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BACKGROUND 
   The present invention relates generally to subterranean well construction, and more particularly to plugs, plug systems, and methods for using these plugs and systems in subterranean wells. 
   Cementing operations may be conducted in a subterranean formation for many reasons. For instance, after (or, in some cases, during) the drilling of a well bore within a subterranean formation, pipe strings such as casings and liners are often cemented in the well bore. This usually occurs by pumping a cement composition into an annular space between the walls of the well bore and the exterior surface of the pipe string disposed therein. Generally, the cement composition is pumped down into the well bore through the pipe string, and up into the annular space. Prior to the placement of the cement composition into the well bore, the well bore is usually full of fluid, e.g., a drilling fluid. Oftentimes, an apparatus known as a cementing plug may be employed and placed in the fluid ahead of the cement composition to separate the cement composition from the well fluid as the cement slurry is placed in the well bore, and to wipe fluid from the inner surface of the pipe string while the cementing plug travels through it. Once placed in the annular space, the cement composition is permitted to set therein, thereby forming an annular sheath of hardened substantially impermeable cement therein that substantially supports and positions the pipe string in the well bore and bonds the exterior surface of the pipe string to the walls of the well bore. 
   In some circumstances, a pipe string will be placed within the well bore by a process comprising the attachment of the pipe string to a tool (often referred to as a “casing hanger and running tool” or a “work string”) that may be manipulated within the well bore to suspend the pipe string in a desired location, including, but not limited to, suspension at or below the sea floor in off-shore operations. In addition to the pipe string, a sub-surface release cementing plug system comprising a plurality of cementing plugs may also be attached to the casing hanger and running tool. Such cementing plugs may be selectively released from the running tool at desired times during the cementing process. The sub-surface release cementing plug system may comprise a bypass mechanism that permits fluids to flow through the plugs at appropriate times. Conventional bypass mechanisms may comprise, for example, a rupture disk, which when punctured, may permit some degree of flow through the plug system. Additionally, a check valve, typically called a float valve, will be installed near the bottom of the pipe string. The float valve may permit the flow of fluids through the bottom of the pipe string into the annulus, but not the reverse. A cementing plug will not pass through the float valve. When a first cementing plug (often called a “bottom plug”) is deployed from a sub-surface release cementing plug system and arrives at the float valve, fluid flow through the float valve is stopped. Continued pumping results in a pressure increase in the fluids in the pipe string, which indicates that the leading edge of the cement composition has reached the float valve and activates a by-pass mechanism built into the bottom plug. After the bottom plug has been opened, the cement composition flows through the float valve and into the annulus. When the top plug contacts the bottom plug which had previously contacted the float valve, fluid flow is again interrupted, and the resulting pressure increase indicates that all of the cement composition has passed through the float valve. It is important that all of the desired cement composition be pumped into the annulus from the pipe string. If not, the cement remaining in the pipe string will have to be drilled out before any further activities can take place. Furthermore, the annulus might not be properly filled with cement, and undesirable formation-fluid migration or failure of the pipe string may result. On the other hand, if the cement is overdisplaced, a lower portion of the annulus might not be properly filled with cement, and undesirable formation-fluid migration or failure of the pipe string could result. Overdisplacement of the cement is considered a worse problem than underdisplacement, as it can be more difficult to correct. 
   Sub-surface release cementing plug systems often have a number of difficulties. For example, a sub-surface release cementing plug system may be damaged when weight is transferred to it while it is being attached to the running tool and/or being inserted into the top of the casing. Such weight transfer may shear the bypass mechanism present in the bottom cementing plug; in such circumstance operations may be performed by removing the bottom plug and continuing the operation by relying solely on the top plug. Another problem is that conventional bypass mechanisms—when activated—may overly restrict the flow of a desired fluid through the cementing plugs. Flow restrictions are problematic because they may generate hydraulic ram effects against subterranean formations intersected by the borehole while the pipe string is being installed, which may result in complications such as hydraulic fracturing of the subterranean formation, for example, which may lead to problems such as lost circulation, differential sticking of the pipe string against the bore hole, loss of well control, difficulty or inability to place a cement composition at a desired location in the annular space, and other problems. Difficulties may also be encountered in releasing the plug sets in a timely and accurate fashion, to ensure that the bottom cementing plug is released in spacer fluid ahead of the leading edge of the cement slurry. The timely and accurate release of cementing plugs via a free fall device (e.g., weighted plastic balls) is particularly difficult in deep wells where the fluid capacity of the drill string may range up to about several hundred barrels. One attempt at solving this problem has been to use a cementing plug system wherein the bottom plug is released by the use of a positive displacement device, e.g., a drill pipe dart. However, this method has been problematic because the dart is captivated within the cementing plug once the plug has landed on the uppermost float valve near the bottom of the well bore and the bypass system has been activated, which may increase the length of the bottom plug and may restrict the flow rate through the bypass mechanism. 
   Cementing plugs must be drilled out of the casing when the cementing operation has been completed. For this reason, the plugs are usually made from materials that are easily drilled. Such materials include some kinds of plastic, aluminum, cast iron, and others. Although generally speaking plastic materials are easier to drill out than metal materials, they generally are subject to rapid erosion when exposed to conditions in the well. 
   Personnel conducting cementing operations often encounter a further problem in attempting to accurately determine the volume of the casing string prior to preparing the cement composition or to deploying a final (“top”) cementing plug. This problem is typically caused by the fact that casing capacity tables are based upon nominal casing inner diameters for a given casing size and weight. Actual casing inner diameters often tend to be slightly larger than these published nominal inner diameters. Accordingly, on long casing strings the actual casing displacement can be significantly larger than the calculated theoretical volume, which may inhibit operators from displacing the final cementing plug to its desired shut-off point—e.g., from reaching and contacting the preceding cementing plugs atop the uppermost float valve near the bottom of the casing. This often prevents the customer from conducting a casing integrity test at the completion of cementing operations, and may result in extended drill out times due to excessive volumes of cement remaining inside the casing. 
