Microstructural Comparison of the Impact of Drilling Fluids Contamination and Casing Corrosion on Wellbore Cement Barriers

Gas leakage through wellbores is one of the major challenges in Oil and Gas Industry related to well cementing and wellbore integrity, whether post CO2 injection or during gas production in hydraulically fractured shales. The inadequate cement bond at the interfaces with formation and/or casing will result in failure of the cement sheath and poor zonal isolation causing hazard to the environment. The mains reasons for poor cement integrity are the drilling fluid contamination, as well as the pipe and casing string corrosions, as both of these create deposition of mechanically inferior and highly porous materials at interfaces. Furthermore, they are big issues in permanent plugging and abandonment as well. P&A requires longer-term isolation comparing to primary cementing. The objective of this paper was to compare the microstructural properties of wellbore cement samples impacted by the drilling fluid contamination versus metal pipe corrosion products. In this research, drilling fluid contaminated, and cement in contact with corroded metal pipe, were compared for changes in chemical composition, and internal structure. Oil-based, synthetic oil and water based types of drilling fluids were applied to the Class H cement slurries (16.4 PPG). Microstructural properties and spatial chemical element distribution were evaluated using Scanning Electron Microscope (SEM), Energy Dispersive X-Ray Spectroscopy (EDS), and Electron Probe Micro Analyzer (EPMA). The results showed that the presence of acid environment and drilling fluid cake would produce leaching of elements and fracture formation, which would result in compromised zonal isolation and wellbore integrity issues.