Downhole detection of gas kick using low-frequency elastic wave: Multiphysics modeling and its implications

Gas kick is a common and high-risk drilling trouble in geo-energy engineering, especially for deep-water drilling and ultra-deep well drilling. The most favorable way of early detection of gas kick is to move the detection from the wellhead or near surface to the downhole. In this paper, a new conception of downhole detection of gas kick using low-frequency elastic wave was put forward. The framework of coupling the multiple physic processes of gas-liquid two-phase annulus flow, bubble migration and low-frequency elastic wave propagation during gas kick was developed. The characteristic responses of low frequency elastic wave to gas kick were quantitatively analyzed based on multiphysics modeling under the ideal case and different levels of background gas (BGG), respectively. The modeling results indicated that: (i) The velocity decreases cliff-like and the attenuation coefficient increases significantly at the gas-kick bubble front. The change magnitudes of the velocity and the attenuation coefficient increase with the gas intrusion rate, and decrease with the pump displacement. The change magnitude of attenuation coefficient decreases with mud density, while the increase of velocity is not sensitive to the mud density. (ii) For the ideal case of the vertical well, the effective detection within the depth section of (2014–2568) meter could identify gas kick about (14.3–29.2) minutes earlier than the conventional pit-gain method. The gas kick could be identified earlier when the deviation angle is smaller and/or the monitoring point goes deeper. (iii) The changing magnitudes and rates of the velocity and the attenuation coefficient decrease with the BGG. The downhole detection of gas kick using low-frequency elastic wave should be reliable when the BGG is lower than 5%. The effects of rock cuttings, phase transition of the gas, and drill-string vibrations should be further investigated in future study.