   An additional problem often encountered with conventional cementing operations relates to the conventional configuration of float valves typically installed at the leading end of casing installed in a well bore. Typically, such float valves have an opening that is relatively small in relation to the inner diameter of the casing. In certain circumstances wherein the casing is disposed horizontally, such as when the casing is installed in a horizontal well, for example, sediment may accumulate along the bottom of the horizontally disposed casing. When a bottom cementing plug is displaced through the well bore, the plug may encounter an amount of sediment that is sufficient to slow the cementing plug&#39;s velocity and stop the cementing plug short of landing against the float valve and sealing against the entire diameter of the casing. This is problematic because the failure of the cementing plug to seal prevents operations personnel from conducting a pressure test on the casing. Furthermore, the problem becomes increasingly problematic as casing diameter increases, because a greater amount of sediment may accumulate due to factors such as decreased fluid velocities (which may permit debris to fall out of suspension) for a given rate of circulation, and because the relatively small inner diameter of conventional float valves in relation to the casing diameter forces the bottom cementing plug to displace the sediment to a greater height in order to propel it through the inner diameter of the float valve, when the casing is disposed horizontally. Sediment may build in front of the bottom plug until the pressure differential required to sustain plug movement exceeds the “opening” pressure of the plug (e.g., the pressure at which the bypass mechanism is activated). At this time cement flow will be established through the plug and over the top of the horizontal, accumulated sediment bed resident between the bottom plug and the upper float valve. When the top cementing plug at the tail of the cement slurry is displaced to the bottom plug, both plugs will continue to displace and push the cement and sediment ahead of the plugs until such time as the compacted sediment prevents the plugs from achieving sealing contact with the upper float valve. The inability of the cementing plugs to establish sealing contact with the float valve will prevent achievement of a pressure shut-off. Accordingly, contaminated cement and sediment may fill the remaining casing below the upper float and/or pass around the end of the casing string, thereby producing what is often referred to as a “wet shoe.” Operators will have no surface indication that the plugs have failed to displace all debris through the float valve, because the landing pressure of the top plug will generally be much greater than the activation pressure of the bottom plug by-pass mechanism. Accordingly, the only indication that a problem exists may be the failure to properly land the top plug, along with the resulting “soft drill out” and/or the failure to achieve an acceptable shoe test after drill out. 
   SUMMARY 
   The present invention relates generally to subterranean well construction, and more particularly, to plugs, plug systems, and methods for using these plugs and systems in subterranean wells. 
   An example of a method of the present invention is a method of separating fluids successively introduced into a passage comprising the step of introducing a plug at an interface of the successively introduced fluids, wherein the plug comprises an outer body and a detachable inner mandrel attached to the outer body. 
   Another example of a method of the present invention is a method of separating fluids successively introduced into a subterranean well bore, comprising the steps of: introducing a first fluid into the well bore through a casing string; introducing a second fluid into the well bore behind the first fluid such that an interface between the two fluids is formed; suspending an assembly comprising a plurality of plugs within the casing string, wherein at least one of the plugs comprises an outer body and a detachable inner mandrel attached to the outer body; and deploying the at least one plug within the casing string at the interface of the first and second fluids. 
   An example of a method of the present invention is a method of cementing a casing string in a subterranean well bore comprising the steps of: placing a cement composition into the casing string, and deploying within the casing string at least one cementing plug comprising an outer body and a detachable inner mandrel attached to the outer body. 
   Another example of a method of the present invention is a method of activating a device in a subterranean well bore, the device comprising a baffle adapter configured to sealingly latch with a cementing plug, the plug comprising an outer body and a detachable inner mandrel attached to the outer body, comprising the steps of: displacing a plug into contact with the baffle adapter so that the outer body of the plug achieves sealing contact with the baffle adapter; and applying a differential pressure across the plug, thereby activating the device. 
   An example of a system of the present invention is a plug system for separating fluids successively introduced into a passage comprising: an assembly comprising a plurality of plugs, wherein at least one plug comprises an outer body and a detachable inner mandrel attached to the outer body; and wherein the plurality of plugs are releasably attached to each other. 
   An example of an apparatus of the present invention is a plug for separating fluids successively introduced into a passage comprising: an outer body and a detachable inner mandrel attached to the outer body. 
   Another example of an apparatus of the present invention is a baffle adapter, comprising an inner bore designed to engage and seal against the outer body of a plug. 
   The features and advantages of the present invention will be readily apparent to those skilled in the art upon a reading of the description of the preferred embodiments, which follows. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the present disclosure and advantages thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawing, wherein: 
       FIG. 1  is a side cross-sectional view of an exemplary embodiment of a three-plug cementing plug system of the present invention. 
       FIG. 2  is a side cross-sectional view of an exemplary embodiment of a two-plug cementing plug system of the present invention. 
       FIG. 3  is a side cross-sectional view of an exemplary embodiment of a baffle adapter of the present invention. 
       FIG. 4  is a side cross-sectional view of an exemplary embodiment of a baffle adapter and catcher tube of the present invention. 
       FIG. 5  is a side cross-sectional view of an exemplary embodiment of a ported collar comprising a baffle adapter of the present invention. 
       FIG. 6  is a side cross-sectional view of an exemplary embodiment of a bypass baffle, which may be used in accordance with the present invention. 
   

   While the present invention is susceptible to various modifications and alternative forms, specific exemplary 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 as defined by the appended claims. 
   DESCRIPTION 
   The present invention relates generally to subterranean well construction, and more particularly, to plugs, plug systems, and methods for using these plugs and systems in subterranean wells. The cementing plugs of the present invention may be placed within a subterranean well bore in a cementing plug assembly comprising multiple cementing plugs. 
   An individual cementing plug may be detached from a cementing plug assembly, and subsequently deployed within the well bore, by contacting the plug with a releasing device; the interaction between the releasing device and a particular plug interrupts fluid flow through the work string and casing, causing a pressure increase sufficient to cause the plug to detach from the assembly. A variety of releasing devices may be used in conjunction with the cementing plug systems of the present invention. Certain exemplary embodiments of the cementing plugs of the present invention may accept a free fall device (such as a weighted ball, for example) as a releasing device. Certain other exemplary embodiments of the cementing plugs of the present invention may accept a positive displacement device (for example, a dart) as a releasing device. 
