MULTI-STAGE CEMENTING OF

A WELL CASING

Background of the Invention

I. Field of the Invention

This invention relates to a device and method for cementing a casing string in a borehole in multiple stages and, in particular, to a device and method for multi-stage cementing of a casing which permits rotation of the upper string independent from the lower string during the second cementation stage in order to ensure full displacement of borehole fluid.

II. Description of the Prior Art

During various oil well production operations, after a borehole has been drilled to a depth of several thousand feet, a steel casing is placed in the well in order to isolate geological formations or zones of productive interest. This casing is cemented into place to prevent unconsolidated formations from sloughing off and to prevent different formation fluids from mixing with one another. In addition, the casing prevents formation fluids from entering the drilled borehole. For whatever reason the steel casing is run, the borehole cannot be isolated properly unless the drilling fluid in the hole surrounding the casing is completely replaced by a hardening, non-porous compound such as cement. During the cementing process, the cement slurry is pumped downhole through the casing string and then back uphole outside of the casing string thereby replacing the mud on the outside of the casing string with cement and bonding the outer periphery of the casing to the borehole wall. A successful cement job is one in which all of the drilling mud is replaced by cement and the cement hardens into a protective sheath before further intrusion of fluids.

Because of the depth of the borehole, the full displacement of the mud by cement along the entire length of the casing has been one of the most difficult of all oil well jobs. If full cementation is not accomplished, vertical fluid migration through channels of undisplaced drilling mud is possible.

Additionally, in longer casing strings the cement slurry may be difficult to place behind the casing in one stage because the fluid density of the slurry column is higher than that of the drilling mud, causing difficulties in forcing the slurry several thousand feet down through the casing and several thousand feet back up the borehole. In order to overcome this limitation, two-stage cementing valves were developed which allowed the cement slurry to be first pumped down the entire casing string and back up the hole to an intermediate point just below the valve. The cementing valve is then opened allowing circulation from the valve upward, shutting off circulation in the casing string below the valve and the remaining cement slurry is then pumped down the casing, through the valve ports into the borehole annulus, and back up the upper portion of the hole to the surface.

Although the two-stage cementation process simplifies cementing longer casing strings, full displacement of the drilling mud cannot be ensured without rotation or reciprocation of the casing string. This is particularly true in a borehole which is not true vertical, or straight, or is elliptical or eccentric in its geometry. While reciprocation of the casing string during the cementation process may be effective in removing mud trapped between the casing and the borehole wall, rotation is preferred since it is able to sweep out the trapped mud by the friction between the string and the mud so that the mud from the hole may be displaced by cement. In shorter boreholes, where only one cementation stage is necessary, the full string is rotated until full displacement of the

drilling mud is accomplished over the entire length of the string.

Full string rotation or reciprocation is also utilized in the two-stage cementation process. While the first stage of the slurry is pumped down the casing and into the lower portion of the borehole, the string is rotated to ensure full mud displacement in the first stage cementation. However, once the first stage of the slurry is allowed to harden, the upper part of the casing above the multi-stage cementing valve could not be rotated during the second stage. This resulted in inadequate displacement of the drilling mud in the upper stage of cement.

Summary of the Present Invention

The present invention overcomes the disadvantages of the previously known devices for multi-stage cementing of a casing string by providing an apparatus and method for multi-stage cementing whereby the upper casing string can be rotated independently from the lower casing string during the second stage of cementation to ensure full displacement of the drilling mud.

The apparatus according to the present invention comprises a swivel mechanism which is connected between the upper and lower casing string in the proximity of the intermediate cementing valve in order to permit rotation of the full string during the first stage of cementation and the upper portion of the casing above the multi-stage cementing valve during the second or final cementing stage. The mechanism includes a lower tubular string portion and an upper tubular string portion telescopically engaged such that the upper tubular string portion can be vertically displaced relative to the lower tubular string portion. In a preferred embodiment, both tubular string portions include engageable splines which cooperate to effectuate rotation of the full casing string yet

disengage upon displacement of the upper string relative to the lower string. Upon disengagement of the splines, the upper tubular string portion and casing are free to rotate independently from the lower tubular string portion and casing. Moreover, because the splines are freely engageable the full casing may be rotated in either direction and the splines may be re-engaged even after disengagement.

