Patent Publication Number: US-11384610-B2

Title: Closed loop drilling mud cooling system for land-based drilling operations

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 14/325,622, filed Jul. 8, 2014, and is hereby incorporated by reference for all it contains. 
    
    
     BACKGROUND 
     1. Field of the Disclosure 
     The present subject matter is generally directed to drilling mud cooling systems, and in particular, to systems and methods that may be used for cooling drilling mud in onshore drilling applications. 
     2. Description of the Related Art 
     During a typical well drilling operation, such as when drilling an oil and gas well into the earth, a drilling mud circulation and recovery system is generally used to circulate drilling fluid, i.e., drilling mud, into and out of a wellbore. The drilling mud provides many functions and serves many useful purposes during the drilling operation, such as, for example, removing drill cuttings from the well, controlling formation pressures and wellbore stability during drilling, sealing permeable formations, transmitting hydraulic energy to the drilling tools and bit, and cooling, lubricating, and supporting the drill bit and drill assembly during the drilling operations. 
     Drilling muds commonly include many different types of desirable solid particles that aid in performing one or more of the functions and purposes outlined above. The solids particles used in drilling muds may have one or more particular properties which make their presence in a given drilling mud mixture desirable and beneficial. For example, some solids particles may need to be of a certain size or size range, which may be useful in sealing off more highly permeable formations so as to prevent the loss of valuable drilling fluid into the formation—so-called “lost circulation materials.” Other solids particles may need to be of a certain density so as to control and balance forces within the wellbore, which may be added to the drilling mud as required to guard against wellbore collapse or a well blowout during the drilling operations. High density particulate materials, such as barium sulfate, or barite, (BaSO 4 ), and the like are often used for this purpose, as their greater unit volumetric weight serves to counterbalance high formation pressures and/or the mechanical forces caused by formations that would otherwise cause sloughing. In still other cases, solids particles may be added to the drilling mud based on a combination of the particle size and density, such as when a specific combination of the two properties may be desirable. Furthermore, the drilling mud in general, and the added solid particles in particular, can be very expensive. As such it is almost universally the case that, upon circulation out of the wellbore, the desirable—and valuable—solids particles are generally recovered and re-used during the ongoing drilling cycle. 
     Once the drilling mud has served its initial purposes downhole, the mud is then circulated back up and out of the well so that it can carry the drill cuttings that are removed from the advancing wellbore during the drilling operation up to the surface. As may be appreciated, the drill cuttings, which are also solids particles, are generally thoroughly mixed together with the desirable solids particles that, together with various types of fluids, make up the drilling mud, and therefore must be separated from the desirable solids particles, such as barite and the like. In the best possible drilling scenario, it is advantageous for the drill cuttings to be substantially larger than the desirable solids particles making up the drilling mud, thus enabling most of the drill cuttings to be removed using vibratory separator devices that separate particles based upon size, such as shale shakers and the like. However, in many applications, a portion of the drill cuttings returning with the drilling mud are similar in size, or even smaller than, at least some of the desirable solids particles contained in the drilling mud, in which case secondary separation devices, such as hydrocyclone and/or centrifuge apparatuses, are often employed so as to obtain further particle separation. 
     There are a variety of reasons why it is desirable, and even necessary, to remove as many of the drill cuttings particles from the drilling mud mixture as possible. A first reason would be so as to control and/or maintain the drilling mud chemistry and composition within a desirable range as consistently as possible. For example, the presence of drill cuttings particles in the drilling mud mixture may have a significant effect on the weight of the mud, which could potentially lead to wellbore collapse, and/or a blowout scenario associated with overpressure conditions within the well. More specifically, since the specific gravity of the drill cuttings particles are often significantly lower than that of the desired solids particles in the drilling mud, e.g., barite, the presence of cuttings particles left in the mud by the typical solids removal processes can cause the weight of the drilling mud to be lower than required in order to guard against the above-noted drilling conditions. 
     The temperature of the drilling mud may also significantly increase as it is being circulated down into and back up out of the drilled wellbore, particularly in high pressure and/or high temperature drilling operations. Elevated drilling mud temperatures can generally cause increased wear and tear on mud circulation equipment, thus potentially leading to premature equipment failure, increased frequency of equipment maintenance, associated shutdown (or non-productivity) time, and/or reduced overall equipment efficiency, thus adversely impacting overall drilling costs. Additionally, high drilling mud temperatures can also have a negative influence on the operation and/or performance of measurement while drilling (MWD) equipment, such as high signal attenuation and the like, or even a loss of communication with the MWD equipment during drilling operations. According, and depending on the specific downhole temperature conditions during drilling operations, the drilling mud must often be cooled prior to it being recirculating back down into the wellbore. 
       FIG. 1  schematically depicts a representative prior art drilling mud system  100  that is used to circulate and treat drilling mud during a typical drilling operation. As shown in  FIG. 1 , a blow-out preventer (BOP)  103  is positioned on a wellhead  102  as drilling operations are being performed on a wellbore  101 . In operation, hot drilling mud  110   h  mixed with drill cuttings  107  is circulated out of the wellbore  101  and exits the BOP  103  through the bell nipple  104 , and thereafter flows through the flow line  105  to the drill cuttings separation equipment  106 . As noted above, depending on the particle sizes of the returning drill cuttings  107  and the degree of particle separation required, the drill cuttings separation equipment  106  may include first stage separating equipment, such as one or more vibratory separators (e.g., shale shakers), as well as second stage separating equipment, such as one or more hydrocyclone and/or centrifuge apparatuses. However, for simplicity of illustration and discussion, the drill cuttings separation equipment  106  has been schematically depicted in  FIG. 1  as a shale shaker device, and therefore will hereafter be referred to as the shale shaker  106 . 
     After entering the shale shaker  106 , the undesirable drill cuttings  107  are separated from the hot drilling mud  110   h  and directed to a waste disposal tank or pit  108 . The separated hot drilling mud  110   h  then flows from the sump  109  of the shale shaker  106  to a hot mud pit or hot mud tank  111   h . Typically, the hot mud tank  111   h  is a large container having an open top so that the hot drilling mud  110   h  can be exposed to the environment. In this way, at least some of the heat that is absorbed by the drilling mud during the drilling operation (e.g., from the surrounding formation and/or from the generation of drill cuttings) can be released to the environment, thus allowing the hot drilling mud  110   h  to naturally cool, as indicated by heat flow lines  113 . 
     In some applications, the temperature of the hot drilling mud  110   h  exiting the bell nipple  104  and flowing to the separation equipment (shale shaker)  106  can be as high as approximately 175° F.-225° F. It should be appreciated that the degree of natural or passive cooling that can take place in the hot mud tank  111   h  is generally limited by the surrounding environmental conditions, such as ambient temperature and/or relative humidity, which can be affected by numerous factors. For example, some such natural cooling factors include the geographical location of the wellbore drilling site (e.g., artic, temperate, tropical, and/or equatorial regions, etc.), the time of year (e.g., the season or month), and even the time of day (e.g., night or day). Therefore, the amount of passive cooling is typically only incremental in nature, e.g., limited to no more than approximately a 5° F. reduction in mud temperature. In such cases, an enhanced degree of mud cooling is often required so as to further reduce the drilling mud temperature to a manageable level. 
