The Effect of Casing-Pipe Roughness on Cement-Plug Integrity

Abstract

Summary In permanently abandoned wells, cement plugs serve as a well-barrier element that is essential for providing long-term zonal isolation. Poorly plugged wells might provide leakage pathways that pose risks to the surrounding environment. It is well-known that the final quality of set cement is influenced by the cement placement, but the influence of downhole-pipe roughness to the hydraulic sealing of a cement plug has not been specifically investigated. This paper presents an experimental study assessing the cement-plug sealing of neat- and silica-cement systems placed in pipes with three different levels of surface roughness. The experimental test uses a small-scale laboratory setup, consisting of a test cell filled with a cement plug, which can simulate realistic wellbore curing and operating conditions. By subjecting the cement plug to gas differential pressure, the setup detects any gas leak through the cement plug and measures the leak rate. As a complement to the experimental study, a computational-fluid-dynamics (CFD) simulation is conducted to study fluid flow around the cement plug. Experimental tests detect an instant gas leak in all samples at low differential pressure (<0.01 bar) through leak sources at the cement/steel interface, presumed to be microannuli. Furthermore, experimental results discover that placing cement plugs in rough-surface pipes could reduce the gas-leak rate, and the most significant reduction was found from samples cured in the roughest pipe. The CFD simulation results, along with pressure-buildup-curve observation, show that the leak path of samples placed in rough-surface pipes tends to be tortuous and less connected, thereby adding resistance for gas to flow.