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The catastrophic events faced by the Oil and Gas industry in the past depict the importance of maintaining the integrity of the well. The cement acts as a crucial barrier throughout the life cycle of the well. The contamination of the cement occurs due to inefficiency in cementing practices and operations. Experimental investigations have been done on the reduction in mechanical properties of different API class cement considering contamination with water-based mud and oil-based mud. This study focuses on analyzing the changes in mechanical properties of API Class C cement on varying the following parameters: OBM contamination (0%, 0.6%, 1.1%, 2.2%, 4.3%) Curing time (4 hrs, 6 hrs, 8 hrs, 1 day, 3 days, 7 days) Temperature (25˚C, 75 ˚C) API recommendations were followed for preparing the cement slurries. The destructive, as well as non-destructive tests were carried out on the cement samples at ambient room temperature to measure the uniaxial compressive strength (UCS) for OBM contaminated class C cement slurries. The general trend observed is that the UCS increases with an increase in curing time and temperature. UCS decreases with an increase in OBM contamination. Logarithmic trends were obtained for UCS vs curing time for different contaminations at a given temperature. Exceptions were observed at lower curing times where contaminated samples showed better results than the neat cement slurries. These observations play a critical role in understanding contaminated cement behavior. This widespread work was carried out only on API Class C cement to provide reliable data for future references. The correlations presented in this paper will help operators estimate the deterioration in mechanical properties of Class C cement in the presence of low OBM contamination. Email: nachiket.arbad@ou.edu & cteodoriu@ou.edu

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.