     When additional mud cooling is required, the hot drilling mud  110   h  is further cooled in a mud cooler, such as the prior art mud cooler  130  shown in  FIG. 1 . In the configuration depicted in  FIG. 1 , a hot mud pump  131  is used to pump the hot drilling mud  110   h  from the hot mud tank  111   h  to a mud coil  132  of the mud cooler  130 . As the hot drilling mud  110   h  passes through the mud coil  132 , a water feed pump  134  is used to pump water  135  from a water tank  136  to an internal spray header  137 , which sprays the water  135  downward over the mud coil  132 . Simultaneously, one or more induced draft fans  133  located at the top of the mud cooler  130  generate an upward flow of air  138  across the mud coil  132 . In operation, the downward spray of water  135  from the spray header  137  and the upward flow of air  138  through the fans  133  acts to cool the hot drilling mud  110   h  flowing through the mud coil  132  by a combination of evaporative cooling and quenching of the coil, as indicated by the heat flow lines  139 . Water  135  sprayed from the internal spray header  134  is collected in a collection tray or collection tank  140  at the bottom of the mud cooler  130 , from which it is then pumped back to the water tank  136  by a water recycle pump  141  for further mud cooling operations in the mud cooler  130 , as described above. Under optimal conditions, a typical prior art mud cooler that is configured and operated in similar fashion to the mud cooler  130  shown in  FIG. 1  can generally achieve a further mud temperature reduction that ranges from 15° F.-20° F. 
     After the above-described mud cooling process, cooled drilling mud  110   c  exits the mud cooler  130 . In some configurations of the prior art system  100 , the cooled drilling mud  110   c  is directed to a cooled mud tank  111   c , where it may be further treated by adding desired solids and/or chemicals so as to appropriately adjust the rheology and/or other characteristics of the mud prior to pumping the cooled drilling mud  110   c  back into the wellbore  101 . Additionally, a further incremental temperature reduction of the mud  110   c  may again occur in the cooled mud tank  111   c  by way of passive cooling  113  to the ambient environment, as previously described with respect to the hot mod tank  111   h.    
     As shown in  FIG. 1 , after the above described separating, cooling, and/or treating operations, the drilling mud  110   c  flows from the cooled mud tank  111   c  to a mud pump  116  through the suction line  115 . In some applications, a mud booster pump  114  may be used to deliver the drilling mud  110  through the suction line  115  and to the suction side of the mud pump  116 . In operation, the mud pump  116  increases the pressure of the drilling mud  110  and discharges the pressurized drilling mud  110  to a standpipe  117 , after which the mud  110  flows through a rotary line  118  to a swivel  119  mounted at the upper end of a kelly  120 . The kelly  120  then directs the drilling mud  110   c  down to the drill pipe/drill string  121 , and the mud  110   c  is recirculated down the drill string  121  to a drill bit (not shown), where it once again provides, among other things, the cooling, lubrication, and drill cutting removal tasks previously described. 
     In other configurations, the system  100  may not include the cooled mud tank  111   c  shown in  FIG. 1 , or the system  100  could be configured to include appropriate valving so that the cooled mud tank  111   c  can be bypassed. In such configurations, the cooled drilling mud  110   c  flows directly from the mud cooler  130  and through the suction line  115  to the suction side of the mud pump  116 , where it is then pumped back into the wellbore  101  as previously described. 
     Additionally, the prior art system  100  can also be configured in such a way so that it can be operated in a mud cooler bypass mode. For example, as shown in  FIG. 1 , appropriate valving can be positioned within the system  100  and operated in such a way as to isolate the mud cooler  130  from the flow of hot drilling mud  110   h  exiting the hot mud tank  111   h . In such configurations, the system  100  can be operated so that the hot mud  110   h  flows directly from the hot tank  111   h  to the cooled mud tank  111   c , e.g., through a mud cooler bypass line  130   b . It should also be appreciated that when a cooled mud tank  111   c  is not provided, or when the cooled mud tank  111   c  is also bypassed (as described above), the hot drilling mud  110   h  will flow directly to the mud pump  116 . Such operational configurations can be used when maintenance is required on the mud cooler  130 , or during drilling operations wherein the temperature of the hot drilling materials mixture exiting the wellbore  101  does not require any additional cooling beyond the incremental passive capabilities of the hot and/or cold mud tanks  111   h  and  111   c.    
     It should be appreciated that, even when a mud cooler  130  is included in the system  100 , various conditions and/or operational parameters can act to detrimentally impact the overall mud temperature reduction capabilities of the system  100 , and can also contribute to an increase in overall drilling costs. More specifically, as noted above, the passive cooling capabilities of the hot and/or cold mud tanks  111   h  and  111   c  are generally significantly influenced by the surrounding environmental conditions at a given wellbore drilling site. For example, in regions where the ambient temperature conditions can be very high (e.g., 100° F. or higher)—such as in Middle Eastern, northern African, southern United States, and/or Central American locations—the passive natural cooling effects obtained from the mud tanks  111   h  and/or  111   c  can be severely limited, such as a maximum of approximately 5° F. reduction in mud temperature, or even less. In similar fashion, such high temperature and/or high relative humidity environments can also reduce the evaporative cooling effects of the mud cooler  130 , such that the maximum temperature reduction achievable under such conditions is no more than approximately 10° F.-15° F., or even less. Therefore, even when the mud cooler  130  is employed as part of the system  100 , the drilling mud temperature can often remain at or above approximately 150° F.-175° F. 
     Additionally, due to the quenching effects of the water spray system (i.e., elements  134 - 140 ) described above, the hot drilling mud  110   h  circulating through the mud coil  132  can often cake up and adhere to the inside surfaces of the coil  132 . Such mud caking effects can reduce the available flow area through the mud coil  132 , thus increasing pressure drop through the coil  132 . Furthermore, the insulating effects attributable to the caked layer of drilling mud on the inside surfaces of the mud coil  132  can also directly reduce the overall heat transfer/cooling capabilities of the mud cooler  130 . Moreover, due to the mud caking inside of the mud coil  132 , the mud cooler  130  must also be bypassed and shut down on a periodic basis for cleaning and maintenance, so that the caked drilling mud can be removed from the coil  132 . Accordingly, during such periodic cleaning and maintenance activities, the only mud cooling provided by the system  100  is the relatively small amount of passive incremental cooling  113  that occurs naturally to the surrounding environment, e.g., from the hot and/or cold mud tanks  111   h  and  111   c.    
     Furthermore, due to the basic evaporative cooling effects of the mud cooler  130 , it should be understood that some amount of the water  135  circulating through the cooler  130  will continuously be lost to the surrounding environment. For example, and depending on the specific ambient conditions in the area where the drilling operations are being performed, as much as 15-20 gallons per minute (gpm), or even more, of the water  135  may be lost to the ambient atmosphere during the operation of the mud cooler  130 . Consequently, the supply of water  135  that is lost to the surrounding environment must periodically be replenished, such as from a portable water tanker  142 , as shown in  FIG. 1 . Furthermore, it should be appreciated that in at least some remote and/or desert-like locations, such as drilling sites located in the Middle East and the like, water is oftentimes a precious commodity that may command a significant price, a situation that may be compounded by the generally high local ambient temperatures. Therefore, the replenishment of significant water losses to the surrounding environment during operation of the mud cooler  130  can have a substantial impact on the overall costs of drilling. 
     Accordingly, there is a need in the drilling industry for a mud cooling system that is less susceptible to the vagaries of the surrounding environmental conditions, and which does not require a continuous replenishment of a cooling water supply. The present disclosure is directed to mud cooling systems and methods of operating the same that may be used to mitigate, or possibly even eliminate, at least some of the problems associated with the prior art mud cooling systems described above. 