   An exemplary embodiment of a cementing plug assembly  90  of the present invention is shown in  FIG. 1 . A first bottom cementing plug is denoted generally by the numeral  10 . First bottom cementing plug  10  comprises outer body  11 . Wiper fins  12  are shown disposed along outer body  11 . In certain exemplary embodiments, wiper fins  12  may be of the floppy or foldable type; such floppy or foldable wiper fins  12  may be particularly useful in tapered casing strings, for example. First bottom cementing plug  10  also comprises receiving portion  18 ; in certain exemplary embodiments, receiving portion  18  is tapered (as illustrated in the top half of  FIG. 1 ). Tapering of receiving portion  18  may permit the cementing plug systems of the present invention to support higher pressures and higher loads during casing integrity tests, among other benefits. First bottom cementing plug  10  further comprises nose  16 , depicted at a leading end of outer body  11 . 
   Detachable inner mandrel  13  is sealed to first bottom cementing plug  10  by seal  58 , and is held in place within outer body  11  by frangible devices  14 . Any type of frangible device may be suitable for use, including shear pins, shear rings, controlled strength glue joints, and the like. At a leading end of inner mandrel  13  is depicted nose  15 , which nose  15  guides first bottom cementing plug  10  into baffle adapter  40  (shown in  FIG. 3 ). In certain exemplary embodiments, nose  15  may be tapered in such a way as to guide first bottom cementing plug  10  into baffle adapter  40  so that nose  16  of outer body  111  seals against receiving portion  44  (shown in  FIG. 3 ) of baffle adapter  40 . In certain exemplary embodiments, both nose  16  of outer body  11  and receiving portion  44  of baffle adapter  40  may be tapered for positive sealing against each other. Among other benefits, positive sealing of nose  16  against receiving portion  44  may permit the cementing plug systems of the present invention to support higher pressures during operations such as conducting optional casing integrity testing. In certain exemplary embodiments, nose  15  comprises longitudinal slots  17 , which ensure that inner mandrel  13  does not obstruct flow at certain times during deployment of the cementing plugs of the present invention. 
   Inner mandrel  13  further comprises inner bore  19 . In certain exemplary embodiments, inner bore  19  may have an inner diameter identical to that of other inner mandrels in the cementing plug assembly; in such exemplary embodiments, inner bore  19  may be configured with a unique receiving profile (such as single lobe unique receiving profile  160  or double lobe unique receiving profile  165  in  FIG. 2 , for example), designed to permit a particular releasing device (e.g., a dart having a nosepiece comprising a matching unique key profile) to locate and lock within it. In certain exemplary embodiments, inner bore  19  may be tapered (as illustrated in  FIG. 1 ) in such a way as to form a “seat” for a releasing device. In certain exemplary embodiments, inner bore  19  may be configured with a receiving profile designed so as to accept a latch-down mechanism on a releasing device (such as a dart having a nosepiece comprising a self-energized “C” ring); an example of such receiving profile may be seen at  170 . 
   In certain exemplary embodiments, first bottom cementing plug  10  may require modifications, so as to permit a particular releasing device to be used; e.g., the length of first bottom cementing plug  10  may need to be altered, or inner bore  19  of inner mandrel  13  may need to be reconfigured, for example. One of ordinary skill in the art, with the benefit of this disclosure, will be able to recognize the appropriate modifications to be made to facilitate use of a particular intended releasing device. 
   A second bottom cementing plug is also shown in  FIG. 1 , and denoted generally by the numeral  20 . Second bottom cementing plug  20  is attached to first bottom cementing plug  10  by frangible devices  51 . Any type of frangible device may be suitable for use, including devices such as shear pins, shear rings, controlled strength glue joints, and the like. Second bottom cementing plug  20  comprises outer body  21 , along which outer body  21  are disposed wiper fins  22 . In certain exemplary embodiments, wiper fins  22  may be of the floppy or foldable type. 
   Detachable inner mandrel  23  is sealed to second bottom cementing plug by seal  99 , and is held in place within outer body  21  by frangible devices  24 . As noted above, any type of frangible device may be suitable for use, including shear pins, shear rings, controlled strength glue joints, and the like. At one end of inner mandrel  23  is depicted nose  25 . When used in a system of cementing plugs, nose  25  of inner mandrel  23  guides second bottom cementing plug  20  into first bottom cementing plug  10 ; in certain exemplary embodiments, nose  25  may be tapered in such a way as to guide second bottom cementing plug  20  into first bottom cementing plug  10  such that nose  26  of outer body  21  seals against receiving portion  18  in first bottom cementing plug  10 . In certain exemplary embodiments, both nose  26  of outer body  21  and receiving portion  18  in first bottom cementing plug  10  may be tapered for positive sealing against each other. In certain exemplary embodiments, nose  25  of inner mandrel  23  has longitudinal slots  27 , which ensure that inner mandrel  23  does not obstruct flow at certain times during deployment of the cementing plugs of the present invention. 
   Inner mandrel  23  further comprises inner bore  70 . Inner bore  70  may be configured to accept a variety of intended releasing devices, including but not limited to a weighted free fall device (such as a weighted ball) or a positive displacement device (such as a dart). For example, inner bore  70  of inner mandrel  23  may be tapered in such a way as to form a “seat” for a releasing device, and to seal against the releasing device. In certain other exemplary embodiments, inner bore  70  may be configured with a unique receiving profile (such as single lobe unique receiving profile  160  or double lobe unique receiving profile  165  in  FIG. 2 , for example) designed to permit a particular releasing device (e.g., a dart having a nosepiece comprises a matching unique key profile) to locate and lock within it. Certain exemplary embodiments of inner bore  70  may be configured with a receiving profile designed so as to accept a latch-down mechanism on a releasing device (for example, a dart having a nosepiece comprising a self-energized “C” ring); an example of such receiving profile may be seen at  175 . One of ordinary skill in the art, with the benefit of this disclosure, will be able to recognize the appropriate modifications to be made to facilitate use of a particular intended releasing device. 
