Abstract:
An apparatus for remotely launching cementing plugs is configured such that the length of a dart is not necessarily the same as the corresponding plug to be launched. Such a design presents operational and ergonomic advantages, particularly in an offshore environment. Methods for launching cementing plugs and cementing a subterranean well are also presented.

Description:
CROSS-REFERENCED APPLICATION 
       [0001]    This application is a divisional application of the U.S. application Ser. No. 12/640,622, incorporated by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The statements in this section merely provide background information related to the present disclosure and may not constitute prior art. 
         [0003]    The present invention is related in general to equipment for servicing subterranean wells. Particularly, the invention relates to an apparatus and method for remotely launching cementing plugs during the primary cementation of a subterranean well. 
         [0004]    Most primary cementing treatments involve the use of wiper plugs that travel through the interior of a tubular body (e.g., casing or liner). When launched, the plugs travel from the top of the tubular body to the bottom, where they become seated. The purpose of the plugs is to separate and prevent commingling of different fluids during their journey through the tubular body. In most cases, operators deploy a bottom plug and a top plug. 
         [0005]    After the tubular body is installed in the wellbore, the annulus between the tubular body and the wellbore wall (or another tubular body) is usually filled with drilling fluid. When the primary cementing treatment commences, the bottom plug is first launched into the tubular body, followed by the cement slurry. The cement slurry may be preceded by a spacer fluid, a chemical wash or both. The function of the bottom plug is to scrape traces of drilling fluid from the internal surface of the tubular body, and to prevent contact between the drilling fluid and the cement slurry. 
         [0006]    The bottom-plug launching and conveyance through the tubular body arises from pressure applied by the cement slurry. When the bottom plug completes its journey through the tubular body, it becomes seated on float equipment installed at the bottom of the tubular body. Continued pumping exerts sufficient pressure to rupture a membrane at the top of the bottom plug, allowing the cement slurry to flow through an interior passage in the bottom plug, exit the bottom of the tubular body and continue into the annulus. 
         [0007]    After sufficient cement slurry to fill the annulus has been pumped into the tubular body, the top plug is launched into the tubular body, and a displacement fluid is pumped behind the plug. The displacement fluid forces the plug through the tubular body. Displacement fluids may comprise (but not be limited to) water, spacer fluids and completion fluids. The function of the top plug is to scrape traces of cement slurry from the internal surface of the tubular body, isolate the cement slurry from the displacement fluid and, upon landing on the bottom plug, seal the tubular body interior from the annulus. Unlike the bottom plug, the top plug has no membrane or interior passage through which fluids may flow. 
         [0008]    A thorough description of the primary cementing process and the equipment employed to perform the service may be found in the following references. (1) Piot B. and Cuvillier G.: “Primary Cementing,” in Nelson E. B. and Guillot D. (eds.):  Well Cementing— 2 nd  Edition, Houston: Schlumberger (2006): 459-501. (2) Leugemors E., Metson J., Pessin J.-L., Colvard R. L., Krauss C. D. and Plante M.: “Cementing Equipment and Casing Hardware,” in Nelson E. B. and Guillot D. (eds.):  Well Cementing— 2 nd  Edition, Houston: Schlumberger (2006): 343-434. 
         [0009]    Wiper plugs are usually launched from a cementing head that is attached to the tubular body near the drilling rig. The tubular body rises from the bottom of the openhole to the rig floor. However, for subsea completions, the problem becomes more complicated, and fluid isolation becomes more and more critical as water depth increases. It thus becomes impractical to launch wiper plugs from the surface. Therefore, the cementing head containing the wiper plugs rests on the seafloor, and the top of the tubular body ends at the mudline. Drillpipe connects the top of the tubular body to the rig floor on the surface. During the cementing process, darts are released into the drillpipe on surface, travel through the drillpipe to the seafloor and, upon arrival, trigger the release of the wiper plugs. 
