A good primary cementation requires careful selection of centralizers and their placement on the string. The centralizer placement algorithm described in the API-10D was corrected and put into a computer program. The standoff value is calculated based on the actual borehole geometry, string data and centralizer performance. The model was enhanced by a newly developed drag force simulation taking the centralizer running force into account. Additionally, the prediction of the expected torque values for rotating liner applications is included.
The key factor for a successful cementation job is the replacement of the mud in the wellbore by the cement slurry. Hydraulic considerations call for the need of a good centralization of the string for all sections in which a good cementation is required. Centralizers have been used for decades to fulfill this job. Throughout the past couple of years, more and more designs of highly inclined, including horizontal, wells incorporate cemented production casing and liner sections. In these cases, the optimum placement of centralizers is achieved by balancing between a high standoff ratio and low drag forces.
A mathematical simulation model is used to calculate the optimum spacing of centralizers to obtain the best standoff at a given borehole location.
This model takes into account relevant factors, such as:
the lateral force at any given location based on borehole geometry, buoyed string weights and tension forces
the centralizer's reaction to these forces, based on test data for each pipe size/hole size combination
the sag between centralizers based on the elasticity of the pipe and a three-dimensional vector analysis of the weight and tension components.
This mathematical model is associated with a torque and drag analysis, utilizing the known running forces of the centralizers and the friction factors that depend on the mud type. This analysis is important in order to evaluate whether the desired centralizer spacing can be run or rotated without creating problems due to high drag forces, or damage to the pipe connections. The equations upon which these models are based and the computer algorithms used are described in this paper.
Various models have been described in the literature to calculate the centralizer placement.
The criteria to select a centralizer pattern should be not only the achieved centralization but, especially in highly inclined wells, the ability to move the string. Thus, drag and torque calculation should be a part of the centralizer placement calculation.
It is important to understand that all mathematical equations and relationships regarding centralizer placement describe a model situation only. The actual standoff in a borehole depends on many different factors. There is no method of actually looking into the well and no tool like a "Standoff-Logging-Tool" to provide this information directly.