Casing Centralization—Planned Compared to Actual: A Kuwait Case Study

Abstract

Several reasons can contribute to poor cementation; however, proper casing centralization can lead to uniform mud displacement with cement and can mitigate some of the cementation problems. Cementing companies design their casing centralizations based on simulators, but they have never been able to validate quantitatively how their planned centralization compares to the actual, the only comparison made in the past was based on the overall cement quality measured by the cement evaluation tools. This can be reflective of the centralization program in some instances; however, in others, attributing quality to centralization can be misjudged or centralization may be mixed with other factors. Not being able to directly quantify and evaluate each element in the casing and cementation program makes it challenging to improve or acknowledge each design. Relating the achieved cement quality to casing centralization gives inconsistent results. In many instances poor cement behind casing is because of other factors such as spacer design, fluid rheology, formation fluids, borehole geometry, etc., and is not only due to poor casing centralization. Casing eccentricity or casing touching outer casing/formation is always observed on cement evaluation imaging logs, as they typically appear in a form of galaxy pattern. However, these galaxy patterns can only be considered as a qualitative indication of the casing being close to a casing or to a formation, as it will be shown later in the paper that galaxy pattern start to show on cement evaluation images when casing centralization is below 60%. The new generation of ultrasonic flexural measurement tools can evaluate cement and at the same time provide third interface echo (TIE) measurement over 360°. The TIE measurement can evaluate casing centralization by evaluating the time between the first casing reflection (mud-casing interface) and the third interface reflection (cement formation interface) (Δt). When all the Δt values across all azimuths are equal, it is an indication of casing being 100% centered, and when Δt is 0 or small (casing touching formation) from any direction then casing is 0% centered. This Δt can also be converted to pseudo annulus thickness by assuming a velocity for the material behind the casing. For an operator to improve their field casing cementation, a campaign was performed for evaluating casing centralization in multiple casing sizes for different well trajectories to set benchmark for future centralization improvements. Each casing had different centralizer size and different patterns.