   Generally, the minor outside diameter of nose  15  of inner mandrel  13  of first bottom cementing plug  10 , and nose  25  of inner mandrel  23  of second bottom cementing plug  20  will exceed the diameter of the opening in the float valve. Nose  15  and nose  25  may be configured in a variety of shapes. For example, nose  15  and nose  25  may be tapered. In certain other exemplary embodiments, nose  15  and nose  25  may alternatively have a rounded or “mule shoe” configuration. In certain exemplary embodiments, inner mandrel  13  of first bottom cementing plug  10 , and inner mandrel  23  of second bottom cementing plug  20  may each have an overall length which exceeds the inside diameter of the casing to prevent inner mandrels  13  and  23  (once released from outer bodies  11  and  21 , respectively) from inverting within the casing as they travel towards the float valve. Preventing a detached inner mandrel from inverting as it proceeds towards the float valve may ensure that the fluid stream flowing towards the float valve flows against the top of the inner mandrel and releasing device restrained therein; among other benefits, this may prevent the fluid stream from causing the premature release from such inner mandrel of a releasing device that does not comprise a latch-down mechanism. 
   Seal  55  seals first bottom cementing plug  10  to inner mandrel  23  of second bottom cementing plug  20 . Seal  56  seals second bottom cementing plug  20  to top cementing plug  30 . In certain exemplary embodiments, seal  55  has an equal or greater diameter than second seal  56 . Among other benefits, this arrangement is useful during the stage of cementing operations when first bottom cementing plug  10  is released, as it may maintain inner mandrel  23  of second bottom cementing plug  20  under neutral or compressive loading during the hydraulic pressuring undertaken before the release of first bottom cementing plug  10 , thereby minimizing the possibility of prematurely shearing frangible devices  24  and  52 . 
     FIG. 1  further illustrates a top cementing plug, shown generally at  30 . Top cementing plug  30  is attached to second bottom cementing plug  20  by frangible devices  52 . Any type of frangible device may be suitable for use, including devices such as shear pins, shear rings, controlled strength glue joints, and the like. Top cementing plug further comprises outer body  31 , along which wiper fins  32  are disposed. In certain exemplary embodiments, wiper fins  32  may be of the floppy or foldable type. 
   Inner sleeve  33  is sealed to top cementing plug  30  by seal  101 . Inner sleeve  33  further comprises inner bore  39 . In certain exemplary embodiments, inner bore  39  of inner sleeve  33  is tapered. Among other benefits, the tapering of inner bore  39  provides a “seat” for a releasing device. Among other benefits, the tapering of inner bore  39  also facilitates the passage through inner bore  39  of certain releasing devices by avoiding a square shoulder that could catch or damage such releasing devices upon their entry into inner bore  39 . In certain other exemplary embodiments, inner bore  39  may be configured with a unique receiving profile (such as single lobe unique receiving profile  160  or double lobe unique receiving profile  165  in  FIG. 2 , for example) designed to permit a particular releasing device (e.g., a dart having a nosepiece comprises a matching unique key profile) to locate and lock within it. Certain exemplary embodiments of inner bore  39  may be configured with a receiving profile designed so as to accept a latch-down mechanism on a releasing device (for example, a dart having a nosepiece comprising a self-energized “C” ring); an example of such receiving profile may be seen at  180 . 
   In certain exemplary embodiments, top cementing plug  30  further comprises lock mechanism  37 . Lock mechanism  37  prevents inner sleeve  33  from moving backward in response to mechanical or hydraulic forces which may be encountered after inner sleeve  33  is activated by contact with a releasing device. In certain exemplary embodiments, lock mechanism  37  comprises a ring which may expand into internal upset  115  when inner sleeve  33  is displaced downward by a releasing device; shoulder area  105  stops the free downward travel of inner sleeve  33 , permitting the ring to expand into internal upset  115 , thereby preventing inner sleeve  33  from moving backward. In certain exemplary embodiments of the present invention, the incorporation of lock mechanism  37  within the cementing plugs of the present invention may, in combination with a second lock mechanism comprised within the releasing device (for example, a releasing dart having a nosepiece comprising a latch down feature) facilitates maintenance of the pressure integrity of the cementing plug system. For example, during events such as when top cementing plug  30  releases from work string  80 , as well as events such as the release of pressure which may become trapped between top cementing plug  30  and an uppermost float valve, or events such as failure of the uppermost float valve, lock mechanism  37  may prevent inner sleeve  33  from dislodging from top cementing plug  30 , and the lock mechanism within the releasing device may prevent the releasing device from dislodging from inner sleeve  33 . 
   Inner sleeve  33  is held in place within outer body  31  by frangible devices  34 . Any type of frangible device may be suitable for use, including but not limited to shear pins, shear rings, controlled strength glue joints, and the like. As illustrated in  FIG. 1 , the top cementing plugs of the present invention (such as top cementing plug  30 , for example), may also be held in place within outer body  31  by a variety of “secondary” release mechanisms. Such secondary release mechanisms may be activated upon the movement of inner sleeve  33  to a “released” position arising from contacting of inner sleeve  33  with a releasing device such as a dart, a weighted free fall device such as a weighted ball, or other known releasing devices. For example, a collet-type secondary release mechanism, such as that denoted generally at  35 , may be employed at the attachment of top cementing plug  30  to work string  80 . Alternatively, a ball-type secondary release mechanism  36  may be used. In certain other exemplary embodiments where a secondary release mechanism is not used, the release mechanisms for each cementing plug may be frangible devices, such as shear pins, for example. Among other benefits, the use of release mechanisms in the top cementing plugs of the present invention may improve the reliability of the cementing plug system, because they permit top cementing plug  30  to be attached to work string  80  by multiple means—e.g., by both frangible device  34  as well as release mechanism  35  or  36 . 
   The inner mandrels of the cementing plugs of the present invention may shoulder against each other in a manner that enables the cementing plug assemblies of the present invention to accept compressive loading without prematurely separating. Inner mandrel  13  of first bottom cementing plug  10 , inner mandrel  23  of second bottom cementing plug  20 , inner sleeve  33  of top cementing plug  30  and work string  80  shoulder against each other at shoulder areas  53 ,  54 , and  57 , respectively. This arrangement directs any compressive loads to which cementing plug assembly  90  might be subjected through inner mandrels  13  and  23  and inner sleeve  33 , rather than direct such compressive loads into frangible devices  14 ,  24 ,  34 ,  51 , or  52 . Optionally, in certain exemplary embodiments, shoulder areas  53 ,  54 , and  57  can be slotted to prevent the hydraulic sealing of inner mandrel  13  and nose  26  of second bottom cementing plug  20  to each other, to prevent the hydraulic sealing of inner mandrels  13  and  23  to each other, or to prevent the hydraulic sealing of inner mandrel  23  in second bottom cementing plug  20  to inner sleeve  33  in top cementing plug  30 . 
