Technical field

The invention relates to improvements in centralisers for wellbore casings.

Background

During wellbore drilling in a formation the wellbore can be reinforced by a casing, also referred to as a casing string. The casing string is formed from a series of casing joints, which are lowered into a section of the borehole and then cemented in place. Cement is circulated down the wellbore from the surface and pushed into the annulus between the casing and the formation, where it is allowed to set to fix the casing in place. A casing string normally includes multiple casing joints that are connected to each other by casing collars. A casing collar partially overlaps the end sections of two adjacent casing joints in the string. Casing joints, sometimes referred to as casing tubulars, are a type of wellbore tubular. Figure 1 shows a prior art arrangement comprising a casing 2 in which two casing joints 4, 6 are connected together using a casing collar 8. In order to ensure that the casing 2 is centralised in the borehole (not shown), the casing 2 is fitted with centralisers 10. The centralisers 10 push the casing 2 away from the surrounding formation (not shown). During casing insertion, the centralisers 10 may experience large forces from the borehole wall. In order to keep them in place, stop collars 12 attached to the casing 2 above and below each centraliser 10 are traditionally used.

Several different centralisers exist on the market today both for traditional casing and liner running and for drilling operations. When running casing into a borehole the centralisers are used to improve cementing operations and also to reduce friction during such operations. When drilling, the centralisers should also protect the casing string from wear. The providers of centralisers also provide stop collars for locking the centraliser in place. The stop collars are bolted to the casing string by a number of bolts or screws through the stop collar body, biting into the surface of the casing to lock the collars in place in order to prevent movement. The existing centralisers used in drilling and completion technology are either costly or lack robustness or both. Time is needed to install them and secure them in place before shipping and placement. They can cause handling problems offshore unless properly planned for. Current stop collar systems are weak, with the exception of new costly designs, and do not secure the centraliser in place during running.

Summary of the Invention The invention provides a tubular coupling device, tubular arrangement and casing string as set out in the accompanying claims.

A preferred embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings.

Brief Description of the Drawings

Figure 1 is a schematic diagram of a section of a casing string in accordance with the prior art;

Figure 2 is a schematic diagram of a section of a casing string in accordance with an embodiment of the invention;

Figure 3 is a schematic diagram of a section of a casing string in accordance with another embodiment of the invention; and

Figure 4 is a schematic diagram of a tubular coupling device according to an embodiment.

Description of Preferred Embodiments

Figure 2 shows a connector 14, or tubular coupling device 14, for fixing together two casing joints 16, 18 of a casing string 20. The connector 14 includes a casing collar 22 and a centraliser 24 mounted on the collar 22. The casing joints 16, 18 are connected together with the casing collar 22 as normal before the casing string is inserted into the borehole (not shown). Because the connector 14 includes a centraliser 24 the need for installing further centralisers is removed or reduced. The centraliser 24 is attached to the casing collar 22, and the collar is fixed to the casing joints 16, 18. Hence, the centraliser 24 cannot move relative to the casing joints 16, 18, and there is no need to install stop collars.

The casing collar 22 can be threaded onto the casing joints 16, 18 using a threaded connection 25, which removes the problem of the centraliser 24 becoming loose during running, i.e. during insertion of the casing 20 into the wellbore. During running, the casing 20 may be rotated. The tubular coupling device 14 is fixed to the casing 20 and rotates with the casing 20. In this embodiment the centraliser 24 is mounted to the casing collar 22 such that the centraliser 24 is fixed both axially along the longitudinal axis of the casing 20, and rotationally about said axis. Hence, the tubular coupling device 14 rotates with the casing string 20.

After the casing string 20 has been lowered into the borehole (not shown), it can be cemented in place by circulating a cement slurry down the casing 20 and up into the annulus between the casing 20 and the borehole wall (i.e. the surrounding formation). The connectors 14 keep the casing string 20 substantially centralised in the borehole, so that an even layer of cement fills the annulus.

Figure 3 shows another embodiment, in which the centraliser 26 has a rotating design. The centraliser 26 is mounted concentrically on the casing collar 28 but is free to rotate relative to the collar 28 and around the longitudinal axis of the casing string 20. The casing collar 28 includes shoulders 30 to stop or restrict axial movement of the centraliser 26 relative to the casing string 20. Axial movement of the centraliser 26 is thereby restricted without the need for external stop collars. The shoulder 30 at each end of the casing collar 28 may be a lip or protrusion directly machined into the collar 28, or a threaded nut for example. During running, the casing string 20 is free to rotate relative to the centralisers 26, which are only fixed axially to the casing collar 28. The facing contact surfaces between the inner, cylindrical body of the centraliser 26 and the outer surface of the casing collar 28 may be made from a suitable low friction material (e.g. a low friction polymer such as Teflon™).

The casing string 20 may be used for drilling, in a so called drilling with casing or drilling with liner operation, in which case rotation of the downhole drill bit (not shown) is transmitted through the casing string 20 from the surface. Using the rotating design illustrated in Figure 3 for such operations can decrease the necessary torque for rotating the casing string 20.

Figure 4 shows a tubular coupling device 14 according to another embodiment. Instead of using a threaded connection, the collar 22 may connect to the casing joints 16, 18 using one of a welded connection 32, a shrinkage connection 32, a hydraulic expansion connection 32, and a hydroform connection 32.

It will be appreciated by the person skilled in the art that various modifications may be made to the above described embodiments without departing from the scope of the invention. For example the connector 14 can be used to connect two wellbore tubulars 16, 18 of a type other than casing joints 16, 18, such as liners or drillstring sections.