A pilot study on time-dependent dissolution of CO2 in oil for prediction of gas kick behaviors in non-aqueous fluids

The influx of gas from formations during drilling or when the well is left undisturbed during tripping, logging, and flow check can dissolve very quickly in Non-aqueous drilling fluids (NAF). The dissolved gas can stay unnoticeable till the gas comes out of solution below bubble point pressure closer to surface. The objective of the paper is to develop a model to predict the time dependent mass transfer of CO2 in oil at sub-critical pressures and validate the model using experimental results. Since CO2 is soluble in oil, the interaction between solvent and solute can help us understand the dissolution and mass transfer mechanism of CO2 in oil. A model has been developed by incorporating factors that drive the interaction and the rate of gas loading into the liquid to predict the time-dependent mass transfer. A 1.5 inch vertical low pressure apparatus is used to conduct experiments by injecting CO2 into pressurized static column of oil. Pressure inside the pipe, and mass of CO2 injected are varied to study their effects on mass transfer. Boundary conditions for this model are provided from experimentally obtained data of volumetric mass transfer coefficient of the injected gas and liquid system at gas injection flow rate. The developed time-dependent model has been validated using the data collected from the tests. The volumetric mass transfer coefficient is found to change with pressure. This model can be extended to experiments under high-pressures to replicate the downhole conditions. The model can be modified to include desorption to predict the loading and unloading of gas in NAF, and gas oil ratios at depths along the annulus in a real well.