         [0010]    After the first dart is launched, cement slurry is pumped behind it. When the first dart lands inside the cementing head, the bottom plug is released. The second dart is launched after sufficient cement slurry has been pumped to fill the annulus. A displacement fluid is pumped behind the second dart. When the second dart arrives, the top plug is released. A brief peak in surface pressure indicates when each wiper plug has been launched. This process is detailed in the following references: (1) Buisine P. and Lavaure G.: “Equipment for Remote Launching of Cementing Plugs into Subsea Drilled Wells,” European Patent Application 0 450 676 A1 (1991); (2) Brandt W. et al.: “Deepening the Search for Offshore Hydrocarbons.”  Oilfield Review  (Spring 1998) 10, No. 1, 2-21. 
         [0011]    Those skilled in the art will understand that process fluids may comprise drilling fluids, cement slurries, chemical washes, spacer fluids and completion fluids. 
         [0012]    Previous plug-launching systems are configured such that the length of the dart must match the length of the plug being launched. The arrival and displacement of the dart inside the cementing head causes a rod and piston to likewise move downward into the plug basket. The distance the rod and piston move downward is equal to the axial displacement distance of the dart. The cementing-plug length may vary depending upon the casing size into which it is being launched. Therefore, it is necessary for the operator to have various sizes of darts available. 
         [0013]    The necessity for the dart length to be equal to the plug length may also pose ergonomic problems. When longer plugs are employed, the length of the dart launching apparatus may be difficult to handle on offshore facilities. 
         [0014]    Despite the valuable contributions of the prior art, it remains desirable, therefore, to provide an improved apparatus and methods for launching various sizes of cementing plugs. 
       SUMMARY OF THE INVENTION 
       [0015]    The present invention fulfills the needs mentioned herein. 
         [0016]    The first aspect of the invention is a system for launching cementing plugs in a subterranean well. The system comprises two portions. The first portion comprises a plug basket that initially contains at least one plug, a dart catcher that contains a hydraulic-liquid reservoir and a piston, ports through which wellbore-service fluids may flow, and a hydraulic liquid inside the reservoir that is in hydraulic communication with the plug basket. The second portion comprises at least one dart. The second portion is initially separated from the first portion. The system is designed such that, upon arrival and subsequent axial movement of the dart inside the dart catcher, the hydraulic liquid is displaced by the piston to a sufficient extent to cause the expulsion of the plug from the system. 
         [0017]    The second aspect of the invention is a method for launching cementing plugs. The method comprises a system for launching cementing plugs in a subterranean well which comprises two portions. The first portion comprises a plug basket that initially contains at least one plug, a dart catcher that contains a hydraulic-liquid reservoir and a piston, ports through which wellbore-service fluids may flow, and a hydraulic liquid inside the reservoir that is in hydraulic communication with the plug basket. The second portion comprises at least one dart. The second portion is initially separated from the first portion. The system is designed such that, upon arrival and subsequent axial movement of the dart inside the dart catcher, the hydraulic liquid is displaced by the piston to a sufficient extent to cause the expulsion of the plug from the system. 
         [0018]    The first portion of the system is installed inside a casing string. Process fluid is pumped into the first portion, and allowed to flow through the flow ports. A dart is launched into the process-fluid stream. Pumping of process fluid continues until the dart lands on the piston inside the dart catcher, blocking process-fluid flowing through the flow ports. Continued process-fluid pumping causes the piston to move downward into the hydraulic-liquid reservoir, forcing the plug to exit the plug basket. 
         [0019]    The third aspect of the invention is a method for cementing a subterranean well. The method comprises a system for launching cementing plugs in a subterranean which comprises two portions. The first portion comprises a plug basket that initially contains at least one plug, a dart catcher that contains a hydraulic-liquid reservoir and a piston, ports through which wellbore-service fluids may flow, and a hydraulic liquid inside the reservoir that is in hydraulic communication with the plug basket. The second portion comprises at least one dart. The second portion is initially separated from the first portion. The system is designed such that, upon arrival and subsequent axial movement of the dart inside the dart catcher, the hydraulic liquid is displaced by the piston to a sufficient extent to cause the expulsion of the plug from the system. 
