Effect of low OBM contamination on long-term integrity of API class H cement slurries – Experimental study

Drilling mud acts as an important primary barrier during the well construction phase, while the integrity of cement sheath is critical throughout the life cycle of the well. The mud-spacer-cement interactions highly affect the long-term integrity of the cement sheath and in other words the well integrity. Even today, the spacers are not capable of displacing all the drilling mud present in the wellbore before performing cementing operations and the cement slurry is contaminated with a small amount of drilling fluid and/or spacers. Oil based mud (OBM) is highly preferred over water-based mud (WBM) as a drilling fluid when drilling in challenging environments, but with current push for more environmentally drilling fluids the Water Base Muds (WBM) are preferred.

The detrimental effects of high OBM contamination (>5%) on the mechanical and rheological properties of API Cement slurries have been fairly studied in the previous decade. This study focuses on the strength development of low OBM contaminated (0.8%, 1.6%, 3.2%, 6.3%) API Class H cement slurries cured for up to 28 days at ambient temperature as well as the elevated temperature of 75 °C. The Ultrasonic Pulse Velocity (UPV) was measured by performing the non-destructive tests before performing the destructive tests which measure the actual Unconfined Compressive Strength (UCS). Based on the 141 samples tested in this study, even for 0.8% OBM contamination the strength of Class H cement samples cured for 7 days at 75 °C was reduced by 23%, and for 3.2% OBM contamination it was reduced by 39%. Novel correlations of UCS vs time and UCS vs UPV were developed to accurately simulate the downhole conditions and predict the long-term integrity of the wellbore cement. A reliable dataset for developing cement data repositories was established which also can be merged with existing databases.