Buoyancy induced convection of riser gas in deepwater drilling operations

Riser gas migration has been an area of interest since the last three decades due to its importance in gas handling in deepwater drilling operations. A previously conducted full-scale test at LSU Petroleum Engineering Research & Technology Transfer (PERTT) well facility indicates, as contrary to traditional belief, significant migration of dissolved gas taking place even when circulation had ceased. In order to understand whether a reduction in density of the underlying contaminated mud resulting from gas absorption is the contributing factor to the above-mentioned phenomenon, a computational fluid dynamics (CFD) analysis was conducted to study the transport of the gas influx while in solution due to buoyancy induced convective mass transfer and simultaneous diffusion. Simulations performed in this study include the hydrodynamics of the upward moving gas cut mud and simultaneous mass transfer of natural gas into the under-saturated drilling fluid. The parameters studied here are the distance traveled of the gas cut mud and saturation levels in the drilling fluid along the length of the riser. The dense phase behavior was shown to have considerable effects on gas loading capacities which in turn affected the density of the gas cut mud, and at pressures upwards of 5,000 psi, the solubility of a natural gas influx can be seen to be infinite in certain synthetic and oil-based drilling fluids. The rate and extent of mass transfer are dependent on drilling fluid density gradients, which in turn are based on gas influx saturation level. Results obtained from this study can help better comprehend migration phenomena of a dissolved influx in oil-based muds in a riser with the BOP shut in and when circulation has ceased.