   The cementing plugs of the present invention may employ a variety of sealing arrangements. For example, a conventional face seal arrangement is shown at  29 . Optionally, certain exemplary embodiments of the cementing plug systems of the present invention may utilize a nose-seal arrangement, such as that shown at  28 , which may be particularly suitable for high-pressure, high-temperature applications. 
   The cementing plug assemblies of the present invention may also be used as two-plug assemblies. Turning now to  FIG. 2 , an exemplary embodiment of a two-plug cementing plug assembly of the present invention is depicted therein, and denoted generally as  120 . Bottom cementing plug  60  is attached to inner mandrel  23  by frangible devices  50 , and is sealed to inner mandrel  23  by seal  59 . Top cementing plug  30  is attached to inner mandrel  23  by frangible devices  52 , and is sealed to inner mandrel  23  by seal  63 . Inner mandrel  23  comprises inner bore  188 . Inner mandrel  23 , inner sleeve  33  of top cementing plug  30 , and work string  80  shoulder against each other at shoulder areas  54  and  83 , thus directing any compressive loads to which two-plug cementing plug assembly  120  might be subjected through inner mandrel  23  and inner sleeve  33 , rather than directing such compressive loads into frangible devices  50 ,  52  and  62 . Optionally, shoulder areas  54  and  83  may be configured to have a profile such that inner mandrel  23  is prevented from forming a face-to-face contact with inner sleeve  33  around their entire circumference, thereby preventing hydraulic sealing of inner mandrel  23  to inner sleeve  33 . In certain exemplary embodiments, such face-to-face contact is prevented by adding longitudinal slots  71  to shoulder area  54  or  83 . In certain exemplary embodiments, longitudinal slots  71  are sized no larger than necessary to permit a well bore fluid to pass between inner mandrel  23  and inner sleeve  33 . In certain exemplary embodiments of the present invention, inner sleeve  33  has a unique receiving profile, such as double lobe unique receiving profile  165 , for example, which may permit a particular releasing device to locate and lock within it. The bottom half of  FIG. 2  also illustrates an exemplary embodiment wherein inner sleeve  33  is held in place within a top cementing plug (e.g., top cementing plug  30 ) solely by frangible devices (e.g., frangible devices  62 ) without employing a secondary release mechanism. 
     FIG. 2  also illustrates that the nose-seal arrangements employed by the cementing plugs of the present invention may be readily modified to include a latch-down feature, where desired. For example, in certain exemplary embodiments, a nose-seal arrangement may comprise latch  145 ; in such exemplary embodiments, a receiving configuration within, for example, a preceding cementing plug (e.g., receiving configuration  155  in bottom cementing plug  60 ) or within a baffle adapter (e.g., baffle adapter  40 ), for instance, will be configured with a profile so as to accept a latch down feature such as latch  145 . Generally, latch  145  may comprise any self-energized device designed so as to engage and latch with a latch down receiving configuration, such as may be present in, for example, a cementing plug, or in a baffle adapter, for instance. In certain exemplary embodiments, latch  145  may comprise a self-energized “C” ring profile that may be attached to a cementing plug of the present invention by expanding the “C” ring profile over the major outer diameter of a nose of an outer body of a cementing plug, so as to lodge in groove  146  on such outer diameter. One of ordinary skill in the art, with the benefit of this disclosure, will be able to recognize an appropriate latch device for a particular application. 
     FIG. 2  further illustrates that the nose-seal arrangements employed by the cementing plug assemblies of the present invention may also, in certain exemplary embodiments, be fitted with one or more seal rings  147  (which may reside within groove  148 ) to enhance sealing. In certain exemplary embodiments of the present invention, seal rings  147  comprise elastomeric “O” rings; in certain of these exemplary embodiments, seal rings  147  may be made from a material such as a fluoro-elastomer, nitrile rubber, VITON™, AFLAS™, TEFLON™, or the like. In certain exemplary embodiments of the present invention, seal rings  147  comprise chevron-type “V” rings. One of ordinary skill in the art, with the benefit of this disclosure, will be able to recognize the appropriate type and material for seal rings  147  for a particular application. 
   Configuring each of the three cementing plugs, and baffle adapter  40  (shown in  FIG. 3 ), with a sealed latch-down feature will, among other benefits, allow the deployed cementing plugs to act as a check valve, permitting the casing string to be installed in the well bore without a float valve. Among other benefits, such a “floatless” installation may be particularly useful in applications where casing is installed in tight well profiles where high ram forces may be encountered during casing installation. An example of a tight well profile is a well bore having an inner diameter that is only slightly larger than the outside diameter of the casing to be installed therein, or only slightly larger than the outside diameter of a casing coupling where threaded and coupled casing is used. Ram forces, e.g., the hydraulic frictional force created by the displacement of well fluids up through the annulus during the installation of casing into the well bore, generally vary proportionately with the clearance between the inner diameter of the well bore and the outer diameter of the casing or the casing coupling; accordingly, the smaller the clearance (such as in a tight well profile) the higher the ram force for a given rate of casing installation. Performing a “floatless” installation reduces the volume of well fluids which must be displaced up through the annulus, thereby desirably reducing the ram forces encountered during casing installation. 
