Cement through side pocket mandrel

Well completion cement may be pumped through a side pocket mandrel that includes parallel rows of filler sections to exclude cement from void space within the side pocket tube. The filler sections are drilled with cross-flow jet channels and surface upsets to stimulate scrubbing turbulence by well working fluid behind a cement wiper plug. The wiper plug includes leading and trailing groups of wiper discs secured to an elongated shaft. The two wiper groups are separated by a distance that permits the leading seal group to gain traction seal before the push seal on the trailing wiper group is lost. A spring centralizer spans a center section of the shaft between the two wiper groups to maintain axial alignment of the shaft as the plug traverses the length of a mandrel.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to methods and apparatus for subterranean well completion. In particular, the invention relates to the manufacture, operation and use of side pocket mandrel tools that accommodate a through-bore flow of cement and enhance a turbulent flow of well working fluid behind the cement wiper plug within the side pocket mandrel as the plug is driven past the mandrel.

2. Description of the Prior Art

Side pocket mandrels are special purpose tubing sections assembled along a production tubing string within a subterranean well for producing fluid such as crude petroleum and natural gas. These special purpose tube sections include relatively short cylindrical barrels (side pockets) in parallel axis alignment with the primary tubular bore axis but laterally off-set therefrom. These side pockets have a bore opening within the tube section interior and an aperture between the barrel interior and the exterior of the mandrel wall. These side pockets constitute receptacles for fluid flow control devices such as valves or property measuring instruments. In the case of valves, fluid flow from the tubing bore into the well annulus or vice versa is controlled.

By means of wireline suspension structures, valve elements may be placed in or removed from the side pockets without removing the tubing string from the well. These flow control options are of great value to well production managers.

Another aspect of well production control that is facilitated by side pocket mandrels is gas lifting. There are many petroleum reservoirs holding vast quantities of petroleum fluids having insufficient internal driving force to raise the native fluid to the surface. Because of the reservoir depth, traditional pumping is not an option. In these cases, the formation fluids may be extracted by means of gas lifting.

There are numerous gas lifting techniques but, in general, a compressible fluid such as nitrogen, carbon dioxide or an external source of natural gas is compressed into the well annulus and selectively admitted into the production tubing bore via side pocket valves. A pressure differential rising of the gas flow within the tubing bore to the surface may be exploited to aspirate a petroleum flow along with the lift gas or to drive a plug along the tubing bore having a column of liquid petroleum above the plug.

When a well is first opened, the reservoir may have sufficient internal driving energy to produce a commercially adequate flow of the formation fluid to the surface. In time, however, that internal energy source may be dissipated long before the reservoir value is depleted. Production experience may anticipate such production developments by positioning side pocket mandrels in the production tube long before the actual need for gas lifted production. When the need for gas lifting arises, the only downhole operations required to begin gas lifting are the wireline placement of the gas lift valve elements in the respective side pockets. When compared to the enterprise of withdrawing and returning several miles of production tubing or coil tubing in a well, wireline procedures are minimal.

Such considerations are more imperative in those cases in which much of the well bore remains uncased. Extremely deep or long, horizontal well bores are examples. For example, a long well bore may be completed with minimum casing length. Below the casing, the raw borehole remains uncased through the formation production face. Completion of the well may include a single “trip” placement of production tube with cross-over and cementing valves. The well annulus between the production tube and borehole wall is cemented above the production zone for isolation. Production flow from the production zone is opened by perforating the production tube and surrounding cement annulus.

Unfortunately, a single trip completion with side pocket mandrels for later gas lifting, for example, has not previously been an available option. Delivery of the cement slurry down the production tube bore unreasonably contaminates the internal labyrinth of the side pocket mandrel.

It is an object of the present invention therefore, to provide a side pocket mandrel that may be cleaned of cement before it sets.

Another object of the invention is a method of single trip well completion that includes pre-positionment of side pocket mandrels that will be operatively available for subsequent gas lift operation.

Also an object of the invention is an apparatus for scouring the flow bore of a side pocket mandrel of cement or other contaminant.

