Well screen

A well screen (101) having a base pipe (103), a filter layer (105) arranged around the base pipe (103), and a discrete mesh outer standoff layer (107) arranged around the filter layer (105) in a first spiral wrap (109) is provided. The first spiral wrap (109) comprises a first succession of loops such that a trailing edge of a next one of the first succession of loops is in contact, or in overlap, with a leading edge of a previous one of the first succession of loops; and to produce a compressive force on the filter layer (105) radially towards the base pipe (103).

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national stage application filed under 35 U.S.C. 371 of International Application No. PCT/SG2009/000067, filed Feb. 25, 2009, which claims priority from Singapore Application No. 200801718-8, filed Feb. 27, 2008, each of which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to a well screen and a method of forming a well screen. The invention has particular, but not exclusive, application in the use of extraction of well fluids such as oil, gas or water.

BACKGROUND OF THE INVENTION

Well screens are generally used in subterranean wells in which it is desirable to extract well fluids such as oil, gas or water from the ground without bringing the debris, for example sand and other soil particulates, up with the fluid.

Conventionally, a well screen includes a length of perforated pipe known as a base pipe, one end of which is connected to a transportation pipe to transport the extracted fluid to the surface of the earth. A filter medium is disposed around the base pipe to prevent the debris in the fluid from entering the base pipe. A protective cover is further disposed around the filter layer to protect the filter medium from abrasion and impact while it is being run into a well bore. There is therefore a clearance (or gap) between the filter medium and the base pipe and/or between the filter medium and the protective cover to facilitate the assembly of the respective components of the well screen.

SUMMARY OF THE INVENTION

The invention is defined in the independent claims. Some optional features of the invention are defined in the dependent claims.

By providing an outer standoff layer in a spiral wrap in accordance with one or more of the techniques described below, clearances between the filter layer and the discrete mesh outer standoff layer and/or between the filter layer and the base pipe may be reduced or even removed. This is because a compressive force is produced on the filter layer radially towards the base pipe. This may provide greater mechanical strength/rigidity than hitherto available from known well screens and the discrete mesh outer standoff layer may provide the filter layer with higher burst strength and/or increased resistance to collapse in circumstances of excessive well pressures when in situ during fluid extraction and/or during pumping of well completion fluids. Additionally, the filter layer (which is often made up from multiple layers) can be reduced either in thickness or in mechanical strength of material due to the additional support provide by the outer standoff layer and the compressive force. Thus, cost of the elements of the well screen—particularly the filter media—can be reduced.

Additionally, the orientation of the outer standoff layer provides increased mechanical support under bursting conditions in the well screen, typically the most difficult mechanical property of the well screen to achieve.

For example, a discrete mesh inner standoff layer may be arranged around the base pipe in a spiral wrap to space the filter layer from the base pipe. The arrangement may be such that the discrete mesh inner standoff layer is in contact with the base pipe. Because clearances between the base pipe and the discrete mesh inner standoff layer may be removed, support may be provided to the filter layer to minimise collapse of the same.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1is a layout diagram illustrating a first well screen. As shown, a portion of a well screen assembly101has a base pipe103having perforations104, a filter layer105arranged around the base pipe103, and a discrete mesh outer standoff layer107of width107aarranged around the filter layer105in a spiral wrap109. InFIG. 1, the detail of the mesh is not shown for the sake of clarity. However, the mesh of outer standoff layer107can be, for example, a welded mesh or a woven mesh, as long as there is a flow path for flow of fluid from outside outer standoff layer107, through filter layer105and through perforations104of base pipe103. The spiral wrap109of outer standoff layer107comprises a succession of loops (in this example, each loop being defined by a 360-degree revolution in the direction117around the circumference of the base pipe103, offset by angle θ119from a direction normal to the longitudinal axis of the well screen101). A trailing edge111aof a next one of the succession of loops is in contact, or in overlap113, with a leading edge111bof a previous one of the succession of loops. That is, the edges of successive loops of the spiral wrap109touch one another so that they are “flush” presenting a substantially uniform surface height on the filter layer105or touch in overlap—preferably a slight overlap113—with one another. In addition, the spiral wrap109produces a compression force on the filter layer105in a direction radially towards the base pipe103.

