Centralisers

This invention relates to centralisers.

During the construction of oil and gas wells a bore is drilled into the ground. Casing is then lowered into the bore and the annular space between the casing and the bore filled with cement.

It is important that the casing is held substantially centrally in the bore while the annular space is filled with cement and for this purpose centralisers are mounted on the casing as it is lowered into the bore.

Usually wellbores extend substantially vertically downwardly and the centralisers comprise two spaced apart bands which can be slipped over the casing and a plurality of spring bows which extend between and are connected to the bands.

If part of the wellbore collapses and it is necessary to withdraw the casing then the spring bows simply compress against the side of the casing as they pass through the obstruction after which they expand against the side of the bore.

More recently the demand for deviated drilling has increased significantly. In deviated drilling a branch bore, which can be nearly horizontal, is drilled outwardly from an existing vertical bore.

Many problems arise with deviated drilling. One of these problems is that centralisers with spring bows are not entirely satisfactory since the spring bows will not withstand the lateral forces as the casing turns into the branch bore. Furthermore, the spring bows are generally not strong enough to centre casing in nearly horizontal bores.

One solution to this problem is to use centralisers with rigid members instead of spring bows. Examples of such centralisers are shown in GB-A-2 171 436 and GB-A-682 362. The overall diameter of such centralisers is somewhat smaller than the bore so that said centralisers can be readily moved along the bore. Whilst such centralisers are not as versatile as centralisers with spring bows they do provide a reasonable centralising action.

The disadvantage of such centralisers is that if part of the bore collapses and it is necessary to withdraw the casing some of the rigid members are nearly always broken off as the casing is withdrawn. This is extremely undesirable since the broken fragments can later be carried to the surface and obstruct or damage flow control equipment.

In order to help reduce this problem the present invention provides a centraliser which is provided with at least one rigid member which, when said centraliser is mounted on casing will substantially irreversibly collapse against said casing when subjected to a lateral load of from 5 to 30 tonnes.

Centralisers in accordance with the present invention can readily be distinguished from prior art centralisers which use rigid members since these were either formed by solid castings (GB-A-2 171 436), for example of aluminium or steel, or by very robust steel sections (GB-A-682 362).

Preferably, the casing will substantially irreversibly collapse against said casing when subjected to a lateral load of from 10 to 25 tonnes.

Advantageously, the casing will substantially irreversibly collapse against said casing when subjected to a lateral load of from 18 to 22 tonnes.

In a preferred embodiment the rigid member is of generally U-shape cross section.

In another embodiment the rigid member comprises a crushable material. In,this embodiment the rigid member is preferably provided with a reinforcing member to provide the centraliser with acceptable tensile strength.

The reinforcing member may comprise, for example, a metal such as steel or aluminium although it could also comprise a plastics material.

If the reinforcing member is embedded in the crushable material there will be some adhesion between the reinforcing member and the crushable material. If the crushable material becomes broken for any reason this adhesion will inhibit separation of the broken pieces from the centraliser. If desired the reinforcing member may be provided with protrusions and/or adhesive to improve the adhesion between the reinforcing member and the crushable material and to reduce the probability of any broken pieces separating from the centraliser. More generally stated, it is preferred that the reinforcing member co-operates with the crushable material to inhibit broken pieces of said rigid member separating from said centraliser.

For a better understanding of the present invention reference will now be made, by way of example, to the accompanying drawings in which:

FIG. 1 is a vertical cross-section through one embodiment of a centraliser in accordance with the present invention;

FIG. 2 is a side view of the centraliser shown in FIG. 1;

FIG. 3 is a horizontal cross-section taken on line III--III of FIG. 2.

FIG. 4 is a side view of a second embodiment of a centraliser in accordance with the present invention; and

FIG. 5 is a horizontal cross-section taken on line V--V of FIG. 4.

Referring to the drawings there is shown a centraliser which is generally identified by the reference numeral 1. The centraliser 1 comprises an upper ring 2 and a lower ring 3 which are spaced apart and connected by six rigid members 4 which are welded to the upper ring 2 and the lower ring 3 at their respective ends.

As shown in FIGS. 1 and 3, each rigid member 4 is of generally U-shape cross-section.

In the embodiment shown in FIGS. 1 to 3 the overall diameter of the upper ring 2 and the lower ring is approximately 180 mm. The maximum width of each rigid members 4 is approximately 45 mm and the thickness of the wall of the rigid members 4 is approximately 2 mm.

In use, as casing is being lowered own a wellbore the centralisers 1 are mounted at the desired spacing. Conveniently, the centralisers 1 are secured between two stop collars to prevent them moving along the casing.

In normal use the casing is simply lowered into a bore and cemented in position, the rigid members 4 acting as centring members. If the casing has to be withdrawn for any reason prior to cementing, for example due to collapse of part of the wellbore, the casing is simply raised by the draw works.

When the centraliser 1 is pulled through the obstruction the obstruction exerts a force on the centraliser 1. If the force is sufficient the rigid members 4 irreversibly collapse thereby enhancing the prospects of the centraliser 1 remaining in one piece on the casing.

In trials using centralisers 1 designed to fully collapse with a lateral load of 20 tonnes nearly all the centralisers 1 were recovered at the surface fully collapsed but in one piece.

Various modifications to the preferred embodiment described are envisaged. For example, the rigid members 4 could be of any convenient cross-section, for example semi-circular, arcuate or trapezoidal. If desired, the rigid members could be locally weakened to facilitate their collapse. Thus, for example the rigid members could comprise two generally aligned sections joined by, for example an epoxy resin bonding the sections together but formulated to fail under a desired load. Another alternative would be for the rigid members to comprise two generally aligned sections connected by a shear pin intended to fail at a desired load. Whilst both the alternatives just described are within the scope of the present invention the embodiment shown in FIGS. 1 to 3 is preferred as there is believed to be less chance of parts becoming detached from the centraliser after collapse of the rigid members. In another embodiment, the rigid members could be made of a crushable material, for example a cellular material. Such an embodiment is shown in FIGS. 4 and 5.

Referring to FIGS. 4 and 5, there is shown a centraliser 11 which comprises an upper ring 12 and a lower ring 13 which are spaced apart and connected by six rigid members 14.

Each rigid member 14 comprises a thin reinforcing member 15 of metal, for example steel or aluminium, which is embedded in a crushable material comprising polyurethane foam 16. Each of the thin reinforcing members 15 is welded to the upper ring 12 and the lower ring 13 at its ends.

In use, if the centraliser 11 is withdrawn through an obstruction the polyurethane foam 16 irreversibly collapses thereby enhancing the prospects of the centraliser 11 remaining in one piece on the casing. The polyurethane foam 16 is designed to irreversibly collapse under a lateral load of about 20 tonnes.

Various modifications to the embodiment shown in FIGS. 4 and 5 are envisaged, for example the reinforcing members 15 could be separate and distinct from the rigid members 14 which could simply comprise a crushable material such as polyurethane foam. Whilst preferably made of metal, reinforcing members 15 could also be made of plastics material.

However, the arrangement described with reference to FIGS. 4 and 5 is preferred as it provides a convenient method of mounting the rigid member 14 to the upper ring 12 and lower ring 13.