In addition to providing independent rotation of the upper and lower casing strings, the present invention is readily arranged within the standard casing string at any position along the string. Thus, multiple stages of cementation may be accomplished simply by providing splines of varying frictional engagement or length; and since the mechanism of the present invention performs no function other than to control the rotation of the various stages, the device provides flexibility by allowing placement in the string according to individual requirements. However, in order to reduce equipment costs, flexibility may be sacrificed and the swivel mechanism may be combined with the multi-stage cementing valve into a single tool.

The procedure of the present invention utilizes the apparatus to permit independent rotation of the upper and lower casing _ strings thereby ensuring complete displacement of the drilling mud. Under this procedure, the casing is run into the hole with the swivel, mechanism installed immediately above the multi-stage cementing valve. While pumping the cement slurry through the casing to the bottom of the hole, the full string is rotated for proper cement displacement. Once the predetermined quantity of cement slurry is pumped into the casing, a valve plug is inserted and forced under pressure to the bottom of the string. Thereafter, a valve bomb is dropped down the casing to open the two-stage cementing valve disposed directly below the swivel mechanism. Once the intermediate valve ports are opened, any excess cement

above the valve is pumped out of the hole, and the first stage cement job is allowed to take its initial set. After a sufficient period of time, the splines on the swivel are disengaged by moving the upper string downwardly. Thereafter, the second stage cement slurry is displaced through the upper casing string while it is independently rotated. Once the cement slurry for the second stage is fully mixed and pumped into the casing, a closing plug is released behind the cement, displaced downward to the valve, and the intermediate valve ports are closed. Upon complete setting of the cement the various plugs disposed within the casing string may be drilled out to provide access to the borehole.

Thus, the present invention provides a simple and convenient apparatus and method for cementation of a full casing string which ensures full displacement of the drilling mud.

Other objects, features, and advantages of the present invention will be apparent from the following detailed description taken in connection with the accompanying drawings.

Brief Description of the Drawing

The present invention will be more fully understood by reference to the following detailed description of a preferred embodiment of the present invention when read in conjunction with the accompanying drawing in which the reference characters refer to like parts throughout the views, and in which:

FIGURE 1 is an exploded perspective view of the apparatus embodying the present invention;

FIGURE 2 is a partial cross-sectional side view of the apparatus of the present invention in its engaged state;

FIGURE 3 is a partial cross-sectional side view of the apparatus of the present invention in its disengaged state;

FIGURE 4 is a cross-sectional perspective view taken along line 4-4 of Fig. 2; and

FIGURES 5 through 9 are diagrammatical illustrations of a cross-sectional view of a borehole formed into the earth and depicting the method and apparatus embodying the present invention.

Detailed Description of a Preferred Embodiment of the Present Invention

Referring first to Figs. 1 through 4, a swivel apparatus 10 embodying the present invention is thereshown connected in series arrangement within a tubular casing string 12. The full casing string 12 includes an upper casing string 14 and a lower casing string 16 connected to each other by the swivel apparatus 10. The swivel apparatus 10 includes means for selectively transmitting rotational torque from the upper casing string 14 to the lower casing string 16 yet can be disengaged to provide rotation of the upper string 14 independent of the lower

string 16 while maintaining connection of the string portions as will be subsequently described herein.

The swivel 10 generally comprises a tubular lower string portion 18 and a tubular upper string portion 20. The lower tubular string portion 18 is detachably secured to the lower casing string 16 by way of cooperating threads 22. Similarly, upper tubular string portion 20 is detachably secured to the upper casing string 14 by threads 24. However, it is to be understood that the swivel 10 may be serially connected within the casing string 12 in any known manner or may be combined with other tools to be utilized in the cementation process to be described. Furthermore, the upper tubular string portion 20 includes an upper mandrel 26 and an upper sleeve 28 to facilitate assembly of the swivel 10. The mandrel 26 and sleeve 28 are fixedly connected to each other by means of threads 27 and include seal means 30 which prevents fluid flow between these components of the upper tubular string portion 20. Seal means 30 further resists relative rotation between mandrel 26 and sleeve 28 after these components have been assembled.