         [0020]    The first portion of the system is installed inside a casing string. Drilling fluid is pumped into the first portion, and allowed to flow through the flow ports. A dart is launched into the drilling-fluid stream. Cement slurry is pumped behind the dart. Pumping of cement slurry continues until the dart lands on the piston inside the dart catcher, blocking fluid flow through the flow ports. Continued cement-slurry pumping causes the piston to move downward into the hydraulic-liquid reservoir, forcing the plug to exit the plug basket. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1A  to E illustrate the design and operation of an embodiment of the invention in which hydraulic fluid may flow from the dart catcher into a tubular body connected to the plug basket where A illustrates the fluid flowing through a tubular, a bottom dart is then launched in step B, in step C further fluid is pumped to force the bottom dart, in step D a top dart is launched and in step E further pumping force the top dart to move downward. 
           [0022]      FIG. 2  A to E illustrate the design and operation of another embodiment of the invention that features an expandable fluid chamber and a movable plug basket where A illustrates the fluid flowing through a tubular, a bottom dart is then launched in step B and land on a piston, in step C further fluid is pumped to force the bottom dart and thus the piston downward, in step D a top dart is launched and in step E further pumping force the top dart to move downward. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    When cementing the annular space between tubulars and the walls of a subterranean wellbore, it is usually necessary to minimize or prevent the commingling of the drilling fluid, spacer fluid and cement slurry. Commingling may result, for example, in adverse rheological effects, dilution of the cement slurry and compromised zonal isolation. One way to minimize commingling involves using wiper plugs to separate fluids as they travel down the tubulars. Wiper plugs also clean the inner surface of the tubulars. Most cementing operations involve two wiper plugs: a bottom plug that separates cement slurry from drilling fluid, and a bottom plug that separates cement slurry from displacement fluid. The bottom plug travels through the tubular body (e.g., casing) and lands on float equipment at the bottom end. Continued pumping breaks a membrane in the bottom plug, allowing cement slurry to pass through the plug and enter the annular region around the tubular body. The top plug lands on top of the bottom plug, forcing the cement slurry out of the tubular-body interior, and leaving the tubular-body interior full of displacement fluid. 
         [0024]    The present invention is aimed at simplifying and improving the ergonomics of cementing-plug launching systems. One of the principal features of the invention is that it is no longer necessary for the length of a dart to be equal to that of the corresponding cementing plug to be launched. 
         [0025]    The first aspect of the invention is a system for launching cementing plugs in a subterranean well. The system comprises two portions. The first portion comprises a plug basket that initially contains at least one plug, a dart catcher that contains a hydraulic-liquid reservoir and a piston, ports through which wellbore-service fluids may flow, and a hydraulic liquid inside the reservoir that is in hydraulic communication with the plug basket. The second portion comprises at least one dart. The second portion is initially separated from the first portion. The system is designed such that, upon arrival and subsequent axial movement of the dart inside the dart catcher, the hydraulic liquid is displaced by the piston to a sufficient extent to cause the expulsion of the plug from the system. 
         [0026]    Embodiment of the first aspect of the invention, shown in  FIG. 1 , may have the following characteristics. The first portion further may comprise at least one bottom plug  101  and a top plug  102  in the plug basket  103 . Above the plug basket  103  may be a first tubular body  108  containing a main rod  104  equipped with a rod piston  105 . A dart catcher  109  may be mounted above the first tubular body  108 . The dart catcher may contain a hydraulic-liquid reservoir  106  and a piston  107 . Hydraulic liquid from the reservoir  106  may flow into the first tubular body  108 , causing downward movement of the rod piston  105  and main rod  104 . Above the dart catcher may be a second tubular body  114  containing ports ( 112  and  113 ) through which wellbore-service fluids may flow. The second portion may comprise at least one bottom dart  110 , and the system may further comprise a third portion comprising a top dart  111 . The second and third portions may initially be separated from the first portion. 
