PRIMARY MIGRATION BEHAVIOR OF HYDROCARBONS - FROM LABORATORY EXPERIMENTS TO GEOLOGICAL SITUATIONS THROUGH FLUID-FLOW MODELS

Primary migration is a complex process involving coupled fluid generat ion and mass transfer in low permeable rocks. Joint experimental and m odelling work have been performed in order to better understand this p roblem. Eventually, experimental results have been modelled and extrap olated to geological conditions. Maturation and expulsion experiments have been carried out with water saturated immature Type II source roc ks from the Paris Basin, with sample weights up to 3 kg. Temperatures were stepwise increased from 20 degrees C to 285, 330 or 380 degrees C at constant hydrostatic pressures of respectively 17 or 55 MPa during up to 5000 h. During the experiments, generated fluids expelled out o f the heating cell and were sampled prior to a complete analysis. Mass balances were checked on hydrocarbon gases, liquid hydrocarbons, CO2 and water. Meanwhile, closed system pyrolysis experiments were perform ed on isolated kerogen extracted from the same source-rocks. They allo wed us to derive a compositional kinetic cracking reaction network. It was then possible to tackle the expulsion of fluids out of the source rock: the kinetic scheme provides a guideline to understand the gener ation of fluids from kerogen; the expulsion experiments provide an ins ight into the expulsion mechanisms. No axial stress has been applied t o the samples in the experiments. Therefore, the influence of rock com paction, due to effective stress, was not studied in these experiments . A numerical model of expulsion has been set up based on volume and m ass balance conservation equations in the source rock. The volume is a ssumed to vary as solid kerogen transforms to liquid products and soli d residue. The mass of each compound is kept in balance, as it is crea ted or destroyed through primary and secondary cracking and expelled t hrough multiphase fluid flow. Therefore, depending on pressure, temper ature and composition, fluids in the source rock are split into severa l phases, following a thermodynamical equation of state; all mobile ph ases flow according to Darcy's law. Experiments were modelled, the bes t fit being achieved with the following conditions: all fluids, includ ing water, CO2 and hydrocarbons are allowed to form a multiphase mixtu re; the heavy compounds (resins and asphaltenes) are in a single very viscous phase and are less easily expelled, as most of them are cracke d inside the source rock. Extrapolations to geological pressure and te mperature conditions were performed afterwards, in order to check the phase behaviour and the expulsion efficiency.