     SUMMARY OF THE DISCLOSURE 
     The following presents a simplified summary of the present disclosure in order to provide a basic understanding of some aspects disclosed herein. This summary is not an exhaustive overview of the disclosure, nor is it intended to identify key or critical elements of the subject matter disclosed here. Its sole purpose is to present some concepts in a simplified form as a prelude to the more detailed description that is discussed later. 
     Generally, the subject matter disclosed herein is directed to various new and unique systems, apparatuses, and methods for circulating and cooling drilling mud during wellbore drilling operations, and in particular, for high temperature drilling operations in onshore applications. In one illustrative embodiment, a method for cooling drilling mud is disclosed that includes, among other things, controlling operation of a first closed-loop cooling system to cool a flow of drilling mud when a first temperature of the flow of drilling mud exceeds a first predetermined mud set point temperature, and controlling operation of a second closed-loop cooling system to further cool the flow of drilling mud when a second temperature of the flow of drilling mud that has been cooled by the first closed-loop cooling system exceeds a second predetermined mud set point temperature. 
     In another exemplary embodiment disclosed herein a method for cooling drilling mud includes controlling operation of a first closed-loop cooling system to cool a flow of drilling mud when a first temperature of the flow of drilling mud exceeds a first predetermined mud set point temperature, wherein controlling the operation of the first closed-loop cooling system includes circulating a first cooling fluid through the first closed-loop cooling system and cooling the flow of drilling mud with the first cooling fluid. Furthermore, the illustrative method also includes controlling operation of a second closed-loop cooling system to further cool the flow of drilling mud when a second temperature of the flow of drilling mud that has been cooled by the first closed-loop cooling system exceeds a second predetermined mud set point temperature, wherein controlling the operation of the second closed-loop cooling system includes circulating a second cooling fluid through the second closed-loop cooling system and cooling the flow of drilling mud with the second cooling fluid. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure may be understood by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements, and in which: 
         FIG. 1  schematically depicts a representative prior art drilling mud system; 
         FIG. 2A  schematically depicts one illustrative embodiment of a drilling mud system disclosed herein; 
         FIG. 2B  schematically illustrates another exemplary drilling mud system in accordance with the present disclosure; and 
         FIG. 2C  schematically depicts an exemplary drilling mud cooler that may be used in conjunction with either of the drilling mud systems shown in  FIGS. 2A and 2B  in accordance with one illustrative embodiment of the present disclosure. 
     
    
    
     While the subject matter disclosed herein is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention. 
     DETAILED DESCRIPTION 
     Various illustrative embodiments of the present subject matter are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. 
     The present subject matter will now be described with reference to the attached figures. Various systems, structures and devices are schematically depicted in the drawings for purposes of explanation only and so as to not obscure the present disclosure with details that are well known to those skilled in the art. Nevertheless, the attached drawings are included to describe and explain illustrative examples of the present disclosure. The words and phrases used herein should be understood and interpreted to have a meaning consistent with the understanding of those words and phrases by those skilled in the relevant art. No special definition of a term or phrase, i.e., a definition that is different from the ordinary and customary meaning as understood by those skilled in the art, is intended to be implied by consistent usage of the term or phrase herein. To the extent that a term or phrase is intended to have a special meaning, i.e., a meaning other than that understood by skilled artisans, such a special definition will be expressly set forth in the specification in a definitional manner that directly and unequivocally provides the special definition for the term or phrase. 
     In general, the present disclosure is directed to various systems, apparatuses, and methods that may be used for circulating and cooling drilling mud during wellbore drilling operations, and in particular, during high temperature drilling operations in onshore applications. 
       FIG. 2A  schematically depicts one illustrative embodiment of a drilling mud system  200  in accordance with the present disclosure that may be used to circulate, cool, and treat drilling mud during a typical drilling operation. As shown in  FIG. 2A , a blow-out preventer (BOP)  203  may be positioned on a wellhead  202  as drilling operations are being performed on a wellbore  201 . In operation, hot drilling mud  210   h  mixed with drill cuttings  207  may be circulated out of the wellbore  201  and exits the BOP  203  through the bell nipple  204 , after which the hot mixture flows through the flow line  205  to the drill cuttings separation equipment  206 . As noted previously, the drill cuttings separation equipment  206  may include first stage separating equipment, such as one or more vibratory separators (e.g., shale shakers), as well as second stage separating equipment, such as one or more hydrocyclone and/or centrifuge apparatuses. However, for simplicity of illustration and discussion, the drill cuttings separation equipment  206  has been schematically depicted in  FIG. 2A  as a shale shaker device, and therefore will hereafter be referred to as the shale shaker  206 . 
     After entering the shale shaker  206 , the undesirable drill cuttings  207  may be separated from the hot drilling mud  210   h  and directed to a waste disposal tank or pit  208 . Thereafter, the separated hot drilling mud  210   h  may then flow from the sump  209  of the shale shaker  206  to a hot mud pit or tank  211   h . In some exemplary embodiments, the hot mud tank  211   h  may be a large container having an open top, thereby exposing the hot drilling mud  210   h  to the ambient atmosphere. Accordingly, at least a portion of the heat that is absorbed by the drilling mud during the drilling operations (e.g., from the surrounding formation and/or from the generation of drill cuttings) may be released to the surrounding environment, thus allowing the hot drilling mud  210   h  to cool passively or naturally, as indicated by heat flow lines  213 . 
     In certain embodiments of the system  200 , a hot mud pump  231  may be used to pump the hot drilling mud  210   h  from the hot mud tank  211   h  to a drilling mud cooler  230 , which may hereinafter in some cases be referred to simply as a mud cooler  230 . The mud cooler  230  may include a first stage mud heat exchanger  232   a  that is thermally coupled to a first stage closed-loop cooling system  250  and a second stage mud heat exchanger  232   b  that is thermally coupled to a second closed-loop cooling system  270 . As shown in  FIG. 2A , the hot drilling mud  210   h  may initially flow through the first stage mud heat exchanger  232   a , where at least a portion of the heat contained in the hot drilling mud  210   h  is exchanged with the first stage closed-loop cooling system  250 , and then into the second stage mud heat exchanger  232   b , where a further portion of heat is exchanged with the second closed-loop cooling system  270 , as will be further described in conjunction with  FIG. 2C  below. Thereafter, cooled drilling mud  210   c  flows out of the second stage mud heat exchanger  232   b  and out of the mud cooler  230  for further circulation through the system  200 . Additionally, in at least some embodiments, a control system  295  may be operatively coupled to the mud cooler  230 , and the control system  295  may be adapted to control the operation of the various elements of the mud cooler  230  so as to achieve a predetermined set point temperature of the cooled drilling mud  210   c.    
