Model for calculating the gas volume fraction of a gas-cut wellbore through natural gamma-ray logging

Well blowout – caused by the formation fluids invading a wellbore because of the imbalance of the bottom-hole pressure and formation pressure – is the most hazardous accident in deepwater drilling and well completion engineering. Blowout accidents can be effectively prevented through early gas-cut detection, but detection of gas invasion is challenging. The earlier the gas cut is detected, the smaller the gas loss is. Conventional gas-cut detection methods are based on monitoring the drilling parameter changes at the wellhead; however, because of the large distance between the wellhead and the well bottom, detection through this approach is not timely. In this study, a theoretical model for gas-cut detection through natural gamma ray (NGR) logging is established in which the gas-cut annulus equivalent diameter as well as the influence of a gas cut on the liquid–gas spatial distribution and the absorption capacity of the drilling fluid are considered. This model, which can predict the real-time gas void fraction, was validated experimentally: on observing the variation in NGR logging under various gas-cut conditions, the NGR logging value was found to increase with the increase in the gas void fraction. Moreover, the experimental and predicted void fractions varied by less than 8% on average, evidencing that the model can accurately predict the gas volume fraction in a wellbore after gas cut. The proposed model is expected to serve as a strong foundation for the further development of early gas-cut detection technology.