         [0027]    The internal volumes of the hydraulic-liquid reservoir  106  and the first tubular body  108  may be adjusted such that the axial displacement of rod piston  105  and main rod  104 , resulting from the axial displacement of piston  107 , and the flow of hydraulic liquid into the first tubular body  108 , is sufficient to expel a plug. 
         [0028]    Yet other embodiment of the first aspect of the invention, shown in  FIG. 2 , may have the following characteristics. The plug basket  216  of the first portion may be movable and may initially contain at least one bottom plug  201  and a top plug  202 . The first portion may further comprise a first tubular body  208  through which hydraulic fluid may flow. The first tubular body  208  may be be mounted between the movable plug basket  216  and the dart catcher  209 . Hydraulic liquid may flow from the hydraulic-liquid reservoir  216  into the first tubular body  208  and then into an expandable fluid chamber  217 . Expansion of the fluid chamber  217  upon entry of hydraulic fluid may cause the plug basket  216  to move upward, resulting in the expulsion of a cementing plug ( 201  or  202 ). Above the dart catcher may be a second tubular body  214  containing ports ( 212  and  213 ) through which wellbore-service fluids may flow. The second portion may comprise at least one bottom dart  210 , and the system may further comprise a third portion comprising a top dart  211 . The second and third portions may initially be separated from the first portion. 
         [0029]    The internal volumes of the hydraulic-fluid reservoir  206  and the first tubular body  208  may be adjusted such that hydraulic-fluid movement through the first tubular body  208 , and subsequent filling of the expandable fluid chamber  217  arising from the arrival and displacement of a dart, cause the movable dart basket  216  to move sufficiently upward to expel a plug. 
         [0030]    The second aspect of the invention is a method for launching cementing plugs. The method comprises a system for launching cementing plugs in a subterranean well which comprises two portions. The first portion comprises a plug basket that initially contains at least one plug, a dart catcher that contains a hydraulic-liquid reservoir and a piston, ports through which wellbore-service fluids may flow, and a hydraulic liquid inside the reservoir that is in hydraulic communication with the plug basket. The second portion comprises at least one dart. The second portion is initially separated from the first portion. The system is designed such that, upon arrival and subsequent axial movement of the dart inside the dart catcher, the hydraulic liquid is displaced by the piston to a sufficient extent to cause the expulsion of the plug from the system. 
         [0031]    The first portion of the system is installed inside a casing string. Process fluid is pumped into the first portion, and allowed to flow through the flow ports. A dart is launched into the process-fluid stream. Pumping of process fluid continues until the dart lands on the piston inside the dart catcher, blocking process-fluid flow through the flow ports. Continued process-fluid pumping causes the piston to move downward into the hydraulic-liquid reservoir, forcing the plug to exit the plug basket. 
         [0032]    Another embodiment of the second aspect of the invention is described below. The system selected for launching cementing plugs in a subterranean well may be further characterized by the following. The first portion may comprise at least one bottom plug  101  and a top plug  102  in the plug basket  103 . Above the plug basket  103  may be a first tubular body  108  containing a main rod  104  equipped with a rod piston  105 . A dart catcher  109  may be mounted above the first tubular body  108 . The dart catcher may contain a hydraulic-liquid reservoir  106  and a piston  107 . Hydraulic liquid from the reservoir  106  may flow into the first tubular body  108 , causing downward movement of the rod piston  105  and main rod  104 . Above the dart catcher may be a second tubular body  114  containing ports ( 112  and  113 ) through which wellbore-service fluids may flow. The second portion may comprise at least one bottom dart  110 , and the system may further comprise a third portion comprising a top dart  111 . The second and third portions may initially be separated from the first portion. 
         [0033]    This embodiment may further comprise the following steps. The first portion is preferably installed inside a casing string  115 . A first process fluid is pumped from the surface through the second tubular body  114 . As shown in Step A, process fluid initially flows through ports  112  and  113 , bypassing the rest of the first portion of the apparatus. A bottom dart  110  is launched into the process-fluid stream in the second tubular body  114 . A second process fluid may be pumped behind the bottom dart  110 . After a desired volume of second process fluid has been pumped into the well, a top dart  111  may be launched into the process fluid stream in the second tubular body  114 , followed by a third process fluid. 