SUMMARY OF THE INVENTION

The invention objectives are accomplished by a side pocket mandrel construction having internal guide and flow vane structure along an internal channel that accommodates the physical alignment and clearance of pocket valve elements. The guide and vane structure comprises a plurality of elongated arc sectors within the mandrel interior flanking the side pocket clearance space. Surface relief, upsets and undercuts into the arc sector surfaces stimulate fluid turbulence for flushing residual cement from the mandrel interior. Cross-flow jet apertures within the arc sector bodies enhance the turbulent generation.

The arc sectors are secured to the mandrel wall, preferably by welding through apertures in the tubing wall. These arc sectors are aligned as parallel rails along opposite sides of a tool clearance channel. The tool clearance channel provides a minimum width required by the valve element and kick-over tool to place and remove and valve element with respect to the bore of the side pocket cylinder.

Used in operational cooperation with the present side pocket mandrel is a cement wiper plug having a pair of longitudinally separated groups of wiper discs. The wiper disc groups are separated by a distance that is proportional to the mandrel length whereby the wiper plug is driven by fluid pressure behind either the leading or trailing wiper group as the side pocket section of the mandrel is traversed. Between the two wiper disc groups, is a centralizer to maintain axial alignment of the shaft linking the two wiper disc groups as the mandrel is traversed.

The fluid pressure driving the wiper plug to push the major bulk of cement from the side pocket mandrel interior often is a light, low viscosity fluid such as water. As fluid flow behind the plug traverses the mandrel, a turbulent flow state within the mandrel is induced by critical fluid flow rates over the arc sector surface profiles and through jet channels across the arc sector widths. Such turbulent flow scrubs and flushes the cement residual from the mandrel interior before the cement is permitted to set.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A representative environment of the invention is illustrated byFIG. 1wherein a production tube10is cemented in an open well bore12by a cement annulus collar14. The length of cemented annulus14extends into or through an economic production zone16. After the cement is placed and set, the tube and collar section is perforated by chemically or explosively formed fissures17that extend into the formation16. These fissures17provide fluid flow conduits from the in situ formation zone16into the flow bore18of the production tube10.

Located along the length of the production tube10above the upper face15of the cement collar14are one or more side pocket mandrels20according to the present description. Procedurally, when the tube10is positioned in the open borehole, a measured quantity of cement is pumped down the tube flow bore18. When the measured quantity of cement is in the tube bore18as a standing fluid column, the trailing or upper face of the tubing confined cement column is capped by a wiper plug50such as that illustrated byFIG. 5. The wiper plug is inserted into the tubing flow bore18against the trailing cement face15while the trailing face is at or near the surface or wellhead. The tubing string is reconnected to the working fluid circulation system and water or other well working fluid is pumped behind the wiper plug50to push the cement down the tube bore18and back up the wellbore annulus. Frequently, a plug seat is placed at the terminal end of the tubing string10to engage the wiper plug50and seal the bottom end of the tubing string10.

The exact location of the collar upper face15may therefore be determined with considerable precision. Similarly, the required location of the mandrels20along the length of the tubing string10may also be precisely determined.

Traversal of the wiper plug through each mandrel displaces most of the cement that has entered the mandrel during the annulus cementing operation. Nevertheless, residual cement remains in the mandrel void spaces that are essential work space for inserting and removing side pocket valves, plugs and instruments. Should this residual cement be allowed to set within a mandrel, the utility of the mandrel is essentially destroyed. The inability of the prior art to adequately clean this work space has prevented side pocket mandrels from be used as in the manner previously described. With respect to the present invention, however, as the well working fluid behind the wiper plug50flows through each mandrel of the present invention, the working flow behind the traveling wiper plug induces turbulent velocities and flow patterns within a mandrel to scrub and flush each mandrel free of residual cement.

Referring toFIG. 2, each side pocket mandrel20in the tubing string10comprises a pair of tubular assembly joints22and24, respectively, at the upper and lower ends. The distal ends of the assembly joints are of the nominal tubing diameter as extended to the surface and are threaded for serial assembly. Distinctively, however, the assembly joints are asymmetrically swaged from the nominal tube diameter at the threaded ends to an enlarged tubular diameter. In welded assembly, for example, between and with the enlarged diameter ends of the upper and lower assembly joints is a larger diameter pocket tube26. Axis32respective to the assembly joints22and24is off-set from and parallel with the pocket tube axis34(FIG. 3).