In one construction, the discrete mesh outer standoff layer107is wrapped tight around the filter layer105such that any clearance (or gap) between the discrete mesh outer standoff layer107and the filter layer105, or between the filter layer105and the base pipe103, is minimised or reduced, and preferably removed.

Note that the term succession of loops is not to be understood to be limited to a plurality of complete loops (i.e. revolutions) around the base filter layer105. The inventors have found that it may be sufficient to provide only a single complete loop and a partial further loop. Depending on the width of the band of the filter layer in the longitudinal direction115and the width107aof outer standoff layer107, it may be sufficient to provide only a partial second loop to cover the filter layer105. Thus, a trailing edge111aof the partial second loop of the succession of loops my be disposed in contact or in overlap with a leading edge111bof the first loop.

Similar considerations apply to the making up of the inner standoff layers409,809of, say,FIGS. 4 and 8.

Thus, reduction of clearances between layers provides enhanced rigidity and, in effect, the discrete mesh outer standoff layer107provides support to the filter layer105to counter hoop stresses—i.e. outward radial pressure exerted from the base pipe103to the discrete mesh outer standoff layer107—generated during fluid extraction, whereby the filter layer105may be deformed. If the filter layer105were to be stretched beyond its maximum elongation, it will consequently be damaged. Accordingly, the minimisation of any clearance between the filter layer105and the discrete mesh outer standoff layer107means that the filter layer105may have higher burst hoop strength to overcome the possible hoop stresses during fluid extraction and/or pumping of well completion fluids.

In one construction of the well screen101, opposite edges of a planar piece of the filter layer105are connected to each other by, for example welding, before the filter layer105slides over the base pipe103. Accordingly, the welding quality of the filter layer105may be inspected before it is arranged around the base pipe103. The combination of the base pipe103and the filter layer105is then fed into a spiral mill (not shown) in the direction of arrow115and is rotated in the direction of arrow117about the longitudinal axis of the well screen101. As the well screen101enters into the spiral mill, the discrete mesh outer standoff layer107is wrapped onto the filter layer105at angle θ119from a direction normal to the longitudinal axis of the well screen101. Suitable ranges of angles for θ119are between 10 and 40, or 20 and 30 degrees. Accordingly, the discrete mesh outer standoff layer107is arranged on the filter layer109in the form of a continuous spiral wrap. The relationship between the linear and angular velocities in directions115,117respectively of the well screen can be selected as appropriate to provide a suitable angle at which the outer standoff layer109is arranged on the filter layer105. Additionally, the width of the outer standoff layer109such that it is wrapped “flush” (i.e. flat on the filter layer, an edge of one loop of the wrap touches an edge of the adjacent loop) or in overlap, is selected appropriately. The inventors have found that the smaller the width107aof the outer standoff layer (i.e. the narrower the band of the wrap) and the smaller the angle θ119, the greater the hoop strength (discussed below) will be.

In one construction, outer standoff layer107covers filter layer105completely, thus enhancing mechanical strength and limiting the possibility of damage to the filter layer105.

FIG. 2is a sectional view illustrating a longitudinal, cross-section of layers103,105,107of the well screen101. In this example, because the discrete mesh outer standoff layer107is wrapped tight around the filter layer105, it is seen that there is no clearance (i.e. gaps) between these layers105,107over a substantial length of the well screen101.

FIG. 3is a sectional view illustrating a cross-section I-I of the well screen101. Because the discrete mesh outer standoff layer107is wrapped tight around the filter layer105, it is again seen that there is no cross-sectional clearance (gaps) between these layers105,107.

FIG. 4is a layout diagram of a second well screen illustrating a portion of a well screen401having a base pipe403with perforations404, a filter layer405arranged around the base pipe403, and a discrete mesh outer standoff layer407arranged around the filter layer405. In this respect, the well screen401of this embodiment is similar to the well screen101of the previous embodiment, as described above.