         [0034]    Step B depicts the moment during which the bottom dart  110  lands on the piston  7  inside the dart catcher  9 . The bottom dart blocks fluid flow through ports  112  and  113 . As shown by Step C, further pumping of process fluid forces the bottom dart  110  downward, thereby forcing the piston  107  downward, thereby causing hydraulic liquid from the hydraulic-liquid reservoir  106  into the first tubular body  108 , thereby forcing the piston  107  downward. Movement of the piston  107  forces the main rod  105  into the plug basket  103 , thereby ejecting the bottom plug  101  from the plug basket. The bottom plug  101  may act as a barrier between the first and second process fluids, preventing their commingling while traveling through the interior of the casing string  115 . 
         [0035]    In Step D, the top dart  111  has landed on the bottom dart  110 , once again obstructing fluid flow through ports  112  and  113 . A shown by Step E, further pumping causes the top dart  111  to move downward, thereby causing more hydraulic liquid to flow from the hydraulic-liquid reservoir  106  into the first tubular body  108 , thereby forcing the piston  107  further downward. Movement of the piston  107  forces the main rod  105  further into the plug basket  103 , thereby ejecting the top plug  102  from the plug basket. The top plug  102  may act as a barrier between the second and third process fluids, preventing their commingling while traveling through the interior of the casing string  115 . 
         [0036]    The internal volumes of the hydraulic-liquid reservoir  106  and the first tubular body  108  may be adjusted such that the axial displacement of rod piston  105  and main rod  104 , resulting from the axial displacement of piston  107 , and the flow of hydraulic liquid into the first tubular body  108 , is sufficient to expel a plug. 
         [0037]    It will be understood by those skilled the art that the internal volume of the second tubular body  114  may be less than the amount of second process fluid necessary to fill the annular region surrounding the casing string  115 . In such cases, the second portion of the first aspect of the invention, the bottom dart  110 , will reach the first portion of the first aspect of the invention before the desired quantity of second process fluid has been pumped into the second tubular body  114 . Thus, the bottom plug  101  may be launched before the top dart  111  is launched. 
         [0038]    Yet another embodiment of the second aspect of the invention is described below. The system selected for launching cementing plugs in a subterranean well may be further characterized by the following. The plug basket  216  of the first portion may be movable and initially contains at least one bottom plug  201  and a top plug  202 . The first portion may further comprise a first tubular body  208  through which hydraulic fluid may flow. The first tubular body  208  may be mounted between the movable plug basket  216  and the dart catcher  209 . Hydraulic liquid may flow from the hydraulic-liquid reservoir  216  into the first tubular body  208  and then into an expandable fluid chamber  217 . Expansion of the fluid chamber  217  upon entry of hydraulic fluid causes the plug basket  216  to move upward, resulting in the expulsion of a cementing plug ( 201  or  202 ). Above the dart catcher may be a second tubular body  214  containing ports ( 212  and  213 ) through which wellbore-service fluids may flow. The second portion may comprise at least one bottom dart  210 , and the system may further comprise a third portion comprising a top dart  211 . The second and third portions may initially be separated from the first portion. 
         [0039]    This embodiment may further comprise the following steps. The first portion is preferably installed inside a casing string  215 . A first process fluid may be pumped from the surface through the second tubular body  214 . As shown in Step A, the first process fluid may initially flow through ports  212  and  213 , bypassing the rest of the first portion of the apparatus. A bottom dart  210  may be launched into the first-process-fluid stream in the second tubular body  214 . A second process fluid may be pumped behind the bottom dart  210 . After a desired volume of second process fluid has been pumped into the well, a top dart  211  may be launched into the second-process-fluid stream in the second tubular body  214 , followed by a third process fluid. 