Within the sectional area of the pocket tube26that is off-set from the primary flow channel area18of the tubing string10is a valve housing cylinder40. The cylinder40is laterally penetrated by external apertures42through the external wall of the pocket tube26. Not illustrated byFIG. 2orFIG. 3is a valve or plug element that is placed in the cylinder40by a wireline manipulated device called a “kickover” tool. For wellbore completion, side pocket mandrels are normally set with side pocket plugs in the cylinder40. Such a plug interrupts flow through the apertures42between the mandrel interior flow channel and the exterior annulus and masks entry of the completion cement. After all completion procedures are accomplished, the plug may be easily withdrawn by wireline tool and replaced by a wireline with a fluid control element.

At the upper end of the mandrel20is a guide sleeve27having a cylindrical cam profile for orienting the kickover tool with the valve cylinder40in a manner well known to those of skill in the art.

Set within the pocket tube area between the side pocket cylinder40and the assembly joints22and24are two rows of filler guide sections35. In a generalized sense, these filler guide sections are formed to fill much of the unnecessary interior volume of the side pocket tube26and thereby eliminate opportunities for cement to occupy that volume. Additionally, the filler guide sections35provide a mass object that prevents a cement wiper plug from entering the spaces that the sections35occupy, thereby preventing the wiper plug from becoming stuck in such spaces. Of equal but less obvious importance is the filler guide section function of generating turbulent circulations within the mandrel voids by the working fluid flow behind the wiper plug.

Similar to quarter-round trim molding, the filler guide sections35have a cylindrical arc surface36and intersecting planar surfaces38and39. The opposing face separation between the surfaces38is determined by clearance space required by the valve element inserts and the kick-over tool.

Surface planes39serve the important function of providing a lateral supporting guide surface for the wiper plug50as it traverses the side pocket tube26and keep the leading wiper elements within the primary flow channel18.

Each of the filler guide sections35is secured within the pocket tube26by one or more filler welds49. Apertures47are drilled or milled through the wall of the pocket tube26to provide welder access to the face of the arc surface36.

At conveniently spaced locations along the length of each filler section, cross flow jet channels44are drilled to intersect from the faces38and39. Also at conveniently spaced locations along the surface planes38and39are indentations or upsets46. Preferably, adjacent filler guide sections35are separated by spaces48to accommodate different expansion rates during subsequent heat treating procedures imposed on the assembly during manufacture. If deemed necessary, such spaces48may be designed to further stimulate flow turbulence.

The wiper plug50utilized with the subject side pocket mandrel is schematically illustrated byFIG. 5. A significant distinction this wiper plug makes over similar prior art devices is the length. The plug50length is correlated to the distance between the upper and lower assembly joints22and24. Wiper plug50has leading and trailing wiper disc groups52and54. Between the leading and trailing groups is a spring centralizer56.

As the leading wiper disc group52enters a side pocket mandrel20, fluid pressure seal behind the wiper discs is lost but the filler guide planes39keep the leading wiper group52in line with the primary tubing flow bore axis18. The trailing disc group54is, at the same time, still in a continuous section of tubing flow bore18above the side pocket mandrel20. Consequently, pressure against the trailing group54continues to load the plug shaft58. As the wiper plug progresses through a mandrel20under the compressive force of group54, the spring centralizer56maintains the axial alignment of the shaft58midsection. By the time the trailing disc group54enters the side pocket mandrel20to lose drive seal, the leading seal group52has reentered the bore18below the mandrel20and regained a drive seal. Consequently, before the trailing seal group54loses drive seal, the leading seal group52has secured traction seal.

Although the invention has been described in terms of specified embodiments which are set forth in detail, it should be understood that the description is for illustration only and that the invention is not necessarily limited thereto, since alternative embodiments and operating techniques will become apparent to those of ordinary skill in the art in view of the disclosure. Accordingly, modifications are contemplated which can be made without departing from the spirit of the described and claimed invention.