TX 75083-3836 U.S.A., fax 01-972-952-9435. AbstractMechanical whipstock plugs are often chosen in preference to conventional cement plugs to sidetrack in very strong formations or where a very precise short-radius kickoff is critical, even though the use of downhole tools requires extra time and introduces other risks and concerns to the drilling operations.In this paper we present a new cement system, based on the optimization of the packing volume fraction (PVF) of the dry blend, which results in a set cement with mechanical properties superior to those of regular cements: compressive strength, impact resistance, and toughness. These cements can be used to achieve the same objectives as a mechanical whipstock plug with less risk and offer potential time savings to the overall operations.The advantages of this cement system are demonstrated in a field test in which a kickoff plug was successfully set and used to drill a short-radius hole.
Mechanical whipstock plugs are often chosen in preference to conventional cement plugs to sidetrack in very strong formations or where a very precise short-radius kickoff is critical, even though the use of downhole tools requires extra time and introduces other risks and concerns to the drilling operations. In this paper we present a new cement system, based on the optimization of the packing volume fraction (PVF) of the dry blend, which results in a set cement with mechanical properties superior to those of regular cements: compressive strength, impact resistance, and toughness. These cements can be used to achieve the same objectives as a mechanical whipstock plug with less risk and offer potential time savings to the overall operations. The advantages of this cement system are demonstrated in a field test in which a kickoff plug was successfully set and used to drill a short-radius hole. Introduction Balanced cement plugs have been used for a long time in the oil industry for abandonment and to sidetrack existing wellbores.1 The cement plug is multipurpose and must provide the following:Zonal isolation from the original completion.Sufficient mechanical strength for the bottomhole drilling assembly to sidetrack.Toughness to remain intact throughout the drilling process and the life of the well. Despite the simplistic nature of setting a balanced cement plug, failures do occur. In some cases it may take several attempts to achieve a successful plug. The process has often been broken down into discrete steps and closely studied in order to maximize the chances of success. Typically, reduced-water slurries have been chosen to maximize the compressive strength development while optimizing the properties of rheology, fluid loss, and sedimentation to obtain a stable and pumpable cement slurry. Washes or spacers are used ahead of and behind the cement slurry to minimize the likelihood of slurry contamination. The plugs are typically under displaced so that the dynamics of the fluids in the wellbore when the pumps are shut down are such that the cement level is slightly higher inside the tubing and continues to drop until it balances with the cement level in the annulus. This is confirmed if the pipe is pulled "dry." Hence, this technique is critical to avoid contamination of the center of the plug (if the job was over displaced). The kinetics of the cement hydration reaction are strongly temperature dependent; hence, accurate temperature data are required so that the slurry can be correctly designed and placed and so that it will develop the necessary strength in the required time interval. Slurry placement in the wellbore also has its challenges. Often the slurry density is higher than the density of the wellbore fluid. Despite a homogeneously mixed slurry, the plug can sink or be strung out in the wellbore, resulting in either inconsistent set properties or even no competent plug at all. Packers, bridge plugs, and plug placement tools2 have all been used at some time to improve the chances of the successful placement of the cement plug. Despite all these precautionary measures, and even if a competent plug is obtained, the final set properties may still fall short of those required. A sidetrack plug has the additional requirement of needing to provide resistance greater than that of the surrounding formation for a successful sidetrack. In more competent formations, the higher compressive strength requirements for the plug may require excessive wait on cement (WOC) times. Even then, if the plug is contaminated during placement or there is a misjudgement in the actual wellbore temperature, the necessary strength may never be reached. As a result, another plug (or plugs) must be set, resulting in more products and, more importantly, more rig time; i.e., more costs. Consequently, some operators have also looked at using mechanical devices3 (whipstock plugs) as a replacement or enhancement for the cement plug in these more-demanding situations.
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