         [0040]    Step B depicts the moment during which the bottom dart  210  lands on the piston  207  inside the dart catcher  209 . The bottom dart may block fluid flow through ports  212  and  213 . As shown by Step C, further pumping forces the bottom dart  210  downward, thereby forcing the piston  207  downward, thereby causing hydraulic liquid from the hydraulic-liquid reservoir  206  into the first tubular body  208 , thereby entering and beginning to fill the expandable fluid chamber  217 . Continued pumping and filling of the expandable fluid chamber  217  forces the plug basket  216  to move upward, thereby expelling the bottom cementing plug  201 . The bottom plug  201  may act as a barrier between the first and second process fluids, preventing their commingling while traveling through the interior of the casing string  215 . 
         [0041]    In Step D, the top dart  211  has landed on the bottom dart  210 , once again obstructing fluid flow through ports  212  and  213 . A shown by Step E, further pumping causes the top dart  211  to move downward, thereby causing more hydraulic liquid to flow from the hydraulic-liquid reservoir  206  into the first tubular body  208 , thereby entering and further filling the expandable fluid chamber  217 . Continued pumping and filling of the expandable fluid chamber  217  forces the plug basket  216  to once again move upward, thereby expelling the top cementing plug  202 . The top plug  202  may act as a barrier between the second and third process fluids, preventing their commingling while traveling through the interior of the casing string  215 . 
         [0042]    The internal volumes of the hydraulic-fluid reservoir  206  and the first tubular body  208  may be adjusted such that hydraulic-fluid movement through the first tubular body  208 , and subsequent filling of the expandable fluid chamber  217  arising from the arrival and displacement of a dart, cause the movable dart basket  216  to move sufficiently upward to expel a plug. 
         [0043]    It will be understood by those skilled the art that the internal volume of the second tubular body  214  may be less than the amount of second process fluid necessary to fill the annular region surrounding the casing string  215 . In such cases, the second portion of the first aspect of the invention, the bottom dart  210 , will reach the first portion of the first aspect of the invention before the desired quantity of second process fluid has been pumped into the second tubular body  214 . Thus, the bottom plug  201  may be launched before the top dart  211  is launched. 
         [0044]    The third aspect of the invention is a method for cementing a subterranean well. The method comprises a system for launching cementing plugs in a subterranean well which comprises two portions. The first portion comprises a plug basket that initially contains at least one plug, a dart catcher that contains a hydraulic-liquid reservoir and a piston, ports through which wellbore-service fluids may flow, and a hydraulic liquid inside the reservoir that is in hydraulic communication with the plug basket. The second portion comprises at least one dart. The second portion is initially separated from the first portion. The system is designed such that, upon arrival and subsequent axial movement of the dart inside the dart catcher, the hydraulic liquid is displaced by the piston to a sufficient extent to cause the expulsion of the plug from the system. 
         [0045]    The first portion of the system is preferably installed inside a casing string. Drilling fluid is pumped into the first portion, and allowed to flow through the flow ports. A dart is launched into the drilling-fluid stream. Cement slurry is pumped behind the dart. Pumping of cement slurry continues until the dart lands on the piston inside the dart catcher, blocking fluid flow through the flow ports. Continued cement-slurry pumping causes the piston to move downward into the hydraulic-liquid reservoir, forcing the plug to exit the plug basket. It will be understood by those skilled in the art that the cement slurry may be preceded by a chemical wash, spacer fluid or both. 
         [0046]    Another embodiment of the third aspect of the invention is described below. The system selected for launching cementing plugs in a subterranean well may be further characterized by the following. The first portion may comprise at least one bottom plug  101  and a top plug  102  in the plug basket  103 . Above the plug basket  103  may be a first tubular body  108  containing a main rod  104  equipped with a rod piston  105 . A dart catcher  109  may be mounted above the first tubular body  108 . The dart catcher may contain a hydraulic-liquid reservoir  106  and a piston  107 . Hydraulic liquid from the reservoir  106  may flow into the first tubular body  108 , causing downward movement of the rod piston  105  and main rod  104 . Above the dart catcher may be a second tubular body  114  containing ports ( 112  and  113 ) through which wellbore-service fluids may flow. The second portion may comprise at least one bottom dart  110 , and the system may further comprise a third portion comprising a top dart  111 . The second and third portions may initially be separated from the first portion. 
