Development and Application of a Self-Sealing, Corrosion Tolerant, and Resilient Cementing System for Long-Term Zonal Isolation

Abstract

Abstract An HTHP exploration well in a remote location offshore Malaysia required a special cementing system to address the challenges of aggressive CO2 and H2S gasses, as well as downhole temperature and pressure fluctuations due to well testing activities. The objective of the planned cementing job with the correspondingly developed slurry design was to ensure long term zonal isolation along the 7-in liner (2,373 m MD) with a total coverage of 526 m. To facilitate successful realization of exploratory and DST objective by eliminating any potential undesired crossflow, a self-sealing, corrosion tolerant, and resilient cementing system was developed and customized according to the given wellbore conditons of the upcoming job. The challange also involved carefully engineered well barriers in permanent Plug & Abandanment of this challenging CO2 rich well in HPHT conditions. This paper reviews the successful implementation of the corresponding cementing system starting from its development, through the job's planning phase, to the operational execution and evaluation with results. A pre-job study was executed to design a suitable cementing system, including thorough lab tests, CFD simulation for optimized fluid displacement efficiency, and cement sheath stress modelling to optimize the cements’ 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. In-situ expansion tests of the set cementing system under downhole conditons were performed with a unique test device. The cementing system's corrosion tolerance was evaluated after a 6 months exposure in a CO2 saturated aqueous environment. 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 the 7-in. liner cementing job for effective mud removal and cement slurry placement covering the desired >500 m annular length at temperatures up to 300 °F. Applying the self-sealing additive in the tested cement design 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 long term exposure test to CO2 with the following analysis demonstrated that the designed cementing system provides integrity towards corrosive fluids. 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 in annulus as well as in the cased hole through P & A plugs. The novel cementing concept combines several advanced design features (self-sealing in the presence of hydrocarbons, expansion after setting, corrosion tolerance, and resilient mechanical properties) that improve long-term zonal isolation in critical well scenarios.