Uncertainty in ultrasonic measurements of oil-based mud contaminated cement

Abstract

Cement is one of the barriers between formation fluids and wellbore and must withstand harsh downhole conditions, such as mechanical loads, high and fluctuating pressures and temperatures and chemical corrosion. The main indicator of cement integrity in the early stages of curing is its Unconfined Compressive Strength (UCS), which suggests how much force the cement sheath can withstand until it fails. Once the cement slurry hardens and attains a certain level of UCS, then drilling and other well operations can continue.Acoustic (i.e. ultrasonic) measurements of well cement quality are commonly used in the oil and gas industry to verify if cementing operations have been performed efficiently. In this study, over 100 experiments were conducted to check the reliability of ultrasonic measurements in poor quality wellbore cleaning situations. Comparisons were made between the UCS and the ultrasonic pulse velocity for three scenarios; class C cement without impurities, the same cement mixed with a certain quantity of oil-based mud (OBM), and cement subjected to thermal cycles.The results show that in the first period of the cement curing (between 8 and 24 h), when the cement bond logs are usually run, the Ultrasonic Pulse Velocity (UPV) and the UCS of the pure cement and the cement mixed with oil-based mud were almost identical. This implies that conventional acoustic methods will not detect the failure of wellbore cleaning situations and would evaluate the cement job as being successful, even if it is not. The long-term effects of an improper cementing could be disastrous for the lifetime of the well, as this study will highlight.