Dynamical well-killing simulation of a vertical H₂S-containing natural gas well

Abstract

This work aims to explore the dynamical well-killing process of a vertical H2S-containing natural gas well. A dynamical well-killing model considering an H2S solubility was established to simulate the overflow and well-killing process of a vertical H2S-containing natural gas well. The mass and momentum equations of the coupled model were solved using finite difference method, while the transient temperature prediction model was solved using finite volume method. The coupled model was validated by reproducing experimental data and field data of Well Tiandong #5. The effect of H2S content, mud displacement, drilling fluid density, and initial overflow volume on the dynamical well-killing process of an H2S-containing natural gas well were obtained and analyzed in this work. Results showed that H2S will gasify near wellhead during well killing when casing pressure decreases. To balance the bottom hole pressure, when H2S releases, the casing pressure increases as H2S content increases. As initial overflow volume increases, the annular temperature, annular pressure and the casing pressure increase significantly. When H2S gasifies, the casing pressure applied at wellhead should be higher at lower initial overflow volume to balance bottom hole pressure. In the well-killing process, the annular pressure and temperature decrease as drilling fluid density increases and a lower casing pressure is needed for balancing bottom hole pressure. The casing pressure is lower at a higher displacement for higher friction resistance. Besides, as well-killing displacement increases H2S will gasify at an earlier time. When drilling for H2S-containing natural gas well, early detection of gas kick should be more frequent to avoid severe overflow. Besides, higher displacement and density of drilling fluid should be considered to avoid stratum fracturing and prevent leakage accidents under the premise of meeting drilling requirements.