     As noted previously, after the above-described mud cooling process, the cooled drilling mud  210   c  exits the mud cooler  230 . In certain illustrative embodiments, the cooled drilling mud  210   c  may be directed to a cooled mud tank  211   c , where it may be further treated by adding desired solids and/or chemicals so as to appropriately adjust the rheology and/or other characteristics of the mud prior to pumping the cooled drilling mud  210   c  back into the wellbore  201 . Furthermore, an additional amount of incremental temperature reduction of the cooled drilling mud  210   c  may also occur in the cooled mud tank  211   c  by way of passive cooling  213  to the ambient environment, as previously described with respect to the hot mud tank  211   h . Additionally, while the system  200  shown in  FIG. 2A  depicts the hot mud tank  211   h  as being separate from the cooled mud tank  211   c , it should be appreciated that  FIG. 2A  is a schematic illustration only. As such, in at least some embodiments the hot mud tank  211   h  and the cooled mud tank  211   c  may be separate chambers of a larger common mud tank. Moreover, either or both of the hot and cooled mud tanks  211   h  and  211   c  may be configured to have separate chambers (not shown), such as, for example, chambers that may be separated by overflow weirs and the like so as to thereby maximize the residence time of the drilling mud as it flows through each tank, thus enhancing the passive cooling  213  in the tanks  211   h ,  211   c.    
     As shown in  FIG. 2A , after the drilling mud has been cooled and/or treated as described above, a flow of the cooled drilling mud  210   c  may then be directed from the cooled mud tank  211   c  to a mud pump  216  through the mud pump suction line  215 . In some embodiments, a mud booster pump  214  may be used to pump the cooled drilling mud  210   c  through the suction line  215  and to the suction side of the mud pump  216 . Thereafter, the mud pump  216  may be operated so as to increase the pressure of the cooled drilling mud  210   c  and to discharge the pressurized mud  210   c  to a standpipe  217 , from which the mud  210  may flow through a rotary line  218  to a swivel  219  mounted at the upper end of a kelly  220 . The kelly  220  may then direct the flow of cooled drilling mud  210   c  down to the drill pipe/drill string  221 , after which the mud  210   c  may be recirculated down the drill string  221  to a drill bit (not shown), where it once again may provide the cooling, lubrication, and drill cutting removal tasks previously described. 
     In other exemplary embodiments, the system  200  may not include the cooled mud tank  211   c  depicted in  FIG. 2A , or the system  200  may be configured to include appropriate valving so that the cooled mud tank  211   c  can be bypassed during system operation. In such embodiments, the cooled drilling mud  210   c  may flow directly from the mud cooler  230  to the suction line  215 , where it may then be directed to the suction side of the mud pump  216  and pumped back into the wellbore  201  as previously described. 
     In still other illustrative embodiments, the system  200  of  FIG. 2A  may be configured in such a way so that it can be operated in a mud cooler bypass mode when maintenance is required on the mud cooler  230 . For example, as shown in  FIG. 2A , appropriate valving may be positioned within the system  200  and operated so as to isolate the mud cooler  230  from the flow of hot drilling mud  210   h  that is pumped from the hot mud tank  211   h  by the hot mud pump  231 . Furthermore, in such embodiments the system  200  may be operated so that the hot mud  210   h  flows directly from the hot tank  211   h  to the cooled mud tank  211   c , e.g., through a mud cooler bypass line  230   b . Additionally, it should also be appreciated that in those embodiments wherein a cooled mud tank  211   c  may not be provided, or when the cooled mud tank  211   c  is also bypassed (as described above), the flow of hot drilling mud  210   h  may be controlled so as to flow directly to the mud pump  216 . 
       FIG. 2B  schematically depicts another exemplary embodiment of the drilling mud system  200  that is similar in many respects to the system  200  shown in  FIG. 2A , except that the drilling mud flow between the various components of the system  200  illustrated in  FIG. 2B  has been differently configured. For example, as with the system  200  shown in  FIG. 2A , the system  200  of  FIG. 2B  includes substantially the same major components, such as the wellhead  202  and BOP  203 , the shale shaker  206 , the hot mud tank  211   h , the cooled mud tank  211   c , the mud cooler  230 , and the mud pump  216 . However, rather than circulating the drilling mud from the hot mud tank  211   h  to the mud cooler  230  as shown in  FIG. 2A , the system  200  of  FIG. 2B  is configured so that the drilling mud entering the mud cooler  230  flows instead from the cooled mud tank  211   c , as will be further described below. 
     As with the system  200  of  FIG. 2A , after the undesirable drill cuttings  207  have been separated from the hot drilling mud  210   h , the separated hot drilling mud  210   h  may then flow to the hot mud tank  211   h . However, in some embodiments, the hot drilling mud  210   h  flowing into the hot mud tank  211   h  may be mixed in the tank  211   h  with a cooled drilling mud  210   z  that is flowing from the mud cooler  230  (where it has been cooled as described with respect to  FIG. 2A  above), thus forming the drilling mud mixture  210   x . As previously described, the drilling mud mixture  210   x  may experience some amount of passive cooling  213  while in the hot mud tank  211   h . The drilling mud mixture  210   x  may then flow directly from the hot mud tank  211   h  to the cooled mud tank  211   c , where an additional amount of passive cooling  213  may occur so as to further reduce the temperature of the mud mixture  210   x.    
     As shown in  FIG. 2B , the mud circulation pump  231  may then be used to circulate a portion of the drilling mud mixture  210   x  (identified in  FIG. 2B  as drilling mud  210   y ) from the cooled mud tank  211   c  to the mud cooler  230 , which is configured as described above with respect to  FIG. 2A . Additionally, another portion of the drilling mud mixture  210   x , identified as cooled drilling mud  210   c , is circulated from the cooled mud tank  211   c  through the mud suction line  215  to the mud pump  216 , e.g., by the mud booster pump  214 , and back down the wellbore  201  in the manner described with respect to  FIG. 2A  above. 
     In certain embodiments, after being cooled in the mud cooler  230 , the drilling mud mixture  210   y  may then flow back to the hot mud tank  211   h  as the cooled drilling mud  210   z , where it may then mix with the hot drilling mud  210   h  flowing from the shale shaker  206  so as to form the drilling mud mixture  210   x  as described above. As with the system  200  of  FIG. 2A , the control system  295  may control the operation of the various elements of the mud cooler  230  so as to achieve a predetermined set point temperature of the cooled drilling mud  210   z.    
     When drilling mud is circulated through the system  200  in the manner described above, the residence time of the drilling mud mixture  210   x  in the hot and cooled mud tanks  211   h  and  211   c  may be increased. This is due at least in part to the portion  210   y  of the drilling mud mixture  210   x  that is circulated through the mud cooler  230 , from which it then exits as cooled drilling mud  210   z  and subsequently re-enters the hot mud tank  211   h , where it then mixes with the hot drilling mud  210   h . This increased residence time increases the amount of passive cooling  213  that may occur. Furthermore, the recirculation of a portion  210   y  of the drilling mud mixture  210  from the hot mud tank  211   h , to the cold mud tank  211   c , through the mud cooler  230 , and back to the hot mud tank  211   h  also allows the mud to be cooled more than one time. This mud recirculation thus acts to further reducing the temperature of the cooled drilling mud  210   c  flowing from the cooled mud tank  211   c  and back through the suction line  215  to the mud pump  216  for pumping into the wellbore  201 . 
     In certain illustrative embodiments, the system  200  of  FIG. 2B  may also be configured and operated in such a manner that the cooled drilling mud  210   z  is mixed with the hot drilling mud  210   h  in the cooled mud tank  211   c , rather than in the hot mud tank  211   h  as described above. For example, a bypass line  230   b  and appropriate valving may be positioned between the mud cooler  230  and the hot mud tank  211   h , as shown in  FIG. 2B . During operation of the system  200 , the valving may then be actuated as desired so as to direct the cooled drilling mud  210   z  exiting the mud cooler  230  through the bypass line  230   b  to the cooled mud tank  211   c . Furthermore, the system  200  may be controlled such that this hot mud tank bypass mode is actuated as necessary so as to meet predetermined mud set point temperature for the cooled mud  210   c  flowing from the cooled mud tank  211   c  to the mud pump  216 . 