As is best shown in Figs. 2 and 3, the lower tubular string portion 18 is telescopically received within the upper tubular string portion 20 and in particular the upper sleeve 28. Thus, the upper tubular string portion 20 is slidably movable relative to the lower tubular string portion 18 such that the upper casing string 14 may be longitudinally displaced relative to the lower casing string 12 from their relative positions shown in Fig. 2 to the position shown in Fig. 3. Moreover, the end 32 of the upper mandrel 26 acts as an abutment surface against the lower tubular string portion 18 to limit the downward movement of the upper tubular string portion 20 relative to the lower tubular string portion 18. Conversely, both the lower tubular string portion 18 and the upper sleeve 28 include engagement shoulders 34 and 36, respectively, which limit the upward movement of the upper casing 14

relative to the lower casing 12. Therefore, the components of the swivel 10 have a fixed distance of relative longitudinal displacement which prevents detachment of the upper casing string 14 from the lower casing string 12 without disassembly of the swivel 10.

Referring now to Figs. 1 and 4 , means are provided for selectively transmitting rotational torque from the upper tubular string portion 20 to the lower tubular string portion 18 in order that the upper casing string 14 may be rotated independently of the lower casing string 12. In a preferred embodiment of the present invention, this means comprises engaging splines 40 formed in the upper and lower tubular string portions. These splines 40 preferably include a pair of splines 42 formed on the lower portion 18 and a pair of splines 44 formed on the upper portion 20. These splines form corresponding longitudinal grooves 46 which receive the splines of the cooperating component of the swivel 10. As is shown in the drawings, these splines may be of any desired width as long as the corresponding groove 46 closely conforms to the width of the cooperating spline in order to minimize lost rotational motion. However, in a preferred embodiment, the splines 42 have a substantially smaller width than the splines 44 of the upper tubular string portion 20 in order to reduce manufacturing costs. Moreover, any number of splines may be utilized to accomplish similar results or to vary the frictional engagement between different sets of swivel components disposed at various levels along the casing string 12. Additionally, the length of the splines may be varied to allow for greater longitudinal displacement prior to disengagement of the splines.

Assembly of the swivel 10 allows longitudinal displacement of the upper tubular string portion 20 relative to the lower tubular string portion 18. After placement of the seals 30 and 38 in their retaining grooves 48, the lower tubular string portion 18 is

positioned within the upper sleeve 28 such that the splines 42 engage the grooves 46. The upper tubular string portion 20 can now be assembled by placing the mandrel 26 within the end of the sleeve 28 and rotating the mandrel 26 to engage the threads 27 of each component. The enlarged diameter portion 50 of the lower portion 18 is now positionally captured between the end 32 of the mandrel 26 and the abutment shoulder 36 of the sleeve 28. The swivel 10 can now be connected within the casing string 12 in order to effectuate multi-stage cementation of the string 12 as will be subsequently described.

Referring now generally to Figs. 5 through 9, the swivel apparatus 10 is thereshown disposed within a full casing string 12. The casing string 12 is shown positioned within a borehole 52 formed through the earth in order to investigate possible hydrocarbon reserves. The casing string 12 extends substantially to the bottom 54 of the borehole 52. The casing string 12 includes numerous interconnected components which perform specific functions in the drilling operation. However, the method of the present invention will be described in conjunction with the components which are necessary to utilize the method. Included in the casing string 12 is an intermediate valve 56 having lateral flow ports 58 capable of being selectively opened and closed. Preferably, the valve 56 is disposed just below the swivel 10. Alternatively, the valve 56 can be integrally formed with the swivel 10, particularly the lower tubular string portion 18 to substantially reduce manufacturing costs. If thus configured, the valve 56 will rotate along with the lower portion 18.

