UNDERSTANDING THE PHENOMENON OF DISSOLVED GAS MIGRATION OF GAS IN RISER DURING DRILLING OPERATIONS

The prevention and control of gas kicks is a major concern in the petroleum industry during deepwater drilling operations. The problem is further aggravated when dealing with synthetic and oil-based muds (SOBM and OBM) that can dissolve a gas influx entering the wellbore. Due to the solubility of formation gases in drilling fluids, the gas cut mud resulting from gas absorption has a density lower than that of overlaying unsaturated drilling fluid. Lab scale experimental tests were conducted in order to understand whether buoyancy-induced convection and diffusion attribute to mass transfer of a dissolved influx. Experiments were performed on a low-pressure mass transfer apparatus using carbon-dioxide (CO2) and mineral oil to study the extent of mass transfer due to buoyancy induced convective flow and diffusion. Measurements were made on the axial distance travelled by the dissolved carbon dioxide and gas concentration over the length of a pipe by measuring the mass of gas accumulated in different test sections of the experimental apparatus. This arises due to a concentration gradient developed when contaminated fluid comes in contact with afresh column of drilling fluid. Experimentally obtained measurements made on the mass transfer coefficient are used to tune simulations carried out using a computational fluid dynamic (CFD) software - ANSYS Fluent. This enables us to replicate field scenarios to study the extent of well control issues that could arise when a gas influx enters the wellbore, even when circulation has ceased. Results obtained here can be used as a base case to understand a similar phenomenon occurring when extended to other fluid systems such as that of a natural gas influx in synthetic oil-based drilling fluids.