   Turning now to  FIGS. 3 and 4 ,  FIG. 3  depicts an exemplary embodiment of a baffle adapter, denoted generally by numeral  40 . Baffle adapter  40  may be used with three-plug cementing plug assembly  90  as well as with two-plug cementing plug assembly  120 . Baffle adapter  40  further comprises an insert, which in preferred embodiments is sealed against the body of baffle adapter  40  by cement  45  and seal  41 . Two alternative embodiments of the insert are depicted in  FIG. 3 . The upper half of the section of baffle adapter  40  depicts conventional length insert  47 . The lower half of the section of baffle adapter  40  depicts extended length insert  43 . In certain exemplary embodiments, extended length insert  43  is used, and extends and attaches to the inner diameter of optional perforated catcher tube  42 , as illustrated in  FIG. 4 . In certain exemplary embodiments, the attachment of extended length insert  43  to optional perforated catcher tube  42  is by a threaded connection. In certain exemplary embodiments, baffle adapter  40  can also be configured to accept a latching mechanism on a bottom cementing plug (such as latch  145  depicted on bottom cementing plug  60  in  FIG. 2 , for example); in such embodiments, baffle adapter  40  may comprise a latch-down receiving profile (such as that illustrated in  FIG. 3  at  48 , for example) into which a latching mechanism may latch. In certain other exemplary embodiments, baffle adapter  40  may comprise a unique receiving profile such as single lobe unique receiving profile  49  in  FIG. 3 , for example. In certain exemplary embodiments where a bottom cementing plug having a tapered nose seal arrangement is used, receiving portion  44  may be tapered (as illustrated) so as to promote sealing with the tapered nose of the bottom cementing plug. Among other benefits, positive sealing of receiving portion  44  against baffle adapter  40  may permit the cementing plug systems of the present invention to support higher pressures during operations such as conducting optional casing integrity testing. Baffle adapter  40  has an inner diameter that is relatively wide compared to the inner diameter of the casing string with which it may be used. In certain exemplary embodiments, baffle adapter  40  has an inner diameter in the range of from about 70% to about 90% of the inner diameter of the casing string. Among other benefits, this improves the ability of the cementing plug assemblies of the present invention, comprising baffle adapter  40 , to tolerate buildup of sediment within the casing before the initial displacement of bottom cementing plug  10 . Further, as the cementing plug assemblies of the present invention are used with increasingly large casing strings, the inner diameter of baffle adapter  40  increases proportionately to the increase in the casing string inner diameter. 
   Optionally, the cementing plug systems of the present invention may comprise a single-plug cementing plug assembly. In certain exemplary embodiments of such single-plug assemblies, baffle adapter  40  may be configured to accept a latch-down mechanism on the cementing plug (such as latch  145 , for example, shown in  FIG. 2 ). In certain exemplary embodiments, such a single-plug assembly is used for a “floatless” casing installation wherein the minimum inner diameter of a work string, such as that exemplified by work string  80  in  FIG. 1  or  FIG. 2 , is only slightly larger than the inner diameter of the releasing sleeve of the cementing plug, such as the releasing sleeve exemplified by inner sleeve  33  in  FIGS. 1 and 2 . In certain exemplary embodiments, inner sleeve  33  may comprise a unique receiving profile such as single lobe unique receiving profile  160  in  FIG. 2 , for example. Among other benefits, such an assembly may minimize the pressure drop across the single-plug cementing plug assembly during installation, thereby minimizing ram forces. 
   In certain exemplary embodiments, a baffle adapter  40  may be installed in a casing string one or more casing joints above a float valve—and above an optional bypass baffle (such as bypass baffle  500 , illustrated in  FIG. 6 , for example)—after which the casing string may be lowered into the well bore using a work string. In certain exemplary embodiments of the present invention wherein bypass baffle  500  is placed within a casing string, the centerline of bypass baffle  500  will be coincident with the centerline of the casing string. Generally, bypass baffle  500  may be placed within a casing string at a desired location so as to provide a desired amount of space between the top of a float valve and the leading end of an inner mandrel which may be landed atop bypass baffle  500 . In certain exemplary embodiments, the bypass baffle may be located within a casing coupling above the float valve, or may be located such that solid bottom  505  rests atop the surface of the upper float valve. Among other benefits, the inclusion of a bypass baffle within a casing string may reduce potential turbulence in the fluid region above the float valve, thereby reducing any potential for erosion of the float valve which may exist. Where a detachable inner mandrel of a cementing plug of the present invention (e.g., detachable inner mandrel  13 ) is displaced downhole according to the methods of the present invention, the detachable inner mandrel may land atop bypass baffle  500 —for example, between solid web segments  510 . Fluid flowing through the casing string towards the float valve may flow around both the landed detachable inner mandrel and solid web segments  510  by flowing through slots  515  in between solid web segments  510 . As the outer diameter of bypass baffle  500  may be relatively close to the inner diameter of the casing string, slots  515  may facilitate fluid in bypassing through the top section of bypass baffle  500 , in order to enter the inner diameter of bypass baffle  500  through slots  520 . Fluid may also enter the inner diameter of bypass baffle  500  by flowing through slots in the landed detachable inner mandrel (e.g., slots  17  in detachable inner mandrel  13 ). Fluid flowing through the inner diameter of bypass baffle  500  then exits through outlet  550 . 
   Generally, a float valve will always be present within the casing string. However, in certain exemplary embodiments, the float valve may be unnecessary, for example where all cementing plugs have a sealed, latch-down nose (an example of which may be seen in  FIG. 2 , for example, comprising latch  145  and seal  147 ), thereby facilitating a “floatless” casing installation. 
   The following example describes one exemplary embodiment in which the present invention may be employed. At the interface between the work string and the casing within the well bore, a three-plug cementing plug assembly may be suspended. During well circulation activities prior to introducing a cement composition into the casing, operating personnel may introduce a releasing device, such as a weighted free fall device (e.g., a weighted ball) or a positive displacement dart, into the work string and allow such releasing device to interact with the three-plug cementing plug assembly. In certain exemplary embodiments where a dart is used as the releasing device, inner bore  19  of inner mandrel  13  is configured such that the dart becomes encapsulated within inner mandrel  13  after contact, and does not become dislodged when inner mandrel  13  separates from bottom cementing plug  10 . In certain exemplary embodiments where a weighted ball is used as the releasing device, inner bore  19  of inner mandrel  13  is tapered such that, after inner mandrel  13  separates from bottom cementing plug  10 , the weighted ball cannot become dislodged from inner mandrel  13  under normal circumstances. In this interaction, in one embodiment the releasing device passes through inner sleeve  33  of top cementing plug  30 , through inner mandrel  23  of second bottom cementing plug  20 , and lodges in inner bore  19  of inner mandrel  13  of first bottom cementing plug  10 . In certain exemplary embodiments, inner bore  19  is tapered. The interaction of the releasing device in inner bore  19  of inner mandrel  13  interrupts fluid flow through the work string and casing, causing a pressure increase, which may in some circumstances be detectable by operating personnel, depending on factors such as whether the well bore is hydrostatically balanced at the time. When the internal casing pressure reaches a selected first differential pressure frangible devices  51  are sheared, releasing first bottom cementing plug  10  from second bottom cementing plug  20 . In certain exemplary embodiments of the cementing plugs of the present invention, seal  55  has an equal or greater diameter than second seal  56 . In certain exemplary embodiments, seals  100  and  101  have the same seal diameter, thereby balancing the pressure on inner sleeve  33 , and preventing frangible devices  34  from being subjected to loading. Among other benefits, this arrangement maintains inner mandrel  23  of second bottom cementing plug  20  under neutral or compressive loading during the increase in pressure before the release of first bottom cementing plug  10 , thereby minimizing the possibility of prematurely shearing frangible devices  24  and  52 , which would prematurely deploy second bottom cementing plug  20  and inner mandrel  23  of second bottom cementing plug  20 . 