         [0047]    This embodiment may further comprise the following steps. The first portion is preferably installed inside a casing string  115 . Drilling fluid may be pumped from the surface through the second tubular body  114 . As shown in Step A, drilling fluid may initially flow through ports  112  and  113 , bypassing the rest of the first portion of the apparatus. A bottom dart  110  may be launched into the drilling-fluid stream in the second tubular body  114 . Cement slurry may be pumped behind the bottom dart  110 . After a desired volume of cement slurry has been pumped into the well, a top dart  111  may be launched into the cement-slurry stream in the second tubular body  114 , followed by a displacement fluid. 
         [0048]    Step B depicts the moment during which the bottom dart  110  lands on the piston  7  inside the dart catcher  9 . The bottom dart may block fluid flow through ports  112  and  113 . As shown by Step C, further pumping of displacement fluid may force the bottom dart  110  downward, thereby forcing the piston  107  downward, thereby causing hydraulic liquid from the hydraulic-liquid reservoir  106  into the first tubular body  108 , thereby forcing the piston  107  downward. Movement of the piston  107  may force the main rod  105  into the plug basket  103 , thereby ejecting the bottom plug  101  from the plug basket. The bottom plug  101  may act as a barrier between the drilling fluid and cement slurry, preventing their commingling while traveling through the interior of the casing string  115 . 
         [0049]    In Step D, the top dart  111  has landed on the bottom dart  110 , once again obstructing fluid flow through ports  112  and  113 . A shown by Step E, further pumping may cause the top dart  111  to move downward, thereby causing more hydraulic liquid to flow from the hydraulic-liquid reservoir  106  into the first tubular body  108 , thereby forcing the piston  107  further downward. Movement of the piston  107  may force the main rod  105  further into the plug basket  103 , thereby ejecting the top plug  102  from the plug basket. The top plug  102  may act as a barrier between the cement slurry and displacement fluid, preventing their commingling while traveling through the interior of the casing string  115 . 
         [0050]    The internal volumes of the hydraulic-liquid reservoir  106  and the first tubular body  108  may be adjusted such that the axial displacement of rod piston  105  and main rod  104 , resulting from the axial displacement of piston  107 , and the flow of hydraulic liquid into the first tubular body  108 , is sufficient to expel a plug. 
         [0051]    It will be understood by those skilled the art that the internal volume of the second tubular body  114  may be less than the amount of cement slurry necessary to fill the annular region surrounding the casing string  115 . In such cases, the second portion of the first aspect of the invention, the bottom dart  110 , will reach the first portion of the first aspect of the invention before the desired quantity of cement slurry has been pumped into the second tubular body  114 . Thus, the bottom plug  101  may be launched before the top dart  111  is launched. 
         [0052]    It will also be understood by those skilled in the art that the cement slurry may be preceded by a chemical wash, spacer fluid or both. 
         [0053]    Yet another embodiment of the third aspect of the invention is described below. The system selected for launching cementing plugs in a subterranean well may be further characterized by the following. The plug basket  216  of the first portion may be movable and may initially contain at least one bottom plug  201  and a top plug  202 . The first portion may further comprise a first tubular body  208  through which hydraulic fluid may flow. The first tubular body  208  may be be mounted between the movable plug basket  216  and the dart catcher  209 . Hydraulic liquid may flow from the hydraulic-liquid reservoir  216  into the first tubular body  208  and then into an expandable fluid chamber  217 . Expansion of the fluid chamber  217  upon entry of hydraulic fluid may cause the plug basket  216  to move upward, resulting in the expulsion of a cementing plug ( 201  or  202 ). Above the dart catcher may be a second tubular body  214  containing ports ( 212  and  213 ) through which wellbore-service fluids may flow. The second portion may comprise at least one bottom dart  210 , and the system may further comprise a third portion comprising a top dart  211 . The second and third portions may initially be separated from the first portion. 
