Plug Cementing: Horizontal to Vertical Conditions

Abstract

Abstract This paper presents an in-depth study of cement plug placement that was conducted with large-scale models for the improvement of plug cementing practices and plug integrity. Common hole and workstring geometries were examined with various rheology and density ratios between the drilling fluid and cement. The critical conditions dictating the difference between success and failure for various wellbore angles and conditions were explored, and the mechanisms controlling slurry movement before and after placement are now better understood. An understanding of these mechanisms allows the engineer to better tailor a design to specific hole conditions. Controversial concepts regarding plug-setting practices have been examined and resolved. The cumulative effects of density, rheology, and hole angle are major factors affecting plug success. While the Boycott effect and an extrusion effect were observed to be predominant in inclined wellbores a spiraling or "roping" effect controls slurry movement in vertical wellbores Ultimate success of a cement plug can be obtained if allowances are made for these effects in the job design. provided all other previously published recommended placement practices are followed. Results of this work can be applied to many sidetracking and plug-to-abandon operations. Additionally, the understanding of the fluid movement (creep) mechanisms holds potential for use in primary and remedial cementing work, and in controlling the placement of noncementitious fluids in the wellbore. Problem Definition Works published by Smith, et al. and Heathman, et al. provide general recommendations but do not provide enough details concerning the requirements for maintaining a stable plug under various conditions. In the 2 years since the recommended plug procedures cited in Reference 2 have been implemented, a near-perfect record of successful kickoff plugs has been achieved. However, many near-misses and inconsistencies exist that have been unexplained based on the current understanding of downhole flow mechanics. For example. plug tops have varied with no apparent pattern. and some plugs have drilled softer than desired. Although large excess volumes of cement have been commonly recommended to improve the chances of success in such jobs, these volumes can pose other problems. For example, the plug top may be extremely high. which would result in excessive rig time for drilling new formation, and larger volumes of cement-contaminated mud will likely result. Concerns are also commonly raised about the capability of successfully pulling a workstring out of the resulting long slurry columns before the onset of cement gelation and/or hydration. In light of the regulatory issues discussed by Calvert and Smith, long-term plug stability based on accepted industry standards is highly debatable. The authors found many cases in internal records and from personal interviews where abandonment plugs failed, despite the fact that they were thought to have been properly set according to all regulatory guidelines. The issues and/or relationships investigated under this project dealt with plug stability as a function of–Wellbore angle including vertical, deviated, and horizontal–Hole size–The spotting fluid and wellbore fluid rheologies and densities–Workstring/hole diameter annulus Finally, assumptions historically made about plug stability as a function of compatible and incompatible fluid interfaces and density swapping were brought to light by Heathman, et al. but were not resolved. In fact, many recommendations made in recent publications appear to contradict what some operators report when tagging the location of a plug top. P. 667