Details of the interaction between natural gas and oil in drilling fluids currently not taken into account, will in extreme cases be significant for the safety of drilling and well control operations. The paper describes such effects, in particular time dependence (kinetics) and compositional PVT with dense phase included. The importance of validation and tuning of PVT calculations, even when using state-of-art tools, is demonstrated by integrating new methods in a well control simulator.
We consider sub-models for kinetics (time dependence of gas dissolution and boiling) and compositional PVT for the drilling fluid-natural gas mixture, and study different effects and assumptions numerically by integration in a well control simulator. Available laboratory data are used for model development and tuning of existing software. The dense phase may be important to consider in HPHT wells, where the conditions allow for the drilling fluid-gas mixture to exceed the critical point. This influences the gas absorption capability of the drilling fluid, as well as the density.
The paper illustrates the impact of kinetics and improved PVT calculations through a sensitivity analysis using realistic well and fluid data. Two specific base-oils, a refined mineral oil and a linear paraffin, are used in combination with methane gas. The simulations show how kinetic effects can be important in some cases, both for early interpretation of a kick and for the response seen at surface as gas approaches and enters topside equipment. Furthermore, it demonstrates that dense phase effects can be significant, and that even state-of-art PVT software requires tuning when used with new combinations of oil-base fluids and hydrocarbon gases. Although the effects discussed are small compared to safety margins for many wells, ignorance may cause drilling teams to run into severe risks without knowing in advance for other wells.
Combining advanced PVT models capable of representing dense phase behavior and a kinetics model with hydraulic flow modelling represents a leap forward in simulation of well control events. In addition, the importance of tuning adds valuable knowledge. These elements enable earlier detection and safer handling, thus increasing the safety on the rig.