     As noted with respect to the system  200  of  FIG. 2A  above, in at least some exemplary embodiments, the hot mud tank  211   h  and the cooled mud tank  211   c  may be separate chambers of a larger common mud tank. Furthermore, the cooled mud tank  211   c  may be configured to have separate chambers (not shown), such as, for example, chambers that may be separated by overflow weirs and the like. In such embodiments, the bypass line  230   b  may be configured to return the cooled drilling mud  210   z  exiting the mud cooler  230  to the same chamber of the cooled mud tank  211   c  where the drilling mud mixture  210   x  from the hot mud tank  211   h  enters the cooled mud tank  211   c —i.e., where the mud in the tank  211   c  may be hottest. Furthermore, the cooled mud tank  211   c  may be configured such that the drilling mud  210   y  and the cooled drilling mud  210   c  are drawn from a chamber that is at an opposite end of the tank  211   c  from the chamber where the cooled drilling mud  210   z  and/or the hot drilling mud  210   h  enter the tank  211   c —i.e., where the mud in the tank  211   c  may be coolest. In this way, the residence time of the recirculated cooled mud  210   z  in the cooled mud tank  211   c  may be maximized, thus also substantially maximizing the passive cooling  213  of the drilling mud mixture  210   x . Of course, it should be appreciated that other configurations of the bypass line  230   b  and cooled mud tank  211   c  may also be used, depending on the overall design parameters and/or mud cooling requirements of the system  200 . 
     In some embodiments, the system  200  of  FIG. 2B  may be operated in a mud cooler bypass mode when maintenance is required on the mud cooler  230 . For example, as shown in  FIG. 2B , appropriate valving may be positioned in the flow line between the cooled mud tank  211   c  and the mud cooler  230  operated so as to isolate the mud cooler  230  from the flow of drilling mud  210   y  that is pumped from the cold mud tank  211   c  by the mud circulation pump  231 . In such embodiments, the system  200  may be operated so that the hot mud  210   h  flows directly from the hot tank  211   h  to the cooled mud tank  211   c  and from the cooled mud tank  211  to the mud pump  216 , e.g., without recirculating the portion  210   y  of drilling mud through the mud cooler  230  and/or back through the hot mud tank  211   h.    
       FIG. 2C  is a more detailed schematic diagram of the mud cooler  230  that may be used in conjunction with either of the drilling mud systems  200  depicted in  FIGS. 2A and 2B . As shown in  FIG. 2C , the hot drilling mud  210   h  of  FIG. 2A  (or the drilling mud  210   y  of  FIG. 2B ) initially enters the first stage mud heat exchanger  232   a , which is thermally coupled to the first stage closed-loop cooling system  250  by a first stage cooling liquid  260  that is circulated through both the first stage mud heat exchanger  232   a  and the first stage closed-loop cooling system  250 . In the first stage mud heat exchanger  232   a , a portion of the heat contained in the hot drilling mud  210   h / 210   y  is exchanged with the first stage cooling liquid  260  that subsequently flows through and is cooled by the first stage closed-loop cooling system  250 . The cooling liquid  260  may be any suitable cooling liquid, such as water or a water/glycol mixture and the like. Furthermore, in some embodiments the cooling liquid  260  may be circulated through the first stage mud heat exchanger  232   a  and the first stage closed-loop cooling system  250  by a first stage fluid circulation pump  233   a , as shown in  FIG. 2C . 
     For purposes of the present disclosure and the appended claims, a “closed-loop cooling system” should be understood as one wherein the same cooling liquid, e.g., water or a water/glycol mixture, is continuously circulated through the system without any cooling liquid losses from the system to the environment, and without any cooling liquid being added to the system during normal operations. Accordingly, it should be understood that, unlike the water spray system  134 - 140  that is employed in the prior art mud cooler  130 , a continuous replenishment of cooling liquid  260  is generally not required when the first stage closed-loop cooling system  250  is operated under normal conditions. 
     In operation, the cooling liquid  260  is heated in the first stage mud heat exchanger  232   a  by the hot drilling mud  210   h / 210   y , and the heated cooling liquid  260  exits the first stage mud heat exchanger  232   a  at a temperature  250   h . The first stage fluid circulation pump  233   a  may then pump the heated cooling liquid  260  to the first stage closed-loop cooling system  250 , where it passes through the cooling coil  255  of an air cooled heat exchanger  254 , which may hereafter be referred to in shorthand fashion as an “air cooler” in the following description and in the appended claims. A plurality of induced draft cooling fans  256  mounted on the air cooler  254  may then cool the cooling liquid  260  by drawing a flow of air across the cooling coil  255  so as to reject the heat absorbed by the cooling liquid  260  in the first stage mud heat exchanger  232   a  by dissipating the heat to the atmosphere, as indicated schematically by the heat flow lines  259  shown in  FIG. 2C . After being cooled in the air cooler  254 , the cooled cooling liquid  260  may then be circulated out of the first stage closed-loop cooling system  250  and back to the first stage mud heat exchanger  232   a , where it enters the first stage exchanger  232   a  at a temperature  250   c.    
     In some embodiments, the first stage closed-loop cooling system  250  may include a first stage buffer tank  261 . As shown in  FIG. 2C , the first stage buffer tank  261  may be arranged such that the heated cooling liquid  260  passes through the first stage buffer tank  261  after exiting the first stage mud heat exchanger  232   a  and prior to entering the air cooler  254 . In certain embodiments, the first stage buffer tank  261  may be sized such that the residence time of the heated cooling liquid  260  in the tank  261  facilitates an additional nominal drop in the temperature of the cooling liquid  260  of approximately a 1° F.-2° F. before it enters the air cooler  254 . 
     As the cooling liquid  260  is heated by the hot drilling mud  210   h / 210   y  in the first stage mud heat exchanger  232   a , the mud  210   h / 210   y  is also correspondingly cooled by the cooling liquid  260  during their passage through the first stage exchanger  232   a . An intermediate (reduced) temperature drilling mud  210   i  may then exit the first stage mud heat exchanger  232   a  and pass to the second stage mud heat exchanger  232   b  for additional mud cooling (as may be required) in the manner further described below. In at least some embodiments, the first stage mud heat exchanger  232   a  may be, for example, a plate and frame heat exchanger and the like, which may thus provide large contact surface areas and high turbulence of the fluids flowing therethrough, thereby maximizing the overall heat transfer coefficient between the cooling liquid  260  and the hot drilling mud  210   h / 210   y . However, it should be understood that other types of heat exchangers may also be used for the first stage mud heat exchanger  232   a  depending on the various overall design parameters of the mud cooler  230 , such as the required mud temperature drop, mud flow rate, size and/or space limitations on the mud cooler  230 , and the like. 
     In certain other embodiments, the size and/or configuration of the air cooler  254  may also be similarly adjusted based on the various design parameters of the first stage closed-loop cooling system  250 . For example, the quantity and flow rate capacity of the induced draft fans  256  and the tube size and/or surface area of the cooling coil  255  may be optimized based on the anticipated ranges of the ambient operating conditions (e.g., ambient temperature and/or relative humidity, as previously described), the size and/or space limitations of the mud cooler  230 , and the like. 
