A new method for early gas kick detection based on the consistencies and differences of bottomhole pressures at two measured points

摘要

Early gas kick detection is an important measure to ensure well control safety. In accordance with the fluctuation of the bottomhole pressure during early gas kick, the gas kick can be detected rapidly and early with downhole measurement tool. At present, erroneous diagnosis often occurred when gas kick detection was performed using pressure fluctuations of a single measured point. In this paper, firstly, considering the water hammer effect, a wellbore pressure fluctuation model at the initial stage of gas kick was established to accurately describe the transient annular pressure variations. The model was solved using the composite difference scheme and was verified using a laboratory gas kick experiment. Secondly, a dual-measured-points (DMP) method for early gas kick detection was presented based on the consistencies and discrepancies of bottomhole pressures at two measured points. The distance between two measured points was optimized. Simulation results indicated that the annular pressure at the measured point increased and periodically changed with time in the initial stage of gas kick, and then declined approximately linearly after the gas-liquid interface reached the measured point. The distance between the two measured points was mainly dominated by the borehole diameter, and increased with the decrease of borehole diameter. The gas kick confirmation time (GKCT) obtained using DMP method was 2-7 min and the corresponding gas kick confirmation volume (GKCV) was 0.3-0.5 m(3). The GKCT diminished with the increase of the formation permeability and bottomhole negative differential pressure, and with the decrease of the borehole diameter and well depth.