Dissolution of Light Hydrocarbons in Drilling Muds, Prediction of the Nature of Reservoir Fluids Based on Gas Shows

The development of real-time measurement techniques to be used on field while drilling is of major interest for oil companies. Such techniques not only provide more data to assess the potential of a reservoir or well, but are also very cost effective as they do not require stopping the drilling. As such, Gas While Drilling enables to measure content of the light hydrocarbon of the drilling on field and then through an analysis procedure, to assess the characteristic of the well or reservoir. The aim of this project is to understand and model the link between the reservoir fluid encountered at the bottom-hole and the related gas show that will appear at the surface. The background of the project is presented in Chapter 1. Creation of a thermodynamic model has required an advanced study of the phase equilibria between water (as a basis for the widest spread muds) as well as liquid and gas hydrocarbon phases in wide temperature and pressure ranges: from reservoir to atmospheric conditions. A comprehensive collection of experimental data on water-hydrocarbon equilibria was created, and several most widely applied equations of state were tested for this purpose. The results of the analysis of the available experimental data and modelling the hydrocarbon-water equilibria are described in respectively Chapter 2 and Chapter 3. After initial stage of collecting the information and describing the process in terms of governing physical mechanisms, it has been decided to restrict further modelling with the simplest thermodynamic model, involving different phase equilibria between the mud and hydrocarbon phases under different thermodynamic conditions corresponding to propagation of the fluids from a well bore to a gas trap. Other mechanisms (kinetics of dissolution, adsorption, complex hydrodynamics etc.) were assumed to be of a lesser importance for the “zero-order” description of the process. The scheme of the GWD process was developed, involving two- and three-phase equilibria, which should have been computed on the basis of the selected thermodynamic models. This scheme was implemented into BEST, the in-house process simulator of Total and is presented in Chapter 4. The model developed contains the three adjustment parameters accounting for our incomplete knowledge of the processes occurring in the well bore (Alpha_1), between the well head and the gas trap (Alpha_2), as well as inside the gas trap (Alpha_3). In the course of the project, it was uncovered that part of the gas might be transferred in the mud, being not in the dissolved state (as was initially assumed), but as micro-bubbles stabilized by natural surfactants present in the mud. In order to verify presence and importance of such micro-bubbles, an industrial experiment was carried out. Although this experiment indicated possibility and importance of the micro-bubbles, their quantitative inclusion into the model requires a further study. This information was important for proper determination of the coefficient Alpha_2, as shown in Chapter 5. Analysis of the experiment with the two gas traps carried out in the Eni E&P (cf. Chapter 6) has provided important information about the value of Alpha_2 responsible for degassing in the gas trap. The created software was tested by comparison with available industrial GWD data. The process of testing was highly interactive, since in the course of the comparison necessity for more data had become necessary, and simultaneously the model was changed and adjusted. Due to roughness of both the data and the model, only the qualitative agreement between them was possible to be achieved for some most well documented cases. In sum, the project has resulted in an important advance in understanding and modelling of the GWD process. For more detailed and more precise modelling, further work is required.