A Step Change in Cementing Mitigating Sustained Casing Pressure

Abstract

Wellbore construction practices are complex. Achieving dependable zonal isolation is a critical and challenging process to optimizing asset life and minimizing future well intervention. Sustained casing pressure challenges related to poor zonal isolation are well documented and can impact production that may require significant remedial well intervention. The need to address these challenges called for revisiting cementing operational practices and lead to the development of a Basis of Design (BoD) document as a tool to help manage well design practices and standards. As common industry practice to prevent fluid migration, slurry designs should be gas tight. To avoid unwanted wellbore fluids to migrate to surface, two things are required: less time to initiate migration and the lack of a flow path. With analysis of the current cement slurries, designs were unable to perform successfully on inflow tests due to low temperature of the zones to be cemented, increasing the transition time for a slurry to move from liquid state to solid phase. With extensive laboratory testing it was concluded that current designs were not addressing the bulk shrinkage phenomenon of cement, which could lead to the creation of micro annulus creating conduits for fluid migration. This paper will discuss the detailed analysis and testing of the current and new designs and techniques validated by laboratory tests and field executions (Cement Bond Logs) to prevent fluid migration and ensure that dependable long-term zonal isolation was delivered. Mud displacement mechanics needed to be optimized to reduce the risk of mud on the wall phenomenon. This included the design of spacers that increased the annulus mud displacement efficiency, improved standoff and finally optimum displacement rates to create sufficient annular velocity. A comprehensive look at all the pertinent steps in the construction of the well starting from the drilling phase, through cement job design, preparation and execution was required to ensure that the best practices are adopted to achieve the best results. Slurry selection, spacer formulation, centralization, hole cleaning and excess volumes were all at the center of the improvements that were necessary to achieve optimum results. A BoD document was developed as a roadmap for cementing to further enhance wellbore integrity. It formalized the planning, design and job execution practices and specified the slurry design, placement, and verification criteria for each casing section. Several cement jobs have been executed using the newly implemented practices that resulted in excellent zonal isolation results, which were verified through cement integrity logs. Since the implementation in all subsequent wells, no casing-casing annulus pressure issues have been reported.