The quantitative description of gas-cut degree in deepwater drilling

Abstract

Abstract Through the analysis of gas-cut features in deepwater drilling and shortages of existing gas-cut detection methods, the feasibility of early detection of gas cut at the bottom of riser was demonstrated, and a method was proposed for quantitative description of gas-cut degree in deepwater drilling based on ultrasonic monitoring at the bottom of riser. The problems of Doppler ultrasound gas-cut detection method was analyzed and the experimental device of gas-cut monitoring at the bottom of riser based on the ultrasonic transmission was built, which was used to analyze the sound attenuation characteristics under different conditions of void fraction. The solutions for using ultrasound to monitor gas-cut situation at the bottom of riser was proposed. Combined with the gas-liquid two-phase model of wellbore annulus in deepwater drilling and the formation pressure prediction method with credibility, the inverse calculation method of gas-cut degree in wellbore was established, which was based on the monitoring data of gas cut at the bottom section of riser. This method could detect the gas cut about four minutes in advance compared with conventional methods, and the gas cut occurring moment, the time left for gas to reach the wellhead, the total overflow rate at any moment, and the void fraction in different depth could be accurately determined based on the acoustic response data of the bottom of riser.