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API TR 10TR4-2008 pdf free download

API TR 10TR4-2008 pdf free download.Selection of Centralizers for Primary Cementing Operations.
1 Introduction The proper centralization of the casing for primary cementing has long been a critical step in quality cementing. Lack of proper centralization can lead to severe cementing problems, including lack of zonal isolation and improper casing support. The goal of this document is to provide the petroleum industry with information for three types of centralizers, their selection and application, and their advantages and limitations. 2 Benefits of Centralization When performing primary cementing jobs, the casing should be centralized in the wellbore for three reasons: 1 . to help get the casing to bottom (this includes reduction of the potential for sticking of the string); 2. to help move the casing during mud conditioning and during the cementing job; 3. to provide an optimal path for fluid flow during mud conditioning and cementing allowing for effective mud removal to achieve zonal isolation. Field experiences, numerous large-scale experiments and computer simulations have shown that poor casing centralization can be detrimental to the cement job, particularly in narrow annuli. Therefore, a good centralization program should aim for high levels of standoff, which produces improved mud removal, particularly across critical areas of the wellbore, that is, those areas where isolation is required. It is imperative the user investigate the standoff at all points, especially between the centralizers. 2.1 Definition of Standoff Standoff is defined by API/ISO documents (e.g. ISO 1 0427-2) as the smallest distance between the outside diameter of the casing and the wellbore. The standoff ratio is defined by the same documents as the ratio of standoff to the annular clearance for perfectly centered casing expressed as a percentage (%). Annular clearance for perfectly centered casing is the wellbore diameter minus the casing outside diameter divided by two. Figure 1 illustrates standoff and annular clearance.
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.

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