Experimental investigation of deterioration in mechanical properties of oil-based mud (OBM) contaminated API cement slurries & correlations for ultrasonic cement analysis

Well construction, well abandonment and restoration of well integrity are the reasons at a broad level why cementing jobs are performed. During well construction phase, the primary objective of cementing job is to replace the drilling mud in the annulus with pure cement. Intermixing of the drilling mud and the cement slurry produces unpumpable mixture, especially in the case of OBM. As literature review suggests the displacement efficiency of spacers and pre-flushes used to displace the drilling mud is not 100%, cement slurries are contaminated with drilling mud left behind the casing. Recently, several research groups have performed studies to understand the strength development phenomenon of OBM contaminated cement slurries, but they failed to document properly the experimental procedures used and correlations for the strength development of OBM contaminated cement slurries. The primary aim of this study was to develop a standard laboratory experimental procedure for understanding the strength development of OBM contaminated cement slurries and develop correlations for future references. This extensive study was carried out on API Class C cement samples cured at room temperature and elevated temperature. Destructive as well as non-destructive tests were performed on the contaminated as well as uncontaminated cement samples to measure the uniaxial compressive strength (UCS) and the ultrasonic pulse velocity (UPV) respectively. Results obtained from these tests showed deterioration in the strength of cement samples as the OBM contamination increases. Correlations for early strength development of OBM contaminated cement slurries were developed using the test results. These correlations developed would help the operators in better estimation of the strength developed by the OBM contaminated cement slurries. In other words, better estimation of weight on cement (WOC) time using the more accurate strength development correlations.