     As noted above, after the intermediate (reduced) temperature drilling mud  210   i  has exited the first stage mud heat exchanger  232   a , it may then enter the second stage mud heat exchanger  232   b , which is thermally coupled to the second stage closed-loop cooling system  270  by a second stage cooling liquid  280  that is circulated through both the second stage mud heat exchanger  232   b  and the second stage closed-loop cooling system  270  for further cooling, as may be required. In the second stage mud heat exchanger  232   b , a portion of the heat contained in the intermediate temperature drilling mud  210   i  may be exchanged with the second stage cooling liquid  280 , which subsequently flows through and is cooled by the second stage closed-loop cooling system  270 . As with the first stage cooling liquid  260 , the second stage cooling liquid  280  may be any suitable cooling liquid, such as water or a water/glycol mixture, and the like. Furthermore, as shown in  FIG. 2C  the cooling liquid  280  may be circulated through the second stage mud heat exchanger  232   b  and the second stage closed-loop cooling system  270  by a second stage fluid circulation pump  233   b.    
     It should be appreciated that the term “closed-loop cooling system” as applied to the second closed-loop cooling system  270  may be understood in similar fashion as to how that term is applied to the first stage closed-loop cooling system  250  and described above. Accordingly, the second closed-loop cooling system  270  is also one wherein there is typically no loss of cooling liquid  280  from the system  270  to the environment, and where the addition of any further amount of cooling liquid  280  the system  270  during normal system operation is generally not required. 
     In the illustrative embodiment depicted in  FIG. 2C , the cooling fluid  280  may be heated in the second stage mud heat exchanger  232   b  by the intermediate temperature drilling mud  210   i , after which the heated cooling fluid  280  may exit the second stage mud heat exchanger  232   b  at a temperature  270   h . The second stage fluid circulation pump  233   b  may then pump the heated cooling fluid  280  to the second stage closed-loop cooling system  270 , where it passes through and is chilled by an evaporator  271 . In some embodiments, the evaporator  271  may be part of a refrigeration system that includes first and second refrigeration chiller units  270   a/b , as shown in  FIG. 2C . The first and second refrigeration chiller units  270   a/b  may include respective cooling coils  272   a/b , as well as several other refrigeration unit components as will be described in further below. In certain embodiments, the heated cooling fluid  280  may be chilled as it flows through the evaporator  271  by exchanging heat with a refrigerant  290  that is passing through one or both of the cooling coils  272   a/b . After being chilled in the evaporator  271 , the chilled cooling fluid  280  may then be circulated out of the second stage closed-loop cooling system  270  and back to the second stage mud heat exchanger  232   b , which it may then re-enter at a temperature  270   c.    
     As the cooling fluid  280  is heated by the intermediate temperature drilling mud  210   i  in the second stage mud heat exchanger  232   b , the intermediate temperature mud  210   i  is also correspondingly cooled by the cooling fluid  280  during their respective passage through the second stage exchanger  232   b . Accordingly, cooled drilling mud  210   c / 210   z  may exit the second stage mud heat exchanger  232   b , where it may then be circulated through the system  200  as previously described (see,  FIGS. 2A and 2B ). Additionally, as noted with respect to the first stage mud heat exchanger  232   a  above, in certain illustrative embodiments the second stage mud heat exchanger  232   b  may also be a plate and frame heat exchanger, although it should be understood that other types of heat exchangers may also be used for the second stage mud heat exchanger  232   b , depending on the overall design parameters of the mud cooler  230 . 
     As noted above, the heated second stage cooling fluid  280  exiting the second stage mud heat exchanger  232   b  may be chilled in the evaporator  271  by a refrigerant  290  passing through at least one of the dual cooling coils  272   a/b . As shown in  FIG. 2C  and noted above, in at least some exemplary embodiments of the present disclosure, the cooling coils  272   a/b  disposed in the evaporator  271  may be one of several components of the respective first and second refrigeration chiller units  270   a/b , which may also include respective compressors  273   a/b , respective condensing coils  275   a/b  disposed in a condensing unit  274 , and respective expansion devices  278   a/b . Additionally, in at least some embodiments, the first and second refrigeration chiller units  270   a/b  may also include respective flash tanks  277   a/b , as will be described in further detail below. Furthermore, it should be understood that the refrigerant  290  may be any appropriate type of refrigerant known in the art, such as, for example R134A (1,1,1,2-tretrafluoroethane) and the like, although other types of refrigerants may also be used. 
     In an exemplary embodiment wherein the refrigerant  290  is passing through both of the cooling coils  272   a/b , after the refrigerant  290  has exchanged heat with and chilled the second stage cooling fluid  280  in the evaporator  271 , the refrigerant  290  exits the respective cooling coils  272   a/b  as a warm low pressure vapor  290   a . Thereafter, the warm low pressure vapor  290   a  may enter the suction side of a respective compressor  273   a/b , where the pressure and temperature of the refrigerant  290  are both increased and the refrigerant exits the compressors  273   a/b  as a high pressure superheated gas  290   b . In certain illustrative embodiments, the compressors  273   a/b  may be, for example, rotary screw compressors and the like, although it should be understood that other types of compressors may also be used, depending on the specific design parameters and desired operational characteristics of the refrigeration chiller units  270   a/b  of the second closed-loop cooling system  270 . 
     After exiting the discharge side of the respective compressors  273   a/b , the high pressure superheated gas  290   b  may then enter the respective condensing coils  275   a/b  of the condensing unit  274 . A plurality of induced draft cooling fans  276  mounted on the condensing unit  274  may then cool the high pressure superheated gas  290   b  by drawing air a flow of air across each of the respective condensing coils  275   a/b , thereby rejecting the heat that is absorbed by the refrigerant  290  from the cooling fluid  280  in the evaporator  271  as well as the heat that is added to the refrigerant  290  in the compressors  273   a/b  by dissipating the heat to the atmosphere, as is schematically depicted by the heat flow lines  279  shown in  FIG. 2C . After being cooled in the condensing unit  274 , the cooled refrigerant exits the respective coils  275   a/b  as a high pressure subcooled liquid  290   c , which may also include some amount of vapor. 
     In some embodiments, after the high pressure subcooled liquid refrigerant  290   c  has exited each of the respective condensing coils  275   a/b , it may then be circulated to the respective expansion devices  278   a/b —which may be, for example, expansion valves or metering orifices and the like—where the pressure of the refrigerant  290  may be dropped in a controlled manner so as to create low pressure subcooled liquid refrigerant  290   e . The low pressure subcooled liquid refrigerant  290   e  then passes back to the evaporator  271 , where it vaporizes into the warm low pressure gas  290   a  as it absorbs heat from the second stage cooling fluid  280 , as previously described. In other embodiments, such as when a respective flash tank  277   a/b  may be included in the first and second refrigeration chiller units  270   a/b , the high pressure subcooled liquid refrigerant  290   c  may first pass through the respective flash tanks  277   a/b , and any refrigerant vapor  290   d  mixed with the liquid refrigerant  290   c  coming from the condensing unit  274 , or that may flash off of the liquid refrigerant  290   c  in the flash tanks  277   a/b , may then be redirected back to the respective compressors  273   a/b  for compression and subsequent re-cooling through the condensing unit  274 . Thereafter, the high pressure subcooled liquid  290   c  passes from the flash tanks  277   a/b  to the expansion devices  278   a/b  and on to the evaporator, as described above. 
