Obtaining Open-hole Cement Plugs On The First Attempt

Abstract

Abstract The four common reasons for setting open-hole cement plugs are reviewed. Factors which influence the condition of a cement plug, as well as the various plugging techniques, are outlined. The effects of auxiliary equipment, spacers, flushers, drilling fluid and the cement slurry are discussed individually with emphasis on techniques and materials that eliminate the uncertainty of plug placement. A recommended job sequence for a typical cement plug placement is presented. Supportive data in the/arm of case histories from Western Canadian wells are reviewed. Introduction There are many reasons why an operator may decide to set a cement plug in his well. Regardless of the reason, he wants a plug that will satisfy the required conditions on the first attempt. Unfortunately, this is not always obtained. Many people consider the setting of a cement plug to be one of the simplest operations performed on a well. This attitude is easy to acquire if the only cement plugs previously placed were those for abandonment purposes. Often abandonment plugs are recorded "as found" when they will support a few points on the weight indicator. The stresses placed on a cement plug for sidetracking or testing, however, are much more demanding. Cement that is diluted and contaminated with drilling fluids will not gain sufficient strength to support DST tools or a whipstock, or prevent the bit from re-entering the original hole. Setting an open? hole cement plug for sidetracking or testing requires planning and precise execution or it can develop into an expensive, timeconsuming operation. This paper will review the main purposes for setting openhole cement plugs and the well conditions that are often present during the placement operation. The various techniques that are used to place cement plugs and the problems that are encountered are discussed. Field-proven techniques and equipment used to ensure competent cement plugs are presented in point form and discussed in detail. Finally case histories of plugging procedures and results on wells in Western Canada are given (Appendix). Purposes of Wellbore Plugs A cement plug is set in an open wellbore for one of at least four reasons. These are:to provide a solid bottom and plug while sidetracking the drilled hole, while bypassing unrecoverable junk or while straightening a crooked hole(l);to seal open hole below a casing seat or zone to be tested;to overcome a lost circulation problem;to abandon a dry or depleted well. During a drilling operation, problems may develop which make it desirable to sidetrack the drilling. This can be due to loss of junk in the hole, such as tools or pipe, which cannot be recovered by fishing methods. Sidetracking may also be necessary to ensure that the drilling target is reached. Regardless of the reason for sidetracking, the cement plug in the original wellbore should be harder to drill than the formation around it. The cement must be given adequate time to allow it to gain sufficient compressive strength before the sidetracking operation begins.