Apparatus and method for launching plugs in cementing operations

An apparatus for use in launching cement plugs in a well cementing operation, comprising: a cylinder (130); a piston (110) slideably received in the bore of the cylinder; and an actuator, operable by the piston, for launching a plug from the apparatus into the well; wherein the cylinder has a resiliently mounted latching member (132) positioned in the wall thereof and biased to project into the bore of the cylinder; and the piston has a profiled outer surface defining a recess (114) into which the latching member (132) can project to hold the piston in position in the cylinder.

TECHNICAL FIELD

This invention relates to apparatus and methods for launching plugs in cementing operations of the type found when constructing wells in the oil and gas industry. In particular, the invention relates to the use of a latching mechanism for controlling the movement of a piston in a plug launcher.

BACKGROUND ART

In the construction of oil and gas wells, it is occasionally necessary to cement a liner or casing in the well to provide stability and zonal isolation. In such processes, it is common to use plugs to separate different fluids pumped along the tubing or casing. Such plugs are usually installed in a basket located in cementing equipment lowered into the well. The plugs are launched from the basket by means of darts pumped from the surface.

A known cement plug launching tool (see U.S. Pat. No. 5,890,537) is shown inFIGS. 1-3. The body32of the launching tool includes an upper tubular housing40whose upper end is threaded to the mandrel of the liner setting tool, and whose lower end is threaded at41to a spacer tube42. A sleeve valve44which is slidable in the bore of the housing40is biased upward to a normally open position with respect to ports38by a coil spring46.

A piston50connected to a drive rod36slides in the bore of the spacer tube42which is connected to the upper end of a cylinder tube55. An lower piston58is formed on the rod36and slides within the bore60of the cylinder tube55which is filled with a suitable hydraulic oil. The piston58has an outer diameter that provides a selected clearance with respect to the wall of the bore60such that, as the piston is forced downward with the rod36, a metering effect is created which retards the rate of downward movement.

The lower end of the cylinder tube55is connected to the upper end of the basket33which initially houses the upper and lower wiper plugs34,35, and is provided with a plurality of longitudinal slots68that receive radial stop pins70which extend from the outer periphery of a drive flange75that rests on top of the upper plug34. A head71on the upper end of the upper plug34receives the inner ends of several radially extending shear pins73on the drive flange75to releasably couple the plug34to the flange.

In operation and use, the liner is run and suspended by a hanger from a point near the lower end of the casing which is below the wellhead. The plug launcher tool is connected to the lower end of the mandrel, and the wiper plugs34and35were previously loaded into the basket33. The drive rod36is in its upper position where the piston58is at the upper end of the oil chamber60. The ports38in the housing40are open so that fluids can flow therethrough. A dart launcher is provided at the surface.

In order to cement the liner in place, cement slurry is pumped in through the dart launcher, and then a valve is opened to release a lower dart101. Pressure is applied to the top of the dart101to force it through the valve and down into the drill pipe ahead of the cement. Eventually the dart101enters the housing40, passes into the bore of the valve sleeve44, and to a position where its nose bumps against the drive head50of the rod36. Since the elastomer cups of the dart101seal off the bore of the valve sleeve44, pressure causes the sleeve valve to shift downward against the bias of the coil spring46, and in so doing, partially close off the radial ports38.

Pressure on the dart101applies downward force to the rod36and causes it to shift downward in the body32, thereby driving both the upper and lower wiper plugs34and35downward. Such movement is slowed by the action of hydraulic oil that meters upward through the clearance between the piston58and the inner wall of the cylinder60so that shock loads are dissipated. When the pins70on the drive plate75reach the bottoms of the slots68as shown inFIG. 2, downward movement of the upper plug34is stopped. However the lower plug35will have been ejected from the bottom of the basket33and into the bore of the liner. At about the same time as the stop pins70encounter the bottoms of the slots68, the top cup of the dart101clears the bottom of the sleeve valve44so that the ports38are re-opened as the sleeve valve is shifted upward by the coil spring46. Pumping of cement is continued until the desired number of barrels of cement has been placed within the liner.

When the proper amount of cement has been pumped into the running string, the upper dart100is forced into the drill pipe, followed by whatever fluid is being pumped behind it. The dart100travels down through the running string, the mandrel, and into the housing40. When the cups of the dart100enter the valve sleeve44and seal off its bore, the valve sleeve shifts downward to close off the lateral ports38. The dart100then engages the lower dart101, so that applied pressures force the drive rod36further down in the body32as shown inFIG. 3. The pins73are sheared so that the drive disc66on the lower end of the rod32passes through the plate75and forces ejection of the upper wiper plug34from the bottom of the basket33. The metering of oil past the piston58again slows or retards downward movement of the rod32so that ejection is smoothed. When the top end of the dart100clears the bore of the valve sleeve44, the valve sleeve again opens, as before, so that displacement fluids flow around the outside of the launcher assembly and through the annular space between the basket33and the inner wall of the liner. A positive indication of the launching of wipers plugs34and35from basket33is shown by an increase in pumping pressure at the surface location resulting from the cushioned travel of piston58for both plugs34and35. The shearing of pins73for upper plug34additionally increases the pumping pressure for upper plug34. For example, the increase in the pumping pressure may amount to about 1500 psi for lower plug35and to about 3000 psi for upper plug34.

