Cement Plug Sealing Studies of Silica Cement Systems

A cement plug is widely applied for permanent abandonment phase to provide long-term zonal isolation against fluid flow. Maintaining cement plug integrity is a challenging task, and loss in cement sealing poses risks to the surrounding environment and surface safety. It is well-known that the cement performance is affected by cement material and downhole conditions. Nevertheless, investigations linking these influencing factors with the sealing of cement plugs are still limited, especially with the lack of proper equipment in the past. In the present work, a small-scale laboratory setup has been constructed to test the sealing ability of a cement plug. It has unique features that can simulate plugging operations at the downhole conditions and preserve the cement curing condition. By testing using this setup, it is possible to measure the minimum differential pressure required for gas to flow across the cement plug and the gas leak rate. The silica cement mixture was selected as the plug material, prepared using silica flour. Investigation of silica cement under the influence of expanding agent additive and various curing temperature was carried out. It was found that adding an expanding agent improved the sealing of cement plugs. Moreover, samples cured at a high temperature were less resistant to gas flow with the leak path observed at the cement/steel interface, indicating debonding.