Casing Centralizer

Types of Casing Centralizer

Three designs dominate. The bow spring centralizer consists of curved steel springs welded or hinged to two end collars. It is flexible, so it compresses through tight spots and then springs back, and it is the standard choice for most vertical and moderately deviated wells. The rigid centralizer has a solid body with straight or spiral blades machined or cast into it. It provides much higher restoring force and is used in horizontal and highly deviated wells where the casing tends to lie on the low side of the hole. The solid body centralizer is a variant of the rigid type with a fully continuous body and no welded components, offering the highest strength and the best performance in the most demanding wellbores, at a higher cost. Selection depends on hole angle, dogleg severity, casing weight and the required standoff.

Spacing and Placement

Centralizer spacing is normally determined by a cementing simulation that balances standoff against drag. The general principle is that centralizers are placed most densely across the producing interval and any section where cement isolation is critical, and less densely elsewhere. In vertical wells, spacing of one centralizer per joint is typical. In deviated wells spacing must be reduced, and in horizontal sections a rigid or solid centralizer every joint or every other joint may be required. The critical check is drag: too many centralizers, or centralizers with too high a restoring force, can increase running drag to the point where the string cannot be run to depth. Always confirm the spacing calculation against the actual hole geometry before ordering quantities.

Centralizers: standoff against the hole condition

A centralizer does not carry load; it holds the casing away from the wall so cement can circulate all the way round the string. Which type is used follows from the hole condition, the deviation and whether the string has to pass a restriction, and the restoring force is the figure that has to be specified rather than the outside diameter alone.

Standards, inspection and delivery

Download Casing Centralizer data sheet (PDF) One page: supply range, grades and key sizes, what to state on the enquiry, and the inspection documents that come with the order. The standard is the governing document.

What to state on the centralizer enquiry

Six things decide the type and the quantity, and they are all fixed before a mill can quote.

• Casing outside diameter and weight, so the centralizer is sized to the pipe it clamps to
• Hole size, or the inside diameter of the previous string where the centralizer runs inside casing
• Type: bow spring, rigid or solid body
• Quantity, or the spacing where the project has already run the cementing simulation
• Material and finish: the standard steel body, or a non-welded or corrosion-resistant option where the well calls for one
• Standard and certificate: whether the order has to be certified to API Spec 10D, and the certificate type

Where the project has not fixed the spacing, order against the hole geometry and the running conditions rather than a rule of thumb: the quantity that will actually run to depth is the one the simulation supports.

Frequently asked questions

Bow spring or rigid centralizer?

The bow spring centralizer is the default: it compresses through tight spots and springs back, it is lighter and it costs less, and it suits vertical and moderately deviated wells. Rigid and solid body centralizers are specified where the casing tends to lie on the low side of the hole, in horizontal and highly deviated sections, because they hold a higher restoring force. The choice follows the hole angle, the dogleg severity and the standoff the cementing design needs.

Does the centralizer have to match the casing size?

Yes. A centralizer is sized to the casing outside diameter it clamps to, and on the other side to the hole or the previous string it has to stand off from. A mismatch either fails to hold the standoff the design assumes, or adds drag without adding clearance, so the casing size and weight belong on the enquiry rather than being left to the supplier.

How is a centralizer held in position on the casing?

By stop collars set on the casing at the spacing the design gives, with the centralizer between them, or by an integral or hinged collar arrangement on the centralizer itself. Because the position is set on the pipe, the stop collar and the centralizer are ordered together rather than separately.

Where to go next

• Casing thread protector – what protects the threads to the rig floor
• Casing coupling – how couplings are specified and made up
• Casing pup joint – the short joints used to land the string