud line  14  is connected into the system  10 . 
     Cuttings from the shale shaker are transferred into the system of FIG. 1 by way of a mass flow sensor  13 , which is preferably a screw-type conveyor or auger, into an inlet line  54 . Mass flow of the cuttings into the system is determined by a load sensor  15  and the scroll rate of the conveyor. In this way, the contribution of the shale shaker to the total low gravity solids processed by the system can be determined. Combining this determination with the low gravity solids processed by the remainder of the system provides an indication of total cuttings, and thus an indication of the effectiveness of the mud system in flushing cuttings from the hole. This also provides an indicator of when a sweep needs to be performed on the hole, and for how long. 
     The principle components of the system will now be described. Supply of drilling mud enters the system from the mud line  14  into a storage tank  16 , although it should be understood that a plurality of such storage tanks are preferably used. Drilling mud from the storage tank  16  is directed through a supply line  18  into a first positive displacement pump  20 . 
     Mud is pumped by the pump  20  into the inlet of a first mass flow sensor  22  by way of a supply line  24 . The operation of the mass flow sensor will be described below with regard to FIGS. 3 a,    3   b,  and  3   c.  The mud is then pumped from the mass flow sensor  22  by a pump  26  into a first stage centrifuge  28 . As previously described, the first stage centrifuge is controlled to separate the desirable, heavy components which have been added to the drilling mud, while passing the lighter weight cuttings. 
     As viewed in FIG. 1, a solids discharge  30  from the centrifuge  28  is on the left, and a liquids discharge  32  is on the right. The solids discharge  30 , including the high value, high gravity solids, is returned to the tank  16 , thereby restoring the high gravity solids to the system for further use. The liquids discharge  32  is directed to a second mass flow sensor  34 . The mud is then pumped by a pump  36  into a second stage centrifuge  38  which is controlled to remove low gravity solids, i.e. cuttings, from the mud. As before with regard to the first stage centrifuge  28 , a solids discharge  40  from the second centrifuge is depicted on the left in FIG. 1 and a liquid discharge  42  is depicted on the right. The solids discharge  40  is directed to a disposal line  44  for discharge. It should be understood that, although the solids discharge disposal line  44  is shown as a single line, the system may include a number of such discharge lines over the side or into a capture system. The liquid discharge  42  is directed to a third mass flow sensor  46 . 
     From the third mass flow sensor  46 , the now substantially clarified drilling mud is pumped by a pump  48  into a line  50  where the mud may be directed to the tank  16  and/or to a fourth mass flow sensor  51  and then to the suction of a booster pump  52 . The booster pump  52  directs the flow to a cuttings inlet line  54  where cuttings from the shale shakers are received. The cuttings inlet line  54  flows into a cuttings dryer  56 , as previously described. The solids from the cuttings dryer  56  are directed to a solids discharge line  58  and to the disposal line  44  for discharge, and the liquids from the cuttings dryer  56  are directed to a liquid discharge line  60  and to a fifth mass flow sensor  62 . Alternatively, the cuttings dryer  56  may be provided with a discharge line  61 , separate from the disposal line  44 , to direct its solids discharge for disposal. From the mass flow sensor  62 , the mud is pumped to a third stage centrifuge  64 . The solids from the third stage centrifuge  64  are directed to a solids discharge line  66  and to the disposal line  44 . The liquids from the third stage centrifuge  64  are directed to a liquids discharge line  68  into a fourth mass flow sensor  70 . The mud is then pumped by a pump  72  over a line  74  back to the tank  16  for further use. 
     It should now be appreciated that the mass flow sensors provide a direct measurement and indication of the operation of the system. For example, the difference between the mass flow through sensor  22  and the sensor  34  provides a direct measurement of the solids discharged into the discharge line  30 . Similarly, the difference between mass flow sensed by the sensor  34  and the sensor  46  provides a direct measurement of the solids discharged from the discharge line  40 . These measurements can also be translated into a direct measurement of the efficiency of the system in removing low gravity solids from the drilling mud and savings realized by use of the system. 
     FIG. 1 also shows an alternative embodiment for monitoring the performance of the system. The solids discharges for any or all of the centrifuges  28 ,  38 ,  54 , and  64  may be directed to a mass flow sensor. The solids discharge of the centrifuge  28  may be directed to a mass flow sensor  29 , which is preferably a screw type conveyor or auger with a load cell  31 , to measure the high gravity solids being discharged back to the tank  16 . Similarly, the solids discharge of the second stage centrifuge  38  may be directed to a mass flow sensor  39  with load cell  41  for measuring solids discharged from the centrifuge  38  overboard. A mass flow sensor  65  with load cell  67  may be provided for centrifuge  64 , and a mass flow sensor  57  with load cell  55  may be provided for the cuttings dryer  56 . With each of the mass flow sensors  29 ,  39 ,  65 , and  57 , efficiency of each of the centrifuges and the cuttings dryer may be determined by summing the mass flow through sensors at the liquids discharges with the mass flow through sensors at the solids discharges to thereby calculate the total influent, and then calculate the solids removal rate. 