As is discussed above, the most common method of latching components inside a pipe is to use shear pins, shear screws or the like. A known force is applied to the shearing component, which subsequently fails mechanically and releases the downhole component that had been latched inside the pipe. Such a system causes debris from the broken shearing components to remain inside the pipe and also requires that the shearing components be replaced after each use. The downhole components, once unlatched, can not be re-secured inside the pipe until it has been recovered back to the surface.

It is an object of the invention that allows the latching of a piston in a plug launching system without the need for shear pins to be used. The invention is based on the use of resiliently biased latching members.

DISCLOSURE OF THE INVENTION

A first aspect of the invention provides an apparatus for use in launching cement plugs in a well cementing operation, comprising:a cylinder;a piston slideably received in the bore of the cylinder; andan actuator, operable by the piston, for launching a plug from the apparatus into the well;

wherein

the cylinder has a resiliently mounted latching member positioned in the wall thereof and biased to project into the bore of the cylinder; and the piston has a profiled outer surface defining a recess into which the latching member can project to hold the piston in position in the cylinder.

The profile in the outer surface of the piston preferably comprises angled surfaces to allow the piston to engage the latching member to progressively move it out of the bore of the cylinder and the piston is moved past the latching member. The profile typically comprises first and second angled surfaces on opposite sides of the recess.

The angled surfaces can be arranged such that the force required to move the piston so that the latching member passes over the first surface and engages in the recess reaches a first maximum, and the force required to move the piston so that the latching member disengages from the recess and passes over the second surface reaches a second maximum.

Preferably, the first and second surfaces have different profiles such that the first and second maxima are different. The second maximum is typically greater than the first.

Multiple latching members can be provided around the cylinder bore.

The latching member preferably comprises spring dogs attached at one end to the cylinder and including a locking formation defined at the other, free end for engagement in the recess.

A second aspect of the invention provides a method of launching a plug in a well cementing operation using an apparatus as claimed in any preceding claim, the method comprising:initially forcing the piston along the bore of the cylinder until the latching member engages in the recess; andsubsequently forcing the piston further along the bore of the cylinder so as to detach the latching member from the recess; wherein movement of the piston is transferred to the plug by means of the actuator.

An increasing force, provided by the latching members, slows the movement of the piston down before the latching members engage in the recess of the piston, bringing it to a hard stop.

The step of forcing the piston along the bore of the cylinder preferably comprises pumping a dart along a pipe connected to the cylinder until the dart contacts the piston, and applying fluid pressure above the dart to move the dart and piston along the cylinder bore.

In this case, the step of initially forcing the piston along the cylinder bore can comprise pumping a first dart into contact with the piston, and the step of subsequently forcing the piston along the cylinder bore can comprise pumping a second dart into contact with the first dart.

The method preferably comprises measuring the pressure of the fluid used to move the darts and detecting a first pressure maximum indicative of engagement of movement of the latching member in the recess, and detecting a second maximum indicative of detachment of the latching member from the recess.

Further aspects of the invention will be apparent from the following description.

MODE(S) FOR CARRYING OUT THE INVENTION

This invention provides a piston system that replaces that shown inFIGS. 1-3discussed above. In particular, the piston and cylinder arrangement58,60, and the shear pin and slots68,70ofFIGS. 1-3are replaced by the system described below.

The apparatus and its operation are shown inFIGS. 4A-4F. The apparatus comprises three portions. The first portion comprises the following elements. A first plug401and a second plug402are located inside a plug basket403. A plug piston404, located above the plug basket403, is driven by a main rod405. A drive piston406is attached at the top of the main rod405. The drive piston406and the upper portion of the main rod405are encased inside the bore of a cylinder407. The cylinder407contains a latching mechanism that comprises at least two spring dogs408and409. Above the drive piston406, there are ports410and411through which wellbore service fluids may flow. This first portion of the apparatus is initially installed inside another tubular body412. The tubular body may comprise casing. The second portion of the apparatus is a first dart413. This second portion of the apparatus is initially separated from the first portion. The third portion of the apparatus is a second dart414, and is also initially separated from the first portion.