         [0054]    This embodiment may further comprise the following steps. The first portion is preferably installed inside a casing string  215 . Drilling fluid may be pumped from the surface through the second tubular body  214 . As shown in Step A, the drilling fluid may initially flow through ports  212  and  213 , bypassing the rest of the first portion of the apparatus. A bottom dart  210  may be launched into the drilling-fluid stream in the second tubular body  214 . Cement slurry may be pumped behind the bottom dart  210 . After a desired volume of cement slurry has been pumped into the well, a top dart  211  may be launched into the cement-slurry stream in the second tubular body  214 , followed by a displacement fluid. 
         [0055]    Step B depicts the moment during which the bottom dart  210  lands on the piston  207  inside the dart catcher  209 . The bottom dart may block fluid flow through ports  212  and  213 . As shown by Step C, further pumping may force the bottom dart  210  downward, thereby forcing the piston  207  downward, thereby causing hydraulic liquid from the hydraulic-liquid reservoir  206  into the first tubular body  208 , thereby entering and beginning to fill the expandable fluid chamber  217 . Continued pumping and filling of the expandable fluid chamber  217  may force the plug basket  216  to move upward, thereby expelling the bottom cementing plug  201 . The bottom plug  201  may act as a barrier between the drilling fluid and cement slurry, preventing their commingling while traveling through the interior of the casing string  215 . 
         [0056]    In Step D, the top dart  211  has landed on the bottom dart  210 , once again obstructing fluid flow through ports  212  and  213 . A shown by Step E, further pumping may cause the top dart  211  to move downward, thereby causing more hydraulic liquid to flow from the hydraulic-liquid reservoir  206  into the first tubular body  208 , thereby entering and further filling the expandable fluid chamber  217 . Continued pumping and filling of the expandable fluid chamber  217  may force the plug basket  216  to once again move upward, thereby expelling the top cementing plug  202 . The top plug  202  may act as a barrier between the cement slurry and the displacement fluid, preventing their commingling while traveling through the interior of the casing string  215 . 
         [0057]    The internal volumes of the hydraulic-fluid reservoir  206  and the first tubular body  208  may be adjusted such that hydraulic-fluid movement through the first tubular body  208 , and subsequent filling of the expandable fluid chamber  217  arising from the arrival and displacement of a dart, cause the movable dart basket  216  to move sufficiently upward to expel a plug. 
         [0058]    It will be understood by those skilled the art that the internal volume of the second tubular body  214  may be less than the amount of second process fluid necessary to fill the annular region surrounding the casing string  215 . In such cases, the second portion of the first aspect of the invention, the bottom dart  210 , will reach the first portion of the first aspect of the invention before the desired quantity of second process fluid has been pumped into the second tubular body  214 . Thus, the bottom plug  201  may be launched before the top dart  211  is launched. 
         [0059]    It will also be understood by those skilled in the art that the cement slurry may be preceded by a chemical wash, spacer fluid or both. 
         [0060]    For all aspects of the invention, the hydraulic liquid may comprise a member of the list comprising: water, mineral oil, glycols, esters, polyalphaolefins or silicone oils and mixtures thereof. 
         [0061]    For all aspects of the invention, the subterranean well may be a member of the list comprising: an oil well, a gas well, a geothermal well, a water well, a well for chemical-waste disposal, a well for enhanced recovery of hydrocarbons and a well for carbon sequestration. 
         [0062]    The preceding description has been presented with reference to presently preferred embodiments of the invention. Persons skilled in the art and technology to which this invention pertains will appreciate that alterations and changes in the described structures and methods of operation can be practiced without meaningfully departing from the principle, and scope of this invention. Accordingly, the foregoing description should not be read as pertaining only to the precise structures described and shown in the accompanying drawings, but rather should be read as consistent with and as support for the following claims, which are to have their fullest and fairest scope.