     FIG. 2 depicts another feature that may preferably be included in the system. As previously described, a cuttings dryer  56  receives cuttings from the shakers over an inlet line  54 . Liquids from the cuttings dryer are discharged to the fifth mass flow sensor  62 , and solids are discharged into a disposal line. The mud is then pumped by a pump  72  to a third stage centrifuge  64 . The solids from the third stage centrifuge  64  are discharged to the disposal line  44  and the liquids from the centrifuge  64  are directed to a mass flow sensor  70 . At this stage, the drilling mud typically still contains small quantities of very fine cuttings, and such small quantities of very fine cuttings are generally tolerated. However, these cuttings degrade performance of the mud, and are particularly harmful to the system because the finest cuttings are the most abrasive and have the most harmful effect on the viscosity of the mud. The present invention directs the mud from the mass flow sensor  70  with a pump  74  to a de-sludging high speed vertical disc centrifuge  76 , available from Hutchison-Hayes as model number SEA-1200. The centrifuge accumulates solids in the bowl, and periodically discharges a quantity of solids for discharge to the disposal line  44 . Liquid from the centrifuge  76  is then directed to a mass flow sensor  75  via a line  73 , and then returned to the system by a pump  77  through a line  78 . In the liquid discharge line from the centrifuge  76 , the mud is roughly 0.5% solids, and 99.5% fluid, preferably an oil-based mud. 
     FIGS. 3 a,    3   b,  and  3   c  depict a preferred structure for a bank  80  of mass flow sensors. While FIG. 3 a  shows four such sensors, fewer than four such sensors may be mounted on the frame, such as for example two sensors. To provide perspective, as seen in FIG. 3 a,  the bank of sensors is roughly 7′ high and about 10′ wide. As seen in FIG. 3 b,  the bank of sensors is about 7′ deep. This compact size for the bank of sensors makes is easy to mount all of the sensors on a single platform  82  so that the entire structure may be transported to a drilling rig and mounted thereon. The bank of sensors is supported and surrounded by a frame  84  which includes lifting eyes  86  to assist in transporting the structure. 
     Referring now to FIG. 3 a,  the sensors  22 ,  34 ,  46 , and  70  mount to the frame  84  by means of load sensors  88 . The load sensors continuously monitor the total weight of each sensor. The sensors discharge into their respective pumps  26 ,  36 ,  46 , and  74 , respectively, as previously described with regard to FIG.  1 . The discharges of the pumps are mounted at an angle, as shown in FIG. 3 a,  to minimize the space required for the pumps. Further, the sensors are fed through feed lines  24 ,  32 ,  42 , and  68 , respectively, as shown in FIG.  1 . The feed lines and the pump discharges provide the couplings to the remainder of the system  10 . 
     Each sensor is also provided with two level sensors, a radar level detector  90  and an ultra-sonic level detector  92 , for accuracy is measuring the total volume of fluid within the sensor. 
     Referring now to FIG. 3 b,  a side view bank  80  of sensors is provided. From this view, the sensor  70  and its inlet line  68  may be seen mounted in the frame  84 . Each of the sensors also includes an overflow line  94  and all four overflow lines  94  flow into a common line  96  which flows back to the storage tank  16  to recover the mud. 
     To detect mass flow rate in a particular sensor, a high level sensor  100 , a nominal level sensor  102 , and a low level sensor  104  are provided. With the system operating at steady state, a constant fluid level will be maintained in a sensor. The speed of an associated pump  26 ,  36 ,  46 , or  74  is then decreased by a predetermined fractional amount. The length of time for fluid level to drop between two level sensors is then timed. This measurement provides an accurate fluid flow rate. The total weight of the sensor is also being constantly measured with the load sensors  88 , and together these measurements provide an accurate mass flow rate calculation at steady state. 
     In addition to the major components just described, the system  10  also includes a number of sensor and control components. All of the pumps and centrifuges are powered from a central electrical power plant, with associated motor controllers for their operation. The operating parameters of the pumps and centrifuges, and all of the other sensor elements for frequency, temperature, level, and load, are also monitored at the same central location. The central location preferably comprises a palletized, air-quality controlled control cabin (not shown) so that the power and control components may also be lifted and transported to the work site. The control cabin further includes redundant computers for fault-tolerant operations of the system. The control cabin preferably includes a bidirectional satellite link to a global communications system, such as the Internet, for command, control, and remote monitoring of the system. This link further provides for remote adjustment of the operating parameters of the system in response to the sensed parameters as needed. 
     The principles, preferred embodiment, and mode of operation of the present invention have been described in the foregoing specification. This invention is not to be construed as limited to the particular forms disclosed, since these are regarded as illustrative rather than restrictive. Moreover, variations and changes may be made by those skilled in the art without departing from the spirit of the invention.

Summary:
A drilling mud clarification or reclamation system is provided. High gravity and low gravity solids are removed from the drilling mud in respective centrifugal separator stages. A plurality of in-line mass flow sensors are provided to provide real-time indication of the effectiveness of the clarification of the drilling mud, and to provide control signals to a central control station. The heavier weight components are separated from the mud and returned to the system for further use. The lighter weight components are removed and are discarded to clean the mud. A cuttings dryer is provided to remove oil from cuttings which have been separated from a shale shaker stage. A de-sludging centrifuge is also provided to remove very fine cuttings which may have a harmful effect on the viscosity of the mud.