A detailed view of the drive piston406is presented inFIG. 5. The drive piston406is attached to the main rod405(shown inFIG. 4A) that acts as an actuator to drive plugs from the basket403(shown inFIG. 4A). The drive piston406has a profiled outer surface comprising a bottom section512, a recess514and a top section516. The bottom section512comprises an angled surface518that flares from the bottom end of the piston to a first diameter surface520that has a first maximum diameter B less than that of the cylinder bore. The diameter of the plug then decreases behind the first diameter surface520to the base522of the recess514. The top section516comprises a further angled surface524that flares from the base522of the recess514to a second diameter surface526having a second maximum diameter A that corresponds closely to the inner diameter of the cylinder bore. The second maximum diameter A is greater than the first maximum diameter B. The surface528then reduces somewhat to the top end of the piston that is adapted for contact with the first dart413(shown inFIG. 4A).

FIG. 6shows the drive piston406ofFIG. 5in the cylinder407that forms part of the plug launching system. The cylinder407includes a latching mechanism comprising a series of latching members disposed around its periphery. The latching members comprise spring dogs408and409which are connected to the cylinder407at one end634and have a latching portion636formed at the other which projects through a slot in the cylinder into the bore638. The top edge640of the latching portion636is angled to and the lower edge642is curved away from the top edge640. Thus, the top edge640serves as a locking formation for engagement in the recess514of the drive piston406.

In use, a first fluid is pumped into the tubular body412. The first portion of the apparatus is placed inside the tubular body412. The first dart413is pumped into the first portion of the apparatus in the usual manner to apply a force F to the drive piston406and force it along the cylinder407until the angled surface of the bottom section512engages the top edge of the latching portion636(FIG. 7andFIG. 4B). The force applied to the piston (i.e., the pressure of the fluid used to pump it along the cylinder) is increased and the piston406continues to move, pushing the latching portions636of the spring dogs408and409outward C against the biasing effect of the spring dogs. As shown inFIG. 4B, the downward movement of the first dart413has caused the main rod405and plug piston404to also move downward and begin to expel the first plug401.

As the piston continues to move downward (FIG. 8), the latching portions636pass over the first diameter surface520and flex back inwardly D to engage in the recess514and bring the drive piston to a hard stop. At this point, the first plug401will have been ejected from the basket403(FIG. 4C) and the drive piston406will be held in place by the latching of the spring dogs408and409in the recess514. Furthermore, the first dart413blocks the further passage of process fluid into the cylinder407; consequently, process fluid flow is diverted through ports410and411. A second fluid may be pumped behind the first plug401.

To eject the second plug, the second dart414is pumped from the surface to contact the top of the first dart (FIG. 4DandFIG. 9). The force F is increased until the drive piston406moves, the surface524acting on the top edge540of the latching portions636to force them outwards E until they pass over the second diameter surface526, thereby releasing the drive piston406(FIG. 4E). As the second plug is ejected, the drive piston406moves down and away from the latching portions636. Once the piston is fully past the spring dogs408and409, the latching portions636are allowed to resume their initial positions (FIG. 10andFIG. 4F). The second dart blocks the further passage of process fluid into the cylinder407; consequently, process fluid flow diverted through ports410and411.

Following use, the device can be reset by withdrawing the drive piston406from the bottom of the cylinder407and re-inserting it at the top, or by forcing the drive piston406back up the cylinder407past the spring dogs408and409.

In order to monitor the progress of the plug launching operation, the pressure of the fluid used to pump the darts can be monitored at the surface. The graph inFIG. 11shows the pressure or force, F, against distance travelled during latching and unlatching of the drive piston406as shown inFIGS. 7-10. The first increase in pressure detected at the surface indicates engagement of the drive piston406with the latching mechanism636. The pressure will rise to a maximum as the latching portions pass over the first diameter surface520. The pressure falls as the latching portions pass down into the recess514, indicating that the piston has completed the first movement to eject the first plug401. At this point, the ports410and411will be open and normal pumping can commence. The second dart414is pumped to release the second plug402, and the pumping pressure will begin to rise when the second dart414engages the top of the first dart413. The pressure plateaus again when the latching portions636pass over the second diameter surface526. However, as the second diameter surface526has a larger diameter than the first diameter surface520, a greater force will be needed to move the latching portions636back from the engaged position and release the drive piston406, resulting in a higher pressure detected at the surface. Again, the pressure falls as the latching portions636pass down the surface528, indicating the end of the movement that launches the second plug402.

The use of spring dogs to latch the piston downhole components has advantages of easier maintenance, a smoother mechanism to unlatch the component, increased efficiency and reliability. Unlike the use of shear pins, there is no need to replace any parts after each use.

Various changes can be made within the scope of the invention. Where more than two plugs are to be launched, two or more recesses can be provided in the piston spaced along its surface, or two or more pistons can be used. The darts can be replaced by other release mechanisms such as balls. Other such changes will be apparent.