     In some embodiments, the second stage closed-loop cooling system  250  may also include a second stage buffer tank  281 . As shown in  FIG. 2C , the second stage buffer tank  281  may be arranged such that the heated cooling fluid  280  passes through the second stage buffer tank  281  after exiting the second stage mud heat exchanger  232   b  and prior to entering the evaporator  271 . In certain embodiments, the second stage buffer tank  281  may be sized such that the residence time of the heated cooling fluid  280  in the tank  281  facilitates an additional nominal drop in the temperature of the cooling fluid  280  of approximately a 2° F.-5° F. before entering the evaporator  271 . 
     Additionally, the size and/or configuration of the condensing unit  274  may also be adjusted based on the various design parameters of the second stage closed-loop cooling system  270 . For example, in some embodiments, the quantity and flow rate capacity of the induced draft fans  276  and the tube size and/or surface area of the condensing coils  275   a/b  may be optimized based on the anticipated ranges of the ambient operating conditions (e.g., ambient temperature and/or relative humidity, as previously described), the overall size and/or space limitations of the mud cooler  230 , and the like. Furthermore, while  FIG. 2C  schematically depicts that the condensing coils  275   a/b  are both part of a common condensing unit  274 , it should be understood that, depending on the design and/or layout of the second closed-loop cooling system  270 , individual condensing units may be used for each of the respective condensing coils  275   a  and  275   b.    
     The mud cooler  230  may be adapted to cool drilling mud under a wide range of ambient temperature conditions, such as between a low ambient temperature of approximately 35° F.-40° F. and a high ambient temperature of approximately 120° F.-125° F. Furthermore, the mud cooler  230  may also be adapted to receive and cool hot drilling mud  210   h / 210   y  which has a temperature that ranges as high as approximately 150° F.-200° F. and a mud flow rate between about 300 gpm and 500 gpm, or even greater. In some embodiments, the control system  295  may be adapted to control the operation of the various elements of the mud cooler  230 , e.g., the first and second closed-loop cooling systems  250  and  270  and the like, under such ambient temperature and hot mud flow rate and temperature conditions so that the intermediate temperature drilling mud  210   i  exits the first stage mud heat exchanger  232   a  having a temperature that is between about 145° F.-150° F., and so that the cooled drilling mud  210   c / 210   z  exits the second stage mud heat exchanger  232   b  at a temperature that ranges from about 120° F.-130° F. In such embodiments, the control system  295  may also control the first stage closed-loop cooling system  250  so that the temperature  250   c  of the cooled first stage cooling fluid  260  as it enters the first stage mud heat exchanger  232   a  ranges between about 120° F.-125° F. and the subsequently heated cooling liquid  260  exits the first stage exchanger  232   a  with a temperature  250   h  ranging from 140° F.-145° F. 
     Furthermore, the second closed-loop cooling system  270  may be controlled so that the temperature  270   c  of the chilled second stage cooling liquid  280  entering the second stage mud heat exchanger  232   b  ranges from approximately 55° F.-60° F. and temperature  270   h  of the subsequently heated cooling liquid  280  exiting the second stage exchanger  232   b  is between about 65° F.-70° F. 
     As noted above, the control system  295  may be configured and/or programmed to control the operation of the mud cooler  230  under a variety of operating conditions, including varying ambient conditions, varying hot drilling mud temperatures and/or flow rates, and/or varying cooled drilling mud set point temperatures, and the like. Following is a description of one illustrative drilling mud cooler control methodology that may be used by the control system  295  to achieve a desired temperature of the cooled drilling mud  210   c  by adjusting the amount of drilling mud cooling that is provided by the mud cooler  230  through a sequentially staged operation of the first and second stage closed-loop cooling systems  250  and  270 . 
     As an initial step in controlling the operation of the mud cooler  230 , a predetermined mud set point is established as the target temperature of the cooled drilling mud  210   c  exiting the mud cooler  230  (in the case of the system  200  of  FIG. 2A ) or of the cooled drilling mud  210   c  exiting the cooled mud tank  211   c  (in the case of the system  200  of  FIG. 2B ). In some embodiments, the mud set point temperature may be programmed into the control system  295  through an appropriate human/machine interface (HMI) system  296 , such as a control panel, computer screen and keyboard, and/or any other appropriate HMI system known in the art. In some embodiments, the mud set point temperature may be in the range of about 120° F.-140° F., whereas in at least one embodiment the mud set point temperature may be approximately 135° F., although it should be appreciated that other mud set point temperatures may also be used, depending on the overall operational requirements of the system  200  and the mud cooler  230 . 
     During operation of the mud circulation system  200  (see,  FIGS. 2A and 2B ), the control system  295  continuously monitors the incoming temperature of the hot drilling mud  210   h / 210   y  flowing through the flow line  205 . When the temperature of the hot mud  210   h / 210   y  exceeds the mud set point temperature, the control system  295  controls the operation of the first and second closed-loop cooling systems  250  and  270  so as to sequentially stage on and off as required in order to lower the temperature of the cooled drilling mud  210   c  down to at least the targeted mud set point temperature. For example, during an early phase of a drilling operation, the temperature of the hot drilling mud  210   h / 210   y  returning from the wellbore  201  may initially stay below the mud set point temperature, e.g., 120° F., when the wellbore  201  is initially relatively shallow and has not yet reached wellbore depths having high formation temperatures, and/or the amount of heat generated by the actual crushing or shearing of rock remains relatively low. In such early-phase low temperature drilling operations, both the first and second closed-loop cooling systems  250  and  270  may remain in a cooling standby mode until such time as the temperature of the mud returning from the wellbore, i.e., the hot drilling mud  210   h / 210   y , rises above the mud set point temperature. Once the temperature of the hot drilling mud  210   h / 210   y  exceeds the mud set point, the control system  295  may then initiate operation of the first and second stage closed-loop cooling systems  250  and  270  in sequential stages based upon the overall cooling requirements necessary to bring the drilling mud temperature of the cooled drilling mud  210   c  at least down to the predetermined drilling mud set point temperature. Therefore, the control system  295  may initially start up the first stage closed-loop cooling system  250  so as to begin cooling the hot drilling mud  210   h / 210   y ; however, the second stage closed-loop cooling system  270  may remain in the cooling standby mode until additional mud cooling capacity is required, as will be further described below. 
     In some embodiments, operation of the first stage closed-loop cooling system  250  is initiated by first starting up the cooling fans  256  of the air cooler  254 . In certain embodiments, the cooling fans  256  may be started up sequentially by the control system  295  with a fixed time delay between the startup of each fan  256 , such as approximately 10 seconds, so as to minimize any spiking of the power requirements imposed on the power system (not shown) that is used to supply power to the mud cooler  230 . After all of the cooling fans  256  have been brought on line, the control system  295  may then initiate operation of the first stage fluid circulation pump  233   a  so as to ramp up the flow rate of the first stage cooling liquid  260  through the cooling coil  255  of the air cooler  254  to approximately the maximum normal operating capacity of the first stage pump  233   a . In this way, the cooling capacity of the first stage closed-loop cooling system  250  may be substantially maximized so that the second stage closed-loop cooling system  270  may remain off line and in cooling standby mode until the cooling capacity of the first stage closed-loop cooling system  250  is no longer sufficient to keep the mud temperature of the cooled drilling mud  210   c  at or below the predetermined mud set point temperature. 
