Multiphase flow modeling of gas intrusion in oil-based drilling mud

Abstract

As the oil-based drilling mud is widely used in the development of oil and gas in deep water, how to model gas intrusion in the oil-based drilling mud accurately becomes a key issue for well control. An unsteady-state multiphase flow model considering gas dissolving and separating out is developed in this work, which includes both saturated oil-based drilling mud condition and unsaturated oil-based drilling mud condition. In this work, under low gas intrusion rate condition, the oil-based drilling mud is unsaturated at the bottom annulus and becomes saturated near the wellhead. The mass concentration of dissolved gas in oil-based drilling mud is proposed in the new model to describe gas dissolving and separating out in the wellbore. A corresponding numerical solution method is built. With an enriched data bank, the new gas solubility correlation is developed by using the nonlinear least square method and the average predicted error decreases from 24.1% to 9.3%. Groups of gas solubility experiments are performed to validate the new correlation and discrepancies between predicted results and experimental values are 5.22%, 8.06% and 4.33% for three types of oil-based drilling muds. The influences of dissolved gas on pit gain, bottom hole pressure and gas distributions along wellbore are studied according to the field case. The gas dissolving results in the pit gain increasing slowly, the bottom hole pressure decreasing slowly, the cross-sectional void fraction decreasing and the gas transportation speed lowering down. Compared with water-based drilling mud, the moment of variants of pit gain is delayed and the response time for gas kick becomes short, which further indicates that the gas kick in the oil-based drilling mud is hard to be detected.