API TR 10TR4-2008 pdf download.Selection of Centralizers for Primary Cementing Operations.
2.2 Casing Centralization and Centralizing Devices Casing centralization requires mechanical devices (centralizers) to keep the casing away from the wellbore and/or from the cased sections of the well. Significant issues include: 1 . the centralizer must provide enough load support to overcome the normal forces tending to lay the casing against the formation wall, particularly in deviated holes, horizontal holes and through doglegs; 2. enough centralizers should be used to provide good casing centralization over the needed intervals (including at points between the centralizers); 3. it is normally assumed (however not always the case) that the formation can provide enough support for the tools (minimum centralizer embedment).
3 General Discussion 3.1 Centralizer Types Available The industry has developed three main types of centralizers: bow-spring, rigid, and solid. 3.1.1 Bow-spring Centralizer The bow-spring centralizer is composed of flexible spring bows (heat-treated steel springs) attached to two collars. By design the bows are flexible enough to allow passage of the centralizer through restrictions but are also expected to provide standoff in enlarged hole sections. The springs come in various shapes and dimensions. The uncompressed outside diameter (OD) of a bow-spring centralizer may be much larger than the nominal hole (bit) diameter; thus, this type of centralizer can potentially “centralize” the pipe in moderately washed-out zones. Double- bow centralizers are also available. The double-bow centralizers can provide good restoring forces with low starting and running forces. Double-bow centralizers have a lesser maximum OD than conventional bow-spring centralizers and might sacrifice standoff in enlarged holes. Double-bow centralizers may also be considered semi-rigid, as will be discussed later in this document.
3.1.2 Rigid Centralizer Rigid centralizers are made using non-flexible bands attached to collars. The bands are not designed to flex, and therefore, tend to maintain a constant OD. The centralizers exhibit minimal (or no) flexibility, but may have some ability to deform in hole restrictions, depending on their construction. Several types of rigid centralizers are available from manufacturers. A subclass within the rigid centralizer category can be made for certain type centralizers. For example, the double bow-spring centralizer is considered a bow-type but may also be considered a “semi-rigid” centralizer. This is because the bows of these centralizers flex, but after small deflection, they become essentially rigid.
The selection of the proper centralizer for a particular well application is a critical engineering consideration. The goal of the centralizer program should be to optimize the centralization of the casing in the wellbore to aid in proper mud removal and achieve zonal isolation. Depending on a number of design criteria, the proper centralizer for a particular application may be a bow-spring, rigid, or solid centralizer. In any given well, there can be application for all three types of centralizers, and only by evaluating all available data can the proper centralizer(s) be selected. Not all centralizers available to operators are of high quality, and the user is cautioned to consider construction quality of the device when selecting a centralizer. There are API/ISO documents available with methods to test the quality of bow-spring centralizers (i.e. ISO 1 0427-2). Similarly, API TR 1 0TR5 defines additional methods to test the quality of rigid or solid centralizers. Bow-spring centralizers are commonly used in cementing operations and can provide a high level of standoff. Double-bow centralizers can also offer good standoff with reduced levels of running forces. There are many situations where bow-spring centralizers will not perform as required because of large normal and/or running forces, such as found in high dogleg situations.