     In certain embodiments, the control system  295  may operate the first stage closed-loop cooling system  250  at substantially a constant maximum cooling capacity as described above—i.e., based on the maximum flow capacities of the cooling fans  256  and the first stage fluid circulation pump  233   a —and only bring the second stage closed-loop cooling system  270  on line and out of cooling standby mode as may be required to provide additional mud cooling. Furthermore, the first stage closed-loop cooling system  250  may be operated continuously at the maximum capacities noted above until the drilling conditions and/or the ambient atmospheric conditions are such that the temperature of the hot drilling mud  210   h / 210   y  flowing through the system  200  drops by a predetermined number of degrees below the mud set point temperature, such as by approximately 2° F.-4° F. When such a hot drilling mud temperature condition occurs, the control system  295  may then shut down the first stage closed-loop cooling system  250  so as to conserve power. The first and second closed-loop cooling systems  250  and  270  may then both remain in the cooling standby mode until such time as the temperature of the hot drilling mud  210   h / 210   y  rises back up to and/or above the predetermined mud set point temperature, at which time the first stage closed-loop cooling system  250  may be brought back on line so as to provide the requisite mud cooling. 
     In other illustrative embodiments, when the first stage closed-loop cooling system  250  is being operated continuously at substantially the maximum flow rate and cooling capacities noted above and the temperature of the cooled drilling mud  210   c  exiting the mud cooler  230  in the system  200  of  FIG. 2A  (or the cooled mud tank  211   c  in the system  200  of  FIG. 2B ) rises above the predetermined mud set point temperature, the control system  295  may then operate to initiate startup of the second stage closed-loop cooling system  270  so as to provide additional mud cooling capacity and to bring the temperature of the cooled drilling mud  210   c  down below the mud set point temperature. Such an increased temperature of the cooled drilling mud  210   c  may occur for a variety of reasons. For example, the moving mud temperature at the bottom of the wellbore  202 —generally caused by a combination of the formation temperature and the heat generated by the drilling operation—may rise above temperature level that the mud cooler  230  is capable of lowering below the predetermined mud set point temperature by operation of the first stage closed-loop cooling system  250  alone. Additionally, the ambient conditions of the environment surrounding the mud cooler, e.g., the ambient temperature and/or relative humidity, may have changed in such a manner as to reduce the efficiency and/or overall cooling capability of the first stage closed-loop cooling system  250 , such as change from nighttime drilling operations to daytime drilling operations. Moreover, in some embodiments, a combination of both the mud temperature and ambient environment parameters may contribute to the rise in the temperature of the cooled drilling mud  210   c  above the predetermined mud set point temperature. 
     In operation, when the control system  295  initiates startup of the second stage closed-loop cooling system  270 , the first refrigeration unit  270   a  of the second stage closed-loop cooling system  270  will be initially brought on line so as to handle the additional cooling requirements needed to address the increase in temperature of the cooled drilling mud  210   c . In order to reduce overall power consumption to the mud cooler  230 , the operation of the first refrigeration unit  270   a  will ramp up gradually and/or incrementally only so as to meet the necessary cooling requirements to reduce the temperature of the cooled drilling mud  210   c  down to at least the mud set point temperature. On the other hand, the second refrigeration unit  270   b  may remain off line and in standby cooling mode until such time as the additional cooling capacity provided by first refrigeration unit  270   a  alone cannot meet the cooling needs of the mud cooler  230 . In other words, second refrigeration unit  270   b  of the second stage closed-loop cooling system  270  will not brought on line and off of cooling standby until the overall mud cooling that is provided by the first stage closed-loop cooling system  250  and the first refrigeration unit  270   a  is insufficient to keep the temperature of the cooled drilling mud  210   c  at or below the predetermined mud set point temperature. In this way, not only may the control system  295  be adapted to conserve power by sequentially staging the operation of the first and second stage closed-loop cooling systems  250  and  270 , the control system  295  may also be adapted to further conserve power by sequentially staging the operation of the first and second refrigeration chiller units  270   a/b  of the second stage closed-loop cooling system  270 . 
     In certain exemplary embodiments, the control system  295  may be adapted to control each of the first and second refrigeration chiller units  270   a/b  at or below a predetermined maximum percentage of the refrigeration unit&#39;s capacity so as to optimize the efficiency of the refrigeration chiller units  270   a/b  and thereby minimize overall power consumption. For example, in at least some embodiments, the control system  295  may control the first and second refrigeration chiller units  270   a/b  so that each operates at or below no more than approximately 75% of the maximum refrigeration capacity. Accordingly, in such embodiments, when the first refrigeration unit  270   a  of the second stage closed-loop cooling system  270  is operating alone at approximately 75% of its rated capacity and the temperature of the cooled drilling mud  210   c  exiting the mud cooler exceeds the predetermined mud set point temperature, the control system  295  may then operate to bring the second refrigeration unit  270   b  on line, i.e., off of cooling standby mode, while maintaining the operation of the first refrigeration unit  270   a  at a substantially constant 75% of rated capacity. As with the controlled operation of the first refrigeration unit  270   a , the control system  295  may then also control the operation of the second refrigeration unit  270   b  by ramping up gradually and/or incrementally only as needed to meet the additional cooling requirements necessary to reduce the temperature of the cooled drilling mud  210   c  down to at least the mud set point temperature. 
     As the overall cooling requirements of the mud cooler  230  decrease, e.g., as the ambient temperature, and/or the temperature or flow rate of the hot drilling mud  210   h / 210   y  decreases, the control system  295  may be operated so as to shut down, i.e., take off line, each of the various components of the mud cooler  230  in a reverse sequence to that used to bring the component on line as set forth above. For example, the control system  295  may be used to gradually or incrementally ramp down the operation the second refrigeration unit  270   b , eventually take the second refrigeration unit  270   b  off line to standby cooling mode, as the mud cooling requirements decrease. Thereafter, the first refrigeration unit  270   a  may be ramped down and taken off line to standby cooling mode in similar fashion. The first stage closed-loop cooling system  250  will then be controlled by the control system  295  so as to perform at substantially maximum cooling capacity until the temperature of the hot drilling mud  210   h / 210   y  entering the first stage mud heat exchanger  232   a  drops below the mud set point temperature by the previously noted predetermined number of degrees, e.g., by approximately 2° F.-4° F. as described above. 
     As a result, the subject matter disclosed herein provides details of various systems, apparatuses, and methods that may be used for circulating and cooling drilling mud during wellbore drilling operations, and in particular, during high temperature onshore drilling operations. Furthermore, in some illustrative embodiments, a control system  295  may be used to adjust the amount of drilling mud cooling that is provided by the mud cooler  230  through a sequentially staged operation of the first and second stage closed-loop cooling systems  250  and  270  by bringing the second stage closed-loop cooling system  270  on line only as required to provide additional mud cooling capacity. Additionally, the control system  295  may also be used to sequentially stage the operation of the first and second refrigeration chiller units  270   a/b  of the second stage closed-loop cooling system  270  in a similar fashion, i.e., by bringing the second refrigeration chiller unit  270   b  on line only when the drilling mud cooling requirements so dictate. In this way, the control system may be adapted to optimize power consumption across all stages of the mud cooler  230  operational cycle. 
     The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. For example, the method steps set forth above may be performed in a different order. Furthermore, no limitations are intended by the details of construction or design herein shown. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.