Gas Kick Simulation for Offshore Gas-Hydrate Reservoir Drilling

Abstract

Abstract In this research, a new gas kick simulator which incorporates the fully coupled heat and mass transfer model to describe the gas kick developing process, has been developed considering the unsteady gas–liquid–solid multiphase flow. Then, a series of numerical simulations are described to make a detail understanding of the gas kick developing process focusing on the variation of gas void fraction, pit gain and bottom hole pressure at various construction parameters. The numerical simulation results show that: fluid flow in annulus will change to gas–liquid–solid flow due to hydrate dynamic decomposition. What's worse, if there is no wellhead pressure control device, i.e. the wellhead pressure is equal to 0 MPa, the gas volume fraction and pit gain can reach 0.68 and 3.12 m3 at the wellhead, respectively. While, if the wellhead pressure increases to 0.5 MPa, gas fraction and pit gain only reach to 0.28 and 1.54 m3, respectively. Therefore, managed pressure drilling technology should be applied to hydrate reservoir drilling. On the other hand, if the mud inlet temperature is lower than 17.5 °C, which means a lower rate of hydrate decomposition, the gas volume fraction at wellhead can be safe and controllable. Compared with wellhead backpressure and inlet temperature, drilling fluid density has few effects on the gas void fraction in annulus. To sum up, in order to ensure safety and efficiency in hydrate layers drilling, a lower inlet temperature of drilling fluid and a higher wellhead pressure, such as 17.5 °C and 2 MPa, which mean a lower hydrate decomposition rate, can achieve the gas volume fraction at wellhead less than 10%. This paper provides some meaningful guidance in hydrate formation drilling process.