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Abstract Successfully achieving primary cementing objective within a very narrow pore-fracture pressure window is a significant challenge for the industry. During the cementing operation, variations in the equivalent circulation density (ECD) above or below the formation's pore/fracturing gradients can lead to wellbore losses or gains. An additional phenomenon that may be observed in these types of operations is wellbore ballooning, where wellbore fluids enter the formation under dynamic overbalance circulating conditions, and then flow back into the wellbore once pumps are stopped, dynamic circulating pressure is removed, and bottom hole pressure (BHP) is reduced, mimicking indications of a kick but without continuous influx of formation fluids. Designing a proper cement job to achieve zonal isolation and meet primary objective in such a scenario requires this challenge to be addressed and a fit-for-purpose solution to be developed to limit any post placement flowback that may compromise the cement integrity and zonal isolation. A 7-in. liner was run inside an 8 ½-in. open hole (OH) that had been drilled to section depth with a very tight drilling pressure window between gains from and losses to the formation. Losses and subsequent ballooning related flows were observed during the drilling phase. To determine the exact drilling window available, dynamic flow tests were conducted before the cement job to simulate ECD, and the available operational pressure window was identified between 13.4 - 14.6 lbm/gal. This data was used as input to a proprietary cementing software simulator to model the dynamic pressures expected during the cement job. Due to the small available window, conventional rules for density hierarchy, which keeps +/- 1.0 lbm/gal density difference among each fluid in cementing jobs, could not be followed. Also, due to the relatively small annular clearance, dynamic fluid friction pressures and optimization of liner stand-off played a critical factor in the job design. A high-pressure lightweight cement slurry was developed in the laboratory, aiming to withstand bottom hole formation pressure at 5,500 psi and achieve tight ECD control to minimize losses to the formation. The slurry system was optimized to be placed safely in the wellbore without jeopardizing the critical cement properties required for the annular isolation across the complete production liner. Multiple simulations and sensitivity tests were run to provide the optimized placement of the cement slurry and avoid losses or gains throughout cement placement. As even very small volumes of loss and subsequent flowback could contaminate the cement slurry, the density of the spacer ahead of the lead slurry was also optimized and enhanced through the inclusion of loss circulation materials, specifically engineered fibers, to further mitigate potential losses to larger fractures. The 7-in. liner cement job was conducted as planned with the liner rotated throughout the cement job to enhance the mud removal and improve cement bond. No losses or ballooning were observed throughout the entire cement job. Playback simulation showed the cementing job was executed as designed. formation integrity test (FIT) and cement bond log (CBL) confirmed shoe integrity up to 6,000 psi and zonal isolation requirement across the target formations. Proper planning and analysis of dynamic pressures and accurate operational control allowed us to achieve the objectives of this challenging cementing job. Close collaboration between the operator and cementing service company was fundamental for the success of this job. Fluids selection, intensive software simulation, laboratory testing, proper execution, and final evaluation will be presented in this paper.
Kondo et al. (Tue,) studied this question.