Advances in Cement-Plug Procedures

Abstract

Operators set cement plugs for a variety of reasons: abandonment, sidetracking, lost-circulation control, or remedial work. However, as important as plug cementing can be to the overall success of a well program, the process often is performed without regard to wellbore conditions. The additional rig time and material costs quickly add up as multiple plugs are set. The industry average is approximately 2.4 attempts per kickoff. Studies have highlighted many of the problems encountered in plug cementing. Implementation of systematic, controlled procedures has established a near 100% success rate where used for establishing kickoff plugs. Recommended Plug Procedure Recent work has determined that cement plugs require the same degree of planning as primary cement jobs, plus special considerations for plug stability. A joint team of operators and a cementing service company evaluated the problems and mechanics of plug cementing. The team developed a recommended plug procedure (RPP) to alleviate problems. The procedure provides a reliable plug-setting procedure and develops a method to monitor success rates. The RPP addresses communication, temperature estimation, mud removal, slurry design, slurry volume, and plug stability. Understanding the relationships between these variables is vital to the success of spotting a balanced cement plug. Job Design Communication. To design a cement-plug job properly, communication among all personnel responsible for the job design is essential. It is important that personnel discuss options for each job so that assumptions are not made about equipment availability and/or capabilities. Sharing data from the field is also important. Detailed pre-/postjob data sheets have helped to build a database that has been instrumental in finding the causes of both successes and failures. Temperature Estimation. One of the most important design variables is placement time, which is governed by pumping rates and bottomhole circulating temperatures. In most cases, little attention is paid to plug-cementing temperatures; most laboratories use published cementing schedules. At times, temperature estimates are also adjusted upward to increase the safety factor, resulting in over-retardation. The use of computer simulation or temperature surveys for temperature predictions can help ensure that the slurry will not be over-retarded. Also, a more realistic waiting on cement (WOC) time can be estimated if compressive-strength thermal-recovery schedules are slow enough to match wellbore conditions, reaching American Petroleum Inst. bottomhole static temperature only after 24 to 36 hours have elapsed.