Once the full casing string 12, including the swivel 10 and the valve 56, has been run into the borehole 52, as shown in Fig. 5, the multi-stage cementation process can be initiated. As the full string 12 is rotated, the first stage of cement 60 is displaced through the interior 62 of the casing 12 and out flow ports 64 in the end of the

casing 12. With continued surface pressure, the cement 60 will be caused to flow back up the borehole 52 exteriorly of the casing 12 as shown in Fig. 6. In order to ensure constant flow, the cement 60 is immediately followed by first plug 66 which acts as a wiper to force all of the cement 60 down the interior 62 of the casing 12. Back pressure on the plug 66 is continued until the plug 66 reaches the float collar 68. This float collar 68 includes a constricted baffle 70 which receives the plug 66 to shut off fluid flow through the end of the casing 12. Since the volume of cement 60 necessary to complete first-stage cementation can be approximated, the height of the column of cement 60 exteriorly of the casing 12 will preferably reach just below the lateral ports 58 of the valve 56 once the plug 66 reaches this point as shown in Fig. 7. Thus, the first stage of cement 60 has been properly displaced while the full casing string 12 is being rotated to ensure proper displacement of the drilling mud and bonding of the casing to the borehole wall by the cement.

Referring still to Fig. 7, a second plug or "bomb" 72 is dropped into the casing 12 to open the lateral ports 58 of the intermediate valve 56. The force of the plug 72 causes the seal ring 74 which covers the ports 38 to move downwardly, thereby permitting fluid flow therethrough. With the first stage of cement 60 properly displaced and the ports 58 opened, the cement 60 is allowed to take at least an initial set prior to initiating second-stage cementation.

After allowing the cement 60 to partially set, the upper casing string 14 is moved downwardly in order to disengage the splines 40 of the swivel 10. In the preferred embodiment, it will be necessary to longitudinally displace the upper casing string 14 approximately 18 inches in order to disengage the cooperating splines 40 of the swivel 10. However, the required displacement will be dependent upon the length of

the splines 40. With the splines disengaged, the upper string 14 can now be rotated and/or reciprocated independently of the lower casing string 16. Thus, while rotating and/or reciprocating the upper string 16, the second stage of cement 80 is displaced through the interior 62 of the upper casing string 16 as shown in Fig. 7.

Referring now to Figs. 8 and 9, as the second-stage cement 80 travels down the interior 62 of the casing, it will be caused to flow through the lateral ports 58 into the borehole 52. Since the first stage of cement 60 has set just below the valve 56, as the cement 80 flows out of the ports 58, it will be forced back up the borehole 52 exteriorly of the upper casing string 14 as shown in Fig. 8. As with the first stage, the second stage of cement 80 is followed by a third or closing plug 82 which ensures complete displacement of the cement. When the closing plug 82 reaches the valve 56, it engages inner sleeve 84 and forces it downwardly to once again close the lateral ports 58 as shown in Fig. 9. Again, since the required volume of cement 80 can be approximated, upon completion of the second-stage cementation, the height of the cement column exterior of the casing will extend to the ground surface. Moreover, during this second stage, the upper casing string 14 has been continuously rotated and/or reciprocated to ensure proper displacement of the cement 80.

Thus, the method and apparatus of the present invention allows multi-stage cementation of a casing string while permitting independent rotation and/or reciprocation of the upper casing string during the second stage of cementing. Although the method of the present invention has been described in conjunction with two stages of cementation using only one swivel apparatus, it is to be understood that any number of swivels can be utilized thereby allowing for multi-stage cementation. Moreover, the method described herein effectively

simplifies the subsequent drilling or milling out of the hole since a majority of the cement has been forced into the borehole exterior of the casing. Thus, the present invention reduces the costs associated with cementation of the casing string while providing superior isolation of the casing to effectively isolate geological formations in the well.

The foregoing detailed description has been given for clearness of understanding only and no unnecessary limitations should be understood therefrom as some modifications will be obvious to those skilled in the art without departing from the scope and spirit of the appended claims.

We claim: