Overcoming the Challenges for Cementing Salt Zones in Deepwater Top Hole Drilling in Brazilian Pre-Salt Wells: A Well Containment Barrier Approach

Abstract

When a surface casing is set in front of salt zones, challenges such as hole enlargement and salt dissolution need to be addressed in the cementing design. This study compares the several possible solutions for drilling and cementing in deepwater top hole drilling with exposed salt formation, with the general objective to provide valuable insight on the relevance of cement integrity on well containment analysis in deepwater wells. Several studies reported in the literature have evaluated the different approaches that industry follows on drilling and cementing top hole sections when salt formations are present. In addition to a review on these practices, through a detailed analysis of field data from hundreds of wells in Santos basin pre-salt wells, we identify the main aspects of cement sheath quality and its ability to withstand well containment loads from cement slurry formulation to the drilling practices and geological conditions. From data in real scale, a new and robust design and its field results are presented. From the analytical study of several possibilities, it is shown that well geometry aspects - such as inclination, tortuosity, and hole enlargement – shall attend minimum requirements to obtain the desired cement sheath quality. These requirements were able to be established through extensive field data analysis and simulations for cementing in different conditions. When those requirements are not achieved, even the highest efforts on the sole cementing design does not mitigate the loss of well integrity. Another set of important conclusions are concerned the cementing slurry design and centralizer selection and placement: salt concentration in cement slurry, bond strength between salt formation and cement slurry stands as the key characteristic and the salt-based cement slurry with this property provided the best for specific field results. Since the industry is continuously discussing the best drilling and cementing practices on salt formations, this paper provides compiled results from a large field dataset on top hole drilling in deepwater wells. Furthermore, both theoretical and applied engineering considerations on the salt-based cement slurry design, its mechanical properties, and the placement environment (hole diameter, geology, and casing centralization) are addressed to bring on a discussion on general guidelines for cement sheath quality in well containment analysis.