Technical Committee Drilling and Completion Technology
A Study on Transport Phenomena and Adhesion between Cement, Mudcake and Formation and their Importance for the Sealing Quality of the Cementing of the Annulus
In this project the physico-chemical processes occurring during the interaction between cement, drilling mud filtercake and formation rock as well as the mechanical properties (adhesion strength of the cementing system) were studied utilizing three different drilling fluid formulations and cement compositions which are representative for German wells. As drilling fluids, a bentonite/fresh water (tw), a bentonite/saltwater (ts) and a formate (fx) system were used. Synthetic ceramic core samples were employed as simulated reservoir rock. At first, radial mud filter cakes were deposited on the core samples by circulating the individual drilling fluids at 40°C (tw) and 110°C (ts, fx), resp., and 70 bar pressure. The cores with the freshly prepared filter cakes were then flushed with spacer, cemented and cured in an autoclave for 3, 8, 14 and 28 days as well as 2 and 6 months at 70 bar (tw) and 300 bar (ts, fx) pressure, resp. Finally, the core samples were analyzed chemically, microscopically and with respect to the fracture properties. The thickness of the mud filtercakes varied (tw: 1 – 7 mm, ts: 6 – 7 mm, fx: < 3 mm), and was found to be independent of curing time. The thickness of the filtercakes was mainly influenced by the mud fluid loss values and the effectiveness of the spacer. In all systems, significant contamination of the cement with drilling mud, never vice versa, was observed. This contamination reduced adhesion between mud filtercake, cement and rock formation. At the cement/mud cake interface, micro cracks were found which present a weak point in the binding force between these two constituents. No evidence for migration of water transport between mud filtercake and hardened cement was obtained. However, a correlation between the absolute water contents present in both phases was observed.