Effective spacer design is a critical criteria for cement to develop an excellent bond between cement-casing and cement-formation. Applications of the spacer are mud displacement, mud cake removal, and to prevent cement contamination with drilling mud. Furthermore applications can be altering the formation and casing wettability, especially when drilling operations involves drilling with oil-based mud, where the production zone will be most likely oil-wet. Oil wet formation can reduce cement bonding with formation, which could lead to pressure communication between hydraulic fracture stages and poor zonal isolation. Therefore, the objective of the study is to investigate the impact of adding surfactants to cement spacer design under reservoir temperature, where the surfactants will help to mitigate the oil base mud impact on cement bond performance, reduce compatibility issues with drilling mud, change formation – casing wettability from oil-wet to water-wet, and reduce interfacial tension (IFT) between the spacer and oil-based mud to have a better mud displacement.
Core samples from Wolfcamp were used to conduct the study. The core samples were cut to small chips and age in (8.8 ppg, 12.5 ppg) oil-based mud and crude oil under reservoir temperature for ten days to mimic horizontal- drilled section of the oil well. Then, the chip's wettability was measured using the captive bubble method. After that, the chips were immersed in a spacer mixture of (water and surfactant). Three different surfactants were used to conduct the study (Anionic surfactant, Nonionic surfactant, and Zwitterionic surfactant). After the chips were exposed to the aforementioned spacers, the chip's wettability was measured again to examine the effect of surfactants on altering the wettability. Moreover, the interfacial tension (IFT) between the spacer and crude oil was determined. Oil-based mud and spacer compatibility tests were conducted to examine the mixability issue with the presence of surfactants in the spacer design. Additionally, mud removal efficiency using different surfactants was examined using beaker and rotor cleaning tests.
The results demonstrate the benefits of adding surfactants in the cement spacer design. Where spacer without surfactant showed compatibility issues particularly at high OBM density, having surfactants in spacer design resulted in an adjustment of the formation wettability from oil-wet to water-wet. Anionic surfactant showed the best performance among all the surfactant types that have been examined. Also, having surfactant in the spacer design will reduce compatibility issues between oil base mud and spacer significantly. Moreover, more surfactant volume will be required when displacing heavy oil-based mud to prevent compatibility problems. Regardless of the outperforming of non-ionic surfactants in alternating the chip's wettability and reducing IFT, remarkable compatibility issues were detected when non-ionic surfactants were added to the spacer recipe.
