First Implementation of a Resilient and Self-Sealing Cementing System to Optimise Long-Term Zonal Isolation in Challenging Offshore Wells - Review of 3 Case Histories in Malaysia

Abstract

Three consecutively drilled, highly deviated, high-temperature offshore wells in Malaysia with multiple thin payzones and planned close perforation distances required the use of a self-sealing cementing technology with optimized mechanical properties to minimize the risk of unwanted fluid communication that would necessitate immediate abandonment. This paper reviews the successful implementation of the corresponding cementing system starting from its development, through the jobs' planning phases, to the operational executions and evaluation with results. A pre-job study was executed to design a suitable cementing system, including thorough lab tests, CFD simulations for optimized fluid displacement efficiencies, and cement sheath stress modelling to optimize the cement mechanical properties. A novel HPHT multi-function test cell (MFTC) was used to measure in-situ ability of fractured cement specimens to seal hydrocarbon flows under the given simulated downhole conditions. Yard tests were conducted to ensure quality and field applicability of the engineered cementing system. With the help of the simulations and lab tests, the mud-spacer-cement fluid trains could be optimized for fluid friction and density hierarchies during each 7-in. liner cementing job for effective synthetic oil-based mud removal and cement slurry placement covering the desired >1,000 m annular length in highly deviated wells with low frac gradients at temperatures up to 300 °F. Combining the self-sealing additive with a mineral fiber in the tested cement designs significantly reduced the Young's modulus of the set cement while the tensile strength and the Poisson's ratio improved, resulting in a higher resilience of the cement sheath to survive the anticipated downhole stresses from the completion program. The crack-and-seal tests demonstrated the cement system could repeatedly seal cracks or micro-annuli up to at least 0.15 mm (0.006-in.) due to the swelling of the self-sealing additive in the presence of a hydrocarbon flow at differential pressures below 1,000 psi and temperatures up to at least 300 °F. The expansion of the set cement was also optimized to enhance the bond to formation and pipe. The evaluation of the pre-job planning, job execution as per program, post-job simulated pressure matches, positive ultrasonic cement logging results, and the fact that no issues have been observed during or after the job indicated well-isolated intervals. The novel cementing concept combines several advanced design features (self-sealing in the presence of hydrocarbons, expansion, resilient mechanical properties) that improve long-term zonal isolation in critical well scenarios.