Improved Zonal Isolation in High-Temperature Offshore Wells with an Advanced Lightweight Cement Design - Gulf of Thailand Case Histories

Abstract

The Gulf of Thailand is characterized by shallow-water wells that are hot and deviated, with low fracture gradients and bottomhole static temperatures of 450°F (and some wells approaching 520°F). Drilling depths average 12,000 ft measured depth (9,000 ft vertical) and fast drilling practices are used for slimhole monobore completions, resulting in daily cementing jobs from each of the offshore rigs. Many jobs are executed in batch mode and are performed offline on the production deck under the rig floor. As a consequence these fast-paced, streamlined, and simultaneous offshore activities require a high degree of rig integration as well as logistically and operationally simple high-performance cementing systems to avoid expensive rig delays. The performance demands on the cement slurries include low density, uncomplicated on-the-fly mixing, low fluid loss, gas invasion control, and stability under the very hot wellbore conditions. This paper will present an advanced lightweight seawater-based cementing concept using a single cement blend with only 3 to 4 primary liquid additives (including a multifunctional polymer) to adjust all primary cement jobs for the entire wellbore. A sophisticated lab testing program (such as analyzing slurry gas invasion) was conducted for the innovative cementing concept according to the required demands on cement slurry design and the given harsh wellbore conditions in the Gulf of Thailand. Tests revealed that the developed cementing systems meet all operator and well requirements despite their relative low densities of 13.3 to 14.0 ppg with high water content. The advanced lightweight cementing design was successfully pumped in 311 Thailand offshore wells during 2012, and its performance was directly compared to previous cementing systems used in more than 1,000 wells in the same fields from 2009 to 2012. Lab test results, pre-job planning, cement job execution and cement bond logs are evaluated and discussed. The case histories conclude that the advanced lightweight cement design significantly improved the quality of zonal isolation in wellbores. Valuable design-effect relationship elements found in this study (such as the impact of the multi-functional polymer on slurry stability and gas control) will be discussed for applications on other upcoming and challenging drilling projects in Asia.