In this embodiment, however, the well screen401additionally comprises a discrete mesh inner standoff layer409arranged around the base pipe403, which spaces the filter layer405from the base pipe403. Thus, the discrete mesh inner standoff layer409may provide support to the filter layer405, thereby minimising collapse of the filter layer405during fluid extraction. If the filter layer405were stretched beyond its maximum elongation due to possible collapse, it will consequently be damaged.

Inner standoff layer409also enhances flow properties by spacing the high open area filter layer from the lower open area base pipe. This spacing allows for more uniform flow through the filter layer rather than directly over base pipe perforations.

FIG. 5is a layout diagram illustrating the discrete mesh inner standoff layer409being spirally wrapped around the base pipe403. Like the discrete mesh outer standoff layer107previously described, the discrete mesh inner standoff layer409is arranged around the base pipe403in a spiral wrap501(in this example, each loop being defined by a 360-degree revolution in the direction507around the circumference of the base pipe403, offset by angle θ509from a direction normal to the longitudinal axis of the well screen101). A trailing edge503aof a next one of the succession of loops is also in contact, or in overlap505, with a leading edge503bof a previous one of the succession of loops. That is, the edges of successive loops of the spiral wrap501touch one another so that they are “flush” presenting a substantially uniform surface height on the filter layer base pipe403or touch in overlap—preferably a slight overlap505—with one another. Additionally, the discrete mesh inner standoff layer409contacts the base pipe403.

FIG. 6is a sectional view illustrating a longitudinal, cross-section of the well screen401. Because the discrete mesh inner standoff layer409is in contact with the base pipe403, it is seen that there is no longitudinal clearance between the discrete mesh inner standoff layer409and the base pipe403over a substantial length of the well screen401.

FIG. 7is a sectional view illustrating a cross-section II-II of the well screen401. Because the discrete mesh inner standoff layer409is in contact with the base pipe403, it is again seen that there is no cross-sectional clearance between the discrete mesh inner standoff layer409and the base pipe403.

In one method of constructing the well screen401, the discrete mesh inner standoff layer409may be introduced onto the base pipe403using the spiral mill, as previously mentioned. Thereafter, the filter layer405slides onto the discrete mesh inner standoff layer409, after it has been welded along its longitudinal edges. The discrete mesh outer standoff layer407is then spirally wrapped around the filter layer405in the same way as previously described.

Because the discrete mesh outer standoff layer407is spirally wrapped tight around the filter layer405such that a compressive force is produced in a direction radially towards the base pipe403, any clearance between the discrete mesh outer standoff layer407and the filter layer405, between the filter layer405and the inner standoff layer409, or between the inner standoff layer409and the base pipe403is minimised/reduced or removed. Accordingly, the filter layer405may be provided with not only a higher hoop strength, but also greater resistance to collapse during fluid extraction.

FIG. 8is a layout diagram illustrating a third well screen800, additionally comprising a protective shroud (or cover801) spirally welded over outer standoff layer807. Shroud801has perforations801ato allow fluid flow in an inwards radial direction towards the base pipe. In this well screen, base pipe803, inner standoff layer809, filter layer805and outer standoff layer807are made up as discussed above with respect toFIGS. 1 to 7, where inner standoff layer809is wrapped tight against base pipe806and outer standoff layer807is wrapped tight over filter layer805, which has been slid on over inner standoff layer809. Additionally, protective cover801slides over the discrete mesh outer standoff layer807, and the protective cover801is then swaged on the discrete mesh outer standoff layer807. The swaging process provides a means to compress the protective cover801radially on the discrete mesh outer standoff layer807. The swaging process may be effected so that the protective cover801is compressed on the discrete mesh outer standoff layer807in a uniform manner, which maintains the cross-section shape of the protective cover801but reduces its diameter. This reduction in the diameter of the protective cover801consequentially removes any clearance which previously existed between the discrete mesh outer standoff layer407and the protective cover801.

It should be appreciated that the invention has been described by way of example only and that various modifications in design and/or detail may be made without departing from the spirit and scope of this invention.