Improved Method of Setting Successful Cement Plugs

Summary Setting openhole cement plugs has historically been a difficult task. Often, several attempts are required to set a cement plug before one is obtained that has sufficient strength for side track and is at the intended depth. It is our opinion that unstable flow behavior, resulting from the heavy weight of an unset cement plug resting on top of a lightweight mud, is one of the main causes of plug failure. The common practice of using open-ended drillpipe or tubing to spot the plug is also a major contributor to unsuccessful plug setting. This paper presents a placement technique and materials design that have resulted in a successful plug on the first attempt in many field cases where previous plug setting was difficult. Several case histories are presented. To examine the instability problem associated with cement plugs, an experimental investigation was conducted. This investigation consisted of setting cement plugs of known densities and rheological properties on top of drilling muds having known properties. The experiments were conducted using lucite pipe to simulate hole or casing and using copper tubing as the drillpipe. In each test, the hole was initially filled with drilling mud. Then, cement was displaced through the simulated drillpipe to displace drilling mud up the annulus. The ability of the cement plug to remain where it was placed was observed visually and correlated with other experimental observations. This paper includes the results of the experimental paper includes the results of the experimental investigation. The main conclusion of this investigation is that the stability and quality of a cement plug for side tracking or any other purpose can be greatly improved through the use of our new placement technique. Introduction Early in 1980, a research project was directed toward determining the possible causes of repeated failures in setting cement plugs, particularly side track plugs, as reported for some operating areas. Those reported failures seemed more common when plugs were placed in wells with drilling muds weighing less than 10 lbm/gal [1198 kg/m ]. As part of the investigation, a detailed literature study was initiated. More than 150 articles dealing with many aspects of setting cement plugs were screened. For detailed study, a smaller group of papers that appeared to have the most relevant information was selected. Those articles and a few others are given in the General References at the end of this paper. The chosen group of articles gave a consensus concerning techniques and slurry design considerations needed for successfully setting cement plugs (a list of those considerations that we concur with is given in Appendix A). After investigating several of the reported failures, it was concluded, with some exceptions, that those industry-recognized techniques given in Appendix A were being followed. However, plug failures were still occurring. In a previous paper, Beirute investigated the behavior of a liquid cement plug once it is left alone in the well. By means of laboratory experiments and a mathematical model, he showed that heavy cement slurries placed on top of lightweight muds form unstable interfaces which because of the adverse gravitational forces may cause the fluids to flow, contaminating the plug or causing it to move downward and, therefore, contributing to the plug job failure. His observations when running his experiments correlate quite well with field descriptions of reported cement plug failures. Since those observations are considered quite significant, they are given in Appendix B.A series of experiments were designed to investigate the phenomenon of cement plug stability in the wellbore. The idea was to determine whether this was a major contributor to cement plug failures, particularly when setting plugs by the balanced plug method, without using a bridge plug or any other form of support "table" below the plug. Also, if plug instability was found to be a serious cause, of failure, it was the intention of the investigation to discover ways to control the problem. Experimental Work The experiments were conducted in a simulated wellbore constructed out of clear lucite tubes approximately 18 in. [46 cm] long and 1.75 in. [4.4 cm] ID as shown in Fig. 1. Some tests were run in longer tubes (approximately 3 ft [0.9 m] long). To simulate the drillpipe, 0.5-in. [1.3-cm] copper tubing was used. The drilling mud and cement slurries that were used in the experiments were mixed as closely as possible to those used in the field. A low-solids, nondispersed (LSND) drilling mud that weighed 9.0 Ibm/gal [1078 kg/m ] was used in the experiments. This type of mud is used extensively and was chosen since it represented the greatest density difference between the cement slurry and the drilling mud that would be encountered in the majority of situations where cement plugs would be set. A list of the fluids used in the experiments is given in Table 1. The initial experiments were conducted to simulate placement techniques as closely as possible. placement techniques as closely as possible. JPT P. 1897