   Having been released from second bottom cementing plug  20 , first bottom cementing plug  10  travels down through the casing until it encounters baffle adapter  40 , interrupting fluid flow once again and causing another pressure increase. This pressure increase signals the operating personnel that first bottom cementing plug  10  has traversed the length of the casing. The time difference between pressure increases, in conjunction with the known pumping rate, may be used by operating personnel to measure a volume of fluid in the system. For example, where a free fall device such as a weighted ball is used as the releasing device, the time difference between pressure increases may be used to measure the volume in the casing string. Where a positive displacement device such as a dart is used as the releasing device, the time difference between the release of the positive displacement device and pressure increases in conjunction with the known pumping rate may be used to measure the total volume of fluid in the system, e.g., the volume in the drill pipe plus the volume in the casing string. Among other benefits, the deployment of first bottom cementing plug  10  during circulation activities enables operating personnel to more accurately determine the amount of displacement fluid that will be necessary to properly displace the anticipated cement slurry by comparing the calculated casing volume based upon nominal inner diameters of the pipe string with the volume measured to have been actually displaced downhole between the two pressure increases. Operating personnel may then increase the differential pressure across seal  58  to a selected second differential pressure sufficient to shear frangible devices  14 , release inner mandrel  13 , and restore fluid flow through the relatively large inner diameter of outer body  11  of first bottom cementing plug  10 . Inner mandrel  13  will fall through baffle adapter  40  onto a bypass baffle (e.g., bypass baffle  500 , illustrated in  FIG. 6 ) installed above the float valve or, alternatively, into perforated catcher tube  42 . In either case longitudinal slots  17  in nose  15  of inner mandrel  13  assure that inner mandrel  13  does not substantially undesirably interfere with fluid flow. The inclusion of a bypass baffle above the float valve protects the float valve and minimizes potentially high fluid turbulence at the interface between nose  15  of inner mandrel  13  and the top of the float valve assembly. 
   When operating personnel subsequently introduce a cement composition into the work string, they also introduce a releasing device. In certain exemplary embodiments, the releasing device is a positive displacement releasing device, such as a dart, although other releasing devices, such as a weighted ball, may be used. Generally, the releasing device is pumped down through the work string at the leading edge of the cement composition. It then passes through top cementing plug  30 , and lodges within inner bore  70  of inner mandrel  23  of second bottom cementing plug  20 , thereby interrupting fluid flow. Next, the differential pressure may be increased across seal  56  to a selected third differential pressure, shearing frangible devices  52 , and releasing second bottom cementing plug  20  from top cementing plug  30 . In certain exemplary embodiments, the differential pressure may be increased across seal  56  naturally by virtue of the hydrostatic imbalance across the releasing device; in certain other exemplary embodiments, the differential pressure may be increased by actions taken by operating personnel. The cement slurry is pumped down through the casing with second bottom cementing plug  20  at its leading edge until second bottom cementing plug  20  contacts, and seals against, first bottom cementing plug  10  which had previously contacted and sealed against baffle adapter  40 . Fluid flow is again interrupted. Differential pressure across seal  99  may then be increased to a selected fourth differential pressure, thereby shearing frangible devices  24  and releasing inner mandrel  23  from outer body  21  of second bottom cementing plug  20 . This reestablishes fluid flow through the relatively large cross-sections of outer body  21  of second bottom cementing plug  20  and outer body  11  of first bottom cementing plug  10 . Inner mandrel  23  passes through outer body  21  of second bottom cementing plug  20 , outer body  11  of first bottom cementing plug  10 , and baffle adapter  40 , falling onto a bypass baffle installed above the float valve or, alternatively, into perforated catcher tube  42 . In either case, optional longitudinal slots  27  in nose  25  of inner mandrel  23  may assure that inner mandrel  23  does not substantially undesirably interfere with fluid flow. 
   When a desired volume of cement slurry has been placed into the work string, operating personnel release a releasing device at the trailing edge of the cement slurry. In certain exemplary embodiments, the releasing device may be a positive displacement device, such as a latch-down type dart. In certain other exemplary embodiments, other types of releasing devices may be used, including but not limited to a weighted ball. The releasing device may be pumped down through the work string at the trailing edge of the cement slurry. The device will interact with inner bore  39  of inner sleeve  33  of top cementing plug  30 , which inner bore  39  may in certain exemplary embodiments be tapered, so as to provide a sort of seat for the releasing device. Fluid flow is interrupted, and the resulting pressure increase signals operating personnel that the trailing edge of the cement slurry has arrived at the casing. Increasing the differential pressure across seal  100  to a selected fifth differential pressure shears frangible devices  34 , releasing inner sleeve  33  in top cementing plug  30 . Inner sleeve  33  travels down from a first position to a second, “released” position within outer body  31  of top cementing plug  30 , shouldering off at shoulder point  105 . 
   Optionally, a variety of “secondary” releasing mechanisms may be employed within top cementing plug  30 , to ensure that top cementing plug  30  does not prematurely detach from work string  80  (for example, by accidental, premature shearing of frangible devices  34 ). Such secondary release mechanisms include, but are not limited to, a collet-type releasing mechanism  35  or a ball-type releasing mechanism  36 . For example, in embodiments where collet-type releasing mechanism  35  is used, inner sleeve  33  may travel down to its “released” position such that the upper end of collet fingers  96  are no longer backed by inner sleeve  33 , thereby allowing collet fingers  96  to flex inwardly and become disengaged from a collet retainer, which collet retainer may comprise split ring  111  (which retains lobes  95 ) and outer case  94 . The collet retainer is initially in interference fit with lobes  95  at the upper end of collet fingers  96 . Generally, inner sleeve  33  remains in sealing contact with the inner bore of the releasing mechanism, and, in certain exemplary embodiments, inner sleeve  33  latches into the second, “released” position by engagement of a lock mechanism  37  into internal upset  115 . In certain other exemplary embodiments, not shown on  FIG. 1 , the lower end of inner sleeve  33  may be configured as collet fingers having a square shoulder at the back of an external upset lobe, wherein such collet fingers may be initially compressed within the minor bore of a collet body, and then, upon being contacted with a releasing device, spring out and latch into internal upset  115 . 
