Maintaining zonal isolation for the lifetime of an oil and gas well much depends on the quality of the cement/mudcake/formation interfaces. In this study, we evaluated the properties of the mudcake/cement and mudcake/formation interfaces and their effects on the bonding strengths and sealing quality of the cement. As water-based drilling fluids, a clay/fresh water (cw), a clay/saltwater (cs) and a formate brine-based (fm) system were used. Synthetic ceramic core samples were employed to simulate reservoir rock. In the experiment, at first a radial mud filtercake was deposited on the core sample by circulating the respective drilling fluid at 40 °C (cw) and 110 °C (cs, fm), resp. and 70 bar pressure in a flow loop. The cores were then flushed with a biopolymer-based spacer, cemented and cured in an autoclave for 3, 8, 14 and 28 days and 2 and 6 months (cw: 70 bar: cs, fm: 300 bar pressure). Finally, the core samples were analyzed chemically, microscopically and with respect to their fracture properties.
The thicknesses of the filtercakes varied significantly (cw: 1 – 7 mm, cs: 6 – 7 mm, fm: < 3 mm) and were found to be independent of curing time. The properties of the filtercakes were mainly influenced by the fluid loss values from the drilling fluid and by the effectiveness of the spacer. In all systems, significant contamination of the cement with drilling mud, never vice versa, was observed. Low fluid loss of the mud produced thin filtercakes, less contamination of cement with mud and high adhesive strength. In adhesion strength testing, cracking always occurred at the cement/mudcake interface which appeared to be the weak zone. No evidence for migration of water (pore solution) between filtercake and hardened cement was obtained. Generally, the experiments demonstrate that spacer effectiveness is critical for efficient bonding and zonal isolation.