   Upon being released by the shearing of frangible devices  34  (and, by the release of an optional secondary release mechanism where such is used), inner sleeve  33  moves from a first position to a second “released” position, which permits the release of top cementing plug  30  from work string  80 . In certain exemplary embodiments, both the releasing device (e.g., a positive displacement dart, for example) and inner sleeve  33  comprise latch-down type devices. For example, inner sleeve  33  may comprise as receiving profile designed so as to accept a latch-down mechanism on a releasing device, as may be seen from the exemplary embodiment illustrated at  180  in  FIG. 1 . In such exemplary embodiments, top cementing plug  30  remains a pressure barrier, which may be useful should problems be experienced with a float valve, for instance. The cement composition travels down through the casing with top cementing plug  30  at its trailing edge until top cementing plug  30  reaches second bottom cementing plug  20 , which had previously in this example reached first bottom cementing plug  10 , which had itself previously in this example reached baffle adapter  40 . Fluid flow is again interrupted, signaling operating personnel that the trailing edge of the cement composition has arrived at baffle adapter  40 . 
   A two-plug cementing plug system of the present invention may be used for a variety of purposes, including, but not limited to, instances where a calibration of the amount of requisite displacement fluid is not needed, or instances where separation of more than two phases of fluid within the well bore is not needed, for example. Generally, the two-plug cementing plug system may be employed through the use of procedures similar to those described above for the three-plug cementing plug system, except that the step of using a first bottom plug to calibrate the interior volume of the casing, is omitted. 
   Among other uses to which the cementing plug systems of the present invention may be put, certain exemplary embodiments of the cementing plug systems may be used to activate other devices used in subterranean well bores. For example, a baffle adapter, such as baffle adapter  40 , may be included within ported collar  200  in the place of a conventional plug seat, as shown in  FIG. 5 . Ported collar  200  is typically located in the casing string one or more casing joints above the upper-most float valve, and comprises exposed ports  210  through side wall  220 , which ports  210  may permit fluid flow when opened so as to allow the casing to rapidly fill to reduce ram effects during casing installation in tight hole conditions. In certain exemplary embodiments, such ported collar  200  will further comprise inner sliding sleeve  230  located within ported collar  200  above ports  210 , which may allow flow through ported collar  200  until a desired time. In certain exemplary embodiments, flow is allowed through ports  210  until such time as a bottom plug is landed to “close” the collar and direct all further flow down through the casing and out around the shoe. In certain exemplary embodiments, inner sliding sleeve  230  would generally comprise inner bore  240 . In certain exemplary embodiments, inner bore  240  may be configured so as to provide a “seat” for a bottom cementing plug. Inner bore  240  may optionally be configured in certain exemplary embodiments so as to comprise a unique receiving profile (such as single lobe unique receiving profile  260 , for example, which is illustrated in the upper half of  FIG. 5 ), designed to permit a particular releasing device (e.g., a dart having a nosepiece comprising a matching unique key profile) to locate and lock within it. In certain other exemplary embodiments, inner bore  240  may optionally be configured with a receiving profile designed so as to accept a latch-down mechanism on a releasing device (such as a dart having a nosepiece comprising a self-energized “C” ring, for example); an example of such receiving profile may be seen in the lower half of  FIG. 5 , at  255 . Inner sliding sleeve  230  may be attached to ported collar  200  by, for example, frangible device  250 . A cementing plug of the present invention (comprising a detachable inner mandrel attached to the outer body of the plug by a frangible device or the like) may be landed on baffle adapter  40  within ported collar  200  so as to seal within the seat provided by inner bore  240 . As pressure within the casing increases to a first differential pressure, frangible device  250  within ported collar  200  is sheared, thereby displacing inner sliding sleeve  230  within ported collar  200  so as to seal off ports  210  in the side wall. As pressure within the casing increases to a second differential pressure, the frangible device attaching the inner mandrel to the cementing plug is sheared, displacing the inner mandrel and permitting fluid flow to resume through the cementing plug. 
   While the use of the cementing plugs of the present invention in sub-surface release applications has been described, other embodiments of the present invention may advantageously employ these cementing plugs as conventional surface-release plugs. For example, a surface-launched bottom cementing plug comprising a detachable inner mandrel in conjunction with a baffle adapter and bypass baffle of the present invention may prove particularly useful in horizontal well applications, to mitigate potential problems with the accumulation of a bed of solids in the horizontal section of the well. Among other benefits, surface launched bottom cementing plugs with detachable inner mandrels may be useful to an operator in applications where it is desirable to employ a bottom cementing plug that may be modified at the surface to perform a particular function as needed; such modifications may comprise replacing a frangible device installed in such bottom cementing plug that shears at a particular pressure with a frangible device that shears at a different pressure more suitable for the particular task to be performed. 
   Therefore, the present invention is well-adapted to carry out the objects and attain the ends and advantages mentioned as well as those which are inherent therein. While the invention has been depicted, described, and is defined by reference to exemplary embodiments of the invention, such a reference does not imply a limitation on the invention, and no such limitation is to be inferred. The invention is capable of considerable modification, alternation, and equivalents in form and function, as will occur to those ordinarily skilled in the pertinent arts and having the benefit of this disclosure. The depicted and described embodiments of the invention are exemplary only, and are not exhaustive of the scope of the invention. Consequently, the invention is intended to be limited only by the spirit and scope of the appended claims, giving full cognizance to equivalents in all respects.

Summary:
A method and plug for separating fluids in subterranean wells is provided. The plug enters a passage at an interface of successively introduced fluids. The plug comprises an outer body and a detachable inner mandrel attached to the outer body. An Assembly comprising a plurality of plugs may also be used, in which case the plurality of plugs releasably attach to each other.